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Full text of "Electroplating; a treatise on the electro-deposition of metals"

ELECTROPLATI NG 



A TREATISE ON THE 

ELECTRO-DEPOSITION OF METALS 

WITH A CHAPTER ON METAL-COLOURING 

AND BRONZING 



BY 

WILLIAM R. BARCLAY, A.M.I.E.E. 

SILVER MEDALLIST, CITY AND GUILDS OF LONDON INSTITUTE 
LECTURER ON ELECTROPLATING IN THE UNIVERSITY OF SHEFFIELD 

AND 

CECIL H. HAINSWORTH, A.M.I.E.E, 

ASSISTANT LECTURER IN ELECTRICAL ENGINEERING IN THE 
UNIVERSITY OF SHEFFIELD 



ILLUSTRATED 



LONDON 

EDWARD ARNOLD 
1912 

[All rights reserved] 



3 



PREFACE 

THIS book has been written primarily as a handbook for the 
practical electroplater, in the hope that the modern practical 
man will also be or will become, at least in some degree, a 
scientific student, for the days of " rule- of -thumb " are 
quickly passing. In our opinion it is essential that a 
practical book on such a subject as the present, to be of any 
real value, shall be written from the scientific standpoint. 

The art of the electro-deposition of metals arose in the 
scientist's laboratory, and its growth was fostered largely by 
the patient work of trained scientific experimentalists ; 
nearly all the important improvements that recent years 
have witnessed have resulted directly or indirectly from 
theoretical research, and it is not too much to say that the 
hope of the future lies almost entirely in this same direction. 

On the other hand, it has not been sought to produce 
either a purely scientific treatise or a laboratory manual 
these can be obtained if desired ; the authors have, in these 
pages, endeavoured to combine, along with a simple exposi- 
tion of theoretical principles, the results of their practical 
experience. 

While it is indisputably true that no book can take the 
place of workshop training and practice, it is also true that 
the man who would be a master of the art of electroplating 
must possess considerable workshop experience, and at the 

282167 



IV 



PREFACE 



same time be a thorough student of the scientific principles 
upon which it rests. Such a man, to quote the late Prof. 
Wm. James,* " need have no anxiety about the upshot of his 
education. ... If he keep faithfully busy, ... he can with 
perfect certainty count on waking up some fine morning to 
find himself one of the competent ones of his generation." 

In its general plan the book is framed partly on the 
courses of lectures delivered to students in the technical 
classes of the University of Sheffield, and partly on the 
syllabus in electro-metallurgy of the City and Guilds of 
London Institute. On these lines the book would be 
inadequate unless some account of the elementary principles 
of electrical engineering were included, and an endeavour 
has been made to present in a concise manner an outline of 
the electrical principles involved, together with an explanation 
of the terms used in connection with them, so that the 
whole subject may be better understood. It is hoped, there- 
fore, that such matter will assist students and workers in 
the art of plating, and help to render as easy as possible 
what appears from our experience to be a thorny part of the 
subject. 

The various formulae recommended for solutions and 
directions for carrying out processes of electro -deposition are 
in nearly all cases those which are in actual use in workshop 
practice and are not merely laboratory experiments. 

The scope of the work does not permit of a very full 
treatment of the science of electro-chemistry, and those who 
wish to go more deeply into this fascinating and rapidly 
developing branch of science are referred to larger works, 
such as Dr. Allrnand's "Principles of Applied Electro- 
chemistry." f 

* " Talks to Teachers on Psychology " (Longmans), p. 78. 
t London : Edward Arnold, 1912. 



PREFACE v 

We have to acknowledge gratefully the assistance 
rendered in various ways by our colleagues Dr. Turner, and 
Messrs. G. B. Brook, F. Mason, and F. W. Bissett. 

We are also indebted to Messrs. W. Canning & Co. ; the 
D.P. Battery Co., Ltd. ; the Chloride Electrical Storage Co., 
Ltd., for the use of blocks illustrating types of cells and 
plant ; and lastly, to our friend Mr. E. H. Crapper for his 
kindness in reading the proofs. 

Finally, we should like to take this opportunity of acknow- 
ledging the debt of gratitude which we both owe to our old 
teacher, Mr. Byron Carr, the first and for over twenty 
years lecturer on electroplating in the former Sheffield 
Technical School, now the Department of Applied Science 
of the University of Sheffield. 

W. E. B. 
C. H. H. 
THE UNIVERSITY, 

DEPARTMENT OF APPLIED SCIENCE, 

ST. GEORGE'S SQUARE, SHEFFIELD. 

October, 1912. 



CONTENTS 

CHAPTER PAGE 

"- I. FUNDAMENTAL CHEMICAL PRINCIPLES ... 1 

- II. FUNDAMENTAL ELECTRO-CHEMICAL PRINCIPLES . . 15 

- III. FUNDAMENTAL ELECTRICAL PRINCIPLES ... 29 

- IV. QUANTITATIVE ELECTRO-DEPOSITION . . ..61 

- V. PRIMARY AND SECONDARY CELLS .... 74 
VI. THE DYNAMO 98 

~ VII. PLANT USED IN ELECTROPLATING . . . .117 

- VIII. PREPARATORY PROCESSES 147 

IX. THE DEPOSITION OF SILVER 172 

X. THE DEPOSITION OF GOLD 217 

XI. THE DEPOSITION OF COPPER 244 

XII. THE DEPOSITION OF NICKEL 270 

XIII. THE DEPOSITION OF IRON AND COBALT . . . 297 

XIV. THE DEPOSITION OF ZINC AND CADMIUM . . . 309 

XV. THE DEPOSITION OF LEAD, TIN, AND ANTIMONY . 325 

XVI. THE DEPOSITION OF PLATINUM AND PALLADIUM . 337 
XVII. THE DEPOSITION OF BRASS AND OTHER ALLOYS . 344 

XVIII. FINISHING PROCESSES 359 

XIX. METAL-COLOURING AND BRONZING .... 366 

APPENDICES 

1. THE ASSAY OF SILVER VOLHARD'S METHOD . . . 383 

2. WEIGHT OF DEPOSIT ON SILVER-PLATED ARTICLES . . 384 



viii CONTENTS 

PAGE 

3. CALCULATION OF THICKNESS OF ELECTRO-DEPOSITS . . 385 

4. To ASCERTAIN CAPACITY OF PLATING VAT . . . 386 

5. TESTING POLARITY AND DIRECTION OF CURRENT . .386 

6. FIRST- AID IN CASES OF POISONING 387 

7. METRIC SYSTEM OF WEIGHTS AND MEASURES . . . 390 

8. WEIGHTS AND MEASURES 391 

9. USEFUL DATA 392 

10. SPECIFIC GRAVITIES OF METALS 393 

11. SOLUBILITIES OF COMMON SUBSTANCES IN WATER . . 393 

12. PROPERTIES OF SOLID COPPER CONDUCTORS . . . 394 

INDEX 395 



ELECTROPLATING 

CHAPTER I 

FUNDAMENTAL CHEMICAL PRINCIPLES 

THE study of the science and practice of electroplating 
and the deposition of metals must, like the study of all 
other branches of applied science, begin with the funda- 
mental facts relative to matter and force, and the theories 
which have been deduced from these facts. 

Matter, Changes of Matter, Force. We are all 

more or less familiar with the changes which matter and 
by matter is meant everything which possesses " mass," i.e. 
bulk and weightis continually undergoing, and also with 
the effects which are being produced by " force." Force, of 
course, is invisible, but we know of its presence by its 
effects on matter in the way of changes of one kind or 
another. Scientists usually regard the changes which 
matter undergoes as of two kinds, physical and chemical, 
though it must be said that the dividing line between the two 
is often not at all distinct; indeed, some physical change 
always accompanies a chemical change. 

Water, in its varied forms, furnishes a familiar and very 
good example of these changes of matter. We know water 
under three distinct conditions, (a) in its normal state as 
a liquid, (b) in the form of ice a solid, (c) in the form of 
steam gas or vapour. Under each of these conditions it is 
absolutely different in form and appearance, yet always the 



ELECTROPLATING 



same in ultimate composition, external conditions of tem- 
perature and pressure determining under which of the 
three conditions it shall exist. These changes are purely 
physical there is no alteration in the essential nature of the 
substance. 

On the other hand, if we pass an electric current through 
water (slightly acidified to render it conductive), and per- 
form the experiment in a suitable apparatus, we shall find 
that we are able to change the water slowly into two gases, 
which subsequent experiment would show to be quite 
different in their properties, both from each other and from 
water itself in any of its forms. Here we have a chemical 
change, a change which the student will at once observe to 
be of a different character to the physical changes previously 
illustrated. 

It will be observed, moreover, that the "forces " at work 
in these changes are entirely different. Physical or, as they 
are sometimes termed, " mechanical " agencies, such as heat 
or pressure, alter the form or appearance or position of 
substances, whereas chemical forces alter the essential com- 
position of the substances. The latter are called "forces" 
by analogy from the former, but are really quite distinct in 
nature. It must, however, be borne in mind, as has been 
previously pointed out, that these changes often merge into 
each other, and it is often difficult, if not impossible, to 
draw a sharp line of distinction. Forces which are purely 
physical, such as are due to heat, often induce or bring 
about chemical action, and therefore chemical change. 

Constitution of Matter. All substances found in 
nature may be divided into two classes, " elements " and 
" compounds." 

Elements are those substances such as oxygen, hy- 
drogen, copper, silver, mercury, gold, carbon, etc., which 
have never yet been decomposed or split up into any other 
kind of matter. This class, however, is also subdivided 
usually into two sections, metals and non-metals. 

The principal characteristics of the first section are, 



FUNDAMENTAL CHEMICAL PRINCIPLES 3 

that they are good conductors of heat and electricity, that 
as a rule they are fairly malleable and ductile substances, 
i.e. they can be hammered or rolled into sheets and drawn 
into wire ; with the one exception of mercury they are all 
solid bodies at ordinary temperatures and pressures, and as 
will be observed later they all act as cations when under- 
going electrolysis (see page 24). 

The non-metals are extremely varied in their character- 
istics ; conversely to the metals they are comparatively poor 
conductors of heat or electricity, and from an electro- 
chemical point of view act oppositely to the metals in 
electrolysis. The element hydrogen, however, forms an 
exception to this rule. 

The principal metals are aluminium, antimony, bismuth, 
copper, gold, iron, lead, magnesium, manganese, mercury, 
nickel, platinum, silver, tin, and zinc. 

The principal non-metals are chlorine, fluorine, hydrogen, 
nitrogen, and oxygen (all gases at ordinary temperatures 
and pressures), bromine (a liquid), and carbon, iodine, phos- 
phorus, and sulphur (solids). 

Compounds are substances composed of two or more 
elements, or, in other words, substances which can be split 
up into other kinds of matter, as, for example, common 
salt (into sodium and chlorine), water (into hydrogen and 
oxygen), copper sulphate (into copper, sulphur, and 
oxygen). 

Modern science regards matter under all conditions, 
whether solid, liquid, or gaseous, as being made up of 
innumerable particles of two orders or types, to which re- 
spectively the names " atoms " derived from a Greek term 
meaning indivisible particles and "molecules "signifying 
" little heaps " have been given. 

The atom is denned as the smallest particle of matter 
which can take part in a chemical change, and atoms 
usually exist in a state of chemical combination with other 
atoms, either of the same or of some other kind. Molecules 
are particles usually of a larger order, and consist as a rule 



4 ELECTROPLATING 

of more than one atom. The molecule is denned as the 
smallest particle of matter which can exist in a free state, or 
perhaps better, the smallest particle in which the original 
properties of any substance are retained. For example, if 
one drop of water was taken and divided, and subdivided, 
until a point was reached where further division was quite 
impossible so long as the substance was still to possess all 
the chemical properties of the bulk, we should then have 
arrived at the molecule. If, however, chemical forces were 
brought to bear, this molecule could be again divided, but in 
this case it would be into the essential constituents of the 
substance, or, in other words, into atoms of the elements 
hydrogen and oxygen respectively. 

This view of matter is due directly to an Italian chemist 
Avogadro (1811), but it is also the indirect outcome of 
what is known as the atomic theory, which, though originally 
propounded more than two thousand years ago, is really due, 
so far as modern chemistry is concerned, to John Dalton 
of Manchester (1808). It had already been observed that 
substances which could be decomposed into other sub- 
stances, or kinds of matter, were of invariable composition. 
Water, for instance, when decomposed, was always found to 
consist of eight parts by weight of oxygen, and one part by 
weight of hydrogen. From this and many other similar 
facts, Dalton argued that matter must be made up of atoms, 
or minute particles, having always the same relative weight 
or mass. This theory affords an explanation of the funda- 
mental principle of chemical science, termed "the law of 
definite proportion" which means that wherever a chemical 
change occurs in matter, whether it be a separation or a 
combination, the relative weight of material liberated or 
used up is a definite quantity, and is always the same for 
every particular substance. One example, that of water, 
has already been cited ; another simple illustration is that of 
hydrogen chloride, which when decomposed is always re- 
solved into 1 part of hydrogen, and 35-5 parts of chlorine, 
by weight. Conversely, whenever these substances are 



FUNDAMENTAL CHEMICAL PRINCIPLES 5 

brought together and chemical forces applied, they always 
combine in these proportions, to form hydrogen chloride. 

A further principle of almost equal importance is that 
named the law of multiple proportions. Though the combina- 
tion or separation of compounds is always definite, yet in 
many cases the same elements can combine in more than 
one proportion to give rise to other kinds of matter. 
Hydrogen and oxygen, for example, when combined in 
the proportion by weight of 1 to 8 form water; if, how- 
ever, these two elements are combined in the proportion 
of 1 to 16, which under certain conditions can be done, an 
entirely different compound results, viz. hydrogen peroxide. 
It is indeed a fairly common occurrence in nature, that the 
same elements combine in different proportions, and give 
rise to different compounds or forms of matter. But Dalton 
showed that these proportions have always a simple relation- 
ship or ratio to each other ; e.g. if a certain element, A, is 
found to combine with a fixed weight of a second element, 
B, in more than one proportion, the different weights of A 
which so combine always bear to each other a simple arith- 
metical ratio, such as 1 : 2 or 1 : 3, and so on. That is, 
the combining weights are simple multiples of one another. 

This principle may perhaps be made clear by reference 
to two well-known elements, oxygen and nitrogen, which 
combine in five different proportions, giving the compounds 
shown below : 



Name. 


Parts by Weight. 


Formula. 


Patio of 
N toO. 


N 





Nitrous oxide . . . 


28 


16 


N,O 


7:4 


Nitric oxide . . . 


14 


16 


NO 


7:8 


Nitrogen trioxide . . 


28 


48 


NA 


7:12 


Nitrogen peroxide 


14 


32 


N0 2 


7:16 


Nitrogen pentoxide . 


28 


80 


N 3 5 


7:20 



It will be observed from the last column, that the re- 
spective ratios of oxygen to each of the other members of 



6 ELECTROPLATING 

the series are as 1:2:3:4:5. These facts and a vast 
number of similar ones can best be explained, so far as our 
present knowledge goes, by the atomic theory of matter, and 
its assumption of the existence of the minute particles termed 
atoms and molecules already referred to. 

Chemical Symbols, Formulae and Atomic Weights. 
The atomic theory, in addition to being an aid to some 
understanding of the chemical changes in matter, has led 
to the introduction of a system of symbols, which enables 
these changes to be readily expressed both qualitatively and 
quantitatively. A chemical symbol, often the first or first and 
some other letter of its English or Latin name, has been 
assigned to every element, and as all substances are either 
elements or combinations of elements, we are enabled to 
express briefly the composition of any substance by means 
of these symbols, e.g. the letter H represents hydrogen, 
O oxygen, Ag, silver (Latin, argenium), Hg, mercury (Latin, 
hydrargyrum), K, potassium (Latin, Jcalium), Na, sodium 
(Latin, natrium). 

These symbols have not only a qualitative but a quanti- 
tative meaning. Though it is impossible at present to assign 
an absolute weight to any atom, it is possible by the study 
of the compounds of atoms to determine their weight rela- 
tively to each other. This has been done, and a system of 
relative weights of the elements has been compiled, known 
as atomic weights. 

Up to recent years hydrogen, as the lightest known 
element, was taken as unity, and the weights of all other 
atoms were regarded as so many times that of hydrogen. 
Eecently, however, it has been found, that more exact values 
can be obtained by taking oxygen, to which an atomic weight 
of 16 is given, as the standard of comparison. In the 
accompanying table of atomic weights, this standard has 
been adopted. On this basis of comparison hydrogen is 
slightly above unity, being 1-008, but for practical pur- 
poses round figures are usually taken as given in the last 
column. 



FUNDAMENTAL CHEMICAL PRINCIPLES 



TABLE I. 

LIST OF THE COMMONER ELEMENTS WITH THEIR SYMBOLS AND 
ATOMIC WEIGHTS. 

Oxygen = 16. Hydrogen = 1-008. 



Element. 



Symbol. 



Atomic Weight. 



SECTION I. METALS. 



Aluminium .... Al 27 '1 

Antimony Sb 120-2 

Arsenic j As 74*96 

Barium Ba 137-37 

Bismuth Bi 208-0 

Cadmium i Cd 112-4 

Calcium ! Ca 40-07 

Chromium .... Cr 52-0 

Cobalt Co 58-97 

Copper I Cu 63-57 

Gold j Au 197-2 

Iron i Fe 55-84 

Lead Pb 207-1 

Magnesium .... Mg 24-32 

Manganese .... Mn 54'93 

Mercury Hg 200-6 

Nickel ' Ni 58-68 

Palladium i Pd 106-7 

Platinum Pt 195-2 

Potassium i K 39-1 

Silver I Ag 107'88 

Sodium J Na 23-0 

Tantalum ! Ta 181-5 

Tin I Sn 119-0 

Zinc Zn 65*37 

SECTION II. NON-METALS. 

Bromine j Br 79-92 

Carbon I C 12-0 

Chlorine / .... I Cl 35-46 

Fluorine i F 19-0 

Hydrogen i H 1-008 

Nitrogen ! N 14-01 

Oxygen j 16'0 

Phosphorus .... P 31-04 

Silicon Si 28*3 

Sulphur S 32-07 



Usual Value taken. 



27 
120 

75 
137 
208 
112 

40 

52 

59 

63-5 
197 

56 
207 

24 

55 
200 

59 
107 
195 

39 
108 

23 

181-5 
119 

65 



80 
12 
35-5 
19 
1 

14 
16 
31 
28 
32 



8 ELECTROPLATING 

If therefore the symbols described above be regarded as 
representing one atom of the particular element thus 
identified, it will be readily understood that a symbol, in 
addition to indicating the nature of a substance, indicates its 
relative weight. For example, the symbol O not merely 
implies oxygen, but one atom or 16 parts by weight of 
oxygen. By grouping these symbols, therefore, the composi- 
tion of any substance may be expressed, thus : H 2 O (water) 
means hydrogen 2 atoms or 2 parts by weight, oxygen 1 
atom or 16 parts by weight. HC1 (hydrogen chloride) means 
hydrogen 1 atom or 1 part by weight, chlorine 1 atom or 35 -5 
parts by weight. Symbols grouped in this way are known 
as molecular formulse, and represent of course the composition 
of the molecule. The small figures at the right-hand lower 
corner of a symbol signify the number of atoms of that 
particular element. 

The foregoing examples are fairly simple, but some mole- 
cules are much more complex, ammonium sulphate, for 
example, being represented thus, (NH 4 ) 2 SO 4 . In this in- 
stance two elements, one atom of one and four atoms of the 
other, nitrogen and hydrogen respectively, are placed in 
brackets and a small figure 2 immediately follows at the 
right-hand lower corner and outside the bracket ; this implies 
that these two elements form a small group, so to speak, 
inside the molecule, and the formula NH 4 is to be multiplied 
by 2 to arrive at the total number of atoms included in 
this group; in addition the molecule also contains 1 
atom of sulphur and 4 of oxygen. One molecule of 
ammonium sulphate therefore contains in all 2 atoms of 
nitrogen, 8 atoms of hydrogen, 1 atom of sulphur, and 4 atoms 
of oxygen. 

It will thus be seen that by means of its formula and a 
knowledge of atomic weights, the percentage composition by 
weight of any substance may readily be determined. A 
larger figure placed immediately before a symbol or group of 
symbols and on a level with them signifies the number of 
molecules. Thus 2H 2 represents two molecules of water, 



FUNDAMENTAL CHEMICAL PRINCIPLES 9 

2HC1, two molecules of hydrogen chloride, 2(NH 4 ) 2 SO 4 , two 
molecules of ammonium sulphate, and so on. 

Molecules of elementary substances contain different 
numbers of atoms. In the commoner elements they often 
consist of two atoms, and are written down thus : oxygen O 2 , 
hydrogen EL, chlorine CL, etc. The following well-known 
elements have only one atom in the molecule : potassium, 
sodium, cadmium, mercury, and zinc, and are therefore 
written K, Na, Cd, Hg, and Zn respectively. In many cases 
of elements the molecular formula is unknown. 

Chemical Equations. Placed in the form of an equa- 
tion, the symbols explained in the foregoing paragraphs, are 
exceedingly useful in expressing chemical changes. For 
example, the equation, 

2Na -f C1 2 = 2NaCl 

denotes that sodium and chlorine have combined or will 
combine to form sodium chloride, and that this combination 
must take place in the proportion of 46 parts by weight of 
sodium, and 71 parts by weight of chlorine, or 39'3 per cent, 
of sodium, and 60-7 per cent, of chlorine. 

Again Zn + H 2 S0 4 = ZnSO 4 + H 2 . 

The complete meaning of this equation is, that 65 parts 
by weight of zinc added to 98 parts by weight of sulphuric 
acid * (hydrogen 2 -f sulphur 32 -f oxygen 64 = 98) produce 
161 parts of zinc sulphate and two parts of hydrogen. The 
figure obtained by adding up the atomic weights of all the 
atoms forming a molecule is known as the molecular weight 
of the substance. The figure 98 is therefore the molecular 
weight of sulphuric acid, while similarly 161 is that of zinc 
sulphate. It may be advisable to point out that the sign -f 
on the left-h&nd. side of an equation signifies that a chemical 
action has taken place between the two or more substances 
thus connected. 

The extreme usefulness of these equations will be evident 
* In aqueous solution only, however, is this reaction correct. 



io ELECTROPLATING 

as the student proceeds ; by them we are enabled to make 
the most exact calculations regarding the composition of 
any solution used in the electro-deposition of metals, and 
also to express the results of the decomposition of these 
solutions by means of electricity. 

Acids, Salts, and Bases. Compound substances are 
often classified by chemists under three headings, (1) acids, 
(2) salts, (3) bases. 

(1) Acids are usually defined as compounds containing 
hydrogen, from which the hydrogen can be displaced by a 
metal (only, however, in the presence of water). Hydrogen 
is consequently a necessary constituent of an acid, though 
it must be understood that all hydrogen compounds are not 
necessarily acids. 

The following are some well-known acids, and the 
equations accompanying them will show how they may be 
decomposed and made to yield up their hydrogen. 

Hydrochloric acid (HC1) 2HC1 + Zn = ZnCL + H 2 
Sulphuric acid (H 2 S0 4 ) H 2 S0 4 + Zn = ZnS0 4 + H 2 
Nitric acid (HNO 3 ) 2HNO 3 + Mg = Mg(NO ;5 ) 2 + H, 

(N.B. The usual reactions between nitric acid and a 
metal result in the liberation of hydrogen and oxygen 
together forming water. Magnesium is an exception.) 

Acids have the power of turning blue litmus (a well- 
known vegetable compound) red, and this fact furnishes a 
very useful test for the presence of acids. 

(2) Salts are compounds similar in molecular type to the 
acids, and indeed differing from the latter only in the fact 
that the hydrogen is replaced by a metal. The compounds 
shown on the right-hand side of the above equations are 
" salts," the usual definition of a salt being : A compound 
resulting from the reactions between acids and the oxides, 
or hydroxides * of metals, or the metals themselves. 

(3) A base is the term usually given to the oxides and 

* The hydroxide of a metal is its combination with HO. 



FUNDAMENTAL CHEMICAL PRINCIPLES n 

hydroxides of metals, or to any substance having the power 
of neutralizing an acid to form a salt. Examples : 

(Potassium hydroxide) (Hydrochloric acid) (Potassium chloride) (Water) 

KHO + HC1 KC1 + H,O 

(Copper oxide) (Sulphuric acid) (Copper sulphate) (Water) 

CuO + H 2 SO 4 = CuSO 4 + H 2 O 

(Silver oxide) (Nitric acid) (Silver nitrate) (Water) 

Ag 2 + 2HN0 3 = 2AgNO :5 + H 2 O 

It must be observed, however, that the word "base," as 
applied above to oxides and hydroxides, is not literally 
accurate, inasmuch as something is lost from the composition 
of the so-called " base " which is not found in the salt, viz. 
the oxygen or the HO combination, which, as will be 
observed, combines with the hydrogen of the acid to form 
water. Some authorities therefore contend that the word 
" base " should be confined to ammonia, and substances like 
ammonia which really form the base of a salt, and do not 
lose anything, thus 

(Ammonia) (Hydrochloric acid) (Ammonium chloride) 

NH, + HC1 = NH 4 C1 

Valency or Quantivalence. It has been previously 
observed that hydrogen was originally regarded by chemists 
as a standard to which the weights of all the other elements 
are relative. This is so in a sense other than that of atomic 
weights only. An element is said to have a certain 
" equivalent " or equivalent weight, and this is not necessarily 
its atomic weight, though it is always either that or some 
simple ratio thereof. The equivalent of any element may be 
defined as the proportion by weight which combines with or 
replaces one part by weight of hydrogen. Hydrogen is here 
taken as the standard, since it enters into combination in 
smaller proportions by weight than any other element. 

Taking water again as an illustration, we find that 
oxygen combines with hydrogen in the proportion of 8 
to 1 (H 2 O =, in round figures, hydrogen 2, oxygen 16). 



12 ELECTROPLATING 

Therefore oxygen is said to have an equivalent of 8 ; in this 
case the equivalent of the element is half its atomic 
weight. 

The ratio of the atomic to the equivalent weight is known 
as the valency, or " quantivalence " of the element, and may 
be briefly expressed in the following formula : 

atomic weight 
equivalent weight ~ 

Substituting the figures in the example just quoted of 
oxygen, we have therefore 

i = 2 = valency of oxygen. 

In some cases, as has been indicated, the atomic and 
equivalent weights are equal ; for example, chlorine combines 
with hydrogen in equal atomic proportion, thus H 2 -f- C1 2 
= 2HC1. Similarly sodium replaces hydrogen in equal 
atomic proportion, 2Na -f 2HC1 = 2NaCl + H 2 . Obviously, 
therefore, the numbers which represent the atomic weights 
of chlorine and sodium also represent their equivalents, i.e. 
35*5 and 23 respectively. The application of the above formula 
would thus give 1 as the valency. These elements are 
consequently known as univalent. 

Oxygen, on the other hand, is bivalent, while similarly 
elements which have the power of combining with, or 
replacing 3 parts of hydrogen, have a valency of 3, and are 
known as trivalent. These three classes embrace the majority 
of the commoner elements, but there are a few which have 
valencies of four, five, and even six, and are called quadri- 
valent, quinquivalent, and sexvalent respectively. 

Table II. gives the usual valencies of the commoner 
elements. 



FUNDAMENTAL CHEMICAL PRINCIPLES 13 



TABLE II. 

THE USUAL VALENCIES OP THE COMMONER ELEMENTS. 



Valency. 


1. 3. 


3. 


4. 


5. 


6. 




Bromine 
Chlorine 


Barium 
Cadmium 


Aluminium 
Antimony 


Carbon 
Iridium 


No common 
elements 


Molybdenum 
Osmium 




Fluorine 


Calcium 


Bismuth 


Platinum 


have a 


Tungsten 




Hydrogen 


Cobalt 


Chromium 


Silicon 


usual 






Iodine 


Copper 


Gold 




valency of 






Potassium 


Iron 


Nitrogen 




five, but 






Silver 


Lead 


Phosphorus 




occasion- 






Sodium 


Manganese 
Mercury 






ally the, 
following 








Nickel 






show this 








Oxygen 






valency 








Palladium 






Antimony 








Sulphur 




( 


Bismuth 








Tin 






Nitrogen 








Zinc 






Phosphorus 





It must be clearly understood, however, that many of 
the elements in the above table are capable of appearing in 
more than one class. Copper, for instance, is usually a 
bivalent element, but occasionally it enters into combinations 
which are of the univalent class. Thus in cupn'c oxide 
(CuO), one atom of copper replaces two hydrogen atoms in 
the corresponding hydrogen compound, ILO ; here, therefore, 
it possesses its usual bivalent quality. Another oxide of 
copper, cuprous oxide, happens, however, to be known as 
existing, having the formulae Cu.,0, and in this case it is 
obviously univalent, the copper atom being equivalent to one 
hydrogen atom only. 

There are, of course, some elements which do not either 
combine with or replace hydrogen directly. In these cases, 
however, the equivalents have been determined indirectly, 
by observing their replacing power relatively to some other 
element, which has a direct action upon hydrogen. 

This subject of valency possesses great significance from 
the electro-chemical point of view, as will be shown later. 



i 4 ELECTROPLATING 

Laws of Conservation. Two great laws of matter, 
the truth of which has been recognized as the result of long 
and patient scientific research, must now be mentioned and 
briefly explained. 

The first of these is the law of The Conservation of Mass. This 
law is a broad generalization based on experience, which means 
that in all changes of matter, whether it be a combination of 
elements, or a decomposition of compounds, no mass is either 
gained or lost. All that can happen in any such change or 
series of changes is a rearrangement of atoms or molecules. 
Matter cannot be either created or destroyed. 

The second of these laws is that of The Conservation of 
Energy. This is another broad generalization, confirmed 
by innumerable experiments, meaning simply that energy 
can neither be created nor destroyed. Its form may be 
changed. It may have been stored up for ages, and then 
liberated to manifest itself in some other form. It may 
exist in one place as heat energy, and from this form it may 
be changed to electrical energy, and again in turn to 
chemical energy, but its quantity remains exactly the 
same ; throughout any number of such changes, it neither 
increases nor diminishes. 



CHAPTER II 

FUNDAMENTAL ELECTRO-CHEMICAL 
PRINCIPLES 

Chemical and Electro-Chemical Action. In the study 
of all chemical changes of matter it is essential to bear 
in mind that no such changes can be effected without the 
aid of chemical force and also of energy in some form or 
other. This fact becomes especially evident to the electro- 
metallurgist or electro-plater, whose chief study is necessarily 
the decomposition or separation of chemical compounds. In 
all cases of chemical change there is evidence that energy is 
being either expended or developed. This is shown by the 
absorption or evolution of heat in many ordinary cases of 
chemical combination or decomposition when there is no 
question of electrical causes or effects. In the majority of 
cases of elements combining to form compounds there is an 
evolution of heat, and energy is being developed or liberated. 
'Hence, in order to decompose these compounds when they 
are formed, as much heat, or a corresponding quantity of 
energy in some other form, must be applied or expended. 

The form of energy which the electro-depositor or electro - 
plater applies for this object is electrical, but the work 
actually done is chemical ; hence the term " electro-chemical 
action." 

A simple case of the electro-deposition of a metal from a 
solution of one of its compounds, will furnish an illustration 
of this action and assist the reader in grasping this most 
important principle. 

Suppose a depositing vat, containing a solution of copper 



16 ELECTROPLATING 

sulphate (CuSOJ, is connected up, in a manner which will 
be explained later, to the connecting wires or " leads," as 
they are sometimes termed, of a dynamo. The copper 
sulphate solution is, as a result of the passage of electricity 
from the dynamo, decomposed, and metallic copper is 
deposited. Now it will be fairly obvious that in this case 
electrical energy is delivered into the vat by the dynamo at 
work. The energy thus delivered is in part expended as an 
equivalent of the heat energy originally evolved when copper 
sulphate was formed by the union of Cu and SO 4 , and it is 
only by virtue of this that deposition or liberation of metallic 
copper takes place. 

As will presently appear, exactly the same result can be 
brought about by using means other than the dynamo for 
producing electrical energy, and at this point it will be 
convenient to study briefly the action of a simple voltaic cell, 
using it as an illustration of electro-chemical action and the 
inter-convertible nature of energy. Such a cell is constructed 
by immersing two plates of zinc and copper respectively in a 
dilute solution of sulphuric acid and water. Pure zinc is not 
soluble in dilute sulphuric acid (though impure zinc is 
exceedingly so), but if a sheet of pure zinc and a sheet of 
copper are both immersed in a vessel containing dilute 
sulphuric acid, and a metallic connection is made between 
the two sheets (Fig. 1), it will be observed that while there is 
no apparent action at the surface of the zinc, the liquid itself 
is decomposed, and a large number of small bubbles of 
hydrogen gas collect on the surface of the copper. If also 
the zinc sheet was carefully weighed at the beginning and at 
the end of the experiment, it would be found that it had 
lost weight ; part of it therefore must have been dissolved. 
The resulting action may be described thus : 

Zn + H 2 S0 4 = ZnSO 4 + H 2 

which is obviously an instance of chemical change. The 
agency by which it is brought about is, however, electrical, 
for on investigation by means of suitable apparatus, it 



ELECTRO-CHEMICAL PRINCIPLES 



would be found that both the metals concerned were in a 
special state, which is described by saying that they are 
" electrically charged," one (the zinc) negatively, the other 
(the copper) positively, and when a complete circuit was 
established through the liquid and through the connecting 
wire, that an electric current passed from the copper to the 
zinc outside the liquid, and conversely inside the liquid, as 
indicated by the arrows in the diagram (Fig. 1). In this 
experiment, we have illustrated the decomposition of 
sulphuric acid into H 2 and SO 4 by means of electrical action, 
and the energy required 
is produced by the com- 
bination of the zinc with 
the SO 4 group or " radicle," 
as it is termed, of sulphuric 
acid. 

But now it must be 
pointed out that not all 
the energy so produced is 
taken up by the simple 
decomposition of sulphuric 
acid. It will be noted that 
the connecting wire be- 




Dilute 

ric 
Acid 



FIG. 1. Simple voltaic cell 



tween the two plates becomes heated considerably. Some 
part, therefore, of the generated energy is occupied in 
producing heat. Now, this spare energy, as it may be termed, 
can be utilized, and may indeed take the place of the dynamo 
in the illustration previously used. To demonstrate this, 
dissolve in another glass cell (similar to that in Fig. 1) a few 
crystals of copper sulphate. This cell will now correspond 
to the copper sulphate vat previously referred to. Immerse 
in it a strip of copper, and (say) a strip of brass, which have 
been cleaned by dipping in dilute nitric acid. Disconnect the 
connecting wire between the zinc and copper in the cell used 
in the last experiment, and connect in a similar manner the 
zinc of this cell to the brass strip in the second cell ; take 
another wire and connect the copper strips in each cell 

c 



i8 



ELECTROPLATING 



together. We have then the arrangement shown in 
Fig. 2. 

The action now observed in the cell containing the zinc 
and copper will be similar to that found to occur in the 
former experiment, and no action will, at first, be observable 
in the other cell. After a few minutes, however, if the 
strip of brass be taken out of the solution and examined, it 
will be found that the whole of the surface which has been 
immersed in the copper sulphate solution, is coated with a 
fine salmon-pink coloured deposit of copper. In this 




Add 

FIG. 2. Simple voltaic cell connected to copper depositing cell. 

experiment, electrical energy has been generated in the 
first cell, and utilized not only in this cell to decompose 
sulphuric acid, but in the second cell to decompose copper 
sulphate (CuS0 4 ), thus liberating the copper and depositing 
it upon the brass strip. The original loss of energy, in the 
form of heat, undergone by copper when combining with 
SO 4 to form copper sulphate, is now restored by applying 
electrical energy, with the result that the copper is recovered 
in its original metallic condition. 

It is evident, however, on consideration of the law of the 



ELECTRO-CHEMICAL PRINCIPLES 



" Conservation of Energy," that an indispensable condition 
of such action is that the amount or quantity of electrical 
energy thus applied must be at least equal to or slightly in 
excess of the amount of energy evolved in the formation of 
the original compounds. Much research has been done in 
the direction of determining quantitatively the amount of 
heat evolved by the elements in thus combining to form 
compounds, and it is now possible to assign to them, what 
may be termed a general order of activity in this respect, 
those at the top of the list evolving a greater number of 
heat units in their combinations than those below. Such an 
arrangement of the commoner metals is given in Table III. 

TABLE III. 

THE COMMONEE ELEMENTS AEEANGED IN OEDEE OF THEIE ACTIVITY 
OF COMBINATION AS SHOWN BY EVOLUTION OF HEAT ENEEGY. 



Combinations ivith 



Oxygen 
0. 


Chlorine. 
Cl. 


Bromine 
Br. 


Iodine 
I. 




Magnesium 


Magnesium 


Potassium 


Potassium 




Calcium 


Potassium 


Sodium 


Sodium 




Sodium 


Sodium 


Calcium 


Calcium 




Potassium 


Calcium 


Aluminium 


Aluminium 




Aluminium 


Aluminium Zinc 


Zinc 




Zinc 


Zinc Cadmium 


Cadmium 




Iron 


Cadmium 


Lead 


Lead 




Cadmium 
Cobalt 


Lead 
Iron 


Copper 
Gold 


Copper 
Gold 




Nickel 


Cobalt 








Lead 


Nickel 








Copper 


Mercury 








Mercury 


Silver 








Silver 


Copper 










Gold 







As will be observed from the typical compounds shown 
above, the order varies slightly according to the nature of 
the compounds formed, some elements having what may be 
termed a special aptitude for forming particular salts. The 
general order, however, is only departed from within com- 
paratively narrow limits. 



20 ELECTROPLATING 

The practical meaning of this feature of chemical com- 
bination is, that wherever two or more combinations of 
elements are possible in any action or series of actions, that 
in which the greatest amount of heat energy is evolved will, 
as a general rule, be effected first. 

In addition, metals occupying a leading position in the 
above arrangement, have usually the power of replacing 
elements lower in the list, in any particular compound ; as 
a consequence they liberate the latter and often deposit 
them in a metallic condition. For example, if a strip of 
metallic zinc is placed in a solution of copper sulphate, the 
heafc energy evolved in the combination Zn -f- SO 4 being 
higher than that of Cu + SO 4 , the zinc will dissolve and 
form ZnSO 4 , and as a consequence metallic copper will be 
liberated on the surface of the zinc immersed, thus 

2Zn + 2CuS0 4 = 2ZnSO 4 + 2Cu. 

A similar result will be obtained, if iron is used instead of 
zinc. This principle is the basis of all the " simple immer- 
sion " processes for the deposition of metals to which refer- 
ence will be made later. Effects of this order may also be 
obtained in the case of fused or melted substances, as well 
as with substances dissolved in water. Silver may, for 
example, be readily liberated from fused silver chloride, by 
placing in the chloride a few small pieces of metallic zinc, 
according to the equation 

Zn + 2AgCl = ZnCL 2 + 2Ag. 

The Electro-chemical Series. In electro-chemistry 
another arrangement of the elements is made, which has 
great practical importance in the deposition of metals. This 
is known as the Electro-chemical Series, being an order or 
arrangement of the metals showing how they are electrically 
related to each other, when placed in solutions which have 
the property of conducting electricity. It will have been 
observed in the experiment illustrated in Fig. 1, that when a 
circuit was completed, the electric current passed inside the 
liquid from the zinc to the copper. This result naturally leads 



ELECTRO-CHEMICAL PRINCIPLES 



21 



us to consider the current as originating at the zinc. If, 
therefore, we consider a flow of electricity as analogous to a 
flow of water, which for present purposes we may do, then 
we may legitimately consider the zinc as being as it were 
at a higher level or, as it is termed, at a higher potential (see 
Chap. III.) than the copper. Similarly, if any other pair 
of unlike metals were placed in sulphuric acid as the con- 
ducting liquid, it would be found in all cases where a current 
was produced, that one metal was at a higher potential than 
the other. 

TABLE IV. 

ARRANGEMENT OF THE PRINCIPAL ELEMENTS IN ELECTRO-CHEMICAL 

SERIES. 



/Potassium 


/Sodium 




Calcium 




Magnesium 




Aluminium 




Manganese 




Zinc 




Iron 




Cadmium 




Cobalt 




Nickel 




Lead 


Positive , 


Tin 


Elements \ 


Bismuth 




Copper 




Silver 




Mercury 




Palladium 




Platinum 




Iridium 




Gold 




Hydrogen 




Antimony 




Carbon 




, Arsenic 



Negative 



\ ALBOlllU 

\Phosph 

Iodine 

Bromine 

Chlorine 



orus 



Elements ^ Nitrogen 
Sulphur 
. Oxygen 



In this order, any single 
element is electro- 
negative to any one 
placed above it, and 
positive to any below 
it. 

Negative elements are, 
in electrolysis, always 
given off at the anode, 
or positive electrode. 
Positive elements are 
given off at the 
cathode. 



22 ELECTROPLATING 

Experiments of this nature have been made, with the 
result shown in Table IV., in which the principal metals are 
placed in such an order that if any two of them are taken, 
the current will flow within the cell from the higher to the lower, 
the higher metal being termed electro-positive, and the lower 
electro-negative. It must be clearly understood, however, that 
the terms " electro-positive " and " electro-negative " are only 
relative. Thus, if two metals are taken almost from the middle 
of the list, e.g. gold and tin, although both are considered 
electro-positive, yet the lower one, gold, would necessarily 
be electro-negative to the other if placed in the same solu- 
tion. As in the case of the table of heat evolution, which, as 
might be expected, the present table closely resembles, the 
order varies slightly with different solutions, but the general 
arrangement holds good for most solutions. 

Electrolysis. Terms employed in connection there- 
with. Electrolysis is the term used to describe the opera- 
tion of decomposing by electricity any substance, whether in 
solution or in a state of fusion (i.e. molten), and in this con- 
nection other terms are used which may here be defined and 
explained. 

(a) Electrolyte is the term applied to substances dissolved 
in a liquid undergoing decomposition, or to any liquid which 
can be decomposed by electricity. All liquids or solutions 
may be divided into two classes, electrolytes and non- 
electrolytes. The former are conductors of electricity, and 
during conduction are decomposed. The latter class in- 
cludes liquids that either do not conduct electricity at all, 
such as oils, paraffin, turpentine, etc., or, if conductive, are 
not decomposed, such as mercury. 

(#) Electrodes are the plates or conducting mediums, by 
means of which electricity enters or leaves an electrolyte. 
That which is at the higher potential and by which the 
current enters is termed the ANODE, that which is at a lower 
potential and by which the current leaves is termed the 
CATHODE. 

(c) Ions, unions and cations. The meaning and use of 



ELECTRO-CHEMICAL PRINCIPLES 23 

these terms will be understood, by a brief consideration of 
the chief points of the theory of electrolysis as given in the 
following section. 

The Theory of Electrolysis. It may be said that the 
distinguishing feature of electro-chemical or electrolytic 
action, as contrasted with chemical action, is that the pro- 
ducts of the former only appear at the surface of the elec- 
trodes, the anode and cathode respectively, whereas the 
products of the latter action permeate the entire mass. In 
order to explain this fact and other phenomena of electro- 
lysis, the molecules which make up an electrolyte are re- 
garded as existing, at least partly, in what is termed a 
" dissociated " condition, i.e. they are not simply molecules 
in the mere chemical acceptation of the term, nor even 
atoms, but particles endowed with a special nature, by 
reason of which they are called " ions " a term due origin- 
ally to Faraday, and derived from a Greek word meaning 
" moving " or " going." 

The nature of the difference may be explained by an 
example. For instance, when crystals of copper sulphate 
are dissolved in water an electrolyte is formed, and when 
the solution is complete, it is assumed that some of the 
molecules of the salt, CuSO 4 , become dissociated into what 
may be termed a metallic part or radicle, and an acid part 
or radicle, the word " ion " being applied to both. It is 
obvious, however, that Cu and S0 4 respectively do not exist 
merely as chemical individvals. " Cu " is the chemical 
symbol for metallic copper. " SO 4 " is a compound of sulphur 
and oxygen, which is not known to exist in a free state. The 
ions of a solution of copper sulphate must therefore differ 
from their atomic or molecular constituents in some im- 
portant essential, and from considerations which need not 
here be entered into, this difference is regarded as consisting 
in their possessing in the ionic state an electrical charge, which 
has both a qualitative and quantitative value. The Cu 
section of the molecule with its charge is then known as 
cuprion, and the S0 4 section with its charge as sulphion. 



24 ELECTROPLATING 

The former is charged with positive electricity, the latter 
with negative. 

To a reader unfamiliar with electrical matters, this may 
require some further explanation. The theory of electrical 
science supposes all bodies to be charged with equal amounts 
of positive and negative electricity, which normally neutralize 
one another, and thus no state of electrification is exhibited 
externally. The act of electrifying a body is to separate the 
positive and negative charges ; the body then exhibits the 
phenomena of " electrification." For example, a rod of 
sealing wax may not exhibit any signs of electrification ; but 
rub it with a piece of dry flannel and then present it near 
to some bits of paper, bran, or sawdust; the latter are 
attracted towards the rod. 

Now, even in this simple experiment it can be shown 
that after rubbing, the rod and the flannel are in different 
states ; the rod is said to be negatively charged, and the 
flannel positively charged ; thus the act of rubbing may be 
looked upon as a means of separating the positive and 
negative charges. Further, a positively charged body attracts 
a negatively charged body, and repels a body which is posi- 
tively charged like itself. That is, charges of opposite " sign " 
attract one another ; charges of like sign repel one another. 

Now, as will be more fully explained later, the terminals 
or poles of a voltaic cell are in the state which is described 
as being electrically charged, the one positively and the 
other negatively. When, therefore, they are connected to 
the two electrodes of the depositing cell, and these become 
positively and negatively charged, they will exert an attrac- 
tion on the oppositely charged ions, the positive electrode 
or anode on the negatively charged ions, and the negative 
electrode or cathode on the positively charged ions. Hence 
the positive ions move to the cathode plate, and are therefore 
called cations; the negative ions move to the anode plate, 
and are therefore called anions. In our instance the positively 
charged cuprions of Cu are the cations ; the negatively 
charged sulphions of S0 4 are the anions. 



ELECTRO-CHEMICAL PRINCIPLES 25 

Now, when these moving ions touch each their respective 
electrode by which they are attracted, they give up their 
charge and immediately return to their natural chemical 
state. The cuprion losing its electrical charge becomes 
simply metallic copper, and deposits itself as such on the 
surface of the cathode. The sulphion, SO 4 , chemically com- 
bines with the metal of the anode and forms copper sulphate. 

Cathode Ion Ion Anode 



Before 
After 



Cu SO 4 

<- -> 

Cu S0 4 

SO 4 4- Cu = CuSO 4 



As the charges carried by the ions are, from the above, 
of an opposite kind to that on the electrodes to which they 
migrate, some neutralization, takes place, and the action 
would soon cease were it not for the fact that the cell or 
battery tends to maintain the electrodes in a charged state, 
i.e. to keep up the potential difference (see p. 33) between 
them. Thus, so long as the action proceeds, electricity is 
drawn from the battery, and as it is termed a current 
" flows " round the circuit. 

The following diagram (Fig. 3) will perhaps make the 
matter clearer, the signs + and denoting positive and 
negative electrical charges respectively. 

Electrolysis continues, therefore, so long as the electrodes 
are recharged from the source of current, and so long as any 
ions remain to be discharged ; in the present instance the 
ions are continually replenished in the solution by means 
of the action of the sulphion S0 4 , which being liberated at 
the anode, combines with it to reform CuSO 4 , and so enables 
the process of deposition to continue, by furnishing successive 
series of dissociated ions. 

Laws of Electrolysis. As has been already observed, 
the ions of an electrolyte not only possess an electrical 
charge of a definite quality, but also of definite quantity. 
Faraday, whose brilliant genius laid the foundations of the 
science of electro-chemistry, investigated this part of the 



26 



ELECTROPLATING 



subject exhaustively, and formulated certain laws or prin 
ciples, which are now considered fundamental. 




(A) (B) (C) 

FIG. 3. Diagram to illustrate the dissociation theory of electrolysis. 

(A) Ions in motion but possessing no definite direction. (B) On electro- 
lysis ions in motion in definite directions. (C) Illustrating action 
at electrodes. 

NOTE. If the anode is not soluble, the S0 4 attacks the water present, 
and liberates oxygen with the formation of sulphuric acid, thus 
2S0 4 -f 2H 2 = 2H 2 S0 4 + 3 . 

These laws may be summarized thus : 

I. The weight of any substance liberated or de- 
posited from an electrolyte is directly proportional 
to the quantity of electricity flowing through the 
circuit. 

II. The weights of different substances liberated 
or deposited by the same quantity of electricity are 
proportional to their respective chemical equi- 
valents. 

In the light of the " ionic " theory of electrolysis, the 
first of these laws may be also stated as follows: The 
number of ions liberated, or in other words, giving up their 
electrical charge, is directly proportional to the quantity of 
electricity flowing through the circuit. If, therefore, a de- 
finitely measured quantity of electricity, flowing through 
an electrolyte, is found to deposit one gram of the metal 
concerned, then double this quantity of electricity, flowing 



ELECTRO-CHEMICAL PRINCIPLES 27 

through the same electrolyte, will result in the deposition 
of two grams. How a " quantity " of electricity is measured 
will appear later. 

The meaning of the second of these laws is that the 
actual weight of metal deposited from a solution, depends 
not only upon the current, but upon the nature of the metal, 
i.e. if the same quantity of electricity is passed successively 
through solutions of silver, copper, gold, and nickel, the 
weight of each metal deposited will bear the same ratio to 
the others as their respective chemical equivalents. 

This law is of extreme importance to the electroplate!*, 
and it may also be advisable to point out, that because of it 
the question of the valency of metals assumes first-rate 
significance, for it is evident from this law, that the weight 
of any metal liberated in electro-chemical action depends not 
only on its atomic weight, but also on its valency. 

Suppose two electrolytes, containing, for example, silver 
and copper respectively, were electrolyzed by the same 
current ; it would be found that the proportion of silver 
liberated to that of copper would be as 108 : 31-75, which of 
course agrees with Faraday's law (II.). Now, the respective 
atomic weights are 108 and 63-5. If, therefore, the ions of 
silver and copper were simply regarded as the chemical 
atoms Ag and Cu, a serious theoretical difficulty would arise. 
When consideration is given to the valencies of the two 
metals, however, the apparent discrepancy is overcome by 
regarding the bivalent copper ion as carrying a double 
electrical charge, corresponding to its valency, viz. 2, while 
the univalent silver ion carries only a single charge. The 
copper ion therefore demands, proportionately to the silver, 
twice the charge at the electrodes to enable it to be dis- 
charged, with the result that the weight of copper obtained 
is relatively only half its atomic weight, while the correspond- 
ing amount of silver obtained is equal to its atomic weight. 
Further, it will be found that the elements of greater valen- 
cies behave similarly ; trivalent ions carrying three electrical 
charges, quadrivalent four, and so on. 



28 ELECTROPLATING 

Indeed, from the electro-chemical point of view, valency 
means simply the number of electrical charges associated 
with the elements, when in solutions undergoing electrolysis. 
Arithmetical illustrations of Faraday's laws, which will 
further elucidate their meaning, will be given in Chapter IV. 



CHAPTER III 
FUNDAMENTAL ELECTRICAL PRINCIPLES 

To those engaged in the work of plating, or kindred pro- 
cesses, a grasp of the fundamental principles of electricity is 
becoming more and more essential. 

Whenever electricity is used for lighting, traction, electro- 
plating, electrotyping, the working of machinery by means 
of electric motors, etc., it is the so-called " electric current " 
which is the agent, or to speak more strictly it is the 
electrical energy associated with the " flow " of electricity 
which in doing the work accomplished is converted into 
some other form of energy. In all cases where electricity is 
the agent doing work, one or other of the properties or 
effects resulting from the " flow " of an electric current is 
utilized, and it is only through these properties that work 
can be done. The properties of an electric current must 
therefore first be considered. 

Properties of an Electric Current. From the pre- 
ceding chapter it will have been gathered that a current of 
electricity has the property of " electrolysing " or decompos- 
ing compound solutions called electrolytes. This effect is 
generally spoken of as the CHEMICAL EFFECT. 

There are, however, two other effects, namely, the 
Thermal or Heating effect, and the Magnetic effect. 

Although the chemical effect is the one which is of 
primary importance to the electroplater, a knowledge of the 
others is necessary in order better to understand the working 
of electricity, so that they will first be briefly mentioned. 



30 ELECTROPLATING 

The Magnetic Effect. If a wire through which a 
" current " is said to be " flowing " is held in almost any 
position near to a pivoted magnetic needle at rest, the 
needle is deflected, thus showing that a mechanical force 
has acted on the needle, and this force is of the same nature 
as that which would be exerted on the magnetic needle by 
another magnet. We see therefore that the " current " has 
a magnetic effect. 

Again, a piece of soft iron if dipped into iron filings will 
exert little or no attractive effect upon them. But when a 
wire carrying a current is coiled round the iron in a close 
spiral of many turns, the iron behaves quite differently, and 
will readily pick up a mass of the iron filings ; it is " magne- 
tized," and this magnetic state has been brought about by 
the current flowing spirally round the iron. 

The Thermal or Heating Effect. Whenever a cur- 
rent flows through a conductor, electrical resistance is over- 
come, and since this resistance is analogous to friction, heat 
is produced. If the rate of production of heat is sufficiently 
rapid, the conductor becomes quite warm to the touch, or 
even has its temperature raised to the point of incandescence 
as in an ordinary electric glow lamp. 

Before dealing in greater detail with the properties and 
effects of electric currents, it will be advisable to get a clear 
understanding as to what is meant by the flow of electricity 
in an electric circuit, and to consider the electric circuit in 
general, so as to explain the meaning of some of the terms 
used in connection with electrical apparatus. 

The Electric Circuit. An electric circuit is the com- 
plete path which an electric current traverses, and in which 
electrical energy is transformed into other kinds of energy. 
It contains essentially the " generator " or source, the appa- 
ratus to be worked, and the necessary transmitting and 
distributing wires connecting the whole together to form a 
continuous conducting path. 

Every electric circuit containing a generator at work is 



FUNDAMENTAL ELECTRICAL PRINCIPLES 31 

divisible into two portions, the internal and external portion. 
The internal portion is the path through the generator from 
one of its terminals to the other ; the external portion is the 
path from one terminal through the apparatus worked by the 
current to the other terminal. Thus in Fig. 4 (a) when 
the switch is closed, the part from D to A through the 
dynamo is the internal, and the part ABCD the external 
portion. These are frequently called the internal circuit and 
the external circuit respectively. 

As the " flow " of electricity in a circuit is in many 
respects quite analogous to the flow of water through a pipe, 

SwitcJb 
_j_ ^2 _ Stop-cock 



-Wire 



Wire 





FIG. 4. The electric and hydraulic circuits compared. 

the analogy will be helpful. When a battery or direct- 
current dynamo is joined up as shown in Fig. 4 (a) in an 
incomplete or " open " circuit (" open " because the switch is 
" off"), it may be likened to a pump (Fig. 4 (Z>) ) with its 
inlet D and outlet A connected by a pipe, in which a stop- 
cock turned to the "off" position is interposed, the whole 
being filled with water. Working the pump will produce a 
difference of water pressure between the two sides of the 
stop-cock, that on the left being, say, greater than that on 
the right. Mark these + and respectively. This differ- 
ence of pressure will depend on the " water- moving force " 



32 ELECTROPLATING 

of the pump. Obviously, however, no water will flow so 
long as the stop-cock is " off," but on turning the cock " on," 
the pressure difference will set the water in motion, and a 
flow will be maintained so long as the pump is at work. 

Potential and Difference of Potential. Eeferring 
now to the electric circuit, and accepting the statement that 
all bodies contain within them electricity "at rest," then 
when the dynamo is working, or the battery is charged, the 
wire AB connected to the + terminal of the generator is in 
a different physical state to the wire CD connected to the 
terminal. From the electrical standpoint the wire AB is 
described as being at a higher electrical potential or as having 
an electric potential which is positive, while CD is at a lower 
potential or is said to have a negative potential. Thus when 
the generator is working there is a difference of electrical 
potential between any point on AB and any point along CD, 
but the electricity is still "at rest," since the conducting 
circuit is interrupted by the switch. 

The term "electrical potential " is perhaps rather puzzling, 
but its meaning may be illustrated by the term " pressure " 
used in a mechanical sense. For instance, if the pressure 
of the steam in a boiler is measured by a pressure gauge, 
the gauge indicates the pounds per square inch above atmo' 
spheric pressure, which inthe case cited is taken as the zero 
of pressure for practical purposes ; in other words, the gauge 
indicates the difference of pressure between the absolute 
boiler pressure and the atmospheric pressure. Similarly, 
the electrical potential at any point in an electric circuit is 
for practical purposes reckoned as the difference between 
the electrical potential of the point in question and that of 
the earth which is arbitrarily taken as the zero of potential. 

For most purposes, however, the actual potential at a 
point in a circuit is of little or no moment, and it is only a 
knowledge of the difference of potential between two points 
which is of vital importance, since this is the cause of the 
electricity being set in motion. Electricity and water at rest 
are of no commercial value so far as doing work is concerned, 



FUNDAMENTAL ELECTRICAL PRINCIPLES 33 

but when in motion they at once assume commercial im- 
portance, for both are capable of doing work. 

Electromotive Force. Returning now to the case of 
Fig. 4 (a), when the circuit is completed by the closing of 
the switch, the potential difference (expressed in an abbre- 
viated form by the letters P.D.) existing between A and D 
sets electricity in motion, starts the " current " in fact. But 
the dynamo or battery is a machine or apparatus devised 
for the express purpose of maintaining the potential difference 
across its terminals ; hence while it is operative a continuous 
flow of electricity results, just as in the case of the pump 
which maintains a difference in pressure between the dis- 
charge and suction pipes. The function of an electrical 
generator is therefore to set up an dectricity -moving-force, 
termed the electromotive-force (in abbreviated form expressed 
by the letters E.M.F.). 

Common usage has introduced such expressions as 
" electricity- generating station " ; "a dynamo generates elec- 
tricity," etc. Nobody would say, however, that the pump 
in Fig. 4 (b) generated water, and, therefore, strictly speak- 
ing, expressions such as the above are incorrect. The cell, 
battery, or dynamo generates the E.M.F. which sets the 
electricity in motion, and so they may in a sense be said to 
generate an electric current, but they do not generate the 
electricity which is thus moved. 

The manner in which an E.M.F. is set up by cells, or 
dynamos, is dealt with in Chapters V. and VI. 

Rate of fall of P.D. Consider now Fig. 5 (a) in which 
AiBi is a horizontal pipe of uniform bore, to which are 
attached at points along its length open-ended vertical 
glass stand-pipes Tj, T 2 , T 3 , T 4 , T 5 , the end A, being attached 
directly to the discharge pipe M of a centrifugal pump, while 
B! is connected to the suction side of the pump through a 
return pipe B^Dj, on which there are similar stand-pipes 
not shown on the drawing. S : is a stop-cock. The electrical 
equivalent is depicted in Fig. 5 (#), analogous parts being 



34 



ELECTROPLATING 



similarly lettered. The electrical circuit consists of a dynamo 
corresponding to the pump, a switch S 2 corresponding to the 
stop-cock, and conductors MA 2 , A 2 B 2 , and B 2 C 2 D 2 correspond- 
ing to the pipes. In both circuits it will be assumed that 




FIG. 5. The rate of fall of (a) by hydraulic pressure, or (&) electric 
potential in circuit of uniform resistance. 

the points A x or A 2 and M are close to one another or con- 
nected by pipes or wires of large area as shown, and likewise 
points D! or D 2 and N, so that virtually A 2 and D 2 are con- 
nected to the terminals MN of the dynamo, while in the 



FUNDAMENTAL ELECTRICAL PRINCIPLES 35 

water circuit A t and D t are joined to the discharge and 
suction ends respectively of the pump. 

Let the entire pipe circuit now be filled with water to a 
level, say, halfway up all the glass tubes, i.e. to G r All the 
water is then at rest, its surface being at atmospheric pressure 
which forms our zero starting-point from which to measure 
pressures. In the case of the electrical circuit the electricity 
is already within it and at rest. 

Now let the stop-cock S x be closed, so that the pipe line 
is interrupted, and let the pump be started. It will be found 
that the water will rise in all the stand-pipes on A^ to 
exactly the same height, the line E^ joining the tops of 
these columns being horizontal. Conversely in the stand- 
pipes on C^Di it \\i\lfall to a uniform level. Since the water 
cannot circulate owing to the stop-cock being closed, it still 
remains at rest (except for the whirling going on in the 
pump which may for our purpose be disregarded), but at 
different levels on the discharge and suction sides respec- 
tively. The extra height to which the water is forced up in 
the stand-pipes T lf T 2 , etc., is a measure of the water pressure 
above the atmosphere at those points where they are con- 
nected, and the vertical pipes could be replaced by ordinary 
pressure gauges which would register the pressure above 
the zero of the atmosphere (corresponding to the higher or 
positive potential in the electrical case). On the other side 
the fall of the water in the stand-pipes would measure the 
suction, and these pipes could be replaced by vacuum gauges, 
registering the fall of pressure (corresponding to the lower 
or negative potential in the electrical case). 

Evidently, then, in the case illustrated the distribution 
of pressure in each pipe is uniform ; it has one uniform value 
in AjBu and another uniform value in C^. Also, it is clear 
that the difference of pressure between any point of AjB, 
and the return pipe is a constant. 

Analogously to this in the electric circuit (Fig. 5 #), the 
electrical potential at all points from M to the switch rid 
A* and B 2 is exactly the same so long as the switch So is 



36 ELECTROPLATING 

"off," and a similar remark applies to the potential at all 
points from S 2 to D 2 , via C 2 . But the potential of the portion 
MA 2 B 2 is higher than that of S 2 C 2 D 2 , if M is the positive 
terminal of the dynamo. This P.D. could be measured by 
means of a suitable voltmeter an instrument for measuring 
difference of potential, and electrically analogous to a boiler 
pressure gauge. Such an instrument would indicate that 
the P.D. was a constant, providing that one of its terminals 
be joined to any point on the conductor MA 2 B 2 , the other to 
any point on the conductor S 2 C 2 D 2 , and that no change 
except the moving of the voltmeter wires be made. Hence, 
when a generator is running, so long as the circuit is 
" open," the P.D. between the conductors leading from its 
terminals is a constant quantity, and further, this constant 
quantity is equal to the E.M.F. developed by the generator. 

Now let the stop-cock Sj be fully opened, and let a steady 
stream of water be allowed to flow through the pipe of 
Fig. 5(), in the direction A x to B lt The pipe being full 
throughout, the whole of the work of the pump is expended 
in forcing water round the circuit, and in doing this work 
the total difference of pressure between inlet and outlet is 
absorbed. The height of the water in the stand-pipes will 
then be different in the different pipes ; those nearer the end 
A! will indicate a greater pressure than those more remote 
towards B 1} and the level in the stand pipes on CiDj will 
fall as we approach N. It follows, therefore, that the water 
pressure at points in the pipe diminishes in the same direc- 
tion as that in which the stream flows. As the pipe- AjB, 
has been assumed straight and of uniform cross section, the 
tops of the water columns in the stand pipes will be found 
to lie all in one straight line E^^^Jj, but sloping. If 
the length d\d. 2 along the pipe A^ equals the length d$ 3t 
the difference between the height of water in the stand-pipes 
T 2 and T 3 is the same as that between stand-pipes T 3 and T 4 . 
Also, if rf^ be n times ^B^ the difference between F^ and 
^B! is n times that between H^s and IjB^ In other words, 
when a steady stream of liquid foics through a uniform pipe the 



FUNDAMENTAL ELECTRICAL PRINCIPLES 37 

difference in pressure between any two points is proportional to 
the distance between those points, and this is true whether the 
tube AB is horizontal or inclined. 

Again, if the stop-cock Sj be partially shut the rate of 
flow of water is diminished and the pressure distribution 
altered ; the statement above (in italics), however, still holds 
good, but the slope of the pressure line will now be, say, EjK,, 
and the pressure difference between, say, d^d* will be less 
than formerly. 

A restriction made in the bore of the tube, say, between 
cl./h (Fig. 6) diminishes still further the rate of flow, and the 
pressure line may now 
be B 3 F 3 G 8 H 8 I a . 

Now, the differ- 
ence in pressure be- 
tween any two points 
of the pipe is depen- 
dent upon the rate of 
flow of the water, and 
the Motional resist- 




A, 



FIG. 6. Fall of pressure in circuit not 
ance offered by the having uniform resistance. 

pipe to its passage. 

As, however, in the example taken the rate of flow is exactly 
the same at all points along the pipe when the stream is 
steady, the explanation of the greater difference of pressure 
between T : , and T 4 than between T, and T., must be put down 
to the extra resistance introduced by the restriction between 
d./l y The rate of flow of water in the pipe circuit, however, 
depends upon the resistance of the pipe as a whole, and the 
difference of pressure or " head " between the discharge and 
suction pipe. 

Analogously in the electric circuit when the conducting 
path is completed by the closing of the switch, a current of 
electricity results, flowing in the direction MA 2 B 2 C 2 N, that is, 
from the + terminal of the generator round the external 
circuit to the terminal, and through the internal circuit 
from to -f . The potential at points along the conductor 



38 ELECTROPLATING 

is no longer uniform, but falls in the direction M to N, i.e. 
in the direction in which the current flows. Assuming A 2 B., 
to have a constant cross-sectional area, the fall of potential 
is indicated by the full straight line E a A 2 , representing to 
scale the potential at A 2 with respect to zero or the earth's 
potential ; similarly the heights to the full straight line re- 
present the potentials at d lt d 2 , etc. If, therefore, the length 
d^L the length d 2 d- 3 , then the P.D. between di and d 2 = the 
P.D. between d and cl 3 , and from the same reasoning, if 
^B., = n times d^l.^ their respective P.D.'s are in the same 
proportion. In other words, when a steady current of electricity 
flows through a uniform wire the P.D. between any two points is 
proportional to the length of the conductor between the two points. 

In the electric circuit this is true whether the wire is straight 
or bent so long as its area is not altered, and whatever be its 
position. The statement could be verified by means of a volt- 
meter placed across d^l^ d^d*, or other points along the wire. 

Again, the rate of flow of electricity in a circuit where 
there is only one path provided for the passage of electricity, 
viz. MAsjBsjS.jC.jD.jN, is the same at all points, for if ammeter* 
instruments for measuring the rate of flow be inserted 
at various points in the circuit, they will all indicate the 
same value. 

Resistance and Ohm's Law. Now, with a metal 
conductor at a constant temperature, innumerable experi- 
ments have shown that the rate of flow is directly pro- 
portional to the P.D. , between the ends of the conductor, 

P D 

and that the ratio V-?i * s a constant quantity, a re- 
rate of now 

lationship first announced by Dr. Ohm in 1827. 

This constant quantity is called the electrical resistance of 
the conductor, while the rate of flow of electricity is ex- 
pressed as the current. Thus the relationship enunciated by 
Dr. Ohm may be written 

P D 

-v = Resistance, 
current 

and is known as Ohm's Law. 



FUNDAMENTAL ELECTRICAL PRINCIPLES 39 

The use of the term " resistance " having now become 
customary, all circuits or parts of a circuit are regarded as 
possessing obstructive properties, so that the P.D. existing 
between two points must be looked upon as the electrical 
pressure used up in forcing the current against the resist- 
ance offered to its passage between these points. 

The introduction of an extra resistance (the equivalent 
of the restriction in the water circuit of Fig. 6) in a circuit 
containing a generator of fixed E.M.F. will reduce the 
current, and there will be a redistribution of the potential 
and of the P.D.'s across the vaiious parts, just as in the 
water circuit. 

The analogy between water circuits and electric circuits 
is a useful one, but like most analogies it must not be 
pressed too far, since there are certain points of difference. 
Electricity, for example, is not a material substance like 
water, and consequently cannot be strictly looked upon as 
"flowing" in the same sense as water flows; the word 
" flow " is merely a metaphor, yet by its aid, probably a 
better grasp of certain electrical phenomena may be obtained 
than by any other explanation. 

Electrical Units and their Definitions. Although 
electricity is not a material substance, yet nevertheless some 
means must be adopted in order to express the magnitude 
of the various quantities used in electrical science in ways 
similar to those adopted in other sciences. For example, 
the quantity of water contained in a tank may be expressed 
by using the unit, the gallon, and further, if a pipe be 
inserted and the water allowed to run out, the rate at which 
the water runs out may be expressed as so many gallons 
per minute, or pints per second. Here the gallon has been 
adopted as the unit of quantity, and the gallon per minute 
as the unit rate of flow, the latter expressing, of course, the 
rapidity with which the water flows from the tank. 

So with electricity, units are required to express quantity 
of electricity, and rate of flow. As the presence of an 
electric current is only manifested by its properties, such 



40 ELECTROPLATING 

units must be based on one or other of the effects mentioned 
at the beginning of the chapter and on the magnitude of 
these effects. 

For reasons which need not be entered into here the 
practical definitions of the above units are based on the 
chemical effect. 

DEFINITION. Unit quantity of electricity is that quantity 
which, when passed through a solution of silver nitrate in 
water will deposit 0-001118 gram of silver, and is called the 
Coulomb. 

DEFINITION. Unit rate of flow of electrkity or the current 
is that unvarying current which when passed through a 
solution of silver nitrate in water will deposit silver at the 
rate of 0-001118 gram per second ; it is thus the rate corre- 
sponding to the passage of a coulomb per second, and is 
called the Ampere. 

If the rate of flow, i.e. the current, be multiplied by the 
time for which it lasts, the product must give the total 
quantity of electricity that passes in the given time, the 
relationship between the above units may therefore be 
expressed as follows : 

Quantity _ current in time in 
in coulombs ~ amperes seconds 

Symbolically Q = I x /, 

where Q = quantity in coulombs, 

I = current strength in amperes, 

t = time during which the flow lasts in seconds. . 

The coulomb, however, is a very small unit, so a secon- 
dary unit called an ampere-hour is often employed for 
practical purposes. 

Since 1 ampere flowing for 1 second = 1 coulomb, 
then 1 3600 seconds = 3600 coulombs. 

But 3600 seconds = 1 hour. 

/. 1 ampere flowing for 1 hour = 3600 coulombs, 
or 1 ampere-hour = 3600 coulombs. 



FUNDAMENTAL ELECTRICAL PRINCIPLES 41 

Examples. 1. A plating vat has a current of 50 amperes 
flowing through it for 6 hours, what quantity of electricity 
passes through the vat ? 

Q = I X t 

Substituting, we get Q = 50 x 6 = 300 ampere-hours, 
or Q = 50 x 6 x 3600 = 1,080,000 
coulombs. 

2. One thousand three hundred ampere-hours pass 
through an electric circuit in 10 hours 50 minutes : what is 
the average current ? 

.4 

, -r 1300 ampere-hours 
Substituting, we get I = _ 

6 

= 120 amperes. 

Current Density. For electrolytic purposes current 
density is denned as the amperes per square centimetre, or per 
square inch of area of electrode immersed in the electrolyte. 
The current density, together with other factors which will 
be discussed as occasion arises, has an important bearing 
on the kind of deposit obtained. A very simple experiment 
readily shows this to be the case. Take a little coppering 
solution (see page 252) and immerse in it two clean and 
smooth copper plates of about 4 square inches area to form 
an anode and cathode respectively. Pass a current of about 
one ampere for 5 to 10 minutes. Observe that the copper 
deposited is salmon pink in colour, dull, but smooth. Now 
pass 5 or 6 amperes for a similar period and notice that 
the deposit is much rougher and more crystalline than 
before. 

Resistance and Conductance. All substances, 
whether solids, liquids, or gases, are regarded as possessing 
from an electrical point of view a property which may be 
described from two opposite points of view as either its 
" resistance," or its " conductance," the one being the con- 
verse of the other. In the case of a water pipe of small 



42 ELECTROPLATING 

section, if we try to force through it a large quantity of 
water, we know that the smallness of the bore presents 
considerable resistance to the effort. The pipe might, there- 
fore, be described either as a " good resister " to the flow, 
or as a " bad conductor " of the flow. In the same way 
that property of any substance which resists the flow of 
electricity is called its Resistance, and from the opposite 
point of view the facility offered to the flow is called its 
Conductance. 

All metals are fairly good conductors of electricity, but 
the four metals, silver, copper, gold, and aluminium, stand 
pre-eminent in this respect, their relative conducting powers 
being of the order 1 : 0-92 : 0*67 : 0-56 respectively. Of 
these silver and gold are obviously too expensive to employ 
for electrical conductors, and in consequence, as copper and 
aluminium are relatively cheap, it is usual to find that con- 
ductors are composed of one or other of these metals, copper 
being used to a far greater extent than aluminium. 

On the other hand, substances such as gutta-percha, 
india-rubber, ebonite, mica, glass, porcelain, etc., are extremely 
bad conductors, so much so that they are termed insulators, 
and are used to confine currents of electricity along definite 
conducting paths and prevent leakage. This, in fact, is the 
object of covering electrical conductors with some substance 
which has good insulating properties. Bitumen, oiled paper, 
vulcanized india-rubber, cotton and silk, are among the chief 
insulating materials used for this purpose, vulcanized india- 
rubber being employed to a very large extent for cables, 
while silk and cotton (well varnished) are used for winding 
electrical instruments and machines respectively. 

The unit of resistance is called the ohm, and is defined 
as the resistance offered to an unvarying current of electricity 
by a column of pure mercury having a uniform cross- 
sectional area of 1 sq. mm., a length of 106-3 cms. and a 
mass of 144521 grams at C. 

As a fairly close approximation, 42^ yards of No. 20 
S.W.G. (0-036" diam.) copper wire has a resistance of 1 ohm 



FUNDAMENTAL ELECTRICAL PRINCIPLES 43 

at a temperature of about 15 C. (roughly 60 F.). Other 
equivalents of the ohm are given in Table IX. on page 129. 

The unit of conductance is called the mho, a term 
suggested by the late Lord Kelvin. It may be denned as 
the facility offered to the passage of an unvarying current 
by a column of mercury having the dimensions and par- 
ticulars given above. 

The relationship between these units is as follows : The 
measure of the conductance of a wire or circuit is given by 
the reciprocal of its (resistance ; if R = its resistance in ohms, 

then .= = K, its conductance is mhos, and vice versa ^ = R. 
K j\ 

It is, however, more usual to speak of the resistance of a 
material, rather than of its conductance, and this more 
general usage will be adhered to in the majority of cases 
for present purposes. But from the above relationship, if 
one of the two expressions be known, it is easy to see how 
it may be converted if we wish to express the property in 
question in its second form. 

Two other terms, namely, " specific resistance " or 
" resistivity," and " specific conductance "or " conductivity," 
are frequently employed when dealing with the resisting, or 
oppositely the conducting, property of different kinds of 
material, and as these terms are frequently confused with 
those of resistance and conductance it will be well to state 
their precise meaning. Resistivity and conductivity are 
terms used to denote the resistance and conductance respec- 
tively of 1 cm. (or 1 in.) length of the material having a 
cross-sectional area of 1 sq. cm. (or 1 sq. in.) at C. Their 
numerical values are spoken of as the resistivity in ohms per 
cm. per sq. cm. (or per in. per sq. in.) and the conductivity in 
mhos per cm. per sq. cm. (or per in. per sq. in.), according to 
whether the dimensions are in centimetre units or in inch units. 

These terms therefore denote respectively, the resistance 
and conductance of a specified length of material, of specified 
cross-sectional area, whereas the terms resistance and con- 
ductance are used to express the obstruction and facility 



44 ELECTROPLATING 

respectively offered to the passage of electricity by a material 
of any length, and any cross section. The resistivity of 
copper is less than that of German silver, but it is quite 
possible to have a copper wire of greater resistance than one 
made of German silver. 

The term "resistivity" is of value in calculating the 
resistance of a conductor (as will be seen below), or for com- 
paring the relative resistance of wires composed of different 
materials but of similar length and area. The resistivity of 
copper, for example, is 0-000000614 ohm per in. per sq. in., 
that of German silver 0-00000828 ohm in the same units at 
C. The relative resistances are therefore as 0-000000614 : 
0-00000828 or as 1 : 13-48. Owing to the low order of 
magnitude of the resistivity of metals, it is more usual 
to express resistivity values in microhms ; 1 microhm 

= jf 000 ooo ( one millionfch ) of an ohm - 

Table V. gives the values of the resistivity of the common 
materials used for electrical purposes. 

TABLE V. 

RESISTIVITIES OF METALS AND ALLOYS. 

Microhms at C. 



Metals. 

Silver, annealed . . . 
Copper annealed 


per cm. 
pzr sq. cm. 
. . . . 1-47 . . 
. 1-56 . . 


per inch 
per sq. in. 
. . 0-58 
. . 0-61 


hard drawn . . , 
Aluminium 
Gold 


, . . . 1-62 . . 
, . . . 2-66 . . 
. . . 2-20 . . 


. . 0-64 
. . 1-05 
. . 0-87. 




. 5-75 . . 


. . 2-26 


Wrought iron, mild steel 


. . . . 10-0 . . 
. . 10-92 . . 


. . 3-94 
. . 4-30 


Nickel 


. . . . 12-32 . . 


. . 4-85 


Tin 


. . . 13-05 . . 


. . 5-12 


Lead 


. 20-38 . 


. . 8-0 


Mercury . 


. 94-1 


. 37-0 



Alloys. 

German silver (varies with com- 
position) 21-0 .... 8-3 

Platinoid 41-7 .... 16-4 

Eureka 44-2 .... 17'4 

Ferry 47'2 .... 18-6 



FUNDAMENTAL ELECTRICAL PRINCIPLES 45 

Laws of Resistance. The resistance of a conductor 
depends upon four distinct factors : 

(1) Length. 

(2) Area of cross section. 

(3) Kind of material. 

(4) Temperature; 

to which may be added (5) the degree of purity and the 
hardness or softness of the material, these being really 
special variations that come more properly under (3). 

Taking the effect of the dimensions and kind of material, 
it is found that the resistance is directly proportional to the 
length, inversely proportional to its cross section, and is 
obviously proportional to the resistivity of the material. 

Expressing the above in algebraic form, 

if E = resistance } 

I = length > of the conductor, 

A = area of cross section] 
a = resistivity of the material, 
then R = o- for unit length having unit area, 

E = o- x Hor a length / having unit area, 
a- x I 



and for area A R = 



A 



which is the fundamental equation expressing the resistance 
of a conductor as influenced by conditions (1), (2), (3). As 
fairly reliable data of the resistivity are given in the table 
above, it is possible to calculate the resistance of a given 
piece of wire, or to determine what length of a particular 
wire would be necessary to make a resistance of definite 
value. Owing, however, to the different units which may 
be employed, the law is expressed in more precise forms 
below, 

f \ T) OYobrns per cm./sq. cm.) X '(cms) 

((I) V(ohms) = - r 

^(sq. cms.) 

(b) R( hni8) = 



./sq. cm.) X '(cms.) 



10 6 X A(gq. cms.) 



46 ELECTROPLATING 

, N -D O" (ohms per in. /sq. in.) X '(ins.') 

W E C'"" 5 > = A (sq . in s.) 

/ 7\ T> ^(microhm per in./sq. in.)' X ?(ins.) 

() -K(ohms) = - lf y, * 

1U X A( S q. ms -) 

Example. The two copper leads from a dynamo to a 
plating vat are each 30 ft. long, and composed of wire \ in. 
in diameter. What will be the resistance of these leads ? 
Eesistivity of copper 0-61 microhm per in. per sq. in. 

Taking expression (rf) above, E = 1Q6 x ^ 

0-61 x 30 x 2 x 12 
substituting, E = 



from which E = 0-00895 ohms. 

For practical purposes, tables such as are given on p. 
129 are far more convenient and handy for resistance cal- 
culations, and examples are there given, but nevertheless 
the student should familiarize himself with the matter given 
above. 

With respect to the resistance of conductors as influenced 
by temperature and purity, hardness or softness, little need 
be said here, as they are relatively unimportant to the electro- 
plater. As a general rule the resistance of pure metals, with 
few exceptions, increases about 0-38 per cent, per 1 C. rise 
in temperature. In the case of alloys such as German 
silver, platinoid, eureka, etc., the percentage increase due to 
a rise in temperature is very much smaller. The degree of 
purity has a very-great influence on the resistivity, as may 
be judged by reference to the resistivity table, and a hard- 
drawn wire offers a slightly higher resistance than one which 
has been subjected to an annealing process subsequent to 
drawing. 

Resistivity and Conductivity of Electrolytes. 

Strictly speaking, the resistivity of an electrolyte is the same 
property as that of any other conducting medium. It varies 
with temperature, in many cases decreasing with increase of 



FUNDAMENTAL ELECTRICAL PRINCIPLES 47 

temperature, and thus an electrolyte behaves in this respect 
in an opposite manner to most metals. Since, however, the 
resistivity of an electrolyte is so greatly influenced by the 
degree of dissociation and rate of migration of its ions, and 
comparatively so little influenced by its dimensions, it is 
more convenient to refer to the conductivity, as this expresses 
the ease with which the ions migrate. It will therefore 
readily be understood that the conductivity of electrolytes is 
a more complex problem than that of solid metal conducting 
mediums. At present it is regarded as being due to the 
power of the water or other solvent (called the " dissociant ") 
to break up the dissolved salt into the two kinds of ions, 
which have been already described in Chap. II. 

Unit of Electrical Pressure. It is now necessary to 
introduce the unit of electrical pressure, which has been 
deferred until the ampere and the ohm had received con- 
sideration, in order that the most practical definition could 
be given. On page 38 the relationship known as Ohm's 
Law has been quoted. We had there the ratio 

Potential Difference T> , 

= Resistance. 
Current 

V 

Symbolically j = R 

where V represents the P.D. 

If, then, I and E each be unity, V must be unity, and in 
the practical system of units, the unit of electrical pressure is 
that potential difference which will cause one ampere to flow 
through a resistance of one ohm. It is calleTl the Volt. 

We shall now enlarge upon the above relationship 
between the quantities, pressure, current, and resistance, in 
order that the law may be correctly applied to any particular 
case, and with the recognized terminology. Generally, one 
or other of four expressions will be applicable to most circuit 
conditions. 

I. For part of an external circuit consisting solely of a 
resistance. 



48 ELECTROPLATING 

P.P. (volts) 
Current (amps) = g-g^-^-^ 

I = E 

where I = current through the part considered. 
V = P.D. across 

B = resistance of ,, 

II. For the whole circuit. 

E.M.F. (volts) 
Current (amps) = Tola^I^resTsTance (oh^is) 



III. For the whole circuit ivhen there are two E.M.F Cs acting 
in it a case frequently arising in practice 

T - E ft * 
E 

where E = the principal E.M.F., 
e = the other E.M.F., 
B = the total resistance of the circuit. 

In words, the current is proportional to the resultant 
E.M.F. acting in the circuit and inversely proportional to 
the total resistance. The resultant E.M.F. is the sum of the 
separate E.M.F.'s if they both tend to send current in the 
same direction, in which case the -f sign must be used. On 
the other hand, the E.M.F.'s may oppose one another ; the 
resultant is then the difference between the E.M.F.'s, and 
the sign is used. The direction of the current will always 
be the same as that in which the larger E.M.F. is acting. 

IV. For part of a circuit containing a resistance and an 
opposing or " hack " E.M.F. 



where V = P.D. across the part in question, 
B = resistance of the part, 
e = the opposing E.M.F. 



FUNDAMENTAL ELECTRICAL PRINCIPLES 49 

The following examples may help to elucidate difficulties 
arising from a consideration of the above. 

Examples. (I) A plating dynamo having an internal 
resistance of 0*02 ohm and developing an E.M.F. of 10 
volts, is joined to an external circuit of resistance 0-105 ohni. 
What current will flow in the circuit ? 

E 
From (II) above I = -^ 

W_ 

0-105 + 0-02 
= 80 amperes. 

(2) Two batteries having E.M.F.'s of 4 and 2 volts 
respectively and of negligible resistance, are joined in 
opposition and their free terminals are connected by a wire of 
10 ,ohms resistance. What current will flow through the 
wire ? 

From (III) we have 

_ E f. 
B 

4-2 
Substituting, I = ~^TQ~ 

Since the E.M.F.'s oppose one another, the resultant E.M.F. 
= 4-2 = 2 volts. 

''-A 

1 

= v ampere. 

If the batteries had been joined so that their E.M.F.'s 
assisted each other, the current would have been 



10 
6 3 
= 10 or 5 ampere. 

(3) The copper leads in the example on page 46 were 
found to have a resistance of 0-00895 ohm. If 80 amperes 

E 



50 ELECTROPLATING 

pass along them what will be the fall of potential or " drop " 
in the leads ? 

From (I) V = IR 

.-. V = 80 x 0-00895 
= 0-716 volt. 

Electrical Work, Energy, and Power. When a 
current of electricity flows in a circuit work is done at a 
definite rate and energy is dissipated, and we must now 
introduce units in terms of which these quantities are 
measured. 

The work done in raising a mass of one pound through 
a difference of level of one foot against gravitational attraction 
is taken as the unit of mechanical energy and called the foot- 
pound, the work done being obtained by multiplying the 
mass in pounds by the number of feet through which it is 
raised. 

Somewhat similarly the unit of electrical energy is the 
work done in moving one coulomb of electricity between two 
points in a circuit between which the P.D. is 1 volt. It is 
called the Joule. But as the quantity of electricity conveyed 
by one ampere flowing for one second = 1 coulomb, the unit 
of work or of energy is usually defined as follows : The joule 
is the work done per second by 1 ampere flowing between 
two points in a circuit, when the P.D. between them is 1 
volt. 

The total work or energy expended in t seconds when the 
current is I amperes, and the P.D. V volts, is given by the 
product of these three quantities, 

i.e. Total work done = amperes x volts x time (sees.) 
or No. of joules = I x V x t. 

The joule, however, is much too small a unit for practical 
electrical purposes. It is customary, therefore, to express 
electric energy in terms of a secondary unit, the watt-hour, 
or in terms of the commercial unit called the Board of 
Trade Unit or Kelvin. This latter is the unit by which 
to use the common but inaccurate expression " electricity " 



FUNDAMENTAL ELECTRICAL PRINCIPLES 51 

is bought and sold, and the meters which are installed on 
consumers' premises are designed expressly for the purpose 
of measuring the energy consumed in terms of this unit. 

Power is defined as the rate of doing work, and we are 
familiar with the term horse-power used to express a standard 
rate of doing mechanical work, equivalent to 33,000 ft.-lbs. 
per minute. 

Electrical power signifies the rate at which electrical work 
is done in a circuit. The average rate can always be found 
by dividing the amount of work done by the number of 
seconds taken for its performance. The rate of working, 
however, may not be constant over a large time, and the 
result arrived at in this manner only expresses the average 
rate. But if we multiply together the P.D. and the corre- 
sponding rate of flow, i.e. the current at the same moment, 
the product of the volts and amperes will then give the 
instantaneous rate of doing work, and we obtain the power 
directly. 

The unit of electrical power is the joule per second, more 
commonly termed the watt, and is the power developed or 
absorbed in a circuit when the product volts x amperes = 
unity. 

Thus 1 watt = 1 volt- ampere. 

We see, then, that if I be the current in amperes, V the 
P,D. in volts, and W = the watts, 

W = I x V 
watts = current x potential difference. 

A kilowatt ( = 1000 watts) is also employed as a unit of 
power, where the watt is inconveniently small. 

Example* If the P.D. of a plating dynamo is 10 volts and 
150 amperes flow in the circuit to which it is connected, what 
is its rate of working ? 

W = I x V 
W = 150 x 10 
/* rate of working = 1500 watts. 



52 ELECTROPLATING 

If the dynamo in question were capable of delivering a 
maximum of 300 amperes at the same voltage, what would 
be its capacity, i.e . the maximum power which could be safely 
taken from it for long periods ? 

W = Ix V 
W = 300 x 10 

= 3000 watts 

= 3 kilowatts. 

But as 300 is the maximum current, then 3 kilowatts is the 
maximum power and represents its capacity. 

We may, however, express power in ways other than as 
above, and as shown below. 

Since W = I x V, 

y 

and from Ohm's Law I = where E = the resistance of the 

\i 

circuit across which the P.D. is V, then by substituting this 
value of I in the former expression we get . 

W = X V 



watts = (Potential difference) 2 
resistance 

Again, V = IE, and substituting in the same expression this 
value of V we get 

W = I x IE 

= I 2 E, 
or watts = (current) 2 x resistance. 

It is obvious, then, that providing any two of the three 
quantities, I, Y, E, be known, the power expended may be 
readily determined. 

The connection between these units of work, power, and 
energy may be tabulated as follows : 



FUNDAMENTAL ELECTRICAL PRINCIPLES 53 

1 joule = 1 volt-ampere-second, 
1 watt = 1 volt -ampere, 
1 H.P. = 746 watts, 
1 watt-hour = 1 volt-am pere-hour ; 

but as there are 3600 seconds in one hour 
1 watt-hour = 3600 joules 

Again, 1 kelvin = 1000 watt-hours, 

.-. 1 kelvin = 1000 x 3600 joules 
= 3,600,000 joules. 

From the Law of the Conservation of Energy, it follows 
that when energy is used up in a circuit it must reappear in 
some other form or forms, and to the exact equivalent of 
that supplied electrically. 

In general, for industrial purposes we wish it to reappear 
as either mechanical energy, heat energy, or chemical 
energy. The form in which we get it again, however, 
depends entirely 011 the nature and disposition of the path 
through which the current flows and the actions which 
result ; in other words, on what happens in the apparatus 
when a current passes through it. Some simple illustrations 
have already been given which bear out the statement in 
dealing with the effects of a current (p. 29). But whatever 
the path may be, the flow of a current is always accompanied 
by the generation of heat which warms the conducting 
medium. Heat so produced represents so much energy 
wasted, unless indeed its production is the only thing aimed 
at. But if it is desired to do chemical work in an electrolytic 
cell, energy used up in the production of heat in the cell is 
wasted, since that is not the purpose in view. 

It must, however, be clearly understood that the heat 
energy here referred to is distinct from the heat energy 
absorbed or liberated in chemical reactions. The former is 
produced by the current in overcoming the electrical resist- 
ance of the conducting path, as explained below, while the 
latter is due to the chemical decomposition set up by the 
current. 



54 ELECTROPLATING 

We shall dismiss any consideration of the conversion of 
electrical into mechanical energy, as it does not concern us. 

Heat produced by a Current, Joule's Law. Ke- 
ferring to Fig. 5 (Z>) (p. 34), it is obvious that in the 
elementary circuit there considered there is no device, such 
as an electric motor or an electrolytic cell, for the con- 
version of electrical into mechanical energy and chemical 
energy respectively, yet the circuit absorbs energy. Taking 
two points such as d l d. 2 , we have explained the fact that a 
P.D. exists between them when a current flows. Let the 
P.D. be V volts, the current I amperes, and E the resistance 
of </! d 2 , then the energy used up in the portion considered 
in t seconds is IV or I 2 !U joules. This energy is spent in 
overcoming the resistance and reappears as heat energy. The 
rate of production of heat is therefore IV or I 2 R> joules per 
second. When a circuit acts simply as a resistance, the 
whole of the energy given up by a current flowing through it 
is converted directly into heat. 

From the investigations of Joule, Prof. Rowland, and 
others a relationship between the joules expended and the 
number of units of heat (calories *) produced can be found. 
This relationship is called Joule's Law and is expressed as 
follows : 

H = VRt x 0-24 

where H = number of heat units in calories. 

Rate of doing Chemical Work by a Current. 
E.M.P. set up by Chemical Decomposition. Suppose 
a current of I amperes to be passed through a decomposable 
solution copper sulphate, for example provided with in- 
soluble electrodes, and let V be the P.D. which is maintained 
across them. The rate at which energy is given to the 
arrangement is V x I joules per second, part of which is used 
in doing chemical work, and from previous considerations 

* A calorie is defined as the heat required to raise the temperature 
of 1 gram of water 1 C, when the water is initially at a temperature of 
15 C, 



FUNDAMENTAL ELECTRICAL PRINCIPLES 55 

part is wasted in the production of heat. Let B = the 
resistance of the electrolytic cell ; then 
VI = w + FB 

where w = rate of doing chemical work in joules per second 
or watts. 

I-B = rate of production of heat. 

Dividing the expression by I, we get 



.-. IB = V - 
and 




This is the form of an expression which is not wholly 
unfamiliar, for on comparing it with case IV on p. 48, we 
recognize Ohm's Law. 

XT w rate of doing chemical work (watts) 

Now, as -- = ' and as 

current 

watts ,, iv 

- = volts, Y represents an electrical pressure, and as 

its sign is negative, it must be an opposing or " back " 
E.M.F. one, in fact, acting in opposition to V, the P.D. 
forcing current through the arrangement. Again, if there 

were no chemical work !? = 0, and there would be no oppo- 

sition E.M.F. We see, then, that when a solution is decom- 
posed by a current of electricity, the electrodes being 
insoluble, there is an E.M.F. set lip by the chemical 
decomposition of the solution, which opposes the E.M.F. of 
the source from which the current is derived. Further, let 
the opposing E.M.F. be denoted by e t as was done on p. 48, 

then ^ = e or w = le, i.e. the rate of doing chemical work is 

expressed by the product of the current and the opposing 
E.M.F. produced. 



5 6 ELECTROPLATING 

The method of calculating this E.M.F., together with 
examples, and a consideration of the case when soluble 
electrodes are used is given under the heading " E.M.F. 
required for electrolysis " (pp. 65 ff.). 

Series and Parallel Circuits. There are two general 
ways of joining " elements " * together to form an electric 
circuit, namely, in series, or in parallel; and circuits so 
formed are spoken of as series circuits and parallel circuits 
respectively. These methods of connection are represented 
diagrammatically in Fig. 7, in which the elements E lf E 2 , E, 
are shown connected in series at (a) and in parallel at (ft). 




(a) Series. (b) Parallel 

FIG. 7. Conductors in series and in parallel. 

A simple way of noting the distinction between them is 
to trace the path provided for the passage of electricity from 
one end of the circuit to the other. By doing so it will be 
seen that in a series arrangement there is only one path by 
which a current entering at A can flow to B, and that is by 
passing in succession along the elements B lt E 2 , E 3 . In (ft) 
it is seen that the current has the choice, so to speak, of three 
paths between A and B, and in consequence it divides 'at A 
into three portions, flowing through the three branches 
simultaneously, in a similar manner to that of a river dividing 
at one point into two or more channels which eventually 
unite again at some other point. " In parallel," therefore, 
means that arrangement which provides several paths along 
which current may flow simultaneously from one point to 

* The term "element" is here used to denote any single device 
which can be placed in an electric circuit, such as a vat, an ammeter, 
a voltmeter, a resistance, a cell, etc. 



FUNDAMENTAL ELECTRICAL PRINCIPLES 57 

another. In practical cases (including plating shop vats) the 
circuit connections as a whole conform more closely to Fig. 8, 
but on examination this is readily seen to be a combination 
of the methods outlined above. For instance, between the 
wires AC and BD we have four branches along which current 
may flow from the positive wire AC to the negative wire 
DB ; the elements E lf B 2 , E :! , E 4 are therefore in parallel, 
and we must regard the wires AC and DB as being the 
practical equivalent of the points A and B respectively in 

M AC 

+ // 4- Main or Lead, 




Main or Lead 
N ' B D 

Fie. 8. Parallel circuit with connections to dynamo. 

the theoretical diagram (Fig. 7). The current from the 
dynamo, however, must necessarily pass along the single 
path MA to the elements, returning by the single path BN, 
and then through the machine from N to M to complete its 
circuit ; the main leads must therefore be regarded as being 
in series with the remainder of the circuit. 

It must not be assumed, however, that elements may be 
joined in series or in parallel indiscriminately. There are 
theoretical and practical reasons which 'prescribe to UB 
the most suitable method. Some of these will be fairly 
obvious by considering the characteristic features of the 
series and parallel methods with respect to the resistance, 
the distribution of the current, and the potential. For 
brevity these will be given in the form of a summary. 

Series Circuits. (1) When elements are in series, the 
total resistance is the sum of their individual resistances. 
Thus in Fig. 7 if Ej, E 2 , B : , represent the resistance of the 
respective elements, then 

E (the total resistance) = Ej + E 3 + B,. 



58 ELECTROPLATING 

(2) The current has the same value at all parts of the 
circuit; there is no " loss of current." This point and also the 
next one (3) have been explained in detail in connection 
with Fig. 5, p. 34. 

(3) The P.D. between any two points is proportional to 
the resistance between the points, and is numerically equal 
to the product of current and resistance according to Ohm's 
Law. 

(4) A break, disconnection, or the opening of the circuit 
at any point with a switch interrupts the current through the 
whole of the elements. 

Parallel Circuits. (1) When elements are connected 
in parallel, the resistance of the combination is always less 
than that of any of its elements taken separately. 

Let us suppose that in Fig. 7 the only element present is 

E,, of resistance EI ohms, and therefore of conductance ~ 

v 

Now introduce the element R, of conductance =,-, the total 

E 2 

conductance is then ^- _j_ =g-, and similarly when E.> is also 
K! 1*3 

introduced the total conductance = ^ 4. =- 4. ^ , and so on 

V 1 V. 2 1., 

for any number. The total conductance is, however, the 
reciprocal of their combined resistance, E, therefore 

E = E! + E 3 + R: 

Example : Let Ej = 4 ohms, E 2 = 4 ohms, E., = 6 ohms, 

- 



.-. E = - or 1-5 ohms. 
It is useful to note that when a number of elements each 



FUNDAMENTAL ELECTRICAL PRINCIPLES 59 

having the same resistance are joined in parallel, the resist- 

ance of the combination = Distance ol lone branch 

number of branches 

(2) The division of the total current into the various 
branches is dependent on the conductance of the branches. 
If they are all alike in this respect the current divides 
equally among them. In any case, however, the ratio 
between the current in any branch and the total current is 
equal to the ratio of the conductance of that branch to the 
total conductance of all the branches. 

For example, taking the figures above, what current 
(I,) flows through the branch E^ if the total current (I) is 40 
amperes ? 

Conductance of branch E, = i 

Total conductance = I 

-H 

I, _3 

40 ~~ 8 

from which I, = 15 amperes. 

Similarly L = 15 amperes, 

I, = 10 amperes. 

(3) The fall of potential along each branch is the same. 
For as they are all connected to a common point A at the 
commencement of the circuit, and terminate at the common 
point B, then whatever P.D. exists between these common 
points is also the P.D. across the ends of each branch. 

(4) Any branch may be disconnected by the mere open- 
ing of a switch placed in it, without interrupting the current 
flowing through the other branches. 

It will be clear, then, that it would be inadvisable to work 
two plating vats in series, when the work in them requires 
different currents. If this were done the rate of deposition 
in one would be too slow, or in the other too rapid, and the 
work would be spoilt. The invariable plan in practice is to 
work plating vats in parallel. By so doing any vat can 



60 ELECTROPLATING 

have work put in, or taken out, and further, by the addition 
of resistance to the branch containing the vat the current 
may be regulated to a suitable value, without interfering 
with the deposition going on in other vats. For details of 
the practical arrangement of vat connections see page 123. 



CHAPTER IV 

QUANTITATIVE ELECTKO-DEPOSITION 

IT will now be possible to consider more fully the meaning 
and value of Faraday's Laws. These laws have already 
been stated in Chapter II., but for convenience they are here 
repeated. 

LAW I. The weight of any substance liberated or 
deposited from an electrolyte is directly proportional to the 
quantity of electricity flowing through the circuit. 

LAW II. The weights of different substances liberated 
or deposited by the same quantity of electricity are pro- 
portional to their respective chemical equivalents. 

Electro-chemical Equivalent. From the second law 
the amount of chemical decomposition per coulomb depends 
upon and is proportional to the chemical equivalent of the 
substance liberated. Taking, for example, silver and copper 
(cuprous) and their chemical equivalents as 107'88 and 
31 '78 respectively, then these numbers express the relative 
weights of silver and copper deposited per coulomb from 
suitable solutions, not the actual weight. To connect to- 
gether the chemical and electrical side more closely on this 
point, and materially to assist the electro-chemist, the term 
Electro-chemical Equivalent (E.C.E.) is used. 

This may be denned as the number of grams weight of any 
ion liberated in electrolytic action by one coulomb of electric iff/. 
The distinguishing feature between the chemical equivalent, 
and electro-chemical equivalent, is that the former is a 
numerical ratio, whilst the latter denotes the weight in 
grams, which in the case of any ion is set free by the passage 
of the specified quantity of electricity. 

Eeferring to the definition of the coulomb (p. 40), obviously 
0-001118 is the electro- chemical equivalent of silver when 



62 ELECTROPLATING 

deposited from a solution of silver nitrate in water. Taking 
this value for silver we may readily calculate the E.C.E. of 
other ions as follows : 

Let g, f = E.C.E. of silver, 

jg,. = E.C.E. of another ion, 

/ 

a^ and a = their respective atomic weights, 
v and v. = their valencies. 

Then the number of grams of Ag set free is proportional to 
the chemical equivalent of silver. 

Symbolically & oc ~ s 

Similarly, for the other ion 



from which .=(-' x -') x 



whence by substitution & t may be found. 

Example. What is the electro -chemical equivalent of 
copper ? Given that its atomic weight is 63-57, its valency 
two, while silver has an atomic weight of 107-88, and 
valency one, 

.Q \*7 T 

& (for copper) = -g X jQ^gg X 0-001118 

= 0-0003294 

In the deposition of copper from copper sulphate solu- 
tion a certain amount of free acid (sulphuric acid) is present 
in the bath, and the value obtained by the above calculation 
is higher than that usually taken in practice. The difference 
may be observed by comparison with the value given in the 
following table. 



QUANTITATIVE ELECTRO-DEPOSITION 63 



TABLE VI. 

ELECTRO-CHEMICAL EQUIVALENTS OF METALS. 





E. C 


. E. 


Ozi. 


Ozs. 


Mttal. 


Grams per 
coulomb. 


Grams per 
amp.-hour. 


(avoirdupois) 
per 
amp.-hour. 


(troy") 
per 
amp.-hour. 


Silver 


0-001118 


4-025 


0'1411 


0-1285 


Gold 
Nickel 
Copper (ous) .... 
(ic) .... 
J^inc 


0-000681 
0-000304 
0-000328 
0-000659 
0-000337 


2-452 
1-095 
1-186 
2-372 
1-219 


0-0860 
0-0384 
0-0416 
0-0831 
0-0427 


0-0783 


Iron (ous) 


0-000289 


1-042 


0-0365 




(ic) . 


0-000193 


0-694 


0-0243 




' ,v / 
Lead 

Tin (ous) .... 
(ic) .... 
Cadmium .... 
Cobalt 
Platinum .... 
Palladium .... 


0-001073 
0-000617 
0-000308 
0-000582 
0-0003056 
0-0005057 
0-0005541 


3-863 
2-320 
1-110 
2-097 
1-100 
1-8206 
1-9904 


0-1354 
0-0778 
0-0389 
0-0735 
0-0386 
0-0638 
0-0698 






0-0581 
0-0636 



NOTE. These figures are based on the generally accepted value for 
silver, viz. 0-001118 gram per coulomb. 

Faraday's Laws, with the introduction of the term 
" electro-chemical equivalent," may now be put into equa- 
tional form, and when so expressed the relationship is in- 
valuable for quantitative electro-deposition. 
Let Q = number of coulombs, 
I = current in amperes, 
t = time in seconds, 
W = weight deposited in grams, 
&, = electro-chemical equivalent. 

For 1 coulomb W = g, 
and from Law I W x Q 

Then for Q coulombs W = Q 
But Q = It 

.-. W = I x x / 

W 

or I = 



64 ELECTROPLATING 

The Electro-chemical Unit Quantity of Electri- 
city, the "Faraday." Eeferring again to Faraday's 
Second Law, suppose the same quantity of electricity to be 
passed through solutions of HC1, AgNCX., and CuSO, 
respectively ; then chemically equivalent quantities of sub- 
stances are produced at all the electrodes. At the electrodes 
H, Ag, and Cu will be liberated in the proportion of 
1-008 : 107-88 : 31-78, or as 1 : 107-02 : 31-52 ; thus for every 
one gram of hydrogen, there will be 107'02 grams of silver 
and 31"52 grams of copper, and these numbers may be 
called 1-gram equivalents (i.e. they are the chemical equivalent 
weights taken in grams). In other words, 107-02 and 31-52 
are the equivalent weights in grams of silver and copper 
respectively corresponding to the liberation of one gram of 
hydrogen. Other substances (ions) may be regarded in a 
similar way. 

Now, to liberate 1 gram of hydrogen requires the passage 
of 96,540 coulombs,"" from which it follows that 96,-540 
coulombs are required for the deposition of one gram-equivalent of 
any substance (ion). This fundamental quantity of electricity 
is called by the Germans a " Faraday." 

Consequently, the passage of one faraday through an 
electrolyte is accompanied by the liberation at the anode and 
cathode respectively of one gram-equivalent of new material. 
To render this as clear as possible, take the electrolysis of 
water as another example and pass through it a faraday of 
electricity ; then as the chemical equivalents of oxygen and 
hydrogen are 8 and 1*008 respectively, 8 grams of oxygen 
and 1-008 grams of hydrogen will be set free at the 
electrodes. 

Examples of the application of Faradatfs laics : 

1. Find the current which was used in depositing 
50 grams of silver, the time occupied in deposition being 
45 minutes. The electro-chemical equivalent of silver is 
0-001118 gram. 

* This is obtained by dividing the weight liberated 1 gram by 
the E.C.E. of hydrogen. 



QUANTITATIVE ELECTRO-DEPOSITION 65 

W 



Nowl = 

.tuting th< 
= 45 x 60. 



&.t 

By substituting the given figures, W = 50 ; = 0-001118 ; 

50 



0-001118 x 45 x 60 
I = 16*5 amperes. 

2. What weight of copper would be deposited by a 
current of 16-5 amperes passing through a solution of copper 
sulphate for 45 minutes ? The electro-chemical equivalent 
of copper is 0-00033 gram. 

W I x & x / 

TV J. /\ g*j S\ If 

Substituting the given figures, 

W = 16-5 x 0-00033 x 45 x 60 
.-. W = 14-7 grams. 

3. Find the electro-chemical equivalent of copper from 
the data obtained in Example 2. 

W = I X & X t 

W 

Therefore $, = -? -. 

Now, W was found to be 14-7 grams. I = 16-5, t = 45 x 60. 

By substitution , = -IQ.^^A.K ^~QQ' 
.-. = 0-00033 gram. 

4. During a certain plating operation 321 grams of silver 
are deposited. How many f aradays have been used ? 

107-02 (say 107) grams are deposited by 1 faraday. 

1 gram is deposited by yi ? 
/. 321 grams are deposited by ffy 

= 3 faradays. 

E.M.F. required for Electrolysis. Hitherto in our 
consideration of the relationships existing between electricity 



66 ELECTROPLATING 

and chemistry our attention has been confined to the study 
of the meaning and applications of Faraday's laws. 

These laws are, however, only an expression of one 
feature of these relationships. It is necessary now to 
consider not only the quantity of electricity which passes or 
is moved through an electrolyte, but the total amount of 
work done or energy expended in moving this quantity \ and 
further, not only the amount of chemical action resulting, 
but the intensity or affinity (as it has been termed) of this 
action. In other words, attention must be paid to the 
E.M.F. required in electrolysis as well as to the quantity 
of electricity to obtain an exact amount of electrolytic 
product. 

On page 18 we explained that when an electric current 
is passed through an electrolyte a definite amount of energy 
is used up and a definite amount of work done in the form 
of chemical decomposition, e.g. in an electrolyte of copper 
sulphate the substance is resolved into the products Cu and 
SO 4 . But by reason of the fact that work is done during 
this operation these two products possess a certain potential 
energy, in virtue of which they can re-unite, and if by any 
means they do re-combine, then their potential energy is 
given up in some form or other. This may occur either in 
the form of electrical energy or heat energy or both, but in 
any case it must be re-applied before the substance can be 
again decomposed. 

Briefly, then, the amount of energy produced by combi- 
nation must be equal to that expended in decomposition. 
Thus, suppose the product of combination to be heat energy, 
then : 

Electrical energy} _ CHeat energy of 
of decomposition ) (re- combination. 

This, of course, is only in accordance with what has been 
previously stated in describing the law of the conservation of 
energy. 

It has already been explained (in Chap. II.) that the 



QUANTITATIVE ELECTRO-DEPOSITION 67 

amount of heat absorbed or evolved in chemical reactions 
varies according to the affinity of the substance concerned, 
and a definite value can be attached to every particular 
combination. 

If we take any column of Table III., p. 19, the nearer a 
substance is to the top of the column, the higher as a general 
rule is its heat of combination, e.g. that of zinc is higher 
than that of copper. When, therefore, zinc replaces copper 
in combination with their respective sulphates a certain 
amount of energy is evolved or given out in the form of 
heat and dissipated. The practical result is that a lesser 
amount of energy is required to decompose copper sulphate 
than zinc sulphate. 

These points are obviously of great importance in either 
the theoretical or practical study of the applications of 
electro-chemistry. 

One or two examples of methods of calculation will 
no doubt assist the reader to understand more thoroughly 
this important principle. It has already been explained 
that the quantity of electricity required to deposit or liberate 
one gram-equivalent of any substance is 96,540 coulombs ; 
the practical point under discussion now is, therefore, 
what pressure is required to move this quantity of electricity 
in any electrolytic reaction ? 

Since our basis of calculation is the heat energy evolved 
in any combination, we make use of Joule's Law (page 54), 
from which we get that 

1 calorie = 4-2 joules, 
or 1 joule = 0*24 calorie. 

Now, suppose that, as our first example, a simple 
univalent compound be taken, sodium chloride. 

The number of calories evolved during the combination 
of one gram-equivalent (23 + 35'5 = 58-5 grams) of NaCl 
has been found to be 97,900 calories (see Table VII.). 

Now, the amount of electrical energy equivalent to this 
figure is found by a very simple calculation : 



68 ELECTROPLATING 

If 0'24 calorie = 1 joule, 
then 9 ^|5? = 4 o7,916 

= number of joules equivalent to 97,900 

calories. 

This figure represents, therefore, the total energy required 
to decompose one gram -equivalent of NaCl. 

Now we know that the quantity of electricity required 
is 96,540 coulombs ; 

,, joules 

.-. since volts = - jL 1 - r , 

coulombs 

the electrical pressure required is 

%$-**** 

Another example which may be taken (almost a classical 
one) is acidulated water, H 2 O. 

In this case we have a bivalent compound, and ac- 
cording to Faraday's Laws the number of coulombs 
required for the decomposition of one gram-equivalent is 
96,540 x 2 = 193,080. 

The number of heat units evolved in this combination 
is 68,400 calories (see Table VII.). 

no AC)(\ 

Equivalent in joules = ' = 285,000. 



/. by the formula volts = ~ - r- , 

coulombs 

the pressure required for electrolysis is 

285,000 

193^80 = 1>47 volts ' 

Eecent research has determined the number of heat units 
evolved in a large number of combinations, and particulars 
of these may be obtained from any good text-book on 
Thermo-chemistry, but a few of the best-known compounds 
are given in Table VII. 



QUANTITATIVE ELECTRO-DEPOSITION 69 

TABLE VII. 

HEAT UNITS EVOLVED IN COMBINATIONS OF CERTAIN COMPOUNDS. 



Compound. Formula. No, of Calories. 

Magnesium chloride ! MgCL 217,300 

KOH" 103,200 

KC1 104,300 



Potassium hydroxide 
Potassium chloride 



Sodium hydroxide . 
Sodium chloride 
Zinc chloride . . 
Cadmium chloride . 
Ferric chloride . . 



NaOH 101,900 

NaCl 97,900 

ZnClo 97,200 

CdClo 93,240 

96,100 



Ferrous sulphate FeSO, 235,600 

Nickel sulphate NiSO 4 229,400 

Cupric chloride CuCl 2 51,630 

Cupric sulphate CuS0 4 182,600 

Silver nitrate AgN0 3 28,700 

Gold chloride AuCl 3 22,800 

Water H 2 68,400 

In applying 'these theoretical principles to practical electro- 
plating, and so obtaining results such as are tabulated in 
Table VIIL, it is, however, necessary to point out that they 
are only exactly applicable in cases where insoluble anodes 
are used. If the particular compound formed by the union 
of the liberated product at the anode surface with the metal 
of the anode is soluble, then the anode in such a case is 
spoken of as a soluble anode, in the opposite event as insoluble. 
If now the anodes are soluble in the particular electrolyte 
being decomposed, as, for example, is the case when copper 
anodes are used in the electrolysis of copper sulphate, then 
under perfect conditions of electro-deposition CuS0 4 is re- 
formed by combination of SO 4 with the metal of the anode 
as quickly and to the same equivalent amount as the deposit 
occurring at the cathode. Obviously, therefore, the amount 
of heat of re-formation will equal the amount of decom- 
position, and the minimum voltage in this case is that 
required to overcome the electrical resistance only; theo- 
retically, no E.M.F. is necessary for decomposition. 

This point constitutes the principal difference between 
soluble and insoluble anodes in electrolysis, and it will be 
noted that a higher voltage is required when the latter are 
used than when the former are employed. 



ELECTROPLATING 



TABLE VIII. 

ELECTRODE PRESSURES NECESSARY FOR DECOMPOSITION OF VARIOUS 
SOLUTIONS (Ls BLANC). 



Normal solutions of 


Values 
in volts. 


Normal solutions of 


Values 
in volts. 


Zinc sulphate, ZnS0 4 . 
Nickel NiS0 4 . 


2-35 
2-09 


Sodium hydroxide, NaOH 
Ammonium hydroxide, 
NH 4 OH 


1-69 
1-74 


chloride, NiCl 2 . 
Lead nitrate, Pb(N0 3 ) 2 
Silver AgN0 3 . 
Nitric acid, HN0 3 . . 


18-5 
1-52 
0-70 
1-69 


Cadmium nitrate, Cd (NO 3 ) 2 
Cobalt sulphate, CoS0 4 . 
Sulphuric acid, H 2 S0 4 . 
Hydrochloric Acid, HC1 . 


1-98 
1-92 
1-67 
1-31 



It should be stated that it is not strictly accurate to 
describe the foregoing calculations (as is sometimes done) 
as the determination of the P.D. required for electrolysis. 
What is determined is, strictly speaking, the tendency of a 
specific electrolyte to set up an E.M.F. ; the chemical affini- 
ties described being those which in the primary cell, as will 
be shown in the following chapter, are so manipulated as to 
set up an E.M.F. for external use. Consequently, whatever 
current is used for electrolysis, it must have a P.D. 
sufficiently high to overcome (a) the " back" E.M.F. of the 
electrolyte ; and (b) the mass resistance of the liquid itself, 
or in other words the " R " of Ohm's Law. 

Soluble and Insoluble Anodes contrasted. It will 
at this point be necessary for the sake of clearness to 
consider the difference between the actions occurring in the 
use of soluble and insoluble anodes respectively in electro- 
lysis. 

Taking as an example of the former, one of its simplest 
illustrations, let it be proposed to electrolyse a solution of 
copper sulphate by means of copper electrodes. The actions 
taking place, expressed in the simplest form, are 

Cathode <- Cu I S0 4 -> Anode. 



QUANTITATIVE ELECTRO-DEPOSITION 71 

Cu is consequently deposited as metallic copper, and SO 4 
is left, which, however, is liberated in contact with a fresh 
supply of metallic copper, and we get the re-formation of 
CuSO 4 to undergo the same cycle of change. Therefore the 
chemical changes taking place exactly neutralize each other, 
and no chemical work is done ; consequently no back E.M.F. 
is set up, and the pressure required for electrolysis is that 
needed only to conform to the terms of Ohm's Law. 

On the other hand, suppose that the same electrolyte is 
submitted to electrolysis by means of platinum electrodes. 
In this case the anode is insoluble, but the same reactions 
occur as previously 

Cathode <- Cu | SO, -> Anode. 

Cu is deposited as metallic copper, and S0 4 is liberated 
in contact with the Pt anode. Now, however, it is evident 
that no re-formation of CuSO 4 can take place, and what 
happens is that the S0 4 (sulphion) being liberated resumes 
its normal chemical nature, and instantly breaks up in 
contact with the water of the electrolyte into sulphuric acid 
and oxygen, thus 

S0 4 + H 2 = H a S0 4 + O. 

Here, then, are exactly the conditions necessary for the 
setting up of a back E.M.R, I.e. an E.M.F. whose tendency 
is in the opposite direction to that of the current being 
applied for the purpose of electrolysis ; and consequently 
the P.D. of the latter must be high enough to overcome both 
this and the second factor previously referred to, namely, 
that of the mass resistance. 

This point, known in electrotechnical literature as 
" polarization," will be made clearer as the student proceeds 
to the study of primary cells in the succeeding chapter. 

Reactions at Anodes and Cathodes. Faraday's 
laws apply not only to the reactions due to electrolysis at 
the cathodes, but also at the anodes. In the case of the 
decomposition of water, for example, a definite current will 
liberate a definite amount of hydrogen at the cathode, and a 



72 ELECTROPLATING 

correspondingly equivalent amount of oxygen at the anode 
surface. It will be clear, therefore, that if this amount of oxygen 
completely combined with the metal of the anode, then the 
weight of metal thus taken up would be chemically equiva- 
lent not only to the amount of oxygen, but to that of 
hydrogen liberated by the current's action. 

In the case of an electroplating bath, it is generally sought 
to obtain such a composition of solution that the particular 
compound formed at the anode surface with the metal of 
the anode is soluble in the electrolyte. 

Anode and Cathode Efficiencies. It is also the 
object of the electroplater in designing a solution for a par- 
ticular branch of the electro-deposition of metals not only 
to obtain one which will give a deposit of good quality and 
suited to his requirements, but also one which will be 
efficient, that is, yield an amount of deposit as closely as 
possible approximating to the theoretical yield as given by 
Faraday's laws. These laws have always been found correct, 
but it must be borne in mind that the products of electro- 
lysis are not necessarily one only in each solution. Indeed, 
it rarely happens that this is so ; other products as well as 
the particular metal concerned are set free at the cathode. 
For example, in most solutions hydrogen is liberated in 
addition to the metal, and that portion of the current which 
is occupied in doing this is wasted so far as the prime object 
of electrolysis is concerned. Similarly, even in the case of 
soluble anodes there may appear products of electrolysis at 
the anode which do not combine with it to form a soluble 
compound, and in this event again the current is so far 
wasted from the point of view of solution of the anode metal. 

The general efficiency, therefore, of a plating solution is 
determined by the proportion which the products of electro- 
lysis actually yielded in some given time bear to the theo- 
retical amount that should be yielded by the current passing 
for the time in question, and this proportion may be 
measured both at the anode and cathode. The terms anode 
and cathode efficiencies are thus given rise to. 



QUANTITATIVE ELECTRO-DEPOSITION 73 

In determining the efficiency of an electroplating process, 
tjae following data must be obtained. 

(1) Exact current passing (determined by means of a 
measuring instrument). 

(2) Time of experiment. 

(3) Nett weight of metal deposited (obtained by weighing 
the cathode before and after electrolysis). 

(4) Nett loss of weight of anode (obtained by weighing 
the anode before and after electrolysis). 

If a soluble anode is used it will be of the same metal as 
that deposited, the chemical equivalent will, of course, be 
the same, and consequently the loss of weight of anode 
should be equivalent to gain of weight of cathode, and both 
correspond to the requirements of Faraday's laws. In such 
an event (which would never occur except under very 
special precautions) the efficiency of each would be 100 per 
cent. To calculate the actual percentage of efficiency at 
each electrode, it is only necessary to divide the figure, 
obtained by experiment in each instance, by the theoretical 
figure and multiply by 100. 

Ezamfile. Calculate the cathode efficiency of a zinc 
depositing solution, on the electrolysis of which 10 amperes 
deposited 10'5 grams of zinc in 1 hour. 

Theoretical yield = 10 x 3600 x 0-000337 (E.C.E. of zinc) 

= 12-1 grams. 
Actual yield = 10-5 ,, 

1 0-^ 

.-. efficiency at cathode = =? x 100 = 86-77 per cent- 

' 



CHAPTER V 

PRIMARY AND SECONDARY CELLS 

IT has already been stated that the various forms of energy 
are convertible in accordance with the Law of the Con- 
servation of Energy, and that certain physical and chemical 
changes are accompanied by the evolution or absorption of 
heat. For example, if a stick of solder is bent rapidly 
backwards and forwards, it becomes perceptibly hot at the 
bend, due to the strain put upon it ; the mechanical energy 
expended during the process thus reappears in the form of 
heat. Again, when a small piece of potassium or sodium is 
thrown into water a violent chemical action ensues, owing 
to the great affinity of these elements for oxygen, and the 
evolution of heat is so great that the liberated hydrogen 
spontaneously ignites. Further, the energy imparted in 
effecting both physical changes and chemical actions will 
under certain conditions reappear, in part at any rate, in the 
form now called electrical energy. Obviously, then, electrical 
energy is a form that may be derived from some other form, 
and for practical purposes there are two modes by which 
the transformation of energy into its electrical form may be 
effected, namely : 

(1) By placing certain metals in dilute acids or some 
alkaline oxidizing solution. Such an arrangement is called 
a voltaic cell, and a number of cells joined together is termed 
a battery. 

(2) By utilizing a dynamo driven by some form of 
mechanical prime mover. 

For all work on a large scale, the dynamo is the most 



PRIMARY AND SECONDARY CELLS 75 

economical means, as far as cost per unit is concerned. 
This is in spite of the inefficiency attendant on the con- 
version of, say, coal into electrical energy, through the 
medium of the boiler, engine, and dynamo, and is due to the 
fact that coal is a comparatively cheap commodity. 

In the case of voltaic cells, the conversion is a more 
direct one, and the efficiency greater. The " fuel " is usually 
zinc an expensive one in comparison with coal and this 
is chemically " burnt" by the oxidizing agent in which it is 
placed. This " burning " tends to keep up the supply of 
energy when once the current has been established. 



PEIMAEY CELLS. 

The Simple Voltaic Cell. If a plate of zinc and one 
of copper are immersed in a solution of dilute sulphuric acid, 
so that the metals are not in contact with one another, the 
arrangement forms a simple cell. When such a cell is made 
up, then 

(1) If the zinc is pure, no action whatever is observed to 
take place. 

(2) If the zinc is impure, chemical action is shown by 
the bubbling which ensues. Impure zinc readily dissolves 
in dilute sulphuric acid due to local action (see p. 77), but 
if the zinc be amalgamated, >. coated with mercury, no 
such action takes place. 

(3) No action is so far observed to take place at the 
copper plate. 

(4) But if the two plates are connected externally by a 
wire, it will be found that (a) the zinc plate gradually 
dissolves, (b) a gas hydrogen is given off at the copper 
plate, some of which adheres to the surface of the plate in 
the form of bubbles, (c) a current of electricity passes round 
the circuit, as shown by the fact that when the wire is held 
near to and parallel with a magnetic needle the needle is 
deflected. 

Now, as currents of electricity are always associated 



76 ELECTROPLATING 

with an electricity-moving force or E.M.F., the cell must be 
the seat of an E.M.R, since the wire is quite inert when 
disconnected. Some idea of how the E.M.R is set up will 
not be out of place, and without entering too much into the 
theory or theories which have been advanced, the following 
may assist the reader. 

As previously stated (p. 17), when a plate of zinc and a 
plate of copper are immersed in dilute sulphuric acid, it may 
be experimentally demonstrated that both plates are in a 
state of electrical charge, the copper positively, the zinc 
negatively, and a P.D. exists between them. According to 
modern theory this may be explained by considering (1) that 
part of the molecules contained in solution are dissociated 
into positively and negatively charged ions, " H 2 " and " S0 4 " 
respectively, (2) that at the moment of immersion, owing to 
what may here be termed the " electro-chemical activity " of 
zinc, a few " Zn " ions are sent off into this solution. These, 
like the hydrogen ions, possess positive charges, and the 
zinc plate is made relatively negative. Simultaneously there 
occurs the passage of a few (H) ions to the copper plate, 
which on contact render it positive. Now when the copper 
plate of the cell is made to touch the zinc or, which is the 
the same thing, is brought by a wire into metallic contact 
with it, a current passes and the copper takes the same 
charge as the zinc and becomes negative. Thus more + (H) 
ions are attracted to the copper and coming into touch with 
it give up their charges and again render this plate positive. 
The P.D. is thus maintained, and a current still passes. 
More " Zn " ions now go into solution, the H ions being 
thus further replaced by Zn, and as long as the circuit is 
complete, an E.M.F. is continuously exerted in the direction 
from copper to zinc via the wire. 

The chemical action resulting from the working of a 
simple cell may be expressed as follows : 

Zn -f H 2 S0 4 = ZnS0 4 + H 2 
Zinc and sulphuric acid are therefore used up in the 



PRIMARY AND SECONDARY CELLS 77 

formation of zinc sulphate and hydrogen gas, the latter 
being of course given off at the copper plate. 

It will now be advisable to mention that as some mis- 
understanding frequently occurs through an apparent am- 
biguity in the designation of the "plates" and "poles" of 
primary cells, that plate which dissolves during working 
is generally termed the positive plate or positive element, 
and the other plate is termed the negative plate or 
negative element; whereas the terminal of the latter is, 
according to the direction of the current in the external 
conductor, the positive pole, and the terminal of the former 
the negative pole. Thus the positive plate forms the negative 
pole, and vice versa. To avoid confusion the plate dissolved, 
zinc, in all the cells to be considered will be referred to as 
the lower potential element, the other the higher potential 
element ; there is then no doubt as to which pole is positive 
to the other. 

Local Action. Common zinc contains impurities, such 
as iron, lead, arsenic, etc., and dissolves readily in sulphuric 
acid, an effect which may be ascribed to electrical causes, 
for these impurities, together with the zinc, being in contact 
with the dilute acid, give rise to a number of local currents 
which circulate between the impurities and the zinc. This 
local action is prevented by amalgamating the zinc, and as 
common zinc is always used in the construction of cells, it 
should always be amalgamated to prevent the zinc being 
eaten away more rapidly than corresponds to the rate at 
which electrical energy is developed in the circuit as a whole. 

Polarization. The hydrogen which accumulates on the 
copper plate during working is very deleterious, inasmuch as 
it sets up a back E.M.F., and in consequence weakens the 
E.M.F. available for sending a current. This accumulation 
of bubbles of hydrogen is termed polarization, and the more 
practical forms of primary cells are mainly devices for the 
elimination of this effect. In all modern cells the hydrogen 
is got rid of by placing in the cell some chemical compound 
which contains oxygen, and which will readily give up its 



78 ELECTROPLATING 

oxygen in the presence of hydrogen. Such a substance is 
called a " depolarizer," and the following are the ones used 
in the cells to be considered next : 

(1) Copper sulphate, 

(2) Bichromate of potash, 

(3) Chromium trioxide, 

(4) Nitric acid, 

and several others. 

The Daniell Cell. This cell is made up in a variety of 
forms, according to the class of work for which it is intended. 




FIG. 9. Section of the Daniell Cell. 

C, copper containing vessel ; P, porous pot ; Z, zinc rod ; S, perforated 
copper shelf ; CS, copper sulphate crystals ; H 2 S0 4 , dilute sulphuric 
acid ; CuS0 4 , copper sulphate solution. 

The form shown in Fig. 9 may be taken as being typical of 
one frequently used. The high potential element is a sheet 
of copper bent into a cylindrical containing vessel, holding 
a saturated solution of copper sulphate. In this solution is 



PRIMARY AND SECONDARY CELLS 79 

also immersed a porous earthenware pot containing the low 
potential element zinc and a solution of sulphuric acid 
diluted to a strength of about 1 part of acid to 12 20 parts 
of water. 

Action of the cell. When the cell is in action oxygen is 
given off at the zinc, and the hydrogen ions are transported 
towards the copper plate. The oxygen attacks the zinc, 
forming zinc oxide, which, however, in the presence of the 
acid ultimately becomes zinc sulphate and dissolves in the 
solution. The hydrogen before reaching the copper plate 
comes in contact with the copper sulphate solution, which 
being decomposed forms sulphuric acid and liberates copper, 
the latter being deposited on the copper plate. 

The resulting reactions may be shown by the following 
equations : 

(1) In porous vessel Zn + H 2 SO 4 = ZnS0 4 + EL 

(2) In outer vessel H 2 + CuSO 4 = H 2 SO 4 -f Cu. 

Hence the result of the reactions is such that : 

(1) The zinc is consumed. 

(2) The sulphuric acid in the porous vessel is used up in 
the formation of zinc sulphate. 

(3) The copper sulphate in the outer vessel is changed 
into sulphuric acid. 

(4) Copper is deposited on the copper plate. 

The cell will not polarize so long as the above action 
proceeds, and so long as the copper sulphate solution is not 
allowed to become weak, but to ensure immunity when 
required to work for long intervals, a perforated copper shelf, 
or a muslin bag, containing crystals of copper sulphate is 
suspended in the solution, and these crystals gradually 
dissolve as the solution weakens. In making up Daniell 
cells it is advisable to have the level of the acid solution a 
little higher than that of the copper solution, to prevent the 
latter from too readily diffusing into the vessel containing 
the zinc; for in the event of this happening the zinc is 
attacked, oxide of copper is deposited on it, and the action of 



8o 



ELECTROPLATING 



the cell is interfered with. The use of the porous pot is, in 
fact, to keep the solutions in contact so preserving the 
electrical continuity but yet to prevent their mixing too 
freely. Most of the depolarizers in use will attack zinc, and 
hence they are kept in a compartment separated from the 
zinc by the porous walls of the pot. 

The Bichromate and Chromic Acid Cell. In both 
these cells the depolarizer is chro- 
mium trioxide (CrO ;! ), popularly 
called " chromic acid," as it has a 
strong acid reaction when dissolved 
in water. Formerly this material 
(CrO.) was prepared by the user, by 
acting on potassium bichromate 
(K 2 Cr 2 O 7 ) with sulphuric acid, but 
as chromium trioxide can now be 
purchased ready prepared, it is often 
used in preference to potassium bi- 
chromate. Consequently, as the 
cells in other respects are identical, 
one description will suffice. Figs. 
10 and 11 show two types of the 
cell, the " bottle " and " Fuller " re- 
spectively. The former is useful for 
portable purposes, but for general 
working where the cells are more or 
less stationary, the latter has several 

zinc plate; E, ebonite advantages; it is easier to clean 
cap ; B, bichromate or anc } its p ar t s are easier to replace 
chromium trioxide solu- , , . , .. 

tion. when worn or broken, and it can 

be left set up out of work without 
appreciable wastage of zinc. 

In the Fuller pattern the outer glazed earthenware vessel 
contains the depolarizing solution made up from one or other 
of the following formulae : 




FIG. 10. Bottle form of 
Bichromate Cell. 



PRIMARY AND SECONDARY CELLS 81 

Chromium trioxide .... 2 ozs. 

Sulphuric acid 2 ozs. (by weight). 

Water 1 pint. 

Bichromate of potash ... 2 ozs. 

Sulphuric acid 3-5 ozs. (by weight). 

Water 1 pint. 

One or more carbon plates electrically connected are 
;mmersed in the solution, forming the high potential element. 




FIG. 11. Section of Fuller's Bichromate Cell. 

V, glazed earthenware vessel ; P, porous pot ; cc, carbon plates elec- 
trically connected ; Z, zinc rod ; M, mercury ; B, bichromate or 
chromium trioxide solution ; H 2 S0 4 , dilute sulphuric acid. 

The low potential element is a zinc rod with an enlarged 
base as shown, immersed in a solution of dilute sulphuric 
acid (1 in 10) and standing in a small pool of mercury 
contained at the bottom of a porous pot. The mercury 
ensures the zinc being kept amalgamated automatically. 

Action of the cell : The chemical reaction in the porous 
cell is similar to that of the Daniell, viz. : 



3Zn 



j = 3ZnS0 4 + 3H 2 . 



82 ELECTROPLATING 

The chemical reactions taking place when the hydrogen 
reaches the depolarizing solution are best shown in several 
stages. 

With solution made from potassium bichromate, 

(2) mixing 

K 2 Cr 2 O 7 + 7H 2 SO 4 + H 2 O = 2H 2 CrO 4 -f K 2 S0 4 + 6H 2 SO 4 . 

.^ (chromic acid) (potassium 
^^ sulphate) 

(3) 3H 2 -f 2H 2 Cr0 4 = Cr 2 O, + 5H 2 O. 

(fronv(l)) (chromium 

< trioxide) 

(4) Cr 2 3 + 3H 2 S0 4 = Cr 2 (S0 4 ) :! + 3H 2 0. 



(5) K 2 S0 4 + Cr 2 (S0 4 ) 3 = K 2 Cr 2 (S0 4 ) 4 . 

The net result of the reactions is therefore : 

(1) The zinc is consumed, zinc sulphate formed, and 

sulphuric acid used up. 

(2) The original potassium bichromate and some 

sulphuric acid are changed into chrome alum 
(K 2 Cr 2 (S0 4 ) 4 ). 

(3) Water is substituted for the remaining sulphuric acid. 
When the depolarizer is made directly by dissolving 

chromium trioxide in water, equations (3) and (4) show the 
reactions which take place. 

The Bunsen Cell. The usual form of this cell is 
illustrated in Fig. 12. An outer glazed earthenware vessel 
contains a solution of dilute sulphuric acid (1 in 10) in which 
is immersed a plate of stout sheet zinc bent into a cylindrical 
form, constituting the low potential element. Inside this is 
a porous pot containing the depolarizer strong nitric acid 
and a rectangular carbon block forming the high potential 
element. 

Action of the cell : When the cell is at work the chemical 
reactions may be thus represented : 

(1) In outer vessel Zn -f H 2 SO 4 = ZnSO 4 -f H 2 . 

(2) In porous pot H 2 + 2HN0 3 = 2NO 2 -f 2H 2 O. 

(nitrogen 
peroxide) 



PRIMARY AND SECONDARY CELLS 83 

In working, therefore, zinc and sulphuric acid are used 
up in the formation of zinc sulphate and the liberation of 
hydrogen ; the nitric acid becomes diluted by the formation 
of water, nitrogen peroxide being liberated. This latter is a 
gas, and its formation results in very objectionable reddish- 




FIG. 12. Section of Bunsen Cell. 

V, glazed earthenware vessel ; P, porous pot ; C, carbon plate, Z, zinc ; 
H 2 S0 4 , dilute sulphuric acid ; HN0 3 , concentrated nitric acid. 

brown fumes being given off, especially after the cell has 
been working for a time and the nitric acid has become 
weakened. For this reason such cells should be placed where 
a current of air will carry the poisonous fumes away from the 
user. 

The Edison-Lalande Cell. The chief feature in the 
construction of this cell is in the high potential element, which 
consists of finely ground copper oxide compressed into 
plates and held in a suitable copper framework. Two well 



8 4 



ELECTROPLATING 



amalgamated zinc plates, electrically connected, form the 
lower potential element ; they are arranged one on each side 
of the copper oxide plate (Fig. 13). 
The elements are suspended from 
the lid of a glazed earthenware vessel 
in a solution of caustic potash, made 
up in accordance with the following 
formula.! 

Caustic potash ... 2 Ibs. 
Water 5 pints. 

Action of the cell : When a cur- 
rent is taken from the cell, potas- 
sium zincate is formed, and the 
hydrogen reduces the copper oxide 
to metallic copper. The oxygen 
in the copper oxide serves as a de- 
polarizer. The chemical equations 
are as follows : 
FIG. 13. The Edison- 
Lalande Cell. 

G, glass or glazed earthen- 
ware containing vessel ; Z, 
zinc plate ; CuO, copper 
oxide plate ; F, framework 
supporting copper oxide 
plate ; P, layer of paraffin . 
oil. 




(1) Zn + 2KHO = K 2 ZnO 2 -f H,. 

(caustic potash) (potassium zincate) 

(2) H 2 + CuO = EL,0 + Cu. 

(copper oxide) 

Care and Management of 

Jells. To maintain primary cells 

in good working order the following 



points should be observed : 

(1) Keep the zincs well amalgamated. To amalgamate 
a plate, first clean it by immersion in dilute 
sulphuric acid (1 6) and allow it to gas freely for 
a few minutes. Pour on the cleaned surface a little 
mercury and rub briskly with a swab of rag until 
the surface is covered. New zinc is liable to have 
a greasy surface ; so before attempting amalgama- 
tion, dip it several times in a hot potash solution to 
dissolve the grease, scour well with sand to remove 
the film of potash solution, and afterwards 
thoroughly wash it with water. It may then be 



PRIMARY AND SECONDARY CELLS 85 

immersed in the sulphuric acid and rubbed over 
with mercury as directed above. 

(2) Porous pots when not in use should be left soaking 

in water and not allowed to dry before being 
thoroughly washed, or they will soon fall in pieces. 

(3) After a cell is exhausted, thorough washing and the 

addition of fresh solution will put it in order again. 

(4) Nitric acid is useless in cells when it has turned 

green. Similarly, bichromate solution should be 
thrown away after it has turned a dark colour with 
a greenish tint. 

(5) In making up cells in which the depolarizer is in a 

separate compartment, avoid the possibility of its 
getting to the zinc, by having the sulphuric acid 
solution a little higher than the other (about J inch). 

(6) When diluting H 2 SO 4 with water, slowly add the acid 

to the water, and not vice versa, since a rapid 
evolution of heat takes place during the mixing. 
Allow the mixture to cool before using. 



SECONDARY CELLS OR ACCUMULATORS. 

Principle of the Lead Cell. If two clean lead plates 
be immersed in dilute sulphuric acid, and their extremities 
connected with a low-reading voltmeter, no evidence of an 
E.M.F. is obtained, and no current can be derived from the 
arrangement. But if a current of electricity be sent through 
it for a few minutes from some external source, then, after 
disconnecting the source and again applying the voltmeter a 
reading of about 2 volts will be shown. Further, on examin- 
ing the plates, the anode will have a chocolate coloration 
on its surface, while the cathode is unaltered. What has 
happened is that oxygen and hydrogen have been liberated 
by electrolytic action, the former at the anode, the latter at 
the cathode. The oxygen has combined with the lead sur- 
face of the anode forming lead peroxide (PbO 2 ), while the 
hydrogen at the cathode mostly rises to the surface of the 



86 ELECTROPLATING 

liquid and leaves the plate unaffected. Hence the surfaces 
of the plates have two different chemical compositions ; this 
difference gives rise as in the case of a primary cell to an 
E.M.F., and a current may be drawn from it for a few 
seconds, or until the resulting chemical action forms on both 
plates lead sulphate. The cell is then inert, but the process 
may be repeated theoretically ad i/ifi/iitum, for on again pass- 
ing a current through, the lead sulphate on the anode is 
re-formed into lead peroxide, while that on the cathode is 
reduced to metallic lead. 

Such is the principle of a lead secondary cell or accumulator. 
It differs therefore from a primary cell in that its elements or 
plates have first to be put into the necessary chemical condi- 
tion by electrolysis. In other words, the plates have to be 
" polarized." 

It has been shown that polarization is detrimental to the 
proper working of a primary cell, chiefly on account of the 
back E.M.F. introduced thereby, but in an accumulator 
polarization is directly aimed at. During the chemical con- 
version of the plates by electrolysis a process called 
" charging " the cell the cell itself exerts an E.M.F. which 
is always in opposition to that of the charging source, and 
electricity has to be forced through the cell against this back 
E.M.F. ; consequently a pressure greater than 2 volts per 
cell has to be available for charging purposes. On the other 
hand, after it has been charged and the charging source 
removed, it is this polarization E.M.F. which serves to main- 
tain the current during the discharge of the cell. 

An accumulator may therefore be looked upon as a cell 
in which energy is kept in store to be used as occasion 
requires. The reader should particularly observe that there 
is no accumulation or storing of electricity ; fundamentally in 
forming the cell electrical energy is transformed into chemical 
energy, and when used to supply a current the energy trans- 
formation is merely reversed. 

The Modern Accumulator. Very little need be said 
here on the usual mode of construction ; so many are in use, 



PRIMARY AND SECONDARY CELLS 87 

small cells especially, that their general make-up is well 
known. A brief reference to the plates, however, may be 
advantageous. Two distinct types are in use, namely : 

(1) Plante", or so-called unpasted plates. For +ve plates 
only. 

(2) Faure, or pasted plates. For both -fves and ves. 
The distinction arises from the mode of forming the 
active material on the plates, i.e. the lead peroxide and the 
spongy lead on the positives and negatives respectively. The 
plates are made in the form of grids ingeniously arranged to 




FIG. 14 (A). E.P.S. grids : Faure type of plate. 
(a) positive ; (6) negative. 

bind the material to the grids, and a few representative types 
are shown in Figs. 14, A, B and C. 

For Plant^ plates the lead peroxide is formed from the 
lead grid itself by chemical and electro -chemical means, a 
process which takes some time. 

For Faure plates the chemical formation is accelerated 
by filling the interstices of the grids with a mixture of red- 
lead (Pb 3 4 ) and sulphuric acid, the mere mixing of which 
forms a certain amount of PbO 2 according to the following 
equation : 



Pb ;! O 4 



4 -f H 2 = PbO, + 2PbS0 4 



2H 2 O. 



88 



ELECTROPLATING 



Subsequent electro- chemical action, when they are placed 
in dilute sulphuric acid and joined up as anodes, results in 
the following reaction due to the oxygen liberated, 



PbCX -f 2PbS0 4 



== 3Pb0 2 + 2H.,S0 4 + 2H 2 . 





(a) 

FIG. 14 (B). D.P. plates. 
(a) Positive ; (6) section of positive plate to a larger scale. 

Faure negative plates are pasted with a mixture of litharge 
(PbO) and sulphuric acid, which forms PbS0 4 (lead sulphate), 

PbO + H 2 S0 4 + H 2 = PbS0 4 -f 2H 2 0. 

Electro-chemical treatment, by making them cathodes in 
dilute sulphuric acid, reduces the PbSO 4 to spongy lead, due 
to the action of the hydrogen liberated, thus : 

PbS0 4 + H 2 = Pb + H 2 SO 4 . 

Chemical Changes during Discharge. Assume that 
a cell is fully charged and that a current is being taken from 
it. The direction of the current outside the cell is from the 



PRIMARY AND SECONDARY CELLS 89 
positive or peroxide plate to the negative plate, and vice 




inside.* Owing to the electrolysis of the electrolyte, 
* When referring to the accumulators it is now common practice to 



90 ELECTROPLATING 

oxygen is given off at the negative plate, hydrogen at the 
positive. The spongy lead at the negative becomes oxidized, 
and in the presence of sulphuric acid changed into PbS0 4J 
while the peroxide plate is converted into PbSO 4 , due to the 
hydrogen liberated there. During both reactions sulphuric 
acid is used up and water formed. 

The reactions may be shown as follows : 

At the negative plate 

Pb + H 2 S0 4 + = PbS0 4 + H.O. 
At the positive plate 

Pb0 2 + H 2 S0 4 + Ha - PbS0 4 + 2H 2 0. 

Chemical Changes during Charge. During charging 
oxygen is liberated at the positive and hydrogen at the nega- 
tive plate. The PbSO 4 on the plates is reconverted into 
Pb0 2 and Pb respectively, water is used up and sulphuric 
acid formed, the reactions being 

At the positive plate 

PbS0 4 + 2H 2 = Pb0 2 
At the negative plate 4 

PbS0 4 + H 2 = Pb 

It must be understood that there is still doubt as to the 
precise actions which take place in these cells, but the above 
equations showing the ultimate result are generally accepted. 

Capacity. The capacity of an accumulator is reckoned 
in ampere-hours. Since the product of a current multiplied 
by time is a quantity of electricity, this is the quantity of 
electricity which the cell will give before it is considered to 
be discharged. The capacity may range from 10 to 20 
ampere-hours in small portable cells, to several thousand 
ampere-hours in large stationary cells. The capacity is 

call the element whose pole is positive, the positive element or plate, 
and the one whose pole is negative, the negative element or plate. The 
usage of terms is therefore different from that in the case of primary 
cells (p. 77). 




PRIMARY AND SECONDARY CELLS 91 

dependent upon the amount of active material entering into 
the reactions, and to make it up to the requisite amount it is 
customary to use 2, 3, 4, etc., positive plates, arranged so that 
each of them is between two negatives ; all positives and 
likewise all negatives are connected together, so that they 
form virtually one large plate of each kind, plates of opposite 
polarity being kept completely apart by insulating separators. 
It must not be assumed, however (as is frequently the case), 
that because a cell is marked, say, 60 ampere-hours, that it 
will give 60 amperes for 1 hour, or 30 amperes for 2 hours, 
although it may give 15 amperes for 4 hours. Generally 
speaking, there is a certain maximum rate which ought not 
to be exceeded, otherwise the cell may show signs of decay 
prematurely, and the marking of the cells presupposes that 
the maximum permissible rate of discharge is not exceeded. 




FIG. 15. Method ot erecting large accumulators. 
(GT type ; D.P. cells with bolted connections on single-tier stand.) 

Erection, Care, and Management of Accumulators. 

The general mode of erecting calls of large size for 



92 ELECTROPLATING 

stationary purposes may be gathered from Fig. 15. They 
are placed on a wooden tier protected from the ravages of 
acid and acid spray, by being coated with acid-resisting 
enamel. The cells are supported at each corner on glass or 
earthenware insulators containing oil, and arranged with 
their connecting lugs alternately positive and negative. Con- 
nections between adjacent cells are made either by bolting 
the lugs together with special bolts well protected with 
vaseline, or by welding them together with an oxy-hydrogen 
flame. 

By strict attention to the following points cells may be 




FIG. 16. Portable accumulator in celluloid case. 

kept in good condition for many years, although like other 
things they naturally deteriorate in course of time. 

(1) Never allow them to stand for any length of time in 
a discharged or partially discharged condition. 

(2) If not required for use, do not empty out the acid ; 
give them a full charge periodically. 

(3) Keep the level of the solution well above the top 
edges of the plates, and make up any evaporation by the 
addition of distilled water. 



PRIMARY AND SECONDARY CELLS 



93 



(4) Periodically test the density of the acid and see that 
it complies with the maker's recommendations. 

(5) With small portable cells in celluloid cases (Fig. 16), 
it is advisable to replace the acid once every six months. 
To do this, charge up fully, empty out the acid, wash the 
cells out quickly with distilled water, empty, and immediately 
add fresh acid of proper density, and " gass up " again. 

(6) Accumulator manufacturers send instructions with 
their cells with respect to the charging current, density of 
acid, and other details, which should be adhered to as closely 
as possible. They also will supply suitable acid, but if the 
user wishes to make up his own, it is important to use 
distilled water, and either pure sulphuric acid or the variety 
known as brimstone sul- 
phuric acid. Never be 

tempted to use the com- 
mercial acid, and ordinary 
tap water. 

Charging Arrange- 
ments. The only satis- 
factory method of obtaining 
current for charging is to 
use a dynamo, or to make 
use of the public electricity 
supply mains, if such be fed 




sAAAA 
R 





sAAAA 

R 



Cells 



with direct current. In 
either case the number of 
cells which may be charged 
in series is dependent on 
the voltage of the source; 
2-5 volts per cell must be FIG. 17. Connections and accessories 

n i . . , n for charging accumulators, 

allowed for ensuring a full 

.., ,, , A, A, ammeters ; B, R, variable 

charge with the normal resistances ; D, dynamo, 

charging current. Thus 

suppose that a plating dynamo gives a voltage of 10 
volts, it would be just possible to charge four cells in 
series. The arrangement and connections are shown in 



94 ELECTROPLATING 

Fig. 17. Connect the positive pole of the dynamo through 
a suitable variable resistance, to the positive pole of the cells, 
putting an ammeter and a switch in circuit ; connect the 
negative pole of the cells to the negative pole of the dynamo. 
With all the resistance in circuit, close the switch, and then 
adjust the resistance to give the required current. Keep 
the current constant by readjusting the resistance as 
occasion requires. If the dynamo is of ample capacity, 
another set of four cells could be charged at the same 
time by arranging them as shown below the dotted line. 

E.M.P. of Cells. The following table gives the approxi- 
mate E.M.F. of the various cells considered. 

TABLE IX. 

Kind of cell. Approx. E.M. F. 

Simple cell 1-0 volts 

Daniell 1-07 

Chromic acid 1-95 

Bunsen 1-9 

Edison-Lalande 0-75 

Storage 2-0 

Arrangement of Cells in Series and in Parallel. 

The preceding table shows that the E.M.F. of a single 
cell is only of the order 0*75 to 2 volts, but a larger 
E.M.F. can be obtained by the employment of a number of 
cells and connecting them up in series. Cells are said to be 
" in series " when the negative pole of the first cell is con- 
nected to the positive pole of the second, the negative of the 
second to the positive of the third, and so on. Fig. 18 (a) 
shows four cells connected in this manner. The' thick 
strokes on the diagram represent the negative poles, and the 
thin ones the positive poles. As the E.M.F. of each cell acts, 
in the direction from negative pole to positive pole through the 
cell, we have a number of E.M.F.'s, each of them acting in the 
same direction along the conducting path, and the resultant 
E.M.F. of the arrangement as a whole is the sum of their 
separate E.M.F.'s. Thus four Daniell cells in series would 
have an E.M.F. of 4 x 1'07 = 4-28 volts. 



PRIMARY AND SECONDARY CELLS 



95 



It is clear also from previous considerations that the 
internal resistance of the battery is the sum of the individual 
resistances of the cells composing it. Connecting in series, 
therefore, not only increases the E.M.F., but also the resist- 
ance of the battery. 

It is also important to remember that the E.M.F. of a given 



(a) 




Series -Parallel. 

(2 in series, 2 rou'S\ 

in. parallel. / 

FIG. 18. 

Jcind of cell is the same whatever he its size, but a large cell will 
have a lower internal resistance than a small one. 

Another way of arranging a number of cells is to join 
them " in parallel." To do this all the positive poles are 
connected together to form a common positive, and likewise 
all the negative poles to form a common negative. Fig. 18 (b) 
illustrates the method, using four cells, but any number may 
be added in a similar manner. 

With this arrangement it is very necessary that all the 



96 ELECTROPLATING 

cells should be of the same kind or have the same E.M.F., 
and for preference they should be of similar size. If, for 
instance, the E.M.F. of two of them differs materially, it is 
easy to see that a current, independent of any current in the 
external circuit, will circulate round the closed circuit which 
the arrangement naturally forms between the cells a current 
which serves no useful purpose and wastes the active 
materials. 

A number of similar cells connected in this way virtually 
becomes one cell of n times the size,v where n is the 
number of cells in the battery; current is drawn simulta- 
neously from each of them, uniting and dividing at the 
common positive and negative terminals respectively. 

The E.M.F. of the combination is only equal to that of one 
cell, but the internal resistance of the battery is reduced to 

th of the resistance of one cell. 
n 

Very little need be said here on the relative merits of 
joining cells in series or in parallel, but one or two leading 
principles may be mentioned. Generally the series arrange- 
ment is the best when the external resistance is high, the 
parallel method when the external resistance is low, compared 
in both cases to the resistance of a single cell. 

A third method is also shown in Fig. 18 (c), a series-parallel 
arrangement. It i#, as may be seen, a combination of the 
former ones, consisting of several rows of cells joined in 
series, the rows being subsequently joined in parallel. The 
method enables us to increase the E.M.F. of the battery, but 
at the same time to keep down the internal resistance. It 
is advisable to use similar cells, and ensure that an equal 
number are placed in each row, for reasons given above. 

Uses of Cells. Before finally leaving the cells, it will 
not be out of place to refer very briefly to some of their uses 
in connection with the electroplater's art. 

The Daniell cell may be used for the deposition of copper 
on a small scale from an acid copper solution, and for small 
electrotyping work, such as medallions. The Bunsen is 



PRIMARY AND SECONDARY CELLS 97 

suitable for the deposition of nickel on small articles, or for 
gilding, while the Bichromate may be used for the prepara- 
tion of small quantities of gilding solution by electrolytic 
methods. The Edison Lalande, although of low E.M.F., has 
a small internal resistance, and is capable of sending currents 
of the order of 10 to 15 amperes without much polarization 
for 10 to 20 hours, before the supply of materials is exhausted. 
It may be left standing on open circuit without appreciable 
waste. Such currents, however, can only be obtained with 
external circuits of low resistance. 

Owing to the fact that accumulators may now be obtained 
in a large variety of designs and sizes at a reasonable price, 
and that in most towns means exist for having them 
recharged without much difficulty, they are, for many 
purposes, gradually taking the place previously occupied by 
primary cells. The modern accumulator is a very reliable 
article, and if properly looked after and used in a legitimate 
manner, will work satisfactorily for a number of years. 



CHAPTER VI 
THE DYNAMO 

IN Chap. III. it has in effect been shown that a dynamo is 
primarily a generator of E.M.F., and when at work main- 
tains a P.D. across its terminals, and across the various 
portions of any external circuit to which it is connected. 

In dealing therefore with this important piece of electrical 
apparatus, it will be advisable to explain those portions of it 
which are instrumental in the production of an E.M.F. ; how 
the E.M.F. is set up ; and then to develop our explanation 
into a practical machine. 

Before doing so, however, it will be advantageous to 
introduce a few elementary magnetic and electro-magnetic 
principles. 

Elementary Magnetic and Electro-Magnetic Prin- 
ciples. Every one is more or less familiar with some 
of the very elementary and yet striking properties of a 
magnet. It is well known that either end of a magnetized 
bar will attract and pick up small iron objects, such as nails, 
and cause a compass needle to be violently deflected when 
brought into its vicinity. If the compass needle be pivoted 
in a horizontal position, its ends point respectively to the 
magnetic N. and S., and however much it may be disturbed 
from this position it will swing to and fro and gradually 
come to rest in precisely the same position as before. Further 
investigation leads to the conclusion that the neighbourhood 
surrounding a magnet is in a special condition different from 
the same space when the magnet is removed, inasmuch as 
there is manifested at every point in it a magnetic force. 



THE DYNAMO 99 

The region or space in which magnetic force manifests itself 
is termed a magnetic field t and for purposes of explanation of 
magnetic and electro-magnetic phenomena, a magnetic field 
is regarded as being permeated with " lines of force " lines 
along which magnetic force will act when another magnet is 
brought into the field. A graphical representation of the 
distribution of lines of force in one plane of a magnet, or 
of a combination of magnets, may be obtained by laying the 
magnet or magnets horizontally, placing on top a piece of 
stiff white paper, and then sprinkling the latter with some 
fine iron filings. On gently tapping the paper the filings 
will arrange themselves along definite lines and curves. 
Such a picture for a single bar magnet is shown in Fig. 19. 



/v7'/(/---^)V\ N v. 



FIG. 19. Magnetic lines of force of bar magnet. 

The direction in which each filing arranges itself shows, 
very approximately, the direction along which the magnetic 
force at that point is acting. 

Poles of a Magnet. The magnetic lines of force about 
a magnet appear to emanate from two centres of maximum 
intensity, situated near to the ends of the magnet ; these 
centres are called the poles, the one nearest the end which 
persistently points N. when pivoted horizontally is termed 
the " N.-seeking pole," the other the " S.-seeking pole " ; 



i oo ELECTROPLATING 

usage, however, has now contracted these to " N. pole " and 
" S. pole " respectively. It is also a characteristic that the 
N. pole of one magnet will attract the S. pole of another, but 
repel the N. pole, hence the " first law of magnetism " states 
that " like poles repel, unlike poles attract." 

Direction of Magnetic Lines of Force. Lines of 
force are found to be circuital, i.e. to complete their circuit 
from pole to pole, and to have a definite direction in space. 
By a convention similar to that adopted with respect to the 
direction of flow of a current, this direction is taken to be 
the same as that in which a free N. pole * would move if 
placed so as to be acted on by the magnetic forces. Imagine, 
then, that such a pole is placed near to the N. pole of a 
magnet ; then from the above law, it is obvious that the free 
N. pole would be repelled by the N. pole of the magnet and 
attracted by the S. pole, its motion being along a line of 
force. Consequently it may be said that the direction of 
these lines outside the magnet is from N. pole to S. pole and 
vice versa inside. 

Electromagnets. If a wire be coiled up in the form of 
a long spiral around a rod of soft iron, and a current of 
electricity be passed through the wire, the iron for the time 
being is magnetized, and will exhibit properties similar to 
those described above. Such an arrangement is termed an 
electromagnet, and where strong magnetic fields are essential 
the electromagnet is the only practicable means of obtaining 
them. This arises from the fact that very soft iron and 
certain classes of steel may be temporarily magnetized by 
means of a current to a far higher degree than that to which 
hard steel can be magnetized permanently. 

Polarity of Electromagnet. The polarity of an 
electromagnet is dependent on the direction in which the 

* This is purely an imaginary pole, as the poles of a magnet 
are in reality inseparable. We cannot magnetize a piece of steel so 
that one portion exhibits N. polarity, without some other part ex- 
hibiting S. polarity. 



THE DYNAMQ \ \ tt \ \ \ \ ' 



current circulates spirally round it. Let Fig. 20 represent an 
iron bar overwound with a spiral of wire hereafter called 
a " solenoid " and traversed by a current in the direction 
indicated by the arrow-heads. Then the polarity will be as 
marked in the diagram, and as determined by the following 
rules : 

(1) EIGHT-HAND EULE. Grasp the solenoid with the 
right hand so that the fingers point round it in the same 
direction as the current circulates, then the thumb out- 
stretched at right angles to the fingers points towards the 
N. end. 




FIG. 20. Magnetic lines of force of solenoid. 

(2) CLOCKFACE EULE. If when looking at the end of the 
solenoid the current circulates in the same direction as the 
hands of a clock rotate i.e. clockwise the end looked at is 
the S. pole. Conversely, if the current circulates counter- 
clockwise, the end looked at is the N. pole. 

The dotted lines in Pig. 20 indicate the general dis- 
tribution of the lines of force, and i,t is seen that the 
distribution is similar to that of the simple bar magnet 
illustrated in Fig. 19. 

The Field Magnet of a Dynamo is virtually a large 
electromagnet designed to produce a very large number of 
lines of force, and lead as many of them as possible through 



iq? : ELECTROPLATING 

air gaps between the poles, within which the armature 
revolves. 

Let us suppose that we take the electromagnet of 
Fig. 20 and bend it (the winding included) so that its poles 
come nearer together, as in Fig. 21 (a). Let also its pole 




FIG. 21. a. Two-pole magnet. 

b. Four-pole field built up of 4 magnets. 

c. Four-pole djmamo. 

d. Four-pole field-magnet in perspective. 

ends be made curved, so forming a cylindrical cavity as 
shown. We then have a simple form of dynamo field- 
magnet with two poles, one N. pole, and one S. pole a 
two-pole field, in fact. 



THE DYNAMO 103 

Two-pole dynamos, however, are now obsolete. All 
modern machines are built with multipolar fields having at 
least four poles. We shall therefore confine our subsequent 
explanation to a four-pole dynamo. 

Let us now take four electromagnets like Fig. 20, bend 
them as described above, and arrange them as in Fig. 21 (/>). 
Let the windings be joined as shown to virtually form one, 
taking care that the current circulates so as to produce the 
polarity shown in the figure. We have now eight separate 
poles, but owing to the fact that two adjacent poles are of 
like polarity, viz. two N. poles or two S. poles, these 
adjacent poles act as one, and we have in effect four poles 
arranged alternately N. and S. ; in other words, a four-pole 
field. 

But such a construction is unmechanical. There is no 
reason why the adjacent iron poles which, as already 
observed, are similarly magnetized, should not be combined, 
and the solenoids or magnetizing windings placed where 
they are most effective, i.e. near the poles. It is, therefore, 
an easy stage from Fig. 21 (b) to Fig. 21 (c), which repre- 
sents the arrangement of a modern type of four-pole field 
magnet, while Fig. 21 (d) is the same, but shown in per- 
spective; its outline is thus more clearly defined. The 
iron core in the figure is cylindrical, and it is on this that 
the armature winding is built up, as explained later. 

Fig. 21 (c) also shows by fine full lines the approximate 
way in which the lines of force distribute themselves in the 
air gaps between the poles and the iron core, while the dotted 
lines indicate their mean path through the iron portion of 
the field magnet and core. We may note in particular that 
the direction of the lines in the air gaps are from the whole 
of the curved surface of each pole to the iron core, or 
vice versa,, depending on whether a N. pole or a S. pole is 
referred to. There is a " brush," so to speak, of lines of 
force crossing each air gap. 

The Armature is that portion of the machine in which 
an E.M.F. is set up by rotating wires in the magnetic field 



104 



ELECTROPLATING 



produced by the field-magnet, and before considering the 
armature in detail the underlying principle must be con- 
sidered. 

Let a straight metal bar or wire be rigidly mounted on 
the periphery of the iron core or cylinder (from which it is 
insulated) in such a way that when the core is revolved be- 
tween the poles of the field-magnet (Fig. 21 (d)), the bar 
moves parallel to the axis of rotation (Fig. 22). The bar or 
wire may be made of any metal, but copper is invariably 
used in practice, for reasons mentioned in a former chapter. 





MetalBina 
insulated 

from Shaft 



rial Circuit 



FIG. 22. Single wire on armature of 4-pole dynamo. 

Bearing in mind the way in which the lines of force 
cross the air gaps (Fig. 21 (e)), it is evident that as the wire 
revolves it cuts through the lines of force its length being 
at right angles to them during those periods when it. is 
passing in front of a pole. Now, when lines of force are 
cut in the manner described, there is a P.D. set up between 
the ends of the wire, and thus the cutting of lines by the 
wire generates an E.M.F. in it. 

The direction of the E.M.F. so produced may be deter- 
mined by means of the following rule, due to Dr. Fleming : 

Hold the thumb, first, and second finger of the right 
hand at right angles to one another. Point the thumb in 
the direction of motion of the wire which cuts the lines, 



THE DYNAMO 105 

and the first finger in the direction of the lines ; then the 
second finger points along the wire and indicates the direc- 
tion of the E.M.F. set up. 

Applying this rule to the above case, it is found that 
when the wire moves in front of a S. pole, the direction in 
which the E.M.F. acts along it is opposite to that generated 
in the wire when moving in front of a N. pole. For 
example, let the wire rotate in the direction shown by the 
arrow; then, when it moves in front of a N. pole, the 
E.M.F. acts from ~b to a, and vice versa for a S. pole. 

As these changes in the direction of the E.M.F. occur at 
regular and definite intervals of time, assuming the speed of 
rotation to be constant, it is termed an alternating E.M.F., 
and if for the purpose of obtaining a current in an external 
circuit we arrange matters as shown at the right hand of 
Fig. 22, the current in the circuit will be an alternating 
one, the brushes being alternately positive and negative. 
Further, the magnitude of the E.M.F. (or of the current) 
varies from instant to instant, as illustrated by the graph 
(Fig. 23). 



Volte \ 






PosHwn of Wire 
? with respect to 
JPoLes. 



FIG. 23. Change of voltage with position of wire relatively to 
the poles. 

The above is the fundamental principle of most forms of 
dynamos, but for plating and other purposes the current must 
be direct i.e. must flow only in one direction in the external 
circuit. One brush must therefore always be positive, the 
other always negative. In fact, for direct- current machines 
not only must the above condition be fulfilled, but also to be 



io6 



ELECTROPLATING 



as perfect as possible the voltage across the brushes and the 
current flowing should be as constant as possible at any 
moment during one revolution of the armature, its graph approxi- 
mating to the straight line AB (Fig. 23). 

The former may be accomplished by making the brushes 
interchange their connections with the rings at those 
moments when reversals take place (wire in positions 




FIG. 24. 4-pole winding with 8-part commutator. 

0, 2, 4, 6), which is accomplished in effect by the device 
called the commutator. 

But the E.M.F. generated by one single wire of 
reasonable length revolving in a strong magnetic field, and 
at as high a speed as practicable, is very small ; hence in 
all commercial dynamos there are a number of active wires 
out of which as component elements the armature winding 



THE DYNAMO 107 

is formed, as will be seen later. With an armature having 
a large number of active wires, we can add together the 
E.M.F.'s set up in two or more of the wires by joining them 
in series. Again, by distributing these wires uniformly around 
the core parallel to the original wire, and properly con- 
necting them up to a commutator having a large number of 
segments, we can secure, almost absolutely, the second 
condition mentioned above, viz. constancy of voltage across 
the brushes at any instant during one revolution of the 
armature. 

The method of connecting together the active wires 
constitutes the problem of armature winding. In modern 
practice only drum windings are employed, and although 
there are several distinctive varieties, armatures so wound 
are termed " drum " armatures. 

Generally the active wires are embedded in slots (insu- 
lated) (Fig. 24) formed during the construction of the core. 
The iron core serves 
a double purpose it 
not only concentrates 
the lines of force in 
the direction desired, 
but it also consider- 
ably reduces the 
" magnetic resistance " 
experienced by the 
lines in passing from 

pole to pole across the FIQ 25 __ g commutator . 

air gaps, and incident- 
ally diminishes the energy required for exciting the field- 
magnet. The core is built up of a number of thin iron 
stampings lightly insulated from one another, suitably 
clamped and mounted to revolve with the shaft. 

Let us now consider a more complete drum armature 
having sixteen active wires fixed in an equal number of 
slots in the iron core. Let the wires be joined together 
at the front and back end (the one remote from the 




io8 ELECTROPLATING 

commutator), and also to the commutator segments, as 
shown in Figs. 24 and 26. 

The type of winding adopted is only a simple one for 
explanatory purposes, and it requires a commutator with 
eight segments, an outline of which is shown in Fig. 25, 
but we do not show it in great detail nor the manner in 
which it is mounted to revolve with the shaft. If the arma- 
ture be rotated in the direction of the arrow, then at the 
moment when the active wires are as shown in the figures, 
the E.M.F. in all the wires under a N. pole will be directed 
towards the observer, or from back to front, while in 
those under S. poles it will be in the opposite direction, or 
from front to back. These directions are indicated by the 
points of arrows () and the tails of arrows (x) respec- 
tively. Wires numbered 1, 5, 9, 13, midway between two 
consecutive poles, are in the position of least action, and 
have little or no E.M.F. set up in them. 

Fig. 26 is another diagram of the armature in question, 
supposed to be laid out flat, and likewise the commutator, 
from which we may more readily trace out what we 
require. 

13 
Back 



Front 

15 

Commutator 
Segments 

Brushes 



FIG. 26. Development on the flat of preceding drum armature. 

Now, an examination of the armature winding will 
reveal the fact that it may be: divided up into four groups, 




THE DYNAMO 109 

each group consisting of the same number of wires in series ; 
group A consists of wires numbered 1, 6, 3, 8 ; group B, 
wires 9, 14, 11, 16 ; group C, wires 5, 10, 7, 12 ; and group 
D, wires 4, 15, 2, 13. Suppose next we take each group 
separately and let its E.M.F. be represented by four cells in 
series, each cell having the same E.M.F. as that developed 
for the moment in the wire which it represents. Let also 
the ends of the combinations be joined to metal blocks 
figured to agree with those of the commutator segments, to 
which the ends of each group are connected. We then get 

N<? of Active Wire 

fell representing 
EMFofWire 




16 11 14 9 
FIG. 27. Analogous arrangement of cells. 

a representation of the whole armature, as in Fig. 27, the 
straight arrows showing the direction of the respective 
E.M.F.'s of the groups, and as each group on the armature 
is situated at any moment in a similar position with respect 
to the field-magnet poles, the groups will have equal E.M.F.'s. 
It will now be seen that the blocks 3 and 7 are positive to 
those marked 1 and 5, the latter are therefore negative. 
Let the positive blocks be electrically connected to form a 
common positive, and similarly blocks 1 and 5, to form a 
common negative. We have then in reality the four groups 



i TO ELECTROPLATING 

joined in parallel, and any external circuit placed across the 
common pairs of terminals will receive a current from the 
arrangement as a whole. 

Applying the above to the actual armature, segments 3 
and 7 will be positive, segments 1 and 5 negative, and fixed 
brushes resting on these will collect the current from the 
armature. Consequently four brushes are required, which 
in their relative positions are alternately positive and 
negative, those of like polarity being joined electrically to 
form a common positive and negative respectively, to which 
the external circuit is connected. 

But so far only the conditions at a particular moment have 
been discussed. Let, therefore, the whole armature, together 
with the commutator, move forward, the brushes of course 
remaining stationary, until segments 2, 4, 6, 8 are under the 
brushes. Then other wires occupy exactly the same positions 
as those in the diagram, but the direction of the E.M.F.'s 
will still be as shown, consequently brushes Z> and d will still 
be positive, a and c negative. The same reasoning holds as 
successive segments pass under the brushes. We see, then, 
that the direction of the current in the external circuit is 
always the same. 

In actual practice the brushes always bridge more than 
one segment, for reasons which need not be entered upon 
here, and when the machine is loaded the best sparkless 
position is generally a little in advance of that shown in the 
diagram. It is found by trial, for which purpose the brushes 
of direct-current dynamos are always mounted on a rocker ; 
they may thus be moved backward or forward while the 
machine is working. 

Type of Dynamo for Plating Purposes. Direct- 
current dynamos are usually " self -exciting," that is, they 
supply the necessary current for maintaining the magnetism 
of the field magnet, and according to the method adopted of 
electrically connecting together the field winding, armature, 
and external circuit, machines are spoken of as Series, 
Shunt, or Compound dynamos. The shunt machine is the 



THE DYNAMO 



in 



only type suitable for electrolytic purposes, and the only 
one, therefore, that need concern us here. From Fig. 28 it 
will be seen that the " shunt " winding (field-magnet wind- 
ing F) is connected across the brushes (neglecting the 
rheostat for the moment), and consequently a portion of the 
armature current about 2 or 3 per cent. is diverted 
through this winding and excites the field-magnet. The 
rheostat E is merely a variable resistance for varying the 




(a) (&) 

FIG. 28. Diagram of connections of shunt-wound dynamo, a, arma- 
ture supposed removed from field-magnet. &, conventional repre- 
sentation. 

exciting current. An increase of excitation produces a 
larger number of lines of force, and augments the E.M.F. 
generated. An adjustment of this kind is very desirable, 
since the voltage of a shunt dynamo diminishes as more 
and more current is drawn from the machine. 

As the voltage required to effect the electrolysis of most 
plating solutions is only of the order of a few volts, and as 
the vats are usually supplied with current independently of 



112 



ELECTROPLATING 



one another, a low-voltage dynamo is all that is requisite 
from t^iis point of view. The current, however, will depend 
on the number of vats to be supplied at one time, the kind 
and the amount of work put into them to receive deposits. 




Generally, then, plating dynamos are machines of low 
voltage and high amperage, and a typical modern form of 
four-pole machine is shown in Eig. 29. 

Care and Management of a Dynamo. When in- 



THE DYNAMO 113 

stalling and in the subsequent management of a dynamo, 
special attention should be given to the following points : 

The machine should be fixed in a dry situation, with 
plenty of light, and with sufficient room for proper inspection, 
cleaning, etc. Eemember that it is a vital part of a plating 
equipment, and frequently the whole of the plating is 
dependent on the good working of one machine. 

Put it as near to its work as possible. 

Bolt the machine firmly on a solid and level foundation, 
which for large machines should be made of concrete. 
Vibration is detrimental to the life of a dynamo, and may 
lead to chattering and sparking of the brushes when at 
work. Sparking will rapidly destroy both brushes and 
commutator. 

If the machine is to be belt-driven from a line of shaft- 
ing, see that the dynamo shaft is set parallel with the one 
driving it, and that the two pulleys are in line. In such a 
case it is a good plan to have a fast and loose pulley on the 
line shaft, so that the machine may be stopped independently 
of the main engine. 

All parts of a dynamo should be kept scrupulously clean<, 
free from dust, waste oil, and water ; very special attention 
should be paid to the bearings, commutator, and brush gear. 

Bearings. Keep them well supplied with good oil. Most 
modern machines are constructed with oil ring lubrication, 
but even so they should be inspected periodically to see if 
the rings are working properly. 

Commutator and Brushes. These two parts require 
careful attention. A commutator in good condition presents 
a smooth polished surface of brownish copper, without 
evidence of scratches. A very little vaseline or a preparation 
called "comm bar" may be applied to the commutator 
surface occasionally as a lubricant. 

The brushes should be adjusted by the tension springs to 
make a light but certain contact on the commutator, and 
when two or more brushes are on one spindle they should 
be exactly in line. 



1 14 ELECTROPLATING 

In a four-pole plating dynamo there will be four sets of 
brushes ; these should be spaced so that the angular distance 
between successive sets is the same. 

If the commutator becomes worn or uneven it may be 
filed with a smooth file and polished with fine glass cloth, 
but the only real remedy for a commutator out of truth is to 
take the armature out of the machine and turn up the 
commutator in a lathe. 

Copper dust, which collects on various parts (chiefly the 
brush gear), due to the gradual wear of the brushes, should 
be removed as soon as it is in evidence. It is a good plan 
to use a pair of bellows occasionally, and blow out any dust 
which may have collected in cavities that cannot easily be 
cleaned ; for example, the hollow spaces between the wires 
where they join the commutator segments. 

Electrical Energy from Public Supply Mains. No 

mention has yet been made of the best means of driving a 
dynamo, nor can this be definitely stated, as so much 
depends upon the particular case. 

In most instances the method used would be one of the 
following : 

(1) Driving it from a counter-shaft, driven by the main 
engine supplying all the power requirements of the works. 

(2) Eunning the dynamo by means of an engine reserved 
specially for the power requirements of the plating shop. 

(3) Driving the machine by means of an electric motor, 
direct or belt coupled to the dynamo, the motor receiving 
energy from the private electric generating plant of the 
works, or from the supply mains of an outside power 
station. 

Undoubtedly there is much in favour of the plating shop 
having under its control the prime mover for its power 
requirements, and electric motors offer many advantages. 
They are clean, run very steadily, and when coupled direct 
to the dynamo the combination occupies very little floor 
space, and both are under the supervision of the attendant. 

The question of driving the motor from electric supply 



THE DYNAMO 115 

mains, if such are available, is worthy of attention, especially 
when extensions to existing plant are in contemplation. In 
most towns electrical energy for power purposes can be 
obtained at fairly cheap rates. The question of expense in 
this connection is really not of primary importance. It 
must of course be taken into consideration, but the total 
cost of supplying energy to plating vats is generally a 
comparatively small item compared with other factors in 
the cost of the deposited metal. 

The intervention of the electric motor is necessary, 
because a private or public power plant is designed to 
deliver energy at voltages varying from 100 to 240 volts, 
or thereabouts, and such voltages cannot be applied directly 
to plating plants of large magnitude without a considerable 
waste of energy in resistance a waste which would be very 
much greater than that represented by the inefficiency of 
the combined motor and generator. Besides, in the case of 
the supply being by means of alternating current, direct 
application is out of the question. The motor-generator is 
therefore essential for economical working, and a direct or an 
alternating current motor would be used, depending on the 
nature of the supply. 

Horse-power of Motor-generator. In estimating the 
horse-power of a motor to drive a given plating dynamo, it 
is necessary to remember that the whole of the mechanical 
energy used in driving the dynamo does not reappear as 
electrical energy ; in other words, allowance must be made 
for the fact that the machine has not 100 per cent, effi- 
ciency. 

Generally the efficiency of a plating dynamo fairly well 
loaded may be taken to be about 75 per cent., i.e. f of the 
energy imparted to it reappears in the form desired. A 
machine, therefore, whose capacity is 2-4 kilowatts (8 volts 
300 amperes) will require a motor capable of developing 

8 x 300 100 , o , , , 

X -w- = 4*3 brake-horse-power approximately. 
/ 4b 7o 

Again, the power to drive the motor will be greater than 



1 1 6 ELECTROPLATING 

its brake-horse-power owing to the various losses in con- 
version. Taking an efficiency of 85 per cent., the 4-3 horse- 
power derived above must be increased by ^.- to arrive at 
the horse-power input to the motor. The input will there- 
fore be 4-3 x ^j- = 5-06 horse-power. Expressing this 

electrically, we get 5 '^,*; n 746 = 3-78 kilowatts. This last 
luUu 

figure represents the power taken from the supply mains 
under the conditions assumed, and it is this figure which 
should be used in estimating the cost of supplying energy to 
the vats when use is made of a motor-generator set. 

Thus in the above case 3-78 kilowatt-hours (Board of 
Trade Units) of electrical energy would be used per hour, the 
cost of which works out to 3'78 x 1*5 = 5-67 pence per hour 
if the price per unit supplied, from whatever source, is 1| 
pence. 



CHAPTEK VII 

PLANT USED IN ELECTROPLATING 

IN the preceding chapters details have been given of dynamos, 
accumulators, and other means of obtaining current for 
electro-deposition; the descriptions in the present chapter 
will therefore be confined to what may be termed general 
plant and apparatus required in electroplating establishments, 
and its arrangement. 

Vats. The construction of vats for electroplating varies 
according to the particular chemical properties of the solu- 
tions used. Welded or riveted wrought-iron tanks are the 
most generally useful, but it is obvious that acid solutions 
must not be placed in such tanks without some kind of 
protective coating. For cyanide and nearly all other alkaline 
solutions used in general electroplating an iron tank is, how- 
ever, quite suitable, since iron is unaffected by any alkaline 
cyanide. For the deposition of silver particularly, therefore, 
iron vats are invariably used, usually with a lining inside of 
fine Portland cement in order to secure efficient insulation in 
making electrical connections. This lining is readily put on 
by a skilled plasterer, the inside surface of the tank being 
roughened to assist adhesion. 

A welded iron tank 5 inch in thickness with a cement 

o 

lining of about J to 1 inch is an ideal silver-plating vat. See 
illustration, Fig. 30. 

These vats are, however, only suitable for cold solutions ; 
for hot solutions the best vat is of enamelled iron. Care 
should be taken to see that the enamel is perfectly sound. 



n8 



ELECTROPLATING 




FIG. 30. Welded Iron Vat. 
showing cement lining 



Section 



Such vats are used for hot gilding solutions, brassing and 
alkaline copper solutions, and indeed any alkaline solution. 
Jacketted boilers with good enamelled linings are very useful 
_ for such solutions. 

For acid solutions 
which are usually used 
cold the best class of 
vat is acid - proof 
earthenware, but if 
for reasons of size of 
work or expense this 
is impracticable, a 
strong wood vat with 
a fairly stout lead 
lining may be em- 
ployed. Such vats are very popular and are made largely 
by manufacturers of plating plants, as shown in Fig. 31. 

The joints of the lead lining must always be fused 
and not soldered, and wherever the solution contains free 

sulphuric acid the 
innermost lining of 
thin tongued and 
grooved boards is 
necessary. These vats 
are very largely used 
for nickel-plating and 
for acid-coppering. 

FIG. 31. Wood Vat, lead lined, showing ,, ,, 

also an inner lining of thin match-boarding. thou S h rather ex P en ' 

sive, vat for nickel- 
plating is a welded iron tank lined inside with strong sheets 
of glass joined at the corners by means of marine glue or 
some similar acid-proof cement. 

The only disadvantage of such a vat is the risk of fracture 
of the lining by accidentally dropping the articles to be plated 
when hanging them from the cathode rods. Slate is occa- 
sionally used as a material for lining in a similar fashion, and 




ELECTROPLATING PLANT 



119 



though not so clean in appearance has the advantage of 
being less liable to fracture than glass. 

Vat Framework and Connections. All plating vats 
should be fitted with a strong framework of well-varnished 
wood running round the top edge. Such a framework is 
usually constructed in two parts, the upper part carrying the 
cathode rods, and the lower the anode rods. The former is 
fitted with roller or ball bearings, so that by connection with 
an eccentric shaft the cathodes may be given a gentle 
swinging or " to and fro " motion in the vat. 

The arrangement is illustrated in Fig. 32. 




FIG. 32. Cathode Motion Frame. 

The movement of cathodes in electroplating is a matter 
of great practical importance, as by this means a greater 
current density can be used and consequently more work 
done, and at the same time a fine smooth deposit obtained. 

These points will be intelligible when it is considered that 
such movements of cathodes in relation to the electrolyte 
continually gives to the surface of the deposit a slight 
friction, which to a small extent may be considered analogous 
to burnishing. In the electro-deposition of copper, Cowper- 
Coles has obtained some very striking results by means of 
an extended application of this principle.* 

During recent years, many ingenious devices have been 
introduced in vat fittings with a view to securing agitation of 
* See Journal Institution of Electrical Engineers, vol. 29, pp. 264 et seq. 



120 



ELECTROPLATING 



electrolytes as well as movement of cathodes. One of the 
oldest and most inexpensive of these is the simple mechanical 
agitator devised by von Hiibl. It consists mainly of 
" beaters " or " paddles " rigidly attached to a shaft running 
along the top edge of the vat. This shaft is in turn con- 
nected to an eccentric wheel, and a slow reciprocating move- 
ment is thus imparted to it, and consequently to the 

" beaters." A diagram 
of the arrangement is 
shown in Fig. 33. 

Compressed air has 
also recently been ap- 
plied to the agitation 
of electrolytes with 
considerable success. 
A very good agitator 
of this class is one de- 




FIG. 33. Von Htibl's Agitator. 



signed and manufactured by Messrs. W. Canning & Co. of 
Birmingham, an illustration of which is by permission 
inserted opposite (Fig. 34). 

The main advantage obtainable by the agitation of 
electrolytes is through the consequent continual renewal of 
the solution in the immediate vicinity of the cathodes. 
Under normal conditions of electrolysis, continuous de- 
position of metal from solution is made possible, owing to 
the principle of the migration of ions alluded to in a 
previous chapter. Positive ions in electrolytes constantly 
travel towards the cathode and negative ions to the anode ; 
consequently as one set of ions is decomposed their places 
are taken by another set, which in their turn are decom- 
posed, and so electro-deposition is continuous so long as 
current is passing. The natural rate of migration is, how- 
ever, very slow. Lodge found, for example, that the rate of 
migration of hydrogen ions the swiftest known is only 
about 1-15 centimetres per minute. The normal tendency 
in electrolysis is, therefore, for the liquid round the anode to 
increase in concentration, and that round the cathode to 



ELECTROPLATING PLANT 



121 



decrease. Now, it will be readily understood that when a 
solution is agitated the normal rate of migration of ions is 




Fio.234. Patent Pneumatic Agitator. A, Air compressor. 1 

considerably enhanced, and this tendency to unequal concen- 
tration neutralized, with the result that the conductivity 



122 



ELECTROPLATING 



of the solution is much increased, and a correspondingly 
higher current density made possible, which of course means 
an important saving of time. 

In the consideration of vat connections, however, the 
greatest importance must be attached to the electrical 
arrangements. It is much to be regretted that in many 
plating establishments this point does not receive the atten- 
tion it deserves. In commercial electroplating, where large 
vats are necessary, the anode and cathode connections are 
always on the parallel system (see Fig. 35), and in arranging 
these the ideal is attained when the arrangement permits 




FIG. 35. Method of connecting Anodes and Cathodes in plating vats. 

the current to distribute itself equally in every part of the 
vat. To this end the main conducting bars should be carried 
along all sides of the vat and not merely, as is so often tile case, 
along one side only. This applies to both anode and cathode 
rods. The distribution of current along conductors is exactly 
analogous to the distribution of water along a number of 
different channels. If equality of the distribution of water 
is required, then all the channels or waterways must not 
only be at the same level but of exactly the same size, and 
the same principle applies to the distribution of electricity, 
i.e. it must be made as easy for the current to flow along 
one set of conductors as along another. Where a number 



ELECTROPLATING PLANT 123 

of articles of one kind are being electroplated with any 
metal in one vat, it is manifestly to the advantage of the 
plater's reputation that all should receive an equal deposit, 
and this is impossible in a vat containing a number of 
parallel connections unless the current is evenly distributed. 

The illustration of a quantity of spoons or forks being 
silverplated in one vat may be used to enforce this point. 
If these are all of one quality and size, as is often the case, 
the manufacturer's reputation depends upon each of them 
receiving an equal deposit, and so giving the same dura- 
bility in subsequent use. If the current is not evenly dis- 
tributed, then though the total weight of silver deposited 
may be quite correct, yet some will be overplated and others 
underplated, and this variation may in practice be from 
5 per cent, to as high as 25 per cent. 

To re-emphasize this point, therefore, the main con- 
nections of the vat must nm entirely round its edges, and must 
have a cross-sectional area more than sufficient to carry 
the maximum current required (see Table of Solid Copper 
Conductors for information on this point, p. 394). In most 
vats, as has been observed, there is more than one pair 
of electrodes (anode and cathode); where this is the case 
the rods or conductors carrying these must be of the 
same sectional area, and they should be so arranged that 
the distance between each anode and cathode is as nearly 
equal as possible. Thus in the case of a vat six feet in 
length in which it is proposed to have six anodes, these 
should be placed twelve inches apart, and the respective 
cathode rods exactly midway between them. It is also 
advisable to make more than one connection between the 
main conductors of each vat and the main leads from the 
dynamo, e.g. one at each end of a vat, and in the case of 
long vats also at one intermediate point. 

In large plating establishments where a number of vats 
are in use, the method of their arrangement is always, like 
the internal connections themselves, on the parallel system. 

Figs. 35 and 36 show the method of connecting the 



124 



ELECTROPLATING 



anodes and cathodes in a vat, and the method of connecting 
a number of vats to the main leads from the dynamo or 
source of current. 




The latter diagram also shows the method of arranging 
resistance frames (often called resistance boards in practice), 



ELECTROPLATING PLANT 125 

ammeters and voltmeters, for the measurement of current, 
P.D., and the regulation of the current in the vat circuits. 
These very important adjuncts of a plating shop equipment 
will now be considered. 

Resistance Frames, or Rheostats. Eheostats used 
in electroplating shops for current regulation should be 

(1) simple in design and arrangement ; 

(2) strong and durable ; 

(3) constructed of wire of high resistivity, and of a 
material not readily attacked by fumes. 

The " continuous switch " type of rheostat is the best, as 
the current may be regulated without breaking the con- 
tinuity of the circuit, sparking being thereby avoided. Its 
arrangement should provide easy access to the contacts and 
general connections for cleaning purposes. Fig. 37 illus- 
trates diagrammatically an arrangement in general use, and 
one which fulfils the above requirements. Fig. 38 shows 
the contacts and switch arm in detail. 

The base of the rheostat should always be of slate, or 
similar insulating and incombustible material, and of 
sufficient strength and thickness to carry terminals, 
contacts, and connections, capable of conducting the maxi- 
mum current used. A thickness of from J" to f" is usual. 

Slate is used to a large extent. It is easily drilled and is 
a fairly good insulator, especially when enamelled. Enamel- 
ling, however, is a refinement which is not necessary for 
plating purposes, on account of the low voltages employed. 

The resistances are frequently constructed from plati- 
noid or German silver wire (an alloy of nickel, copper, and 
zinc) wound in open spirals. The authors have found, 
however, that some alloys of this description corrode badly 
in use under average workshop conditions. The best resist- 
ance wires they have tried hitherto for plating practice are 
those obtainable under the trade names " Eureka " and 
"Ferry." These are very pliable wires of high resistivity, 
and have been found to withstand the corrosive fumes and 
atmosphere of the plating shop better than many others. 



126 



ELECTROPLATING 



The number of " contacts " or " stops " in a rheostat is 
usually about seven, but in the case of vats containing a 
larger number of pairs of electrodes than this, it will be 
found very convenient to have at least as many resistances 
as the number of pairs of electrodes in the vat itself. In 




e 




a e 
e 
















SI 




e 





Off 



Slate, 



FIG. 37. ^Resistance Frame. 

this way the current can be regulated according to the 
number in use at one time. 

Very few details respecting the precise number of steps 
advisable, the total resistance, and its subdivision between 
the various contacts can be given here, as so much depends 
upon individual requirements, but a few details of the design, 
electrical arrangement and size of wire to use may be useful. 



ELECTROPLATING PLANT 



127 



As already mentioned, Fig. 37 illustrates a very common 
form which is adaptable for much of the ordinary routine 
work. It consists of a number of resistance coils arranged 
as shown, which are normally connected in series, when 
the switch arm is on contact 1, but which may be cut out of 
circuit one by one by moving the arm over the contacts 
from right to left. Thus with the switch arm on contact 3 
the current enters, say, at terminal T x , passes along the arm 
to contact 3, flows through resistance coils c, d, e, /, and out 




FIG. 38. Details of contact block and switch arm. 

at terminal T ; coils a and 1} are cut out, as there is no path 
via contact 3, through coils # and a after contact 1. When 
the arm is in the " OFF " position, it is obvious that the 
circuit is broken, and therefore no current can floiv. 

In such a rheostat the resistance per step is often un- 
equal, the first (i.e. a) being greater than the second, the 
second greater than the third, and so on. When all the 
coils are in circuit the current is smallest, but increases as 
the coils are cut out by the movement of the switch arm. 
Owing to the gradation of resistance required, coupled 



128 



ELECTROPLATING 



with the fact that the coils towards the left carry a greater 
current than those towards the right, several different 
gauges of wire are frequently used in the making of the 
coils, a thicker wire being employed for the smaller resist- 
ances, i.e. those which carry the larger currents. 

In all cases when a current flows through a resistance, 
energy is dissipated in heating the material, a fact which 
will have been gathered from a previous section, and in 
consequence, the temperature of the substance is raised. 
The rate at which heat is generated in a given wire is 
according to Joule's Law proportional to the square of the 
current, and the temperature of the substance will go on 
increasing until the rate of generation of heat is balanced by 
the rate at which heat is lost by radiation, conduction, and 
convection. In brief, the rate at which the heat can be got 
rid of depends upon the radiating and other properties of 
the material, and upon its environment. It is therefore 
very desirable that wires of suitable size should be used for 
the coils of resistance frames, in order that no excessive 
temperature rise, with its risk of fire or fusion, should result. 

By experiment it has been found that platinoid and 
eureka wire, exposed to the atmosphere in a horizontal 
position, attain the temperature of blood heat (98 F. or 
36'6 C.) when carrying the approximate currents indicated 
in the following table : 



Size. 



TABLE X. 

Current-carrying capacity in amperes. 



W. G. Platinoid.* Eureka.* Ferry f- 


(final temp. 100 C.). 


8 


t 


37 


. 




30-6 








33 


10 


t 


25 






20-37 








23 


12 




15 






12-23 








16 


14 


,: 


10 


t 




8-15 








9-4 


16 




6 


, 




4-89 








6-1 


18 .... 3-3 .... 2-72 








4-4 



Table XI. gives useful information respecting various 
kinds of resistance wire. 

* Compiled from the London Electric Wire Co.'s list. 

t Compiled from the list of Henry Wiggin & Co., Ltd., Birmingham. 



ELECTROPLATING PLANT 



129 



as 



Ohms per 
1000 yds. 



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6 6 6 CM O GO 



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fl o CO CM Oi CO O5 TH 

SS COiOt^COOt- 

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00 O CO CM iH . _ 

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ft CMOO5QOOCO & 

TH CO CO C5 GO CO 

6 6 6 TH TH o 

49 

Q K^ 

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<^ '~> O5 b- O CO 

t- TH CO TH 



s ! 8^S 







a 



130 



ELECTROPLATING 



Ammeters and Voltmeters. Instruments intended 
for the measurement of current are called ammeters, while 
those designed for the measurement of difference of potential 
are called voltmeters. 

The principle upon which a large number of these 
instruments work depends on the magnetic effect produced 

by the passage of a current 
through a fixed coil of wire, on 
a movable soft iron needle. 

The chief advantages of 
moving iron instruments are 
undoubtedly their simple but 
sound mechanical construction 
and their comparative cheap- 
ness. 

The Nalder gravity-con- 

on trol moving iron instrument 

G. oy. Ammeter. , -, . -n. ne\ mi 

is illustrated in Fig. 39. The 

essential features of its construction (Fig. 40) and operation 
are as follows : 





B 



FIG. 40. Interior of Ammeter with moving portion drawn forward 
to show working parts. 

C is a coil of insulated wire wound spirally on a hollow 



ELECTROPLATING PLANT 131 

brass bobbin B, fixed to the base plate of the instrument. 
The moving portion consists of a soft iron wire, or a small 
bundle of wires, W, attached to a steel spindle in such a way 
that the former moves concentrically with the latter and lies 
inside the coil parallel with its axis. The spindle is carried 
in jewel centres, and near one end is fastened the pointer 
P, the counterpoise or control weight CW, and the arm 
carrying the damping vane V, which moves with very little 
clearance inside a damping box D. 

When no current passes round the coil, the control 
weight CW hangs vertically, the pointer stands at zero on 
the scale, and the moving piece of iron W lies close to and 
parallel with a rod of soft iron W x fixed to the framework 
carrying the spindle. 

On passing a current through the coil the adjacent ends 
of the moving and fixed irons become similarly magnetized 
with, say, north polarity at the ends nearest the pointer and 
south polarity at those more remote. There are, therefore, 
two north poles near together at one end of the system, and 
two south poles at the other end ; consequently since like poles 
repel one another, the moving iron W is repelled from the 
fixed iron W lf with a force which is greater the larger the 
current. The moving iron, the control weight, and the 
pointer are therefore turned through an angle. 

On the other hand, a diminution of the current reduces 
the force exerted between the iron pieces, and the action of 
gravity on the control weight brings the movement back and 
thus diminishes the angle of deflection. It is obvious then 
that the angular deflection of the pointer is dependent on 
the current, and thus it is a measure of the current flowing. 
The object of the damping box is to steady the movement 
and help the pointer to come to rest quickly. 

Such an instrument may therefore have its scale gradu- 
ated in amperes by passing definite known currents through 
its coil and marking the positions taken up by the 
pointer. 

The " range " of an ammeter can be extended in many 



132 ELECTROPLATING 

cases by the employment of a " shunt " placed in parallel 
across the terminals of the ammeter. The shunt is a strip 
of metal, of low resistance, which bears a certain definite 
relation to the resistance of the ammeter coil ; by it a certain 
fixed proportion of the total current passing through the 
ammeter and shunt together is shunted past the ammeter. 
Its readings require, therefore, either to be multiplied by 
some factor or to be taken on an alternative scale dependent 
on the multiplying power of the shunt in use. 

The principle of the instrument described may be adopted 
in the construction of either an ammeter or a voltmeter. It 
is essential, however, to point out and make clear the differ- 
ence between them, and under what circumstances an 
instrument whose action depends on a current, may be used 
to measure a P.D. and thus become a voltmeter. 

It may first be remarked that as the force causing the 
needle to deflect is proportional to the ampere-turns 
(current x number of turns) on the coil, it is possible to use 
a small number of turns through which passes a large 
current, or a large number of turns and a small current, 
and yet have the pointer deflected through the same 
angle. 

We may note also that ammeters are always connected 
in series with the circuit (see Fig. 36), and (in the types out- 
lined above) as the whole of the current to be measured 
passes round the coil only a few turns of wire are required. 
There is not much difficulty therefore in comprehending 
that the deflection of the pointer under these conditions is a 
measure of the current. 

Again, the resistance of the coil of an ammeter should 
be as low as possible, otherwise there will be an excessive 
waste of energy in the instrument. For example, if I = 
current passing through the instrument, and E its resistance, 
the energy dissipated in the instrument is I 2 E joules per sec. 
(see page 52), and obviously as I is the current we desire 
to measure, the first factor (I 2 ) is fixed, hence the dissipation 
depends solely on E, and will be as small as possible when E 



ELECTROPLATING PLANT 133 

is as low as possible. A small number of turns is therefore 
an advantage from this point of view. 

A voltmeter, however, is joined across or in parallel with 
the portion of the circuit the P.D. of which is required (see 
Fig. 36), and its resistance must be relatively large compared 
with that of the circuit across which it is placed. One 
consideration which determines this in the case of a 
voltmeter is that its introduction into the circuit should not 
materially alter the resistance between the two points of the 
circuit across which it is applied. Expressed in another 
way, a voltmeter ought not to divert through itself any 
appreciable current from the circuit. From either point of 
view the change which occurs is as small as possible when 
the voltmeter resistance is as high as possible. 

The second consideration is that the power absorbed when 
working should be small, and since this may be expressed as 

V 2 

j (page 52), where V = P.D. applied to the instrument, 

E = its resistance, it follows that for a given value of V, the 
power absorbed diminishes as E is increased. 

The winding of a voltmeter therefore consists of a large 
number of turns of fine wire, through which only a small 
current flows. 

The current (I) which flows through the winding of 

V 

a voltmeter is, according to Ohm's Law, I = ^, V being the 

applied P.D. and E the resistance of the winding, from which 
V = I x E, and it follows that a definite current and conse- 
quently a definite deflection will always be obtained for the 
same voltage, providing that (E) the resistance of the instrument 
remains a constant. It is on this ground that the scale may 
be graduated in volts. For example : Suppose the pointer 
of an instrument whose resistance is 200 ohms to be deflected 
to a certain point on the scale by a current of ~ amp. Then 
as V = IE the P.D. across its terminals would be ^ x 200, 
i.e. 20 volts, and this point may therefore be marked 20, and 
similarly for other points ; the instrument will then read 



ELECTROPLATING 



directly in volts, and hence be a voltmeter. Constancy of 
resistance is therefore important for ensuring the reliability 
of the instrument's indications. 

Ampere-hour Meter for Electroplating." Until 
quite recently the only method of controlling or ascertain- 
ing the amount of metal deposited in a plating bath has 
been to note the average current-flow during any period, 
and the elapsed time. The product of these quantities 
gives the approximate ampere-hours of current passed, and 
from this it is possible to ascertain the amount of metal 
which has been deposited. 




Plating Tank 



FJG. 41. Diagram of ampere-hour meter and signal bell. 

By the use of a special form of ampere-hour meter, 
illustrated in Fig. 41, the former method of watching a -clock 
and ammeter is entirely done away with, remarkable accu- 
racy being obtained simply from the record made by the 
ampere-hour meter. The standard meter as furnished for 
electroplating control has a dial reading in any desired unit 
weights of the metal with which the meter is to be used ; 
for example, dwt. of silver, grains of gold, pounds of copper, 
etc. The meter is equipped with a movable pointer, ope- 
rated by a knob in the middle of the glass window over the 
* From The Metal Industry, May, 1912, by kind permission. 



ELECTROPLATING PLANT 135 

dial, so that the pointer can be set at the amount of metal 
desired for any particular plating operation. For example, 
if twelve dozen spoons are to be silver plated, and require 
100 dwt. of silver, the indicating pointer would be set at 
100 on the dial, after which the large moving hand, operated 
by the mechanism of the meter, would be set at the zero 
point. As current passes through the meter, the large 
hand moves in a clockwise direction around the dial until it 
reaches the pointer, in this case set at 100 dwt., when con- 
tact is made against a pin in the adjustable pointer, thus 
operating through auxiliary leads an electric light or bell, as 
a signal (Fig. 41). 

While the ampere-hour meter has been furnished and is 
being successfully used with all kinds of plating baths, its 
widest application has been with silver and nickel. For 
control of gold plating a special arrangement using two 
meters is used, as the amount of gold ordinarily deposited in 
any operation is very small, a few grains only, in many 
cases. 

The principle and construction of the meter were very 
completely described in The Metal Industry, April, 1909. 

Cleansing and Dipping Tanks. Tanks to contain 
hot caustic potash or soda solutions should always be of 
welded iron. Welded iron tanks are for the purpose much 
superior to either cast iron or riveted ones. The heating 
arrangements may be for Bunsen burners or steam coils. 
If steam is available the latter system is by far the most 
convenient. For electrolytic cleansing the vats should be 
fitted with a strong, well-varnished wood frame, in order to 
carry the anode and cathode rods and provide efficient 
insulation. As in this class of work fairly large currents are 
used, the authors have found it also advisable to mount the 
rod connections on porcelain insulators. 

For acid dips and pickles well-glazed earthenware (Fig. 42) 
is undoubtedly the best material, except in very small work 
where glass can be employed. For a hot dilute sulphuric 
acid pickle the best vat is one of solid lead not less than 



i 3 6 



ELECTROPLATING 



^-inch thick. This, however, must be heated by means of 
steam coils, also of lead, and all joints burnt or fused. 




FIG. 42. Earthenware Rinsing Tank. 

Scratch-brush Lathes and Scratch-brushes. Lathes 
for scratch-brushing are made in two types, single and 
double-ended. See illustrations, Figs. 43 and 44. 

Where a number of 
lathes are required the 
single-ended type is al- 
most invariably adopted, 
so that all brushes rotate 
in one direction. As the 
operators must in scratch- 

^. 43-Si^le scratch-brush lathe, brushing face the end of 

the spindle and not the 
side, it is obvious that a double-ended lathe presents one end 





FIG. 44. Double scratch-brush lathe, 
where, as the operator holds it, the article is met by the 



ELECTROPLATING PLANT 137 

brush at the right-hand side ; at the other end it is met at 
the left-hand side. To the average worker this is very 
confusing. In small plants, however, a double-ended lathe 
is often used, and one end reserved for brushing the insides 
of hollow ware articles. 

The illustrations in Fig. 45 show the type of brush gene- 
rally used for flat work and the outsides of hollow articles- 




FIG. 45. Scratch brush for flat work (about } natural size). 

The complete brush consists of 7 or 9 " knots," as they arc 
called (Fig. 46), mounted on a brass chock, so arranged that 
as the ends wear they can be moved outwards until the 
stock is too short for any further adjustment. The knot 
itself is simply a bundle of perfectly straight lengths of very 
fine wire from 38 to 43 B.W.G. bound tightly together 
by means of thick copper wire closely coiled round it. The 
usual diameterof the knot is inch. 



' ill. :! .J';;,':t ';.!,!' 



FIG. 46. A " knot." 

Other types of brushes for hollow work inside and other 
uses are shown in Fig. 47. 

During use, these are simply screwed on to the pointed 
end of the lathe spindle. 

An important point in connection with scratch-brushing 
is the speed of the lathes. They should not be run from 
the same shaft as polishing lathes, or if so steps must be 



138 



ELECTROPLATING 



taken to reduce their speed. The exact number of revo- 
lutions per minute depends largely on the class of work 
done and on the metal to be plated, but from 1200 to 1500 





FIG. 47. Types of scratch brushes for inside and special purposes. 

revolutions per minute may be taken as the average require- 
ment. If the speed is too slow the brushing is ineffective ; 
on the other hand, if it is too fast the articles are given a 
grained or frosted appearance which interferes considerably 
with the subsequent finishing and polishing processes. 

Polishing Lathes. Lathes for polishing are constructed 
on exactly the same principle as for scratch-brushing, 



ELECTROPLATING PLANT 139 

except that usually only double-ended lathes are employed. 
As has been already intimated, their speed should be greater 
than the scratch -brushing lathes, generally 2000 revolutions 
per minute. Owing to the dusty nature of most polishing 
processes the lathes should always be installed in a shop, 
separate from the cleansing or plating shops, but in their 
immediate vicinity, as in many classes of plating, particularly 
nickel, polishing is closely identified with the other processes 
preparatory to plating. 

Sand-blasting. Another essential part of the plant 
of a thoroughly well-equipped electroplating establishment 
is an efficient apparatus for sand-blasting. Very many 
beautiful and artistic effects in the electro-deposition of 
metals can be simply and quickly obtained by a judicious 
use of such apparatus. 

In addition, the sand-blast is a very efficient cleansing 



/-Compressed/ Air 




FIG. 48. Sand-blasting apparatus. 

A, Sand container, coarse. 

B, fine. 

C, Pumice. 

agent for many kinds of work. There is on the market at 
present a large variety of types of sand-blasting machines, 



140 



ELECTROPLATING 



but a number of these have been designed for use in cleans- 
ing and " fettling " large iron-castings for engineering work, 
and are not at all suitable for the average electroplated s 
purpose. They are usually worked either by steam or com- 
pressed air at very high pressures, and give on most metals 
a surface far too coarse for electroplating requirements. 
The accompanying illustrations (Figs. 48 and 49) show various 
types of machines adaptable for electroplaters. Fig. 49 is 




FIG. 49. Sand-blasting apparatus. 

a Continental type of apparatus very compact, and con- 
venient for use in a limited floor space. It is however only 
suitable for small work such as cups, small bowls, cigarette 
cases, matchboxes, etc., though if confined to this class 
it is very efficient, and has the additional advantage of being 
comparatively inexpensive. For larger work, particularly 
when different grades of " matting" or "graining" are 
required, the type of machine illustrated in Fig. 48 is most 



ELECTROPLATING PLANT 141 

generally convenient. Such types can be readily and con- 
veniently adapted for a large range of work, and not only 
can the pressure be varied but different grades of material 
employed according to the requirements of the moment. 

To obtain the necessary pressures either steam or com- 
pressed air may be employed, but for the classes of work 
with which the electroplater usually has to deal, the latter 
is by far the most convenient. For small jobbing work 
machines fitted with a foot bellows are used, but these 
have only a very limited application. 

The modes of using sand-blasting apparatus and the 
classes of material employed will be described in the follow- 
ing chapter (on preparatory processes). 

General Arrangements of Plant. A properly de- 
signed plating shop should consist of at least three separate 
rooms or sections, each one distinct yet conveniently con- 
necting to the others, so that work may pass from one 
to the other with a minimum loss of time. These rooms 
should also be if possible arranged on the ground floor, 
and be well lit and well ventilated. The two latter points 
are particularly important, not only from the point of 
view of securing successful work, but also of the health of 
the operators. Contrary to what appears to be popular 
opinion, none of the ordinary operations of electroplating 
are of themselves injurious to health, provided only that a 
thoroughly efficient system of ventilation is secured, and 
let it be said that this is also conducive to a high standard 
of work. 

The principal room or section of the building should of 
course be the plating shop proper, containing the plating 
vats and, unless another small room is available, the 
dynamo and electrical instruments. The room immediately 
adjoining this should be reserved for cleansing operations, 
and should contain scratch-brushing lathes, scouring benches, 
sinks, potash and acid dipping tanks, and all solutions for 
processes immediately preparatory to plating proper. 

The third room or section should contain the polishing 



142 ELECTROPLATING 

or finishing lathes. Sand-blasting machines and apparatus 
may be placed either in this latter room or in that for 
preparatory processes ; but in either case a wooden partition 
should be arranged, so that the sand or pumice powder 
which may escape may be confined to as small an area as 
possible, and not allowed to become objectionable in other 
processes. 

Sometimes the dynamo is placed in a recess and 
partitioned off from the vat room, but it is better that the 
operator in charge of the vats should have this machine in 
sight so that any irregularity may be immediately detected. 
If, however, accumulators are used to any extent they 
should be enclosed in a separate room or compartment, since 
in charging they give off fumes which are very objectionable. 
In laying down a plating plant care should be taken to 
arrange the dynamo or sources of current as near to the 
plating vats as possible in order to avoid loss of energy in 
transmission, and also the expense of long lengths of cable 
or connecting wires. The vats themselves should be 
arranged along the sides of the room, sufficiently near to the 
walls to allow the latter to be used for the electrical leads 
and connections, and the eccentric shaft for movement of 
cathodes, or agitating arrangements. In planning the 
position of individual vats relatively to the dynamo regard 
should be paid to the voltages required, e.g. nickel vats 
requiring a high voltage should be nearer to the dynamo than 
the silver ones which only require a very low one. This 
point may be disregarded in small shops, but in very large 
establishments it is worthy of attention. 

A suggested outline plan for general electroplating shops 
is sketched diagrammatically in Fig. 50. 

We have previously mentioned that all electrical arrange- 
ments and connections for plating vats are connected " in 
parallel" The general method of wiring is to carry two 
main cables from the dynamo round the entire length of the 
shop, and if necessary on both sides. Sub -connections are 
then made by jointing short lengths of cable to the mains, 



ELECTROPLATING PLANT 



143 



and connecting these in turn 
to each vat and its resistance 
board and measuring instru- 
ments as shown in Fig. 50. 
Owing to the low voltages 
employed in electroplating, 
however, it is not at all essen- 
tial that these main leads 
should be of insulated cable. 
They may be and often are 
plain bare copper wires solid 
drawn, of sufficient cross - 
sectional area to carry the 
required current, and so long 
as these wires are securely 
fixed on insulated brackets so 
that there is no danger of 
"short circuits" they are quite 
as effective as the much more 
costly cable and often more 
convenient, as by means of 
sliding binding screws the 
sub-connections maybe taken 
off at any point with the 
minimum of trouble and in- 
convenience. In the sub-con- 
nections to vats and resist- 
ance boards it is always 
better to use insulated cable 
owing to the risk of the con- 
nections crossing, and so 
causing " short circuits." 

Working Dynamo and 
Accumulators in Parallel. 
On p. 114 several ways 
were mentioned which are in 
use for driving the dynamo 




j'44 ELECTROPLATING 

supplying current to the vats. Whatever method is adopted, 
it is an advantage to have the speed of the machine as 
steady as possible, since this tends to ensure steadiness of 
the current supplied. The steadiness or otherwise of the 
current is readily noticeable by glancing at the ammeter in 
the circuit. A steady current will produce a steady deflection 
on the instrument, the pointer remaining at rest, but one 
which is the reverse causes the pointer to oscillate to and 
fro. In cases however where a fluctuating current is trace- 
able to an unsteady drive, a battery of large accumulators 
may be run in parallel with the dynamo, as shown in Fig. 
51, but only when the dynamo is shunt wound or has its 




FIG. 51. Connections for dj-namo and accumulator run in parallel. 

field-magnet winding supplied with current from another 
source. 

With this arrangement the fluctuations will almost if 
not entirely disappear, since in the event of the dynamo 
current diminishing, the cells will discharge a current 
approximately equal to the diminution, and so compensate 
for it, while any increase in the dynamo current will go 
(wholly or in part) as a charging current through the cells. 
Cells used in this way are said to be floating on the circuit. 

The voltage of the accumulator must be the same as that 
at which the dynamo usually works, and as the P.D. of a 
single cell is two volts the number of cells to be joined in 
series for the purpose is easily found; an 8-volt dynamo 
would require 4 cells, a 10-volt dynamo 5 cells. 



ELECTROPLATING PLANT 145 

In connecting up care must be taken that the positive 
pole of the cells is connected to the positive main from the 
dynamo, and it is advisable to have a central- zero permanent 
magnet moving- coil ammeter, and also a switch in both 
dynamo and cell circuit as shown in the diagram. By 
means of the switches it is obvious that the dynamo and 
cells may be used separately for the supply of current to the 
circuit, or both together in parallel. In the latter case a 
larger current may be drawn from the combination than it 
would be safe to take from the dynamo or cells used alone. 
The type of ammeter mentioned enables us to observe not 
only the value of the current in amperes in the respective 
circuits in which the instruments are placed, but also the 
direction of the current; for if both are supplying the 
circuit the pointers will, say, deflect to the right of the zero 
mark, whereas if the current in either circuit for any reason 
reverses, the ammeter in that circuit will show a left 
deflection. 

Again, in the section dealing with the deposition of 
alloys, it will be pointed out that the constancy of the 
P.D. acting in the circuit is a most important feature in 
such cases. And as an unsteady current resulting from 
imperfections in the driving arrangement is really caused by 
fluctuations in the value of the E.M.F. generated due to the 
varying speed of the machine, the benefit to be gained from 
the use of accumulators alone or in conjunction with a 
dynamo is obvious. Accumulators have an extremely 
steady and almost a constant P.D. during the major part of 
their discharge. 

Another feature of this combination of dynamo and 
accumulator is the possibility of charging the cells from the 
dynamo, while the latter is also supplying current to the 
vats. Especially is this so when the current required for 
deposition is comparatively small and the dynamo only 
lightly loaded. For instance, suppose we have an 8 volt, 
300 ampere machine and four large cells, and that the work 
in hand only requires 100 amperes. Under such conditions 

L 



146 ELECTROPLATING 

the dynamo is working at J full load, and in general the 
efficiency of the machine would not be at its best. But by 
arranging the four cells in two sets of two in series, as in 
lower part of Fig. 18, and connecting them to the main leads 
from the dynamo as illustrated by the whole of Fig. 51, i.e. 
so that they form two branches across the leads, the cells 
could readily be charged with the 8 volts available. If each 
set were capable of being charged with 100 amperes (the 
current being adjusted to this value by the resistances) we 
should have 100 amperes in each cell circuit, and 100 
amperes going to the vats, or 300 amperes in all. The 
dynamo would then be fully loaded, and working with 
increased efficiency, f of the energy developed being stored 
as chemical energy in the cells, to be used subsequently. 

After charging the cells in this way, it would only be a 
simple matter to arrange all four in series, and connect them 
in parallel with the dynamo as previously observed. More 
than 300 amperes if necessary could then be obtained, both 
cells and dynamo supplying current to the external circuit. 



CHAPTER VIII 

PEEPAEATOBY PEOCESSES 

THE subject of the preparatory treatment of articles prior to 
actual electroplating is of the greatest possible importance. 
It is in the preliminary stages of treatment in the plating 
shop, that three-fourths of the troubles and difficulties inci- 
dental to electro-deposition have their rise ; and in no section 
of the art do care, patience, and skill bring their reward so 
quickly and so completely as here. 

" Absolute cleanliness in all things " should be the 
working motto of the electroplater, whether he deals with 
the noble metals like gold, silver, or platinum, or with the 
more ordinary copper, nickel, or brass. This motto, further, 
should be given a very wide application, not merely to the 
articles dealt with, themselves, but also to the shops through 
which they pass ; the plant, the benches or tables, even the 
floors should be kept as rigidly clean as it is possible to keep 
them. The greatest care in removing grease and tarnish 
from a metallic surface is often completely nullified by a 
dirty scratch-brush lathe, or a little greasy matter on the 
edge of a vat or earthenware rinse-pot. 

In the present chapter, general outlines of methods 
applicable to all metals will be given ; special methods of 
treatment peculiar to one class of work only will be given in 
the chapters relating thereto. 

Before dealing however with the processes belonging 
strictly to the plating department, it may be advisable to give 
a general description of methods employed to render surfaces 
perfectly smooth and regular so that the subsequent " finish " 



148 ELECTROPLATING 

shall possess the smooth gloss and brilliant polish usually 
associated with finished electroplated work. Articles as 
they come from the manufacturers' hands, whether spoons 
or forks, cutlery, flat-ware or hollow-ware in any class of 
metal, and whether made by casting, forging, stamping, roll- 
ing, or by hand, usually retain the marks of the varied 
operations through which they have passed; and all such 
irregularities, file-marks, etc,, must be buffed or polished off. 
This process is usually known as " buffing " or " polishing." 
The operations vary according to the basis metal and class 
of work handled, but consist essentially of treating the 
articles with fine emery powder, pumice powder, Trent sand, 
rotten stone, etc., by means of emery wheels, leather or felt 
discs, bristle brushes, calico dollies and other hand or 
machine tools of a similar nature. 

In the present book it is quite unnecessary to enter in 
detail into the manufacture of these tools or materials, as they 
can be readily and reasonably purchased from manufacturers 
who make a speciality of polishing reagents. A brief out- 
line of the treatment of the principal metals in industrial use 
will therefore suffice in this connection. 

Silver, Copper, German Silver, Brass, and similar metals 
and alloys, are buffed generally on lathes similar in type to 
Fig. 44, p. 136, by holding them firmly, and with an even 
pressure at all parts of their surface, against a leather or felt 
disc screwed on to the lathe spindle. The buffing material 
is in the first instance usually powdered pumice and finally 
finely sifted Trent sand thoroughly mixed with rape or. some 
similar oil. The pumice or sand is allowed to " flow " 
between the article and buff. In the best practice and class 
of work the pumice powder is used for "grounding," i.e. 
smoothing out the coarser marks of the surface, and fine sand 
applied as a secondary or fining-off process. For many kinds 
of work fairly hard bristle brushes are used in a similar manner. 

Britannia Metal, Pewter, and Tin alloys generally are given 
very much the same kind of treatment to the above except 
that only finely sieved sand mixed with oil is used. Pumice 



PREPARATORY PROCESSES 149 

powder is much too keen and abrasive for use on the softer 
metals. 

With regard to the respective use of pumice powder and 
sand in buffing processes, it should be observed that the 
former material has much greater <( cutting " properties than 
the latter. It is therefore an exceedingly useful substance 
for clearing the surface roughness, or grain, of the harder 
metals, particularly nickel and copper alloys. If, however, 
as is often the case for the sake of cheapness, the article is 
not given further treatment the " cutting " marks of this 
material are always discernible, and it is impossible after 
plating whatever metal be deposited to give the work the 
fine mirror-like polish characteristic of really well-finished 
work. 

For articles of any of the above-mentioned or similar 
metals, intended to be plated either with copper, brass, silver, 
gold, and most other metals, the treatment just described is 
sufficient. As, however, such goods always leave the fine 
sand with a surface which though quite smooth is yet dead 
or dull in appearance, they are not sufficiently prepared 
for deposits of nickel or cobalt. These two metals as 
deposited electrolytically possess such a high degree of hard- 
ness that unless the surfaces upon which they are deposited 
are not only perfectly smooth but possess a fairly high 
polish, it is impossible after plating to bring out to the fullest 
extent the brilliant colour and gloss of which they are both 
capable. The materials mainly used for this purpose are 
Sheffield lime, Vienna lime, Tripoli, rouge, crocus, and com- 
positions mainly composed of these substances, applied by 
means of calico mops or dollies, the processes being 
practically a continuation of those previously described. 

Iron and Steel Goods requiring a perfectly smooth and 
bright surface are prepared almost entirely by means of 
emery powder. This extremely useful substance unrivalled 
as a polishing reagent for this class of work is a natural 
product consisting almost entirely of the oxides of iron and 
aluminium. 



T5o ELECTROPLATING 

In the first stages of preparation solid emery wheels are 
generally used, but in the later stages, leather buffs, treated 
with various grades of emery powder, are employed. These 
buffs are really wooden bobs or discs covered on the outer 
edge with leather, of a thickness of from f to f inch. The 
leather covering is secured to the disc by means of glue, and 
the operation must be carefully and skilfully performed, as 
accidents .occasionally happen through the covering breaking 
away from its base, when in use on high-speed polishing 
lathes. 

Before actually using these buffs they must be " dressed," 
as it is termed, with emery powder. This also is an opera- 
tion demanding a little practice and experience ; the outer 
surface of the leather is given a slight coating of thin glue 
spread equally over it. While the glue is still warm, the 
disc, which is held by means of a short rod passed through 
its centre, is rolled backwards and forwards regularly in a 
trough or shallow dish containing the emery powder of the 
grade required. Any irregularities of surface may subse- 
quently be removed by fixing the buff on the lathe and while 
revolving, pressing firmly a piece of lump pumice at its face. 
A number of buffs are thus prepared using various grades of 
the powder, from say No. 60 (fairly coarse) for the earlier 
stages of polishing to No. 120 or 140 (very fine), for the final 
gloss. From time to time the buffs require redressing with 
emery powder, and opportunity should be taken at the same 
time to examine the security of the leather covering on 
the disc. 

For small work and work having many irregularities or 
indentations in the surface, solid leather buffs are used. 
These can of course be turned to any diameter from 1 or 2 
inches upwards and are thus convenient for use in polishing 
hollow articles. 

The present writers have also found a good quality of 
felt, of corresponding thickness to the leather, suitable for 
the covering of wooden bobs for use in obtaining a very high 
polish with No. 140 emery in the final stage of polishing. 



PREPARATORY PROCESSES 



15* 



Very small articles are now often prepared for plating by 
means of what are termed " tumbling barrels " (Fig. 52). 




FIG. 52. Tumbling Barrel. 



Cleansing Processes. After the preliminary treatment 
outlined above, the articles are ready for the processes which 
may be considered as essential parts of the plating opera- 
tions proper. These are (1) cleansing from grease, and 
(2) cleansing from metallic oxides or tarnish. 

(1) Cleansing from grease. This is accomplished mainly 
by the use of boiling solutions of caustic soda or potash 
(strength | Ib. per gallon). These substances have the 
property of converting fatty materials and greases, which 
ordinarily are insoluble in water, into a soap and glycerine, 



152 ELECTROPLATING 

both of which substances are readily soluble in water and 
may then be entirely removed from the surface of the article. 
The process in its chemical reaction is exactly analogous to 
the main operations in soap manufacture. In the latter case 
equivalent weights of caustic alkali and some form of 
vegetable or animal fat are placed in the soap-boiling pan, 
and both substances are entirely neutralized in the pro- 
duction of soap together with free glycerine. 

It is of the utmost importance to remember that this 
operation is a chemical reaction and not simply a case of 
washing off grease in a hot liquid, as some electroplaters 
apparently believe. Each time, therefore, a greasy surface is 
immersed in the cleansing liquid a certain equivalent of 
caustic alkali is neutralized and the solution rendered 
correspondingly weaker. It is, further, important to note 
that the grease is not necessarily washed away even when 
this chemical action is complete. It is simply converted 
from an insoluble compound to a soluble one, which can be 
readily dissolved off in water. During the process therefore 
it is always advantageous to brush the work over from time 
to time to remove the soapy compounds and enable the 
potash to complete its work thoroughly. 

Articles occasionally reach the electroplater which are 
covered with oily matter upon which potash has little or no 
action. This is the case, for example, where goods are coated 
with vaseline or any of the paraffin compounds in order to 
protect from atmospheric action. These substances, and 
indeed all mineral oils, are best removed by means of benzene, 
in which they are perfectly soluble. Articles should be well 
brushed with the benzene, and then scoured with whiting 
made into a thin paste with water, afterwards thoroughly 
rinsed under running water. This treatment will be found 
very effective in dealing with a class of work which some- 
times gives a great deal of trouble. 

The above processes are applicable to all ordinary metals 
and alloys dealt with by the electroplater. It must be 
observed however that tin and lead, and alloys containing 



PREPARATORY PROCESSES 153 

large proportions of these inetals, must not be allowed to 
remain in the potash tank any longer than is absolutely 
necessary to remove the grease, as these metals are attacked 
to some extent by strong alkaline solution. Aluminium also 
should be excluded from these liquids, or at most be given 
but a momentary immersion. The benzene treatment with 
subsequent scouring with lime or whiting will be found the 
best method of removing grease from surfaces of this metal. 

2. Cleansing from oxides or tarnish dipping and pickling. 
After the removal of grease in the potash boil there still 
remains, in the case of most of the metallic surfaces treated 
for plating, a film of oxide or other stain which must be 
completely removed before the article can be given a 
perfectly adherent coating of deposited metal. This is 
accomplished by means of acid dips or pickles, the com- 
position of which varies according to the kind of metal to 
be treated. 

For copper, brass, German silver, and similar alloys, one 
of the best dips is made up as follows : 



Sulphuric acid . . | 10 imperial gallons 
Nitric acid . . . 2 ,, ,, 

Water ! 10 

Common salt . . | 4 ozs.f 



Metric* 

50 litres 
10 
50 
125 gr. 



The sulphuric acid is slowly added to the water in an 
acid-proof earthenware vessel, and the nitric acid and salt 
added when the mixture has cooled. The whole is 
thoroughly stirred before use. 

Sometimes it is desired to bring articles from the dip 
with a decided dead or dull effect. This may readily be 

* Where metric alternatives are added for convenience, it will be 
seen and must be borne in mind by the reader that they are not 
necessarily strict equivalents (unless an = sign is employed), but 
merely give the requisite relative proportions, which is all that is 
necessary for the plater's purpose. 

t In all cases, unless otherwise stated, the avoirdupois ounce and 
pound are used. Troy weight is only used in the case of silver and 
gold and certain of their compounds in Chaps. IX. and X. 



154 ELECTROPLATING 

done by using a dip composed of equal parts of sulphuric 
acid and water to which about a quarter of its bulk of nitric 
acid is added and a small proportion of zinc sulphate (from 
1 to 3 ozs. per imperial gallon, or say from 6 to 18 grams per 
litre). 

A good pickle for these metals is composed of dilute 
sulphuric acid (one of acid to twelve of water). This is 
generally used, prior to dipping, for articles which are badly 
stained. 

A preliminary immersion in a pickle enables the dipping 
acid to act more quickly and effectually. 

Iron and steel goods, particularly those with bright 
surfaces, must not be dipped in strong acids ; these articles 
are usually pickled in dilute sulphuric or dilute hydro- 
chloric acids. A pickle for this purpose, recommended by 
Langbein, which gives excellent results, is made up as 
follows : Add 28 ozs. of strong sulphuric acid to 2J imperial 
(or 3 U.S.A.) gallons of water, dissolve in the mixture 2 ozs. 
granulated zinc, and finally add 12 ozs. nitric acid. Stir 
thoroughly and put aside to cool. Dilute nitric acid itself 
(1 in 20) is also a useful pickle for bright steel goods. 

In the case of the softer metals such as zinc, lead, tin, 
and alloys consisting mainly of these, oxides and stains are 
best removed by scouring with powdered pumice or whiting 
and scratch-brushing ; but in many instances a dip consist- 
ing of a strong solution of potassium cyanide (1 Ib. per im- 
perial gallon or 100 grams per litre) will be found extremely 
useful. 

A similar dip is sometimes used for treating polished 
surfaces of copper or brass which might be injured in strong 
acids. 

If however the cyanide dip is used for polished surfaces 
which are to be nickel-plated, a precaution which must be 
most carefully observed is to rinse thoroughly in clean run- 
ning water in order to avoid contaminating the nickel bath 
with traces of the cyanide liquids. The method of pro- 
cedure which we have found most satisfactory after the cyanide 



PREPARATORY PROCESSES 155 

dip is to rinse well in water, afterwards to immerse the 
articles for a few seconds in very dilute sulphuric acid (1 in 
20), again to rinse quickly, and place immediately in the 
nickel vat. 

Electrolytic Cleansing. This is a modern develop- 
ment which will doubtless ultimately replace the older 
methods of cleansing by simple immersion in potash or 
soda liquids as described above. The fundamental principle 
of this method is to attack and remove the grease or oxide 
from metallic surfaces by means of chemical reactions which 
are made to occur electrolytically. The reader will by this 
be familiar with the fact that whenever an electric current 
is passed through an electrolyte, chemical substances are 
produced and chemical action occurs both at the anode and 
the cathode. It will therefore be readily understood that, 
given a suitable electrolyte, products may be generated at 
the surface of the electrodes which strongly attack either 
grease or oxides, or both. 

A considerable number of particular methods and solu- 
tions for electrolytic cleansing have been published, but the 
literature of the subject is as yet in a somewhat unsatisfactory 
condition, and much investigation remains to be made re- 
lative to the exact nature of the reactions which occur and 
the conditions essential to the most efficient results. 

Some of the earlier experiments in electrolytic methods 
of cleansing appear to have been made by Mr. Cowper Coles 
mainly in the direction of "pickling" iron preparatory to 
electro-zincing, the method adopted being to make the 
articles alternately the anode and cathode in dilute sulphuric 
or dilute hydrochloric acid as the electrolyte. This method 
was very successful in removing both grease and scale from 
such surfaces. 

In 1899 a process was patented on the Continent for 
electrolytic cleansing by means of aqueous solutions of alka- 
line salts. In working this method also the articles to be 
cleaned may be made either the anode or cathode or both 
alternately. For the preparation of iron plates it was 



156 ELECTROPLATING 

directed to use a 20 per cent, aqueous solution of sodium 
sulphate. In the electrolysis of this solution sulphuric acid 
is formed in the vicinity of the anodes and, on the other 
hand, caustic alkali (sodium hydrate) is formed at the 
cathode. For removing oxides and scales, therefore, the 
plate to be treated forms the anode, and for cleansing from 
grease, the cathode, the opposite electrode in each case being 
also sheet iron. This process is said to be operated on a 
very large scale on the Continent, and is both efficient and 
economical. 

For non-ferrous metals and alloys generally, and also 
brightly polished iron and steel goods in preparation for 
electroplating, the following and similar solutions have 
been strongly recommended : 

Metric.* 



Caustic soda ....... i Ib. 



Carbonate of soda (crystals) . . J Ib. 



250 gr. 
250 ,, 



Sodium cyanide ...... J Ib. 250 ,, 

( one imp. gall. 
Water ..... | or 1J U.S. 5 lltres 

The solution is contained in an iron vat, and may be used 
either hot or cold. The electrical connections include a 
resistance board for current regulation and a reversing 
switch. In this way the current density can be varied, and 
the article made either anode or cathode at will. On im- 
mersion the articles are first made cathodes and a strong 
current passed for a few minutes, the anodes being usually 
iron or carbon plates. This action neutralizes grease, but 
sometimes produces stains which a brief reversal of the 
current, making the articles the anodes, will completely 
remove, and the goods are brought from the vat clean and 
bright. 

The methods of electrolytic cleansing which the present 
writers have found most efficient are as follows : 

* Throughout in the case of such formulae as the above for solutions, 
the basis for the metric alternative has been taken as 5 litres (instead 
of 4-54, the strict equivalent of 1 imp. gallon), but the quantities of the 
ingredients are adjusted to agree therewith. 



PREPARATORY PROCESSES 157 

1. For removing scale and oxide from average cast or 
wrought iron goods, make up as an electrolyte a solution of 
one part strong sulphuric acid to from twelve to fifteen parts 
of water. The articles to be treated are made the cathodes, 
and the anodes consist of strong plates of sheet lead or 
carbon. The voltage used should be not less than 4 volts 
with a current density sufficiently strong to generate gas 
freely at the cathode surface. From 10 to 15 minutes will 
usually suffice to remove all oxide from an average class o 
work. 

A most important saving of time is thus effected, since 
often in ordinary pickling an immersion of several hours 
is required to loosen the scale adhering to these goods. 

2. For German silver, brass, cupro-nickel, and all such 
alloys as well as copper, the electrolyte is made up of a 
simple solution of caustic soda in water. Commercially 
pure caustic soda should contain 78 per cent, of sodium 
hydrate, NaOH, and this should be used in the proportion 
of about | Ib. per imperial gallon of water (or 75 grams per 
litre). The solution should be worked hot in order to assist 
in a complete saponification of the grease. The articles are 
made the cathodes, and anodes may be of carbon or sheet 
iron (we prefer the latter). 

A voltage of 4 or 5 volts is sufficient for ordinary work, 
with a current density of not less than about 12 amperes 
per square foot. The higher the current density, the quicker 
the removal of grease. 

As will be readily understood, the electro-chemical action 
resulting in this case is the rapid liberation at every point of 
the entire cathode area, of nascent hydrogen and sodium ; 
the former assists in the reduction of oxides, the latter, 
attacking the water, forms anew sodium hydrate, which 
immediately neutralizes the grease in the vicinity of its 
formation, and as fresh sodium hydrate is continually being 
formed by the current at every part, even in the deepest 
recesses, of the immersed surface, this reaction is extremely 
rapid and effective. 



158 ELECTROPLATING 

No one who has given this method a thorough trial will 
for one moment doubt its immense superiority to the old 
method of simple immersion in caustic soda or potash with 
a periodical scrubbing of the greasy surfaces with the potash 
or scouring brush. 

At the discretion of the operator, the acid dip may be 
omitted in the case of metallic surfaces treated electrolytically, 
but as it is only a momentary process, and therefore involves 
practically no loss of time, it is advisable in most cases to 
give the articles this treatment as a safeguard. 

, With regard to the electrolytic cleansing or pickling of 
iron or steel goods in acid solutions, an interesting point has 
been observed by several experimentalists which deserves 
mention here. This class of work is very often called upon 
to conform to certain physical or mechanical tests, and while 
before electrolytic treatment they have been found to possess 
the qualities corresponding to these requirements, they have 
been found afterwards to be appreciably changed, and oc- 
casionally have lost some rather important properties. 

The most probable explanation of this unfortunate pheno- 
menon is that the iron has occluded some proportion of the 
hydrogen gas which is always liberated very freely in all 
electrolytic actions of the nature described above. If porous 
castings particularly are allowed to remain for any consider- 
able length of time in contact with hydrogen, in what is 
undoubtedly at the moment of liberation a nascent condition, 
it is in the highest degree likely that sufficient may be 
occluded to affect appreciably its composition and constitu- 
tion, and therefore mechanical properties. Sand-blasting 
(see later, p. 160) has been suggested as an alternative method 
of cleansing surfaces of articles in regard to which this 
difficulty is liable to arise. 

Scouring and Scratch-brushing. These processes 
are very largely adopted, not only in the treatment of articles 
preparatory to plating, but often during plating itself, 
particularly in building up thick deposits, in order to obtain 
perfectly regular and even coatings. Scouring and scratch- 



PREPARATORY PROCESSES 159 

brushing are operations having the same ultimate effect, and 
are used as supplementary to the cleansing methods described 
in the foregoing paragraphs. As the term implies, scouring 
consists of scrubbing the surfaces to be plated by means of 
fine sand, lime, whiting, or precipitated chalk, with either 
bristle brushes or pads of calico flannel, or swansdown. 
Scratch-brushing, on the other hand, consists in brushing, 
usually by machine power, with very fine hard brass or 
German silver wire brushes, using some liquid lubricant 
having organic matter in solution, e.g. stale beer, malt, bran, 
or oatmeal water, or solution of soap wort, dilute vinegar, 
etc., etc. A very dilute decoction of fine pea-meal in water 
will be found effective. 

The apparatus for scratch-brushing has already been 
described (see page 136), as also various types of brushes. 
It must be noted that the wire used in making up these 
brushes must be harder than the metal undergoing treat- 
ment, but not sufficiently so to scratch or otherwise injure 
the surfaces treated. 

It will be understood that scratch-brushing is much more 
severe in its effects than scouring, and consequently for 
highly glazed or polished surfaces the latter operation is 
almost invariably substituted, the scouring material being 
lime or whiting. Scouring must also be resorted to usually 
in treating deep recesses or parts which cannot well be got 
at in lathe scratch-brushing. 

Since both these processes are usually the last through 
which an article passes before immersion in the plating 
liquid, or, in the case of silver deposition, the quicking bath, 
it is of the greatest importance that the fingers be kept 
absolutely clean in handling goods. In the case of work for 
nickel-plating for which scouring is often adopted, a good 
plan is, after thoroughly washing the hands, to rub over them 
a little dry whiting or fine pumice powder, and to repeat this- 
occasionally during scouring operations. 

While on the subject of scratch-brushing it may be well 
to recur to the fact previously mentioned, that this process 



160 ELECTROPLATING 

is often resorted to during plating, in building up thick 
deposits, particularly of copper, silver, or brass. In the case 
of silver, for example, when the deposited metal has obtained 
a thickness of from 0-0025 to 0-003 inch (0-065 to 
0-075 mm.), however smooth the basis metal surface may 
have been originally, the " grainy " crystalline nature of 
the deposit causes a definite irregularity on the surface of the 
plating which if allowed to go on would ultimately render it 
impossible to obtain a perfect polish during finishing opera- 
tions. An extreme illustration of this point may be observed 
on the backs of electrotypes or the surfaces of electrolyti- 
cally refined copper plates (" electrolytic cathodes "). 

A thorough scratch-brushing of the surfaces at the stage 
named will, however, by flattening or grinding off the pro- 
jecting points of these minute crystals of which the deposit 
is composed, render the surface almost as smooth as the 
original basis ; and so enable the operator to proceed to build 
up a further deposit of equal thickness without fear of 
obtaining a final surface too rough for finishing. 

It is often advisable, and, indeed, where soft-soldered articles 
are concerned, necessary, to give work a preliminary film of 
deposit often termed a " striking " or " starting " deposit 
and then scratch-brush, before placing in the vat for the full 
deposit. Starting or striking deposits are usually given with 
a current stronger than the normal, and the effect of this is to 
force the deposit of metal over parts of surfaces, such as soft 
soldered seams or joints, which are less conductive than the 
main surface. Scratch-brushing at this stage has the effect 
of testing the adhesion of the deposit generally and remedy- 
ing any roughness which the strong current may have 
caused at edges or projecting corners. 

For many classes of work, particularly flatware, this pro- 
cess is unnecessary. 

Sandblasting. Amongst processes preparatory to 
electroplating in any of its branches sandblasting must now 
be considered of increasing importance, inasmuch as it 
provides almost ideal means of producing in the preliminary 



PREPARATORY PROCESSES 161 

stages of treatment effects which, in the finished product of 
the electroplater's art, are often exceedingly beautiful and 
artistic. It is now indeed a process not merely of a prepara- 
tory nature, but is, in a large number of instances, used in 
the finishing stages. This latter application will however 
be touched upon in Chapter XVIII., so that only the former 
need be treated here. 

The apparatus required for this process has already (in 
the previous chapter) been fully described, and it only 
remains to be stated in this connection that the type of 
apparatus chosen will be determined by the size and class of 
work to be done. 

It is, of course, well known that sandblasting consists 
essentially in forcing under strong pressure (usually com- 
pressed air) currents of sand or similar abrasive material 
against metallic or other surfaces undergoing treatment ; the 
effect being to give to these surfaces a character varying 
from an extremely slight dull or dead appearance to a very 
coarse-grained or crystalline frosted effect. Whatever grade 
of result is obtained, however, the characteristic nature of 
sandblasting is the perfect regularity of texture and conse- 
quently also uniformity of colour imparted to the surface 
treated. 

In attempting any description of the details of sand- 
blasting processes it should be plainly stated that actual 
figures given with regard to pressures and classes of material 
must be taken, not as exact values, but rather as guides to 
those who may be to a large extent unacquainted with the 
possibilities of these methods. Eequirements, as well as 
conditions, vary so greatly that it is impossible to do more than 
give approximate numbers derived from the experience of 
operators having considerable knowledge of the ordinary 
needs of the trade. 

A brief survey of the possibilities of the types of machines 
previously referred to will show that, broadly speaking, there 
are two methods by which differential treatment may be 
applied, (1) by variation of pressure, and (2) by variation of 

M 



1 62 ELECTROPLATING 

material. In one or other of these directions an almost in- 
finite variety of results can be obtained. In the first case, 
the depth of the blasting effect is regulated. In the second, 
it is mainly the grain or texture which is influenced. But 
both these factors are so interdependent on each other 
that this distinction can only be taken as applying 
approximately. 

It will be fairly obvious that different metals require 
widely differing treatments to obtain even similar effects. 
Iron and steel goods, for example, may be subjected to a 
much higher pressure and coarser material than the soft tin 
or zinc alloys which occasionally have to be treated. The 
former class are usually sandblasted at pressures of from 
20 to 24 Ibs. per square inch, the abrasive material being 
generally a medium or coarse grain of Calais sand. The 
latter can rarely be subjected to a higher pressure than from 
3 to 5 Ibs. per square inch, and only the finer grades of sand 
employed. 

In electro-zincing iron and steel this treatment is now 
often resorted to, instead of dipping, scouring, or scratch- 
brushing. The articles are cleansed from grease in benzene, 
or caustic potash in the usual manner, rinsed in hot water, 
dried, then sandblasted, and after thorough rinsing to remove 
all traces of sand are ready for plating. 

Silver, which perhaps more than any other single metal is 
required to undergo this treatment, is now to a large extent 
treated with pumice powder of various grades, instead of 
sand ; particularly in preparation for " oxidizing " or gilding. 
A finely frosted matte finish, for example, is given to silver or 
electro-silver-plated goods which are intended for subsequent 
colouring or gilding, by blasting with finely divided pumice, 
say No. 60 at a pressure of about 8 Ibs. per square inch. 

A few special modes of treating silver, copper, brass, and 
German silver for particular effects are detailed in the 
following Table XII. 



PREPARATORY PROCESSES 



163 



TABLE XII. 

SANDBLASTING. SILVER, COPPER, BRASS, AND GERMAN SILVER. 

Effect. 



Material. 
No. 54 Calais sand 



Powdered pumice, . 
No. 60 

Powdered pumice, . 
No. 90 

Powdered glass . . 



Coarse Calais sand, . 
about No. 18 



Pressure. 
12 to 15 Ibs. per 
sq. inch 



. 8 to 12 Ibs. per . 
sq. inch 

. 6 to 8 Ibs. per . 
sq. inch 

. 6 to 8 Ibs. per . 
sq. inch 

. 15 Ibs. per sq. . 
inch (momen- 
tary pressure 
only) 



Rather coarse satin-like 
surfaces. Usually termed 
frosting effects. 

Satin-like surfaces, finer 
than above. 



Dull, exceedingly 
matted surface. 



fine 



Similar matte to above, 
but bright. 

Ice-like crystalline sur- 
face, similar to moulded 
glass. 



Partial Frosting. By this term is meant some treatment 
which will leave part of the surface of an article with a 
frosted or satin -like appearance while the remaining part is 
normal. As would naturally suggest itself to any one 
acquainted with the sandblasting of glass, this may be done 
by means of stencils cut from ordinary writing-paper. These 
paper stencils are cut so as to reveal the parts to be frosted, 
and then pasted with glue on to the surface of the article. 
After thoroughly drying, the work is submitted to sand- 
blasting, and all parts left uncovered receive the frosted 
effects. The glued paper can be readily removed subse- 
quently by immersing in hot water. 

A sandblasting apparatus fitted with a very small nozzle 
is often very useful in ordinary cleansing operations for 
treating deep recesses in hollow-ware articles which are 
difficult to clean properly otherwise either by scratch-brushing 
or by scouring ; particularly is this the case where soft solder 
has been used in such recesses. 

Preparation of Aluminium and its Alloys. The 

problem of electroplating aluminium with any other metal 



164 ELECTROPLATING 

has for long attracted the attention of electroplaters, but 
complete success in this direction does not yet appear to 
have been attained. One of the principal difficulties is the 
great affinity of this metal for oxygen. Even when most 
careful precautions are taken effectively to cleanse the 
surface and remove every trace of oxide, the slightest ex- 
posure to a moist atmosphere and even an immersion in an 
aqueous electrolyte is sufficient to form a fine film of alu- 
minium oxide and so to prevent that perfect cohesion of 
the basis metal and its deposited coating which is essential. 
Eepeatedly has it been found, when this metal has been given 
what appeared to be a thoroughly sound coating of copper 
or silver which indeed has stood the test of burnishing (see 
p. 359), that sooner or later, on standing, small blisters have 
appeared here and there over the surface of the article, and 
the deposit rendered absolutely valueless. 

There seems also very good reason to believe that the 
liberation of hydrogen, which always occurs to a greater or 
lesser degree in electrolysis of aqueous solutions, is another 
very serious obstacle to obtaining the perfect adhesion of- an 
electro-deposited metal on aluminium. This point however 
requires further investigation. 

Directions for the preparation of this metal for plating 
can only therefore be considered as suggestive for further 
experiments and research. 

A slight acquaintance with the chemical properties 
of aluminium will suggest the necessity of avoiding strong 
alkalies in preparatory treatment. If the surface is" very 
greasy, benzene should be used in the first place, and sub- 
sequently after rinsing in clean water the articles should 
be passed through a hot solution of cyanide of potassium 
this being the safest alkali to use in this connection, with or 
without the addition of a little ammonia. For removal of 
oxide, dipping in hydrochloric acid of various strengths is 
usually resorted to. 

It should be said however that the successful plating 
of aluminium depends to a considerable degree on the 



PREPARATORY PROCESSES 165 

composition of the electrolyte, probably quite as much or 
more than on the particular preparatory process adopted. 

Preparation of Non-metallic Surfaces for Plating. 

In addition to the well-known metals and metallic alloys 
used in the arts, the expert electroplater is often called upon 
to give deposits of copper, silver, or gold to articles of glass, 
china, wood, vegetable growths, and other substances which 
are non-conductors, and therefore must be rendered conductive 
before any electrolytic deposit can be imparted to them. 

The principles adopted in dealing with this class of 
work may be described under two heads, (a) chemical, (b) 
mechanical. 

Under the former principle the method usually adopted 
is to treat the surface in question with some solution or 
series of solutions which by chemical action will precipitate 
a metallic powder or film, and so give the article superficially 
the conductive property of a metal. 

Under the latter principle surfaces are either brushed 
over with fine plumbago or a mixture of plumbago with a 
very finely divided metallic powder, or " metallized " as 
the process is termed by brushing with finely divided 
silver, copper, or tin powders, after preliminary treatment 
with some solution which will give an adherent base. 
Examples of the plumbago method are found in the treatment 
of gutta-percha moulds for electrotypy (see p. 268), and of 
the metallizing process in the treatment of wood by first 
applying a coating of thin varnish or lacquer, and then, while 
this is still plastic, brushing the entire surface with a plentiful 
supply of copper bronze powder. 

Many different methods have been tried and used with 
more or less success in both the chemical and mechanical 
processes of treatment, but it should be said that success 
depends quite as much on the experience and skill of the 
operator as on the particular method chosen. 

(a) In the great majority of cases the method of chemical 
treatment adopted is to precipitate finely divided metallic 
silver on the surfaces to be treated. The reason for the 



i66 ELECTROPLATING 

choice of silver as a metallizing agent is fairly obvious in 
view of the highly conductive property of this metal. 

One of the best modes of procedure is carried out as 
follows, and is particularly applicable to gelatine moulds 
for electrotypes, vegetable and organic substances, such as 
grass, leaves, flowers, fruit, lace or cotton fabrics, etc. 

The surfaces to be treated should first of all be tho- 
roughly washed with alcohol or benzene to remove all dirt 
and greasy matter, then sprayed with fine jets of water, 
especially in all recesses or undercut portions, and the excess 
water drained off. Then while the surface is just damp, 
carefully pour, over every part required to be metallized, a 
saturated alcoholic solution of silver nitrate. This should 
be previously prepared by dissolving in pure alcohol as much 
silver nitrate as the liquid will absorb the solution of the 
crystals can be assisted by immersing the containing vessel 
in a hot-water bath. Now set the article aside to drain off 
and dry, and when quite dry repeat the operation until it is 
certain that not the smallest portion of the surface has 
failed to receive the silver solution. The next step is to 
reduce the silver in the film of solution, so that the silver 
as the result adheres to the prepared surface, either in the 
metallic or some other form which shall be electrically con- 
ductive. This may be accomplished in two ways, either 
by treatment with phosphorus or some similar substance 
which will reduce the silver nitrate to finely divided metallic 
silver, or by exposing to the fumes of sulphuretted hydrogen * 
(H 2 S), which reduces the nitrate to sulphide of silver, a com- 
pound which is a fairly good conductor of electricity. 

If the above operations are carefully and completely 
carried out, the article treated now possesses a surface 
which will conduct the current and is capable of receiving 
an electrolytic deposit. 

For glass, china, and earthenware, silver is also used as 

* This gas is readily generated by placing on a shallow dish a few 
small pieces of iron sulphide and covering them with dilute hydro- 
chloric or sulphuric acid. The operation must be performed in a 
draught cupboard. 



PREPARATORY PROCESSES 167 

a metallizer, but the method of treatment is somewhat 
different. The articles are first thoroughly cleansed from 
grease in potash or benzene, then immersed for a short time 
in a dilute solution of hydrofluoric acid the containing 
vessel for this acid must be of gutta-percha, since it will 
attack and ultimately dissolve glass rinsed in clean dis- 
tilled water, momentarily redipped in the acid, rinsed again, 
and are then ready for the silver treatment. 

For this purpose two solutions are necessary : 

(1) Dissolve 90 grams of sugar candy in distilled water, 
add 4 c.c. of nitric acid of a specific gravity of 1*22, and 175 
c.c. of alcohol. Make up the bulk to 1 litre by adding dis- 
tilled water. 

(2) Dissolve 1-8 grams silver nitrate in 180 c.c. dis- 
tilled water, add ammonia drop by drop until the precipitate 
which forms is nearly redissolved, then add 0-9 gram 
potassium hydroxide (KOH) dissolved in a little water, and 
again nearly redissolve the precipitate by the addition of 
a few drops of ammonia. 

The article being now ready for immersion, take 10 c.c, 
of No. 1 solution and 180 c.c. No. 2 solution, mix together, 
and immediately immerse the whole surface to be silvered. 
The amount of the two solutions must of course be in- 
creased proportionately if the articles are too large for this 
quantity of liquid. The result of the operation is that a 
film of metallic silver is thrown down by the reaction of the 
organic compound in No. 1 solution with the silver salts n 
No. 2. The preliminary treatment in hydrofluoric acid 
having slightly roughened the surface of the prepared article, 
this film of silver is quite adhesive and forms an efficient 
conducting coating, on which a further deposit may be built 
up electrolytically. 

It should be observed that it is advisable to line the con- 
taining vessel for the above operation with a thin coating of 
white wax, or some similar substance, to prevent as far as 
possible deposition of the silver on this vessel as well as on 
the article immersed. 



1 68 ELECTROPLATING 

Q. Marino has recently taken out several patents for 
the metallization of glass and china and similar surfaces 
preparatory to electroplating, the novel feature of which is 
principally the use of a mixture of cuprous oxide and silver 
nitrate as the metallizing solution. A brief description of 
this inventor's method is given in the following. 

The surfaces to be treated are first rendered slightly rough 
or given a " matte " by dipping in hydrofluoric acid or by 
sandblasting. A cold solution is prepared by introducing 
cuprous oxide into a solution of nitrate of silver whereby is 
formed a grey substance consisting of nitra-tetra-cuprate of 
silver; this substance is dissolved in hydrofluoric acid and 
applied to the surface of the article to be metallized by 
means of a brush. While the surface is still wet, an inti- 
mate mixture of finely divided copper and zinc powder or 
copper with some more electro-positive metal is dusted over 
the damp surface. 

In this way the silver-copper compound is reduced by 
electro- chemical action to the metallic form, and the surface 
of the article thus rendered conductive. The inventor 
however prefers to rub this conducting film briskly when 
dry with a brush until it presents a polished and uni- 
form appearance, thereby facilitating the passage of the 
current. 

Instead of silver, or, as in the foregoing paragraph, silver- 
copper, copper alone may be used as a conducting film, as 
described by F. D. Chattaway, F.K.S., in a paper read 
before the Eoyal Society (Nov. 21, 1907), from which the 
following is abstracted. The method is based on the dis- 
covery of a reagent for the precipitation of copper in a thin 
reflecting metallic film in the same manner as silver may 
be thrown down by organic and some other compounds. 
The reagent found to be successful with copper is phenyl- 
hydrazine. 

The following procedure, which resembles that employed 
in silvering glass, gives a uniformly excellent result. Heat 
a mixture of one part of freshly distilled phenylhydrazine 



PREPARATORY PROCESSES 169 

and two parts of water till a clear solution is obtained. To 
this add about half its bulk of a warm saturated solution of 
cupric hydroxide in strong ammonia. Add next a hot 10 per 
cent, solution of potassium hydroxide (KOH) until a slight 
permanent precipitate of cuprous hydroxide is produced. 
The prepared glass or china surface should now be immersed 
and the liquid, which should ba colourless or pale yellow, 
heated cautiously, when a fine thin coherent, perfectly 
reflecting lamina of metallic copper will be deposited. The 
article should be left in contact with the solution for an 
hour or so before removal ; it should then be washed with 
distilled water and transferred to the electrolytic bath for 
further deposition. 

(b) Articles principally treated by mechanical methods 
are mainly of gutta-percha, vulcanite, wood, and similar 
substances. The former are generally washed with alcohol 
and benzene, sprayed with clean water, dried, then 
thoroughly brushed over either with fine plumbago powder 
or with an intimate mixture of 2 parts by weight of plum- 
bago and 1 part of tin powder. Occasionally finely divided 
silver is used in place of tin. The brushing must be very 
thoroughly done, and continued until the whole surface 
has a smooth metallic lustre. 

Wood should be made thoroughly smooth and cleansed 
by rubbing well with methylated spirit. A thin slow-drying 
varnish (copal varnish) should now be applied to every part 
of the surface to be plated, and, after drying, a second coat. 
When the last coating is not quite dry, but in the condition 
technically known as " tacky," fine copper bronze powder 
should be thinly spread over the varnished surface and 
thoroughly brushed until a smooth coherent metallic film 
is obtained. The bronze powder should be repeatedly 
applied until it is certain that every part is covered. 

A very reliable method of varnishing is first to prepare a 
thin varnish by dissolving J ounce of orange shellac in 1 
imperial pint of denatured alcohol (or 12-5 gr. in 500 c.c.). 
Give the wood one or two coats of this, and afterwards a 



170 ELECTROPLATING 

coating of copal varnish, brushing on the metallic powder 
before the latter is quite dry. 

The principal difficulty in plating wood is that the surface 
is apt to contain pin-holes. This can only be overcome by 
care and thoroughness in the bronze powder treatment. 

P. Marino has recently taken out a patent (Pat. No. 
20,012, Sept. 1911) for 'the preparation of wood, gypsum, 
paper, etc., for electrolytic deposition, of which the following 
is a summary. The article is coated with a solution of an 
alkali silicate, allowed to dry, then painted with a solution of 
60 parts silver chloride, and 100 parts of ammonium fluoride 
in a saturated solution of potassium cyanide. It is then 
treated with a saturated solution of 100 parts of hydrazine 
sulphate and 60 parts of sodium hydroxide. The effect of 
the latter treatment is to reduce the silver contained in the 
silver solution to the metallic form, the article being as a 
consequence covered with a thin film of finely divided 
metallic silver. The film thus produced is made into a 
coherent deposit by friction such as vigorous brushing. 

It will be noted that the principle of the foregoing 
method is also based on the reduction of a silver salt by 
means of an organic reagent, the novelty of the process 
lying almost entirely in the particular reagent chosen. 

" Wiring " Articles for Plating. This is a matter of 
some considerable importance to electroplaters. Some very 
unsatisfactory specimens of electroplating have in our 
experience been traceable to bad electrical contact during 
immersion in the plating vat. Objectionable marks are. also 
often observed in the finished article through carelessness in 
this respect. 

The variety and divergences of size of the goods dealt 
with make it impossible to give detailed directions to meet 
all requirements ; but a few general principles may be laid 
down. 

(1) It is generally advisable to use copper wire of various 
gauges for this purpose. Copper is not only a very good 
conductor of electricity but is very malleable and ductile 



PREPARATORY PROCESSES 171 

and can be bent and twisted into almost any shape required 
without breaking. In the case of wire which has once been 
used, particularly when it has received a deposit of another 
metal and been afterwards stripped, it should be well 
annealed; otherwise annoyance will be caused through its 
becoming brittle. 

(2) It is the best practice to make as many contacts for 
the cathode rod as is reasonably possible. An article of any 
appreciable size is not as a rule satisfactorily plated, that 
is, with an equal distribution of current, with only one 
contact wire. For example, a tea or coffee pot should have 
at least three points of contact : one, say, at the bottom of 
the handle or socket, another wire passed down the spout, 
then upward through the cover opening and secured outside ; 
and the third point of contact, either by a separate wire or 
connection to the first, to the cover. The attachment of the 
cover to the body of the pot by means of the joint is often 
an unsatisfactory one from the electrical point of view. 
With wires thus arranged the whole article is in good 
electrical contact when hung from the cathode rod. Obviously 
the method of wiring must vary according to the shape of 
the article, but the foregoing will serve to illustrate the 
principle. 

(3) On flat ware and plain surfaces where points of 
contact are likely to show marks, the wires must be care- 
fully moved from time to time during deposition, especially 
when thick deposits are being given. 

In many cases copper or brass springs, hooks, skeleton 
frames, or racks for small work are in use, but advantage is 
now generally taken of plating barrels and other mechanical 
arrangements for small work which is to be plated in large 
quantities. Details of these are given in the catalogues of 
dealers in platers' supplies. 



CHAPTER IX 

DEPOSITION OF SILVER 

THIS is probably the most important branch of the electro- 
plater's art, not only from the widespread nature of the 
applications of silver deposition, but also from the beauty 
and perfection of results now obtainable and the intrinsic 
value of the metal itself. 

Properties of Silver. Silver is a beautifully white 
metal capable of taking a brilliant polish. It is very malle- 
able and ductile, being excelled in this respect only by gold, 
than which however it is harder and more tenacious. It 
is unaffected by oxygen at ordinary temperatures, but when 
exposed to air, especially that of towns, it becomes discoloured 
by means of the small traces of sulphuretted hydrogen 
which are ordinarily found in the atmosphere, silver being 
extremely susceptible to the action of sulphur compounds. 
It is readily dissolved by nitric acid and more slowly by hot 
concentrated sulphuric acid, but it is scarcely acted upon by 
hydrochloric acid at any temperature. Silver excels all 
other metals in its power of conducting heat and electrieity, 
and is also the most generally useful of all metals as a 
protective coating for metallic articles of domestic use owing 
to its non-liability to attack by organic substances such as 
fruit and vegetable juices. 

Solution for Deposition. The solution now invariably 
used for electro silverplating is that of the double cyanide of 
silver and potassium in water (formula, KAg(ON) 2 ), though 
for its electro-deposition in refining operations a simple 
solution of silver nitrate (AgNO 3 ) in water is used. 



DEPOSITION OF SILVER 173 

Materials Used Quality and Tests. Before de- 
scribing in detail the methods of making an electro-silver- 
plating bath it will be advisable to deal with the important 
question of the materials used, particularly that of cyanide 
of potassium. This substance is of very great importance 
to the electroplater, entering as it does into the composition 
of so many of the solutions with which he has continually 
to deal. Many attempts have been made to replace it by 
some other reagent less poisonous and offensive in general 
properties, but up to the present without success. It is 
still unrivalled as the principal chemical reagent in the 
practice of electroplating. The question of its purity or 
otherwise assumes therefore first-rate importance. 

Commercial Cyanide of Potassium is usually ob- 
tained as fused cakes or blocks. In its purest form it is 
colourless or nearly so, but the ordinary product is greyish 
white. It has a characteristic smell, closely resembling that 
of bitter almonds. It is perfectly soluble in water, giving 
an alkaline reaction, and very slightly soluble in absolute 
alcohol. It is deliquescent and decomposes rapidly when 
exposed to the atmosphere into potassium hydroxide, potas- 
sium carbonate, and ammonia. It also decomposes to some 
extent if dissolved in hot water; in making solutions of 
potassium cyanide therefore cold water should invariably 
be used. In its decomposition hydrocyanic acid gas (HCN) 
is also slowly given off, and as this is extremely poisonous 
care should be observed not to inhale deeply when using 
cyanides. It should be stored in a dry, cool place in air- 
tight cannisters or jars. 

For use by the electroplater, potassium cyanide is 
commonly prepared by fusing together in an iron vessel 
yellow prussiate of potash, more correctly named potassium 
ferrocyanide, having the formula K 4 FeC 6 N 6 . 3H 2 O, and 
potassium carbonate (K 2 CO 3 ). The reaction of these sub- 
stances in fused mass results in the formation of potassium 
cyanide (KCN), potassium cyanate (KCNO), carbonic acid 
gas (CO 2 ), and metallic iron (Fe), the latter being deposited. 



174 ELECTROPLATING 

In actual manufacture steps are usually taken to de-oxidize 
the potassium cyanate formed, so as to obtain a higher 
percentage of pure KCN. 

The practical details of the process of the manufacture of 
cyanide are as follows: About 25 Ibs. of the prussiate of 
potash, which has been previously finely ground and dried 
at or just over the temperature of boiling water (100 C.), 
are melted together with 8 Ibs. of potassium carbonate in a 
sufficiently large iron pan fitted into a coke-fired furnace 
having a good draught, and so arranged that the heat 
reaches every part of the pan as evenly as possible. In the 
bottom of the pan a taper hole is bored, through which is 
inserted an iron rod whose upper end is shaped into a ring 
for convenience of extracting. After the charge is placed in 
the pan it is covered over to exclude the atmosphere, and 
the heat applied. In a short time, depending on the 
temperature, the greenish colour of the melt changes to a 
porcelain white (the colour is judged by removing a small 
portion and allowing it to solidify) ; then a further 39 Ibs. of 
the prussiate salt are weighed out and added in quantities of 
about 4 Ibs. at a time, waiting until the green colour given 
by one addition is discharged before adding another lot. 
When the final addition has been made and the colour of 
the melt is to the liking of the operator, the pan is removed 
from the furnace, the taper rod withdrawn, and the molten 
contents allowed to run into the casting pan in the form of 
cakes or slabs. Care must be observed in running the 
material that as little as possible of the finely divided iron 
is carried out by the stream of molten cyanide. Immediately 
the substance has solidified, it is broken up and packed in 
air-tight jars or tins. 

Owing to its extreme liability to decompose both in the 
molten and solid state, it is almost impossible to obtain an 
average quality of over 96 per cent., but if the operation has 
been carefully carried out the percentage of purity should in 
no case be below 92. It will be evident however that the 
purity of the final product is largely dependent on the purity 



DEPOSITION OF SILVER 175 

of the original materials used, as well as on the efficiency of 
the methods adopted for deoxidizing the cyanate of potassium 
which, as has previously been stated, is always formed. On 
this latter point a good deal of uncertainty exists, some of 
the methods employed, such as adding small quantities of 
finely divided metallic tin, being of very doubtful efficiency. 

Other methods * for the manufacture of potassium pyanide 
are: 

(1) To heat the completely dehydrated ferrocyanide with 
metallic sodium, thus obtaining a cyanide of higher strength, 
consisting of a mixture of potassium and sodium cyanides : 

K 4 Fe(CN) 6 + 2Na = 4KCN + 2NaCN + Fe. 

Such a product is known commercially as " double salt 
cyanide." 

(2) Beilby's process, in which a fused mixture of potas- 
sium carbonate and charcoal is treated with ammonia, the 
product being a very pure molten cyanide which is filtered 
from the small amount of insoluble matter present and is 
then cast into moulds yielding crystalline cakes of pure 
white cyanide. 

The following are the principal impurities found on 
analysis in commercial potassium cyanide, and usually 
some, if not all, are present in the purest specimens of the 
' salt, viz. potassium cyanate, potassium thiocyanate, potas- 
sium ferrocyanide, potassium sulphate, potassium sulphide, 
potassium carbonate, potassium silicate, potassium formate, 
and the corresponding sodium salts, and often in addition 
calcium and aluminium compounds. None of these im- 
purities are of any value to the electroplater, and some are 
very deleterious. If however the sample used is found to 
contain from 92 to 95 per cent, of pure KCN, then the total 
amount of impurities present is sufficiently low to be dis- 
regarded. It is, therefore, essential for good work that the 
percentage composition of commercial potassium cyanide be 

* See Roscoe and Schorlemmers' Treatise on Chemistry, vol. ii., 
" The Metals," pp. 352-3. 



176 ELECTROPLATING 

determined before it is used for making up an electro-silver- 
plating solution. 

The assay of cyanide of potassium. A thoroughly reliable 
method of assaying a sample of commercial cyanide to 
ascertain the percentage of pure potassium cyanide present, 
known as Liebig's method, is outlined in the following * : 

The theory of the method depends on the fact .that when 
a solution of potassium cyanide is added to one of silver 
nitrate, the first reaction which ensues is the formation of 
silver cyanide according to the following equation : 

AgNO 3 + KCN = AgCN + KN0 3 . 

This occurs in the proportion of their respective mole- 
cular weights, viz. 

AgN0 3 (170)x KCN (65). 

If however the addition of potassium cyanide is con- 
tinued after the precipitation of the whole of the silver, a 
second reaction begins and the silver cyanide which is quite 
insoluble in water is slowly re-dissolved in the excess 
potassium cyanide until the whole of it is held in solution, 
this further action being 

AgCN + KCN = KAg(CN) 2 . 

These reactions, upon which a silver-plating solution 
itself depends, will be more fully explained later. It will be 
evident however, from a study of the foregoing, that if a few 
drops of silver nitrate solution are added to a solution of 
potassium cyanide, a precipitate results which at the" very 
moment of formation re-dissolves in the excess of potassium 
cyanide present, and that this will occur on further additions 
of silver nitrate until the whole of the pure cyanide present 
has been taken up. On this principle depends the method 
which will now be given for the assay of cyanide of potassium 
for the percentage of real cyanide. 

The apparatus required is a fairly delicate assay balance, 

* Extracted from a pamphlet on The Assay of Commercial Cyanide 
of Potassium, by A. H. Allen, late Public Analyst of Sheffield. 



DEPOSITION OF SILVER 



177 



one turning to one milligram or less, preferably O'Ol mg. 
(gram weights should be used), a 100 c.c. burette and stand 
(see Fig. 53), and a flask holding 500 c.c. 

The sample of cyanide, which should weigh not less than 
3 to 4 ounces (say 100 grams), and be a fair representation 
of the bulk, is first of all thoroughly 
powdered in a mortar, and if the assay 
cannot be immediately proceeded with, it 
must be transferred to a perfectly dry air- 
tight bottle or at least kept as completely 
as possible from exposure to the atmo- 
sphere. Now by means of the assay 
balance weigh out with extreme care 6*5 
grams of the powdered cyanide if the 
balance is not provided with glass pans a 
watch-glass must be counterpoised and 
the cyanide placed in this, as it must not 
be allowed to come into contact with a 
metal pan. Carefully transfer the weighed 
powder to the 500-c.c. flask by means of 
a glass funnel placed in the mouth of the 
flask. With a small quantity of distilled 
water now wash every particle of the 
powder into the flask and add a further 




FIG. 53. Burette 
and Stand. 



quantity of water sufficient to dissolve it completely. When 
the solution of the powdered cyanide is complete but not 
before fill up the flask with distilled water, carefully observ- 
ing to fill up just to the mark indicating 500 c.c. on the neck 
of the flask. During the filling of the flask the contents 
must be thoroughly shaken or stirred in order to ensure a 
solution of equal strength throughout. In a similar manner 
a standard solution of silver nitrate must now be made, by 
weighing out exactly 8-5 grams of pure re-crystallized silver 
nitrate, dissolving in distilled water and diluting to 500 c.c. 
of solution just as described for the standard cyanide solution. 
The molecular weight of AgNO 3 being 170 and of KCN 
65, it will be noted that the weighed amounts of both the 



178 ELECTROPLATING 

potassium and the silver salts bear a simple ratio to their 
molecular weights : 

AgNO, + 2KCN = KAg(CN)., + KNO ;! 

.-. 170(AgN0 3 ) oc 130(KCN) 

or 17 oc 13 

or 8-5 x 6-5. 

The next step is to remove from the solution of cyanide 
any impurities which would interfere with the clearness of 
the reaction between silver nitrate and potassium cyanide 
solutions. Fortunately only one of the impurities previously 
mentioned has any effect in this direction, namely potassium 
sulphide, and since this is readily removed it is always 
advisable to assume its presence and proceed accordingly. 
Take a small quantity of pure white lead (lead carbonate) in 
fine powder, about as much as would cover a sixpence, insert 
this powder into the flask containing the cyanide solution and 
thoroughly agitate the liquid ; this will effect the conversion 
of potassium sulphide, if present, into the black insoluble 
sulphide of lead, which will thus be precipitated and may 
subsequently be filtered off. If no black precipitate appears, 
the sample may be considered free from sulphides and the 
filtering process of course omitted. The presence of the 
slight amount of white lead will not in the least interfere 
with the remaining processes. 

The actual estimation may now be proceeded with by 
measuring out exactly 100 c.c. from each of the two 
standard solutions. The silver solution is measured by 
pouring it into the burette, just filling to a little above the 
zero mark, and taking care also that the jet below the top 
is quite filled and free from air-bubbles ; the tap at the bottom 
is then turned, a few drops allowed to escape, and the level of 
the liquid thus brought exactly to zero. The cyanide solution 
may be measured by means of a 100-c.c. pipette and then 
poured into a small conical flask, the pipette being rinsed out 
with a little water which is afterwards added to the solution 
in the flask. This flask, containing the cyanide, is then 
brought under the tap of the burette, and the silver solution 



DEPOSITION OF SILVER 170 

allowed to drop into it very slowly. It will be now observed 
that as each drop of silver solution enters the cyanide a 
slight milkiness is produced, which however immediately 
disappears on shaking or stirring with a glass rod. As the 
addition of silver solution continues, this milkiness disappears 
with greater difficulty until towards the end of the reaction 
vigorous stirring is required to clarify the liquid. This is an 
indication that the cyanide is nearly exhausted. The silver 
nitrate must now be added only one drop at a time, and at 
the moment when a permanent milkiness is produced it 
must be stopped. A little practice is necessary to determine 
this point exactly, but a careful worker will have little 
difficulty in the operation. It is advantageous to place a 
disc of black paper under the flask. 

The point at which the solution in the burette now stands 
must be carefully read off, and will indicate directly without 
further calculation the percentage of real cyanide in the 
sample. Thus supposing it is observed that exactly 90 c.c. 
of silver solution have been added, then the sample tested 
is of 90 per cent, purity. It is advisable however to repeat 
the experiment at least twice, and if any divergence of results 
is observed the process should be repeated until two readings 
are obtained with not more than 1 per cent, difference. With 
careful attention to details a much closer agreement can be 
obtained. 

The quantitative meaning of the process will be made 
clear by a further consideration of the equation given above. 

AgN0 3 4- 2KCN = KAg(CN)o (a soluble compound) + KNO 

170 2 (65) 
relative weights 

Therefore 170 AgN0 3 corresponds to 130 KCN 
and 1*7 ,, ,, 1*3 

In the standard solutions used in the above operations it 
will be noted that 100 c.c. of silver nitrate solution contain 
1-7 grams AgN0 3 and 100 c.c. of potassium cyanide solu- 
tion should contain 1/3 grams KCN if it were pure. 



i8o ELECTROPLATING 

If then the cyanide solution is of 100 per cent, purity 
the two solutions will be chemically equivalent, and 100 c.c. 
of silver solution will be required to combine with 100 c.c. of 
KCN solution exactly. The lesser number which the latter 
amount actually does require is consequently the measure 
of its percentage purity. 

It must however be pointed out that the figures and 
calculations of the foregoing method of assay of potassium 
cyanide are all based upon the assumption that the salt 
under examination is potassium and not sodium cyanide. If 
the latter is present in any appreciable quantity, the results 
of the assay will be high, owing to the fact that the atomic 
weight of sodium is only 23 compared with potassium 39. 
Under these circumstances the results of an assay may show 
a strength of cyanide over 100 per cent. Such a result is 
still of value, in making up a plating solution, as an indica- 
tion of the proportion of ON in a specific amount of the salt. 
On the other hand, however, it is no criterion of the amount 
of impurity present. If the sample under test is presumably 
sodium cyanide alone the amount taken for the standard 
solution for testing must correspond to the molecular weight 
of NaCN (49) instead of KCN (65). 

Silver, " Standard " and " Fine." With regard to the 
only other essential constituent of a silver-plating bath, viz. 
silver, little need be said further than that it is always 
advisable to use " fine " silver which is practically of 100 per 
cent, purity in preference to the ordinary " standard " silver 
which is only 92J per cent. pure. The plating solution may 
be made either from sheet silver by electro-chemical pro- 
cesses or from grain silver or a salt of silver by chemical 
methods. Where the latter methods are used and grain 
silver is employed, the silver is first converted into silver 
nitrate by dissolving in dilute nitric acid, and here it will be 
advisable to point out that at present silver nitrate of the 
highest possible purity may be purchased at a price only 
very slightly higher than the market price of the actual 
content of silver in the salt. Many operators therefore prefer 



DEPOSITION OF SILVER 181 

to buy silver nitrate rather than metallic silver, and thus 
save the considerable amount of labour and possible loss 
incurred in conversion. This course is strongly advised by 
the present writers. 

The amount of silver in silver nitrate is as 108 is to 170, 
thus \~ = 1-574 ounces of silver nitrate contain 1 ounce of 
silver. 

Tests for silver. The following rough tests which may 
readily be performed in the workshop will be found interest- 
ing and useful. 

1. Dissolve a small fragment of the metal to be tested 
in dilute nitric acid. Add a few drops of dilute hydrochloric 
acid or of a solution of common salt ; a curdy white pre- 
cipitate of silver chloride is instantly formed. To confirm, 
add a little strong ammonia and shake vigorously : the pre- 
cipitate is dissolved. If copper or nickel is present, the 
nitric acid solution will be blue in colour, which the addition 
of ammonia will intensify. 

2. A very convenient and approximately reliable method 
of distinguishing between "standard" and "fine" silver 
depends upon the fact that when alloys of silver and copper 
are heated over a Bunsen flame or on a muffle, superficial 
oxidation and consequent discoloration occur, and by this 
means some indication may be obtained as to the proportion 
of copper in certain of these alloys. 

The alloy if not already in the form of sheet should be 
rolled or hammered flat and then very slightly heated until 
discoloration takes place. Too high a temperature must 
be avoided, since that would give different results. 

Table XIII. on the next page gives a classification of the 
colour changes obtained in various alloys.'"" 

In distinguishing between fine silver and the richer silver 
alloys the test is quite unmistakable, but the method ceases 
to be applicable in the case of alloys containing more than 
160 parts by weight of copper per 1000 of the alloy. 
* See also J. Percy, Metallurgy of Silver, p. 157. 



1 82 ELECTROPLATING 

TABLE XIII. 

SdV yihe*allm iS Characters of the surface after heating. 

1000 (i.e. pure silver) . Dull, but quite white. 

950 Uniform grey- white. 

925 Dull grey-white, pinkish-black fillet at edges. 

900 Dull grey-white, black fillet at edges. 

880 Grey, almost black. 

860 do. 

840 Quite black. 

To distinguish silver from other white metals and alloys. 
Make up a test solution by dissolving 30 grains of silver 
nitrate in 1 oz. of distilled water (or 2 grams to 29 grams of 
water) and add a few drops of nitric acid. A drop or two 
of this solution when placed on base metals such as German 
silver and other white alloys instantly gives a brown or 
black stain due to the precipitation of the silver in solution. 
The surface of the metal must be quite clean or the test 
will be ineffective. No stain is produced with fine silver or 
standard silver. Silver alloys containing more copper than 
standard silver give a faint brown stain which increases in 
intensity as the proportion of base metal increases. 

Another very beautiful and delicate test for the same 
purpose is made by dissolving in water in a test tube a 
sufficient quantity of potassium chromate crystals to make 
a strong or saturated solution. Make this solution fairly 
acid by adding a drop or two of strong nitric or sulphuric 
acid. By means of a glass stirring rod, apply one drop of 
this solution to the clean surface of the metal to be tested. 
If the metal is fine or standard silver a bright red stain 
(silver chromate) will be instantly produced. Other metals 
and alloys give either a very faint dirty coloration or none 
at all. 

This test is extremely useful for distinguishing between 
silver and nickel deposits sometimes rather a difficult task 
without some such acid. 

Test for silver nitrate. If silver nitrate is used, the follow- 
ing is a good method of testing its purity. Dissolve one 



DEPOSITION OF SILVER 183 

gram of the salt in 30 c.c. of distilled water, and add 
1 c.c. of pure hydrochloric acid. Heat to boiling point and 
filter off the precipitate, which will contain the whole of the 
silver contents (as AgCl). Then evaporate the remaining 
liquid, the filtrate, to dryness. If the sample tested is per- 
fectly pure, there will be no residue or at most one weighing 
less than half a milligram. 

Methods of preparing Depositing Solutions. The 
methods of preparing silver-plating solutions may, as pre- 
viously indicated, be described under two heads. (A) Electro- 
lytic Methods. (B) Chemical Methods. Very many different 
formulae have been published under both these headings, but 
only those will be described here whose value has been tested 
thoroughly in actual practice. 

(A) Electrolytic, Methods. These methods, though quite 
applicable to many metals other than silver, have been far 
more largely applied to the preparation of silver-depositing 
solutions than to those for the deposition of any other metal. 
This is doubtless due in great measure to the fact that there 
is no possibility of loss of metal in the actual making of the 
solution by these methods. 

The principle involved may be explained thus. When 
two electrodes are placed in an electrolyte and a current is 
passed through it, the anode, if a soluble one, is always 
attacked and dissolved. Consequently the electrolyte gradu- 
ally acquires a considerable metallic content due entirely to 
the solvent action of the products of electrolytic decomposition 
at the surface of the anode. In this way an electrolyte 
which originally contained none of the metal of which the 
anode is composed may become so thoroughly charged with 
this metal as to form a solution from which it may be readily 
deposited. 

The actual method of preparation is as follows : Suppose 
that it is desired to prepare 100 imperial (120 U.S.) gallons 
of solution. To form the electrolyte dissolve in a sufficiency 
of cold water 500 ozs. of potassium cyanide of not less than 
95 per cent, purity. When the whole of the cyanide is 



184 



ELECTROPLATING 



dissolved, pass the resulting solution through a strong calico 
filter of fine mesh. The best method of making and using 
such a filter is to obtain a square wooden frame of the same 
inside measurement as the vat in which it is proposed to 




make and use the solution. Fasten by means of strong 
tacks two thicknesses of strong calico so as to stretch across 
the frame, then filter the cyanide solution directly into the 
vat. When filtered make the solution up to the required 



DEPOSITION OF SILVER 185 

bulk, 100 imperial gallons, by adding clean cold water, pre- 
ferably distilled water. Then arrange the vat for electrolysis 
as shown in Fig. 54. 

The anodes are of course fine silver, and should be 
arranged along the vat at intervals of about 12 ins. as 
illustrated ; they should be rolled to as large an area as the 
size of the vat will allow so as to obtain the greatest possible 
efficiency in electro-chemical action at the anode surfaces. 
On the other hand, the cathodes which may consist of 
copper, German silver, or iron sheet, must be small enough 
to be contained in the porous cells (C) (Fig. 54). The liquid 
in these cells should be potassium cyanide solution of similar 
strength to that contained in the vat itself. The electrical 
connections are made as shown in the diagram, an ammeter 
(A) being placed in the circuits in order to enable the plater 
to form an idea of the progress of the operation. When the 
connections are completed, current is allowed to pass through 
the vat and continued until 200 ozs. (Troy) of silver have been 
dissolved. This may be ascertained both from the ammeter 
readings and by weighing the anodes before and after 
electrolysis. 

The action taking place on the passage of the current 
may be briefly and simply described as follows : 

The electrolyte contains potassium (K) and cyanogen 
(ON) ions, forming respectively cations and anions. On 
electrolysis therefore potassium ions are liberated at the 
cathode. Immediately on liberation, however, potassium 
attacks the water present, forming potassium hydroxide and 
setting free hydrogen, thus : 

2K -f 2H 2 = 2KHO + H 2 . 

The products of electrolysis at the cathodes are therefore 
potassium hydroxide or caustic potash (KHO) and hydrogen 
gas (H 2 ), and as these are enclosed in the porous cell (C, C), 
they are to some extent at least prevented from diffusing 
through the bulk of the electrolyte. 

On the other hand, the anion liberated at the anode is 



1 86 ELECTROPLATING 

cyanogen (ON), which immediately combines with the metal 
constituting the anode, forming silver cyanide (AgCN). This 
compound is insoluble in water, but readily soluble in 
potassium cyanide; so long therefore as the electrolyte 
contains a considerable excess of uncombined potassium 
cyanide, this anode product is immediately dissolved to form 
the double cyanide of silver and potassium [KAg(CN)J, which 
of course constitutes the required depositing solution. 

The complete reaction taking place may be thus ex- 
pressed : 

2Ag + 4KCN + 2H,O = 2KAg(CN), + 2KHO -f H 2 

* It will be obvious therefore that the resulting bath 
contains a considerable proportion of potassium hydroxide, 
even if the liquid in the porous cell is thrown away. As 
the solution is worked however this is speedily converted, by 
the action of the atmosphere and by other secondary actions, 
into potassium carbonate. 

The advantages of this method of making silver-plating 
solutions are mainly : 

1. The avoidance of risk of loss of silver. 

.2. Its comparative simplicity and the fact that it does 
not require chemical experience on the part of the operator. 

The method 'has however several disadvantages which 
claim consideration, viz. : 

1. It is more costly than chemical methods in that it 
necessitates the expenditure of a considerable amount of 
electrical energy. 

(This point assumes great importance where large 
quantities of solution are concerned.) 

2. The composition of the bath is not under such exact 
control as is desirable, particularly in regard to the pro- 
portion of free cyanide present. 

(B) Chemical Methods. SOLUTION I. The first solution 
to be described under this heading and one of the most 
widely used is made up from the following formula : 



DEPOSITION OF SILVER 187 

For 100 gallons of solution : 

Fine silver .... 200 ozs. (Troy) I 6-85 kg. 
Or silver nitrate * 315 I 10'8 

Potassium cyanide Q.S.f 

( 100 imp. galls. I er . A ,., 
Watel ' ).orl20U.S | 5001ltres 

If metallic silver is used it should be in the form of 
grain and must be converted into silver nitrate as follows : 
Place the silver in a sufficiently large acid-proof jar, prefer- 
ably of porcelain or earthenware. Arrangements must be 
made to heat this by means of a water bath so as to obtain a 
temperature nearly equal to boiling water. Pour on to the 
silver pure nitric acid which has previously been diluted to 
twice its bulk with distilled water. As the solution becomes 
warm, a violent chemical action sets in and d$nse brown 
fumes of nitrogen peroxide are evolved with the formation of 
silver nitrate. The resulting reaction is 

6Ag + 8HNO :; = 6AgNO :! + 2NO + 4H. 2 O. 

The amount of nitric acid required may be readily calculated J 
from this equation, if the strength of the acid be known, but 
it is advisable to add only half the required quantity at first, 
and when this is exhausted, which will be observed by the 
cessation of chemical action, the liquid should be poured off 
and set aside for crystallization, and the second portion of 
acid added. When the whole of the silver is dissolved, the 
resulting liquid is poured into a porcelain evaporatmg dish 
and heated at about 100 C. until the liquid shows signs of 
thickening and gives evidence of the formation of crystals on 
the edge. At this point allow to cool and a quantity of 
crystals of AgNO 3 will be obtained. The remaining liquid 

* For convenience, the weight of silver nitrate here and in similar 
cases is given in troy ozs., but in commerce silver nitrate is sold by the 
avoirdupois oz., and this must be taken into account when ordering. 

t Q.S. = a sufficient quantity. 

J 200 ozs. of silver require 85 to 90 fluid ozs. of pure concentrated 
HN0 3 (sp. gr. 1-43). 



i88 ELECTROPLATING 

must be poured off and still further evaporated, and a similar 
process repeated until the whole is crystallized. 

It must however be emphasized that it is not now advis- 
able for electroplaters to attempt the preparation of silver 
nitrate themselves. This salt is now manufactured on such 
a large scale and so economically by silver refiners and 
manufacturing chemists that in the case of any reasonably 
large quantity (100 ozs. or upwards) it can be purchased for 
very slightly more than the value of the metallic silver 
contents; the margin is indeed so small as to scarcely 
more than cover the cost of the nitric acid required, leaving 
out all considerations of time and cost of apparatus on the 
part of the electroplater. 

Having now obtained the silver in the form of silver 
nitrate the operations involved in the making of a silver- 
plating solution may be summed up under three headings. 

(1) The conversion of silver nitrate (AgNOJ) into silver 
cyanide (AgCN). 

(2) The conversion of silver cyanide (AgCN) into the 
double cyanide (KAg(CN) 2 ). 

(3) The addition of a further quantity, of KCN to provide 
free cyanide. 

These operations will now be explained seriatim. 

(1) The conversion of silver nitrate into silver cyanide. 
This is done by precipitating the silver from the solution of 
nitrate in water as silver cyanide by means of a solution of 
potassium cyanide. The reaction is 

AgN0 3 + KCN = AgCN + KN0 3 . 

Now according to this equation one molecule of silver 
nitrate requires one molecule of potassium cyanide in order 
to convert it entirely into silver cyanide. If then the two 
salts are combined in the exact ratio of their molecular 
weights, the operation will be exactly complete. This point 
is extremely important, since owing to the fact that silver 
cyanide is soluble in potassium cyanide there is great risk of 
loss in the operation (in subsequent washing) by the possi- 



DEPOSITION OF SILVER 189 

bilifcy of adding an excess of cyanide solution over that 
required for precipitation of silver cyanide only. From the 
above equation, however, the amount of cyanide required 
may be exactly calculated and the danger entirely averted. 
Taking the molecular weight of the two substances, it is 
observed that 170 parts by weight of silver nitrate require 
65 parts of potassium cyanide in order to precipitate the 
whole of the silver as silver cyanide. In the solution under 
consideration the weight of the silver nitrate is 315 ozs. ; 
then 

170 : 65 : : 315 : x 

x being the weight of pure K.CN necessary to convert 
315 ozs. of silver nitrate into cyanide. 
Calculating out thus, 

315 x 65 1onK/ . N 
x = 17Q = 120-5 (nearly). 

It must be remembered however that the figure so obtained 
applies only to potassium cyanide of 100 per cent, purity. 
As it is impossible for such to be the case, a correction must 
be made to allow for the percentage of impurities. If the 
sample in use by" the operator has been examined as 
previously directed, this correction is easily made, for the 
percentage of purity will be known. 

Suppose it to be 95 per cent., then 

95 : 100 :: 120-5 : y 
y being the actual weight of impure cyanide required. 

Calculating out, we have 

120-5 x 100 
y = - Qg = 127 Troy ozs. (nearly) 

(on the metric alternative of p. 187 the amount = 4-35 kg). 
This weight of potassium cyanide is then dissolved in 
sufficient cold water and added with vigorous stirring to the 
silver nitrate which itself has been dissolved in distilled 
water. In this way the first operation may be conducted 
with confidence and with little or no loss of silver. When 
precipitation is complete the precipitate is allowed to settle, 



1 90 ELECTROPLATING 

and the top liquid, which it will be noted is simple potassium 
nitrate (KN0 3 ), is carefully syphoned off and set aside for 
recovery of the small trace of silver which may possibly be 
present. The precipitate is then thoroughly washed by 
pouring in clean hot water, stirring vigorously, and allowing 
to settle and then syphoning off. The washing should be 
repeated two or three times in order to get rid of all traces 
of the original liquid and leave nothing but the pure silver 
cyanide and a little water. 
The next step is 

(2) The conversion of silver cyanide (AgCN) into the double 
cyanide of silver and potassium, KAg(CN). 2 . [For this 
purpose weigh out a further quantity of potassium cyanide 
of about 250 ozs. (say 7 kg.). Dissolve this in cold, water 
so as to form a solution containing from 10 to 15 ozs. per 
gallon (68*5 to 103 grams per litre), and add slowly with 
constant stirring to the silver cyanide precipitate until it is just 
dissolved. Some little difficulty is sometimes found in 
determining this point owing to the fact that usually a 
certain quantity of insoluble matter is formed, due to 
impurities in the cyanide. A short experience will however 
enable the operator to judge when the solution is complete, 
and if by any chance some particles of silver cyanide remain 
undissolved at this stage they will be brought completely 
into solution in the next stage. 

The final step is 

(3) The addition of a quantity of potassium cyanide to form 
" Free Cyanide" The 'exact amount of " free cyanide " 
required in a silver-plating solution is a point upon which 
expert opinion is still very undecided, and the matter will be 
further discussed later in the present chapter. In making a 
new solution however the safest rule is to add as free 
cyanide an amount of potassium cyanide equal to that used to 
precipitate the silver in stage (1). 

In the particular instance now under illustration there- 
fore 127 ozs. (Troy) of potassium cyanide imust be added 
to the solution obtained at the end of stage (2). 



DEPOSITION OF SILVER 191 

The solution niust now be filtered and afterwards made 
up to the required bulk, 100 imp. gallons, by the addition of 
water. Advantage should be taken of this addition of water 
to wash the filter through in order to carry into the vat any 
soluble matter which may be held in the deposited substances 
on the filter. The solution is then ready for use. 

SOLUTION II. The solution now to be described was 
introduced by one of the authors a few years ago and is one 
which has been tried on a very large scale commercially with 
excellent results. 

The formula is as follows : 

Silver nitrate ..... 315 ozs. (Troy) I 10-8 kg. 
Pure anhydrous sodium carbonate 8 Ibs. (av.) | 4 
Potassium cyanide ........ Q.S. 



"0 litres 



The silver nitrate is dissolved in about 15 imp. gallons, 
(75 litres) of distilled or filtered rain-water and the sodium 
carbonate in a similar quantity in a separate vessel. When 
both salts are completely dissolved, the two solutions are 
added together and vigorously stirred. The resulting re- 
action is the precipitation of the whole of the silver as silver 
carbonate (Ag 2 CO a ). The precipitate after some continuous 
stirring is allowed to settle, the top liquid poured off and 
then thoroughly washed in the manner directed in Solution I. 
After the last washings have been poured off, with as little 
loss of time as possible since the precipitate is very suscep- 
tible to the action of light and air, a solution of potassium 
cyanide is added slowly with stirring until the whole of the 
silver carbonate is dissolved. 

A similar difficulty with regard to the presence of 
impurities in the cyanide will be observed as in the case of 
the dissolving of silver cyanide in potassium cyanide, but 
these insoluble impurities do not interfere with the reactions, 
and by close observation the operator will learn to distinguish 
the point at which complete solution is attained. 



i 9 2 ELECTROPLATING 

A similar weight of potassium cyanide must be added as 
free cyanide as in Solution I., viz. 127 ozs. (Troy), or 4-35 kg. 
on the metric alternative. 

The solution is then filtered and water added to bring up 
the bulk to 100 imp. gallons (or 500 litres). 

So far as simplicity in making is concerned, this solution 
has obvious advantages over No. I., and, as already observed, 
it has proved a very satisfactory solution in actual workshop 
practice. From a theoretical point of view an objection can 
be urged that a bath so made must contain a considerable 
quantity of potassium carbonate, as is indeed evident from 
the chemical reactions involved which are these 



(1) 2AgNO. + Na 2 CO, = Ag 2 CO ;3 + 2NaN0 3 

(washed away). 

(2) Ag 2 C0 3 + 4KCN = 2KAg(CNJ 2 + K 2 CO 3 

(retained in bath). 

The presence of potassium carbonate however in a silver- 
plating solution is not at* all an objectionable feature. 
Indeed, all commercial silver-plating baths contain* large 
proportions of this salt, particularly those which have been 
in use a number of years, and in the course of a long 
experience in the electro-deposition of silver we have 
observed that these old solutions (in use 25 years and up- 
wards) give results in rapidity of working and quality of 
deposit which certainly cannot be obtained from freshly- 
made solutions prepared in, the usual manner, in spite of 
the fact that the latter are made from cyanide of potassium 
of a much higher degree of purity than was obtainable a 
generation ago, and it is at least interesting and suggestive 
that the only notable difference which can be found after 
most exhaustive examinations is in the relatively far larger 
content of potassium carbonate that is possessed by the 
older solutions. In this connection the following typical 
analyses of old silver-plating liquids may be found in- 
teresting : 



DEPOSITION OF SILVER 193 

Solution I. Solution II. 

in use approx. in use approx. 

Contents. 30 years. 10 years. 

Ounces per Ounces per 

gallon. qallon. 

Metallic silver 3-15 . . >48 

copper 0-50 . . 0-17 

Double cyanide of silver and potassium\ ,, on K.AO 

(estimated as KAg(CN),) / ' 

Double cyanide of copper and potassium \ 1 . Q1 n . 41 

(estimated as KCu(CN) 2 ) / * L * 

Potassium cyanate, KCNO 0-35 . . 0*30 

carbonate, K,C0 3 13-05 . . 11-49 

sulphate, K 8 S0 4 0-16 . . 0-23 

chloride, KC1 0-17 . . nil 

cyanide, KCN (free) .... 2-17 . . 1'43 

It will be noted that the content of potassium carbonate 
in solution is in both instances extremely high, and in the 
case of the older liquid more than double that of the most 
important constituent (KAg(CN).,). Both these solutions, 
it may be remarked, are in daily use and give completely 
satisfactory results. 

It must be pointed out, however, that in all probability 
nothing like these proportions of potassium carbonate were 
present originally, the baths having acquired them in 
process of working by the reactions of electrolysis and 
exposure to the atmosphere. Evidently, however, this 
substance is not deleterious, and as the solution described 
in tbe foregoing (No. II.) approximates very closely to an 
old solution in its working properties even when freshly 
made, it is reasonable to suppose that this may be due at 
least in some measure to the presence of the potassium 
carbonate acquired in making. In all probability the latter 
acts as a conducting salt. 

It occasionally happens generally owing to the constant 
use of an excessive proportion of free cyanide in a silver vat 
that in the course of years the amount of potassium carbo- 
nate present becomes so great as to render the solution very 
dense, and as a consequence sluggish and unworkable. 
(This is explained by the tendency of potassium cyanide, on 
exposure to the atmosphere, to become converted into 





1 94 ELECTROPLATING 

potassium carbonate. Obviously, therefore, the more cyanide 
used, the greater the quantity of the latter formed.) 

When this is the case, the difficulty may be overcome by 
adding to the bath a few pounds of barium cyanide dissolved 
in water. The resulting action is the precipitation of a pro- 
portionate quantity of potassium carbonate as barium carbo- 
nate and a corresponding formation of potassium cyanide, 
thus 

Ba(CN), + K,CO ;! = BaCO 3 +2KCN 

(insoluble pptate.) 

This treatment, which is really the conversion of the 
excess potassium carbonate into potassium cyanide, should 
be continued until the bath is restored to a satisfactory 
working condition. 

SOLUTION III. The third solution to be described under 
the head of chemical methods is one very largely used in the 
United States. It is 

Silver nitrate ... 315 ozs. (Troy) | 10-8 kg. 

Hydrochloric acid Q.S. 

Potassium cyanide Q.S. 



Water 



C 100 imp. galls. 



500 litres 



(or 120 U.S. 

The mode of preparing this solution is very similar to 
that described in the case of Solution II. The silver is pre- 
cipitated from a solution of the silver nitrate in water, by 
means of hydrochloric acid, as silver chloride, thus 

AgN0 3 + HC1 = AgCl + HNO ;! 

The silver nitrate is weighed out and dissolved in about 
ten to fifteen gallons of distilled or filtered rain water, and 
hydrochloric acid diluted by the addition of an equal bulk of 
water is added carefully until no further precipitate is pro- 
duced. It is advisable to stir the solution vigorously from 
time to time during precipitation; when this is complete 
allow it to settle, and test the clear liquid by adding a further 
few drops of HC1 to determine whether the whole of the 



DEPOSITION OF SILVER 195 

silver is precipitated. The top liquid is then carefully 
syphoned off, and the silver chloride thoroughly washed by 
means of clean hot water. 

A solution of potassium cyanide, prepared by dissolving 
from 200 to 250 Troy ounces in about 20 gallons of water 
(say, 6-85 to 8-55 kg. in 100 litres), is then added to the 
washed silver chloride until the whole of it is dissolved. (The 
same remarks in reference to impurities apply at this point 
as in the case of Solutions I. and II.) 

The amount of free cyanide added in the case of this 
solution is usually rather larger than in the former solutions 
described, and varies from 150 to 170 ozs. Troy, according to 
the percentage of the cyanide used. When this addition has 
been made the liquid is then filtered in the usual way, and 
the bulk made up to 100 imp. gallons (or 500 litres) by the 
addition of water, which is also passed through the filter in 
order thoroughly to wash it. 

General Remarks on making Silver Solutions. It 

will have been observed that in giving the details of all the 
solutions described under the heading of " Chemical Methods," 
the exact amounts of potassium cyanide required for dis- 
solving the respective silver salts cyanide, carbonate, and 
chloride have not been stated, but have been left to the 
operator to determine by actual experiment in making the 
solution itself. The reason for this is that this amount is 
variable, and in practice is never exactly that required by 
theory. 

This point is particularly exemplified in the case of silver 
cyanide. According to theory the amount required to 
re-dissolve this salt is exactly equivalent to the amount 
which precipitated it from the solution of silver nitrate. In* 
actual practice, however, more than this amount is always 
required ; the extent of difference being greater in pro- 
portion to the extent of impurity in the sample of potassium 
cyanide used and also in proportion to the time occupied in 
the operation. The former factor is important in view of 
the fact that the impurities in potassium cyanide usually 



196 ELECTROPLATING 

consist of salts like the carbonate or chloride which give a 
corresponding precipitate of the silver salt, and as will be 
shown presently such salts if present require a double 
proportion of potassium cyanide to re-dissolve them. The 
latter factor enters into consideration owing to the suscep- 
tibility of silver salts to the action of light. This may be 
explained by an example. Suppose that 134 grams of pure 
silver cyanide are to be dissolved in potassium cyanide, the 
normal action would be 

AgCN + KCN = KAg(CN), 
(134) (65) 

Therefore 65 grams of KCN should be required, but sup- 
posing that this pure silver salt had been left a few hours 
exposed to the action of light and the atmosphere, then part 
of the silver cyanide would have become decomposed into 
some other sub- salt of silver, and before that portion could 
be dissolved in potassium cyanide it would need re-con- 
verting into silver cyanide. Thus part of the 65 grams of 
potassium cyanide would be taken up for this requirement, 
leaving insufficient to complete the solution and conse- 
quently a further quantity would be necessary. 

It must therefore be clearly pointed out that whatever 
salt of silver is used for the early stages of making solutions, 
if that salt is not cyanide, the action of dissolving in potas- 
sium cyanide occurs in two parts. In the first part the 
particular salt is converted into the single cyanide of silver, 
AgCN, and in the second part this is converted into the 
soluble double cyanide of silver and potassium. Thus in the 
case of silver chloride the reactions may be represented as 
taking place as follows 

(1) AgCl + KCN = AgCN + KOI) 

(2) AgCN + KCN = KAg(CN) 2 j 

the results of the reactions being bracketed, since from their 
nature the operator has no means of distinguishing between 
them. 



DEPOSITION OF SILVER 197 

It may be of interest here to observe that during recent 
years silver cyanide has been placed on the market by 
reputable manufacturing chemists, and the operator may 
now, therefore, if he prefers, make a solution direct from 
this salt as bought, by simply dissolving in a solution of 
potassium cyanide. 

It is not advisable to attempt to use a silver solution 
containing a lower proportion of silver per gallon than the 
weight recommended in the foregoing solutions. Many 
workers prefer a greater proportion, but it should be borne 
in mind that the amount of silver in a plating solution is 
equivalent to so much " capital " invested, and it is con- 
trary to sound commercial principles to increase capital 
invested unless there is a reasonable prospect of a propor- 
tionate increase in the returns on capital, and it by no means 
follows that if the proportion of silver in solution in a 
plating establishment is increased, say, from 2 oz. to 3 oz. 
(Troy) per imperial gallon (or 1| to 2J oz. per U.S. gallon) 
there will be an increase in returns of 50 per cent. Indeed, 
it is impossible to obtain such an increase. Eicher solutions- 
do certainly within limits work more quickly than poorer 
ones, i.e. have a higher conductivity if all other conditions 
are equal, but not in anything like the proportions corre- 
sponding to the increased capital expenditure. In fact, it is 
no uncommon experience in practice to find a solution con- 
taining only 2 or 2J oz. (Troy) of silver per imperial gallon 
conducting better and consequently working more rapidly 
than one containing double this proportion of metal. Some 
explanation of this, at first sight, rather perplexing phe- 
nomenon is found in the now generally accepted theory of 
electrolytic dissociation (see p. 23). As the effects of elec- 
trolysis are obtained by means of the dissociation into ions 
of the molecules forming the electrolyte, it follows that one 
of the main factors in the conductivity of a solution is the 
degree of dissociation of the dissolved substance. Now it 
Diay be stated as a general principle of electro-chemistry that 
while the actual conductivity of a solution falls off when it is 



1 98 ELECTROPLATING 

diluted, yet the equivalent or the molecular conductivity 
increases with its dilution.* In other words, the extent to 
which an electrolyte splits up into ions (which alone are 
concerned in carrying the current) increases as the solution 
becomes more dilute up to a certain point. When dissocia- 
tion is complete, however, the molecular conductivity is at 
its highest value. Each solution, therefore, has a point of 
maximum conductivity, and this point falls off with con- 
centration on the one hand or dilution on the other. This, 
in bare outline, is one of the results of modern research into 
the question of the conductivity of electrolytes. The pos- 
sibility, therefore, will be readily understood that, in a solu- 
tion very rich in silver, a large proportion of the molecules 
of the silver salt remain undissociated and consequently 
take no part in the conductance of the current. As a 
matter of fact the presence or addition of other substances 
in the electrolyte may play a much greater part in the 
actual conductivity of the plating solution than an increase 
of the silver compound. This is borne out by practical 
experience. 

To make the matter clearer it may be advisable to 
emphasize the point that electrical conduction is a phe- 
nomenon distinct from that of electrolytic decomposition. 
The two things must not be confounded. AH the dissociated 
ions present in an electrolyte take part in conducting the current, 
but by no means are they all necessarily deposited or liberated at 
the electrodes. 

In the case of electrolytes like that of a solution of the 
double cyanide of silver and potassium, where the actual 
metallic deposit is due to a secondary action (see p. 200) 
and not to a primary one, these principles assume para- 
mount importance. The really essential point is that, given 
a solution of high conductivity, there shall be a sufficiency 
of silver salt in the vicinity of the cathodes to provide 
material for the secondary actions to complete themselves. 
The presence of silver beyond this is valueless and means 
* See K. A. Lehfeldt, Electro-chemistry (Longmans), p. 59. 



DEPOSITION OF SILVER 199 

commercially " unremunerative capital." The seriousness 
of the matter is obvious in cases where the electroplating of 
silver is carried out on a large scale, necessitating the use of 
several thousand gallons of solution. 

Anodes. The anodes used in silver-plating should always 
be of " fine " silver rolled into sheets approximately 0-03 
inch (J mm.) in thickness. Each sheet should be annealed 
at a dull red heat, and before placing in the vat it is advisable 
to rinse well in the potash boil in order to remove any dirt 
or greasy film which may adhere to them. 

Management of Solutions. The good management 
of solutions is one of the most important factors in the 
successful electro-deposition of silver. A silver-plating solu- 
tion properly made and continuously well managed will give 
good results for a very long period. Some solutions which 
are in use to-day in the large plating establishments of the 
principal trade centres have been continuously used for 
upwards of thirty and even forty years. The two main 
points to be emphasized are 

1. The continual and regular adjustments of the pro- 
portion of " free cyanide " present, and 

2. The arrangement of anode surfaces so that the super- 
ficial area of the anode surface presented to electrolytic 
action is approximately equal to that of the cathode surface. 

The first point, the proportion of free cyanide, is one 
upon which, as previously indicated, considerable difference 
of opinion prevails, but the experience of the present authors 
after considerable experiment is that in all cases where the 
silver content is not less than 2 oz. nor more than 4 oz. 
(Troy) per imperial gallon (1 to 3J oz. per U.S. gallon), the 
proportion of free cyanide present should be between 50 and 
80 per cent, of the combined cyanide. E.g. suppose a vat to 
contain 108 oz. (Troy) of silver in solution ; then, from the 
equation previously given, we know that that amount of 
silver will have required 2 x 65 = 130 oz. of potassium 
cyanide in order to convert it into the double cyanide of 



200 ELECTROPLATING 

silver and potassium. The proportion of free cyanide 
present in such a vat should therefore be between 50 and 
80 per cent, of 130 oz. In other words, to find the minimum 
of free cyanide 

100 : 130 : : 50 : x 

or x = 65 
and to find the maximum 

100 : 130 : : 80 : x, 

or x l = 104 
i.e. 65 oz. and 104 oz. respectively. 

It will be observed that the margin allowed between the 
minimum and the maximum points is fairly wide, as the 
exact amount from which the best results can be obtained 
varies somewhat according to local conditions. But it may 
be taken as a safe rule that in the case of a new solution the 
lowest figure should be adopted, and then as the solution 
ages the amount increased until the maximum is reached. 

The necessity for the presence of free cyanide in a 
plating solution may be best explained by a consideration of 
the reactions which occur in the electrolysis of the double 
cyanide of silver and potassium. These are as follows : 

Primarily the electrolyte KAg(CN) 2 is decomposed at 
the electrodes thus 

Ag(CN) 2 liberated at anode. 

K ,, cathode. 

The ON of the compound ion AgCN.CN combines with 
the silver of the anode, and forms AgCN, so that the com- 
plete reaction at the anode may be expressed thus 

AgCN.CN + Ag = 2AgCN. 

At the anode therefore an excess of the insoluble sub- 
stance silver cyanide is formed. 

At the cathode, the simple ion K at the moment of 
liberation attacks the surrounding electrolyte KAg(CN) 2 , and 
the deposit of metallic silver on the cathode is the result of 
the reaction ; thus 

KAg(CN) 2 + K = 2KCN + Ag (liberated). 



DEPOSITION OF SILVER 



201 



The actual deposit on the cathode therefore is really a 
secondary and not a primary effect of electrolysis. 

As a result of the above reactions it will be observed 
that the liquid round the cathode is denuded of its silver 
contents, and on the other hand the anode is rapidly en- 
crusted with insoluble silver cyanide. It is owing to the 
latter effect that the presence of a fairly large quantity of 
" free " cyanide is necessary, in order to dissolve the AgCN as 
quickly as it is formed, and so preserve the anode surface 
clear and metallic. A deficiency of free cyanide always 
results in the anodes becoming dirty and slimy, and con- 
sequently in an increase of the resistance of the circuit. 

2. With regard to the second point in solution manage- 
ment, that of the arrangement of anode surface, little need 
be said further than that if a large amount of work is to be 
done and it is not desired to have a heavy weight of silver 
in stock in the vats as anode, the required surface may 
readily be obtained by rolling the silver sheets as thin as is 
necessary to give the maximum of superficial area required, 
and exposing the whole of the sheet to the action of the 
electrolyte : this can be done by fitting it into a skeleton 
frame of purest iron wire somewhat after the style shown in 
Fig. 55. 



Fm. 55. Framework for holding silver anodes. 

The frame is in electrical contact with the + pole of the dynamo and 
is entirely submerged in the electrolyte. As iron is insoluble 
in cyanide solutions even when conducting the current, such a 
frame will last many years and introduce no impurity into the 
bath. 

With careful attention to these two main points, the 
regular addition of water to make up for loss by evaporation 



202 ELECTROPLATING 

and the maintenance of the temperature at from 18 to 
20 C., uniformly good results will be obtained, and it will 
be found quite possible to work a solution so that its silver 
content scarcely varies more than a few dwts. from year to 
year. It must, however, further be observed that it is 
absolutely necessary to stir the solution thoroughly at least 
once in two days to prevent its separation into layers of 
varying density, and to secure evenness of deposit on the 
cathode surfaces. 

Electrical Conditions in Silver Deposition. The 

voltage required in the deposition of silver from a cyanide 
solution is very low ; and under average conditions of con- 
ductivity of solutions and distance between electrodes, 
should not exceed 1^ volts at the vat terminals. The 
current density generally employed is from 2J to 4 amperes 
per square foot of cathode surface, but the higher figure can 
only be employed when the cathodes are given a gentle 
swinging motion in the vat (see page 119) ; otherwise the 
deposit will become rough and granular, particularly on the 
edges. 

Special Treatment of Metals for Silver-plating. 
The general methods of preparation of articles for plating 
have been given in Chapter V., but the following special 
points require enumeration. 

(1) Copper, Brass, and German Silver. Practical experi- 
ence in depositing silver on these metals has demonstrated 
that the adhesion of the deposit is considerably enhanced 
by coating them with a film of mercury after the usual 
cleansing operations, and before immersion in the silver 
bath. The principal reason for this is that copper and its 
alloys are extremely susceptible to the action of the atmo- 
sphere and oxidize so rapidly that it is almost impossible to 
complete the cleansing processes and transfer to the silver vat 
without having formed during conveyance to the vat a film 
of oxide which would prevent perfect adhesion. The pre- 
liminary deposition by a simple immersion process of a 






DEPOSITION OF SILVER 203 

thin film of mercury prevents this trouble, and incidentally, 
as mercury is more electro-negative than silver, prevents 
any " simple immersion " deposit of silver which it is not 
advisable to have. Hence the process known as QuieUng. 
The term " quicking " is applied to the immersion of a 
metal in a solution containing mercury, during which a 
thin film of mercury is deposited by simple electro-chemical 
exchange. The solution generally used is made up as 
follows : 



Mercuric oxide (red oxide of mercury) . 1 oz. 



31-2 gr. 



Potassium cyanide 1 lb. 0-5 kg. 

f 1 imp. gall. 
Water lor 11 US 5htres 

The potassium cyanide is first dissolved in the water, 
then the mercuric oxide added, and the solution vigorously 
stirred. A black deposit usually occurs which remains un- 
dissolved, but this will quickly settle to the bottom of the 
vessel and may be disregarded. The working qualities of 
the solution should be tested by immersing in it a piece of 
clean, freshly " dipped " copper or brass for two or three 
seconds, when it should become completely covered with a 
clear bright film of metallic mercury. If the deposit is not 
clear and bright, add a little more potassium cyanide. 

It is usually supposed and it is also reasonably probable 
that " Quicking" has the effect of strengthening the adhesion 
of the silver deposit owing to the well-known amalgamating 
properties of mercury, it being said that the latter first 
amalgamates with the basis metal and afterwards with the 
silver deposit on its surface. In other words, that it forms 
a kind of " cement " between the deposit and its basis 
metal. Some investigation upon this point, however, remains 
to be made.* 

(2) " Britannia Metal " and Alloys of Tin, Lead, or Zinc. 
Britannia metal is an alloy containing usually about 90 per 
cent, tin, the remaining 10 per cent, being copper and 

* See Journal of the Institute of Metals, No. 1, 1911, vol. v. p. 222. 



204 ELECTROPLATING 

antimony in varying proportions. The recommendations for 
the preparatory treatment of this alloy for plating will serve 
equally well for similar alloys containing lead or zinc. Suc- 
cessful electro- silver-plating of these metals requires consider- 
able care and experience, and the various points in the 
directions which follow must be carefully attended to in 
order to ensure good results in the adhesion of the deposit. 

Many text-books recommend a preliminary coating of 
copper, but there is no necessity for this, and in practice it 
is rarely if ever resorted to. For preliminary treatment, 
i.e. cleansing from grease, etc., the ordinary caustic potash 
boil is the most effective agent. Sometimes the boil is made 
up of a weaker strength than that for German silver and 
other copper alloys, but the best practice is to use a fairly 
strong solution one containing at least \ Ib. caustic potash 
or soda per gallon and to shorten the time of immersion. 
These metals are rather susceptible to the action of strong 
alkalies, and therefore a prolonged immersion in potash 
would tend to injure seriously the articles ; but practical 
experience in handling these metals has proved that it is 
better in this respect to use a strong boil with consequently 
a shorter immersion than a weak boil which obviously will 
necessitate a longer one. The method of electrolytic cleans- 
ing is very useful in this connection. 

When the articles are free from grease they are usually 
scratch-brushed thoroughly on a soft brush, then rapidly 
passed through another strong potash boil (reserved for this 
purpose), and ivithoiit rinsing transferred to a "striking" or 
" starting " bath. This bath is an ordinary plating solution 
containing a comparatively small proportion* of metallic 
silver and a large proportion of free cyanide, and in addition 
to the usual anode sheets and cathode connecting rods the 
containing vat is usually fitted at one end with a shelf 
covered with a thin sheet of fine silver or copper connected 
with the cathode or negative rod. A strong current is used, 
and immediately the article is completely covered with a 
* From 10 to 15 dwts. per imp. gallon. 



DEPOSITION OF SILVER 



205 



thin film of silver it is taken out, and if of flat work (dishes, 
etc.) is transferred to the ordinary plating vat and the 
deposit built up in the usual manner. Hollow articles, 
however, like teapots, are without being emptied of the 
starting solution, first placed on the silver-lined shelf, and 
while thus in contact with the negative pole, a cylindrical 
piece of sheet silver attached to the positive pole is held 
inside for a few minutes until the inside is as perfectly 
coated as the outside. They are then transferred to the 
ordinary plating vat as in the former case. 

A difficulty often arises in the electro-silver-plating of 
Britannia metal owing to the " cutting " of the surface of 
this soft metal in scratch-brushing. Even the softest scratch- 
brush leaves marks on these surfaces which interfere with 
the subsequent finishing processes. This may be obviated 
by adopting the following method. After cleansing from 
grease, instead of scratch-brushing brush the article over 
by means of a soft bristle jewel brush, with a thin paste 
made up of precipitated chalk or whiting, and water. Rinse 
thoroughly in clean water, pass through strong potash to the 
starting vat, and proceed as before directed. 

3. Iron and its alloys. Iron and steel goods are, after 
cleansing from grease, immersed in an acid dip or pickle of 
25 per cent, hydrochloric acid or 10 per cent, sulphuric acid, 
and then usually coated with a film of copper in an alkaline 
solution (see Chapter XI.) before immersion in the silver 
vat; English operators adopt this method generally as tend- 
ing to give the most reliable results. 

In the United States, however, the following is the 
generally adopted treatment of steel goods, coppering being 
omitted. After the ordinary cleansing treatment in hot 
potash and acid pickles the articles are rapidly passed 
successively through two "striking" baths. The first of 
these is made up by dissolving about 8 oz. of potassium 
cyanide in 1 imperial (or \\ U.S.) gallon of water (50 grams 
per litre) without any silver content whatever. The articles 
are immersed in this and connected to the negative pole of 



206 ELECTROPLATING 

the dynamo, the positive pole being connected to anodes 
consisting of small sheets of silver and copper alternately. 
No appreciable deposit of course results from such a bath, 
but it has the effect of removing every trace of oxide which 
may remain on the surface of the articles. The second 
striking bath to which the articles are immediately transferred 
should contain from 6 to 8 dwts. of metallic silver per gallon 
and a large excess of free cyanide, and may be prepared by 
simply dissolving J oz. ( = 14-17 grams) of silver chloride in 
potassium cyanide solution of a strength of about 6 oz. psr 
gallon (37'5 grams per litre). Silver anodes are used or a 
large copper and small silver anode alternately. After the 
goods are completely covered with a slight film of silver 
they are transferred without further treatment to the 
ordinary silver-plating baths for the deposit required. 

Very successful results can also be obtained in the silver- 
plating of steel goods by giving them a preliminary film of 
brass from the brassing solution described on page 350 
instead of coppering. 

A further method of silver-plating iron and steel which 
is recommended, and appears to be used to some extent on 
the Continent, but was originally introduced in England, 
consists in depositing by separate current a preliminary 
coating of mercury on these surfaces before immersion in 
the plating vat. 

The article is cleaned and pickled in the usual manner, 
then made the cathode for a few seconds in a bath consist- 
ing of a solution of the oxide or nitrate of mercury in dilute 
nitric acid. The liquid should contain from 1 to 2 oz. of 
the metal per gallon, and sheets of carbon are used as the 
anodes. 

Bright Plating. In 1847, not long after the introduc- 
tion and use on a commercial scale of the cyanide solution 
for silver deposition, Mill ward accidentally discovered that 
the presence of a small trace of carbon bisulphide (CS 2 ) in 
the plating vat exercised a great influence on the character 
and appearance of the deposit. Usually the deposit of silver 



DEPOSITION OF SILVER 207 

from an ordinary plating vat is of a dead pearly white 
appearance and somewhat coarse-looking in texture ; the 
addition of carbon bisulphide, however, produces a bright 
lustrous deposit of very pleasing appearance and of a close 
smooth texture. It is difficult to assign any reason for 
this, and curiously enough successful results in ''bright" 
plating depend as much on suitable electric current con- 
ditions as on the correct proportion of CS 2 present. The 
smaller the amount of carbon bisulphide which can be added 
to secure the desired result the better. 

It is usual in silver-plating establishments to reserve one 
vat only for this treatment (unless a large amount of work 
is required) and to add the brightening liquid to this in 
extremely small proportions each day. One of the best 
methods of procedure is to mix together thoroughly, 4 British 
fluid ounces ( = 113-4 c.c.) of carbon bisulphide and 5 British 
fluid ounces ( = 141-7 c.c.) of ether, and store this solution in 
a well-stoppered bottle. Now, for a vat containing approxi- 
mately 180 to 200 imperial gallons, take J oz. of this liquid, 
pour it into a Winchester quart bottle, and fill the bottle up 
with plating solution taken from the vat to be " brightened." 
Shake the contents vigorously for a short time so as to 
obtain a thorough mixture, and then add the whole of this 
solution to that in the vat and stir the vat contents up 
thoroughly. The operation is best performed at the end of 
the day's work, so that the vat may be ready for the follow- 
ing day. It is also an advantage to have two Winchester 
bottles and use them alternately ; the ^ oz. of ether solution 
of OS., may thus be in contact with the plating solution 
24 hours before being added to the vat, and so assist the 
operator in securing the thorough mixture of carbon bi- 
sulphide with the plating solution, which is absolutely 
essential. 

The current conditions required for the " bright " vat 
vary according to local circumstances, but it may be taken 
as a general principle in bright plating that a higher E.M.F, 
should be used than in ordinary silver deposition. 



208 ELECTROPLATING 

Except in the case of very thin films of silver it is not 
advisable to put the whole of the deposit on an article in 
the bright vat. The usual procedure is to put on the major 
portion of the required silver deposit in an ordinary vat and 
transfer to the bright for the last 10 or 15 minutes of de- 
position. 

The problem as to what exactly are the reactions taking 
place in a " bright " vat is an extremely interesting one ; 
but up to the present no very satisfactory solution is forth- 
coming. Carbon bisulphide, though only very slightly 
soluble in potassium cyanide solutions, certainly dissolves 
in the small proportion in which it is present in the ordinary 
bright plating liquid. It does not, however, appear to com- 
bine chemically with the solution, but remains in it simply 
as a dissolved body. Its decomposition, therefore (if such 
takes place), is due to secondary reactions, and a theory 
tentatively put forward is that it may become decomposed 
at the cathode surfaces only by the liberation of the ion K, 
which it will be remembered is the primary product of the 
electrolysis of silver cyanide solutions. That it may be 
decomposed, with the liberation of sulphur at the cathode, 
is apparently borne out by Gore's statement that he found 
the deposited silver of the bright-plating solution to contain 
traces of sulphur. Also that sulphur plays some part in 
the brightening effect seems very probable, as some experi- 
menters have obtained good bright deposits by adding to the 
plating solution various compounds containing sulphur, other 
than carbon bisulphide. Another possible explanation is 
that it may act in a manner analogous to that of an addition 
agent, such as glue, etc. (see Deposition of Copper, Chapter 
XL, p. 248), and alter the character of the deposits, and 
consequently the colour, by affecting the size of the crystals. 

A practical point of great importance to the electroplater 
is, however, the comparatively evanescent nature of the 
effects of CS 2 . This the authors, after considerable observa- 
tion, believe to be due not so much to decomposition as to 
evaporation. This substance is extremely volatile (its 



DEPOSITION OF SILVER 209 

boiling point is 46 C.), consequently bright vats which happen 
by any chance to be exposed to a higher temperature than 
normal require more frequent addition of brightening liquid ; 
on the other hand, where the working temperature of a vat 
is fairly low it is often found advisable to make additions 
only once in two or three days. It should be noted, how- 
ever, that bright vats do not work satisfactorily at very low 
temperatures. 

An important question often raised in practice refers to 
the best method of treating a bright vat which has acquired 
an excess of " bright " liquid ; and a plan sometimes resorted 
to is to work the vat with silver sheets as cathodes with the 
idea that CS 2 would be decomposed and deposited out with 
the silver. This latter is an uncertain point, however, and 
in any case the plan is very inefficient and unsatisfactory. 
A far better method is to either boil the solution or heat it 
above 50 C. for a few hours ; in this way CS 2 , ether, and 
other volatile substances are expelled, and a " bright " vat 
which has been spoilt is restored to perfectly satisfactory 
working as a " bright " or even, if required, re-converted into 
an ordinary " dead "-plating solution. 

The Assay of Silver and Free Cyanide in Solution. 

It is essential to the efficient management of silver-plating 
solutions that the operator should be able from time to time 
to ascertain at least approximately the amount of silver and 
free cyanide contents respectively of a silver bath. The 
following methods are the most suitable for workshop 
practice, requiring the minimum of apparatus and being 
capable of yielding results of a fair degree of accuracy. 

(A) The Assay of Silver in Solution. Take an exactly 
measured quantity of the solution, say 100 c.c., or 5 fluid 
ounces, transfer to a beaker, and dilute by adding an equal 
bulk of water. Now add a considerable excess of strong 
hydrochloric acid, with the object of precipitating the silver 
from solution as silver chloride (AgCl). If only a small 
amount of HC1 is added the precipitate produced will be 
silver cyanide, the effect of the acid being simply to neutralize 



2io ELECTROPLATING 

the KCN in which AgCN is dissolved, so throwing down 
the latter, which is of course insoluble in water. Since it is 
better to get the precipitate as AgCl it is therefore advisable 
to add at least twice as much HC1 as that which appears to 
complete precipitation. Owing to the fumes of hydrocyanic 
acid liberated the process should be conducted in a fume 
cupboard or where a good draught of air is available. Now 
place the beaker and its contents on a hot plate or sand- 
bath and warm gently. This will ensure the solution of 
any copper which may be present, and also assist the pre- 
cipitate to settle. Dilute by adding cold water, pour off the 
top liquid cautiously and wash the precipitate once or twice 
by decantation ; then empty it on to a filter paper folded 




FIG. 56. Method of folding filter paper. 

and fitted into a glass funnel as shown in Fig. 56. The 
precipitate can then be thoroughly washed on the filter by 
pouring hot water on to it ; this is done most conveniently 
by means of a wash bottle, the stream of water being 
directed so as to collect the precipitate to the apex of the 
filter. At this point the silver chloride may be dried, col- 
lected into a porcelain capsule (previously weighed), then 
fused, allowed to cool, the capsule reweighed, and the silver 
content thus estimated from the weight of silver chloride 
(AgCl) obtained, but some considerable experience and skill 
in chemical operations are required for this method. The 
plater will find it much more convenient to obtain the silver 
in metallic form before weighing. Several methods are avail- 
able for this purpose, but certainly one of the best is the 



DEPOSITION OF SILVER 211 

following, which was suggested to the authors by their 
friend Mr. F. C. Robinson (Chief Assay er to the Sheffield 
Smelting Co., Ltd.). 

The precipitate on the filter is thoroughly dried, prefer- 
ably in a steam oven, and transferred to a crucible, the bulk 
by gently squeezing the cone together, and the remainder by 
flattening the paper and gently rubbing one side against the 
other until every particle is detached. The paper itself is 
bound up lightly with a little thin platinum wire and burnt 
so that the ash may be collected and added to the contents 
of the crucible. An amount of dry powdered potassium 
cyanide of about equal bulk to the silver chloride is then 
mixed with the latter and a still further equal amount added 
on the top as a cover. The crucible, covered by a lid, is now 
placed in a muffle or injector furnace and gradually heated 
to a bright red heat. A Fletcher Russell concentric jet 
furnace with a foot-blower is very convenient for this pur- 
pose if a muffle is not available. Failing either, the silver 
may be reduced by means of a large silversmith's blowpipe 
as used for hard-soldering. 

In this way the whole of the silver in the crucible con- 
tents is reduced to the metallic state and is found at the 
bottom of the crucible as a beautifully bright button of silver 
along with clean slag. Before weighing, the button or beads 
should be cleaned in boiling water, dried, and slightly 
flattened. With a little practice assays of an accuracy well 
within 1 per cent, may be obtained by this method. For 
other methods of the assay of silver, see Appendix. 

(B) The Assay of free Cyanide. This is carried out in a 
very similar manner to that directed for the assay of com- 
mercial potassium cyanide (see page 176), the principle of 
the method being the same. Take in a beaker 100 c.c. of 
the plating solution, and, in order to provide a larger bulk 
so that the reaction may be more easily observed, dilute 
with an equal bulk of water. Filter, and by means of the 
burette add standard silver nitrate solution (containing 
17 grams AgNOo per litre) drop by drop until just a faint 



212 



ELECTROPLATING 



milkiness persists in the solution. At this point take the 
burette reading, and the amount in grams of free cyanide in 
the sample tested is this figure multiplied by 0-013 (the 
cyanide equivalent of 1 c.c. standard silver nitrate). 
The following is an actual experiment : 

Amount of solution tested, 100 c.c. 
Standard silver nitrate added, 93 c.c. 
/. amount of free cyanide = 93 x 0-013 = 1-209 grams. 

It will be found very helpful to tabulate regularly the 
results of the above tests on plating solutions somewhat 
after the following fashion : 







Amount of 












combined 




Percentage of 




JVo. of vat 
tested. 


Weight of 
silver. 


KCN calcu- 
lated on the 


Amount of 
free cyanide. 


free cyanide 
to combined 


Remarks. 






formula 




cyanide. 




15 


2-15 


2-59 


1-209 


47 





A convenient quantity of solution to take for examina- 
tion is 100 c.c., and the figures in the above table are 
obtained from such a quantity. If it is desired to know the 
respective weights per gallon, these figures must be multiplied 
by 45-4 (4540 c.c. = 1 imperial gallon), and if further the 
weight is required in Troy ounces instead of grams, the result 
must be divided by 31-1 (the number of grams in 1 oz. Troy). 

To take an example from the above table, let the weight 
of silver per imperial gallon be required in oz. Troy. 

2-15 x 45-4 
Then ^ = 3-14 

/. solution contains 3-14 oz. per imperial gallon or 2-62 oz. 
per U.S. gallon. 

Stripping of old Silver Deposits. The silver coating 

* This calculation is based on the fact that 130 parts of potassium 
cyanide exactly combine with 108 parts of silver to form the double 
cyanide. Therefore multiply column 2 by 130/108 = 1-204. 



DEPOSITION OF SILVER 213 

on old copper, brass, or German silver goods may be dissolved 
off by immersing in the following : 



Concentrated sulphuric acid . . j 11 TJ 8 
Powdered potassium nitrate (saltpetre) . 3 oz. 



5 litres 
93-75 gr. 



The acid is placed in an acid-proof earthenware jar 
which is arranged in a hot-water tank so that the tem- 
perature of the acid can be raised to 70 or 80 C. When the 
acid is warm add the saltpetre, which should be powdered 
as finely as possible, and stir well with a glass rod. In this 
way by chemical action a small amount of nitric acid is 
liberated in the solution. Such a liquid dissolves a silver 
deposit readily and is without action on basis metals com- 
posed of copper or its alloys. Great care must be taken, 
however, to exclude water or even moisture as far as possible, 
since in that case the basis metal is attacked and its surface 
considerably injured. 

Silver coatings on iron and steel, Britannia metal goods, 
or zinc and tin and their alloys generally are best removed by 
making the article the anode in a solution of potassium cyanide 
of 8 oz. per imp. gallon and passing the current through by 
means of small carbon cathodes. The basis metal if iron or 
steel is not attacked in the least, and in the case of the other 
metals only slightly, and if care is exercised scarcely at all. 
Such a solution may be used until the potassium cyanide is 
almost exhausted, as will be evidenced by increasing density 
and sluggish working ; it must then be put aside for the 
recovery of its metal and a new one made up. 

Recovery of Silver from Stripping Solutions. 

From the acid solution above described the silver is recovered 
by first diluting the stripping liquid by pouring it into a large 
earthenware tank which contains two or three times as much 
water as the bulk of the " strip " (the latter must of course 
be added to the water and not the water to the acid), and 
then precipitating the silver by (a) adding a considerable 
quantity of common salt (NaCl), in which case the silver is 



2 14 ELECTROPLATING 

precipitated as silver chloride (AgCl), or (b) suspending in 
the liquid strips of scrap iron or zinc, , thus by electro- 
chemical exchange precipitating the silver as finely divided 
metallic silver on the surfaces of the suspended metal ; from 
which it may be readily removed by simply washing them 
well with a stream of hot water. In either case the silver 
contents of the strip are entirely recovered in a convenient 
form, and if not required for use in the plating shop itself 
may be sold to silver refiners. 

To obtain the silver contents from the cyanide solution 
different methods must be adopted, and by far the best, if a 
dynamo or accumulator is available, is to extract the silver 
electrolytically. This may be done quite easily and con- 
veniently by means of anodes of sheet-iron or carbon, pre- 
ferably the latter, and cathodes composed of very thin sheets 
of silver, about equal in area to the anodes, but as thin as 
practicable. The E.M.R of the current used should be from 
0*75 to 1*25 volts, and a current density of about 6 amperes 
per square foot will be most satisfactory. The silver recovered 
in this way will be found to have a high degree of purity 
and if not required for use may be sold to the refiners on 
assay results. 

An alternative method to the above is to evaporate the 
solution down to as small a bulk as makes it convenient to 
manipulate and add an excess of hydrochloric acid, thus pre- 
cipitating the silver as silver chloride. The operation should 
be performed in the open air so as to lessen the evil effects of 
hydrocyanic acid gas which is evolved. When precipitation 
is complete wash the precipitate by pouring into it a large 
volume of hot water. Stir vigorously, allow the chloride to 
settle, and syphon off the clear liquid. This process should 
be repeated at least twice. Silver chloride obtained in this 
way is quite pure, and may well be used to make up a new 
plating solution by dissolving in potassium cyanide as 
described on pages 194 and 195. 

Silver Deposition by Simple Immersion Pro- 
cesses. These processes, though not coming strictly within 



DEPOSITION OF SILVER 215 

the range of electroplating as commonly understood, yet 
merit, in the case of silver at least, a certain amount of 
attention owing to their fairly wide commercial application 
for superficially coating small articles, such as buttons, pins, 
hooks and eyes, and small springs, with silver. 

The solutions used for this purpose are almost invariably 
cyanide solutions made up in a very similar fashion to those 
for electrolysis by separate current, but containing a much 
smaller proportion of silver. 

Either of the methods previously described may accord- 
ingly be used in the preparation of solutions for this purpose, 
but the amount of silver present should not be greater than 
o oz. (Troy) per imperial gallon (3'9 gr. per litre), and 
for most purposes a lesser amount will be found to work 
more satisfactorily. 

One of the best solutions is made up as follows : 

Silver nitrate ....... -* oz. | 15' 6 gr. 

Common salt (sodium chloride) . i 7-8 ,, 

Potassium cyanide ..... 1^ oz. j 46-8 



Dissolve the silver nitrate in about half a pint of water 
(0'31 litre for the above metric values) and the common 
salt in a similar quantity. Mix the two solutions and stir 
vigorously. Then in the remaining seven pints (4'38 litres) 
of water dissolve the potassium cyanide and mix the whole 
together, stirring meanwhile. The resulting solution after 
boiling for a short time is ready for use, and may be used 
either cold or lukewarm, say 90 or 100 Fahr. At the latter 
temperature it will work more rapidly than in the cold. 

The articles to be treated should be thoroughly cleansed 
from grease and oxide as if for ordinary electroplating. 
Brass and copper goods may be coated directly, but iron and 
steel articles must be given a preliminary film of copper or 
brass in a separate current alkaline bath. Immediately 
before immersion in the silvering solution all work should 



2i 6 ELECTROPLATING 

be rinsed through a strong solution of potassium cyanide. 
Small articles are enclosed in a perforated basket so that 
when they are immersed they may be thoroughly shaken 
or agitated in order to expose every piece to the action of 
the solution. When a satisfactory colour has been obtained 
the goods must be well rinsed in cold water, then passed 
through boiling water and dried out on hot box-wood 
sawdust. 

For certain classes of work silvering pastes are used ; 
the paste being rubbed over the surface of the work to be 
plated by hand with a piece of chamois leather or swans- 
down. A good formula for a paste for this purpose is : 

Silver chloride 1 part by weight 

Cream of tartar .... 2 parts ,, 
Common salt 2 ,, ,, ,, 

Mix together well and add sufficient water to form a stiff 



This process is useless if the surface of the article to be 
treated is not absolutely free from the slightest trace of 
grease or tarnish ; otherwise the deposit is quite patchy and 
of a bad colour. 



CHAPTER X 
DEPOSITION OF GOLD 

ALTHOUGH by no means of such widespread commercial 
importance as the deposition of silver or nickel, the electro- 
deposition of gold is nevertheless a very valuable branch of 
the electroplating industry, and, by reason of the great 
variety of artistic effects which may be obtained, a very 
fascinating one too. Its application also is not altogether 
confined to ornamental purposes, but, of recent years par- 
ticularly, has been extended to the provision of protective 
coatings to the commoner metals in cases where protection 
from acid and other corrosive influences is required. 

Properties of Gold. Gold is a very soft, yellow metal, 
capable of taking a brilliant and pleasing polish. It is the 
most malleable and ductile metal known, and is also a very 
good conductor of heat and electricity, ranking inferior in 
this respect only to copper and silver. It is not acted upon 
by air or oxygen at any temperature, and is therefore par- 
ticularly suited to withstanding atmospheric influences. 
With the exception of selenic acid no single acid is capable 
of attacking or dissolving it, this property being also a very 
valuable one. It is, however, readily dissolved in the 
mixture of hydrochloric and nitric acids known as aqua regia, 
and it is also to some extent soluble in an aqueous solution 
of potassium cyanide. 

In its uses in the arts, gold is usually alloyed with some 
other metal, principally silver or copper, in order to give it 
a measure of hardness and strength which it lacks in its 



218 ELECTROPLATING 

pure state. With certain exceptions which will be explained 
later the pure metal only should be used for electrogilding. 

As will be observed by its position in the order of the 
electro-chemical series, gold is a very negative element, and 
consequently is most easily reduced from its combinations 
by almost every other metal. 

The principal salt of gold is its chloride, Au01 3 , formed 
by dissolving the metal in aqua regia (HC1 3 parts, HNO :{ 
1 part), and from this salt in the first instance all solutions 
of gold for electrogilding are made except those prepared 
by electrolytic methods. 

As in the case of silver, the best solution for the electro- 
deposition of gold is the double cyanide of gold and po- 
tassium in water, and this must be prepared either from fine 
gold or from pure gold chloride. The latter salt, like silver 
nitrate, is manufactured on a fairly large scale, and may 
therefore be readily purchased of a high degree of purity. 

Compounds of Gold. The only salts of gold calling 
for mention here are the chloride and the cyanides. A 
description of gold chloride, together with instructions for 
testing, will be given later. With regard to the combination 
with cyanogen to form cyanides, gold, like silver, readily 
combines with the alkaline cyanides to form double salts. 
Unlike silver, however, two series of double cyanides are 
known, viz. the auro and the auri salts. With potassium, 
e.f/., we may have potassium aurocyanide and potassium 
auricyanide, the respective formulae being : 

Auro . . KAu(CN) 2 . 

Auri . . 2KAu(CN) 4 .3H 2 O. 

Under ordinary conditions of making gold-depositing 
solutions the former salt is formed, but the latter can be 
made and used for electrogilding, as will be explained. 

Tests of Materials. (A) Gold. The exact assay of gold 
and its alloys is an operation demanding considerable train- 
ing and experience ; but as it is often very necessary for the 



DEPOSITION OF GOLD 



219 



clectrogilder to be able to make rough or approximate tests for 
gold, it is hoped that the following hints will be of service. 
Colour alone is misleading in judging the quality of a 
gold alloy, since by careful adjustment of the proportions 
of copper and silver present alloys of low quality are often 
made to bear a close resemblance to those of higher quality. 
The alloys of high and low quality can, however, be usually 
distinguished from each other by using the following " test " 
acids recommended by Wigley, i.e. nitric acid 4 oz., hydro- 
chloric acid | oz., water 3 oz. 

This "acid" with alloys rich in copper gives a green 
solution and copious evolution of gas bubbles, while with 
alloys of high carat the action (if any) amounts only to a 
coloration. The most common of the rough tests for gold is 
the touchstone method. For the following description of this 
method the authors are indebted to Mr. E. A. Smith, of the 
Sheffield Assay Office. 

The method consists in rubbing the alloy to be tested on 
a small block of hard, smooth, dark stone, resembling slate, 
called a fouchstone, and comparing the appearance and colour 
of the streak thus produced with those made by a series of 
small bars of carefully prepared alloys of definite compo- 





FIGS. 57 and 58. Touch needles. 

sition known as " touch-needles " (Figs. 57 and 58). The 
effect of the action of a drop of nitric acid and of dilute 
aqua regia on these streaks is also noted ; the streak from 
the less pure alloy will be more readily acted upon, with the 
production of a more or less green colour, according to the 



220 ELECTROPLATING 

proportion of copper present. Several series of touch- 
needles are usually employed, consisting of alloys of gold 
and copper, gold and silver, and gold, silver, and copper, 
either corresponding to legal standards or in series in which 
the proportion of gold increases by carats or half-carats. 

The valuation of an alloy is made by determining to 
which of the touch-needles the streak it produces most 
nearly corresponds. In order to get correctly the streak of 
the alloy to be tested the surface of the metal should first 
be slightly filed away, as this may have been made some- 
what richer than the bulk of the alloy by boiling with acid 
to remove the base or inferior metal from the surface a 
method often resorted to by goldsmiths to get a " colour " 
on gold articles. 

(B) Gold Chloride. The formula for this salt is gene- 
rally stated as AuCL, ; the commercial salt in its crystallized 
form, however, whether purchased or made in the workshop, 
contains excess hydrochloric acid and water, and is more 
correctly described by the formula, AuCl..HC1.4H 2 0. Accord- 
ing to this formula the percentage of metallic gold in the 
salt is 48, but sometimes a slightly higher proportion is 
found owing to a small loss of HC1 and water which occurs 
in drying the crystals. 

To test for percentage of gold, dissolve J gram of the 
salt in 25 c.c. of distilled water. Add to this pure potas- 
sium hydroxide (a solution in water) until the gold solution 
is distinctly alkaline (test with litmus paper); now add 
5 c.c. of a 10-volume hydrogen peroxide solution, and heat 
at the temperature of boiling water for about an hour. 
The precipitate produced is finely divided metallic gold, 
which should be washed with water rendered slightly acid 
with hydrochloric acid. It must then be collected in a 
porcelain crucible, dried, and carefully ignited. 

The resulting product should weigh not less than 0*24 
gram. 

To test for foreign metals, the filtrate from the above 
should be treated by passing sulphuretted hydrogen gas 



DEPOSITION OF GOLD 221 

through it or by adding strong ammonia and afterwards 
ammonium sulphide. No coloration or precipitate should be 
obtained. 

Varieties of gold chloride containing sodium chloride are 
now largely sold for photographic purposes. These should 
be carefully avoided by the electrogilder. They frequently 
contain only 20 to 30 per cent, of metallic gold, and are 
therefore very misleading. 

(0) Potassium Cyanide. It is of the greatest importance 
that the cyanide used in making up gilding solutions should 
be the purest obtainable. Before using, therefore, it should 
always be tested according to the methods described in 
Chapter IX. 

Methods of preparing Depositing Solutions. Gold 
solutions may, like silver, be prepared by either electrolytic 
methods or chemical methods. With due care both methods 
will give equally satisfactory results. Directions will, there- 
fore, be given for both. 

(A) Electrolytic Methods. To prepare one imperial gallon 
of solution containing 1 oz. (Troy) of gold. Dissolve 4 oz. 
(Troy) potassium cyanide in one imperial gallon of distilled 
water (or 137 gr. in 5 litres to contain 34-2 gr. of gold). 
Pour the solution into' a sufficiently large glass or earthen- 
ware vessel either round or oblong. Place inside this vessel 
a porous cell containing a strong solution of potassium 
cyanide. The level of the solution inside this cell should 
be about the same as that outside, or a little higher. 

The following diagram (Fig. 59) illustrates the arrange- 
ment. 

The anode should be of fine gold, weighing about 1J oz. 
Troy (=46-6 gr.), and rolled to as large an area as the size 
of the vessel will allow. The cathode which is placed 
inside the porous cell is preferably a strip of fine silver of 
the same length as the depth of the cell, and as wide as the 
latter will allow. If current from a dynamo or accumulators 
is not available, the most convenient form of supply is two 
large bichromate or Bunsen cells connected in series. The 



222 



ELECTROPLATING 



E.M.F. required is from 3 to 4 volts. The time occupied 
will of course depend upon the capacity of the cells, and 




FIG. 59. Electrolytic method ot preparing gilding solution. 

V, outer vessel. 
P, porous cell. 
A, anode of fine gold. 
C, cathode of silver. 

the current must be continued until the weight of the anode 
is reduced to about 10 dwts. The progress of the operation 
may be readily ascertained from time to time by weighing 
the anode. 

In plating establishments where the deposition of gold is 
only a comparatively small branch, as is often the case, this 
will be found a very convenient method of preparing solu- 
tions : especially if the operators have little chemical know- 
ledge. The apparatus may be arranged just before leaving 
for the night, and with cells of a fair capacity the solution 
will be complete next morning ; no intermediate attention 
is required, particularly if bichromate cells or accumulators 
are used. 

Before actually using the solution for gilding it will be 
found advantageous to boil it for an hour or so. 

(B) Chemical Methods. In making solutions by these 

\ methods either metallic gold or gold chloride may be used. 

If the former is employed, however, the first stage of the 



DEPOSITION OF GOLD 223 

operation is its conversion into the chloride. This, as will 
have been gathered, is done by dissolving it in a mixture of 
three parts hydrochloric acid and one part nitric acid. 

For this purpose, the gold should be cut up into small 
pieces and placed in a thin conical-shaped glass flask or 
beaker. The acid mixture is then poured on to the gold 
and gentle heat applied by placing the vessel in hot water 
or on a sandbath. A vigorous chemical action ensues, the 
gold being attacked by chlorine which is liberated in the 
interaction of the two acids. It will be found better to add 
a relatively small proportion of acid at first (say 50 to 100 
c.c. for 1 oz. of gold), and when this is saturated, as will 
be observed by the cessation of the chemical action, it may 
be poured off into an evaporating dish, and a further quan- 
tity of acid added according to the amount of gold left. 
In this way an excess of acid is avoided. When the whole 
of the gold is dissolved the solution must be slowly and 
carefully evaporated by heating in a porcelain evaporating 
dish until the liquid shows signs of thickening, when it is 
set aside to cool. When cold the whole mass will consist 
of fine needlelike crystals of gold chloride. Special care 
must be taken, however, not to dry up the liquid in evapo- 
rating, as in that case some of the AuCL product may at 
185 C. be reduced to AuCl, above 185 C. to metallic gold. 
If by any accident this occurs an addition of aqua regia 
must be made as found necessary. If the gold salt is not 
required for immediate use in making up solutions, it may 
be stored in the crystallized form or dissolved in distilled 
water kept in a stoppered glass bottle, and used as needed. 

For the remaining stages of the preparation of electro- 
gilding solution by chemical methods, a number of different 
formulae have been recommended, the chief feature of many 
of them being their complexity. Only three will, however, 
be described here, each of these being thoroughly reliable. 
The second is the most generally used, with varying pro- 
portions of gold content according to the class of work 
done. 



22 4 ELECTROPLATING 

FORMULA I. 
Gold (converted into gold chloride) . 1 oz. (Troy) 



34-2 gr. 
68-4 , 



Or gold chloride 2 oz. ,, 

Potassium cyanide Q.S. 

Water (distilled) .. l -S'^ ' 5 litres 



The gold chloride is dissolved in about a pint of distilled 
water. A solution of potassium cyanide of a strength of 
from 8 to 10 oz. per imperial gallon (50 to 62-5 grams per 
litre) is then prepared, and a portion slowly and carefully 
added to the gold solution as long as a precipitate is pro- 
duced. This precipitate (brownish in colour) is gold cyanide, 
and like silver cyanide it is readily soluble in excess of 
potassium cyanide ; the greatest care therefore must be 
taken to exactly precipitate the gold as cyanide, and not to 
redissolve it. The reaction is 

AuCl 3 + 3KCN = Au(CN), + 3KC1. 

The amount of cyanide required in this reaction may be 
calculated therefore as in the case of the corresponding 
silver reaction if its percentage purity be known. 

After vigorous stirring the precipitate must now be 
washed thoroughly either by decantation or on a filter. 
As the amount of solution is not large, the latter method is 
best. For this purpose fold a circle of filter paper, about 
10 ins. diameter, into four folds. Fit the apex into the apex 
of a 5-in. or 6-in. glass funnel and open in the manner 
illustrated in Fig. 56. 

Pour the solution containing the precipitated gold cyanide 
on to the funnel, the clear liquor will run through and the 
precipitate will be retained in the filter. Wash the precipi- 
tate several times by pouring on a supply of warm water 
and allowing it to run through. When the wash waters 
have been finally drained off, place the funnel in the mouth 
of a large bottle a Winchester will do and continue the 
addition of the potassium cyanide solution previously made 



DEPOSITION OF GOLD 225 

up. The precipitate will thus be slowly dissolved and the 
solution will run through into the bottle. Care must be taken 
not to add more of the cyanide solution than is actually 
required, since many gilding solutions require very little 
" free " cyanide, and the specific amount of this must be 
adjusted according to the class of work to be done. 

The solution must now be boiled and afterwards made 
up to a bulk of one gallon by the addition of distilled 
water. 

FORMULA II. 
Gold (converted into gold chloride) . 1 oz. (Troy) 34-2 gr. 



Or gold chloride 2 



684 



Ammonia, s.g. 0-880 Q.S. 

Potassium cyanide Q.S. 

( 1 imp. gall. I e ,.i 
Water lorUU.S | 51ltreS 

Dissolve the gold salt in about a pint of distilled water, 
or less, not more. When solution is complete, add ammonia 
slowly until no further precipitate is produced (from 2J to 3 
fl. oz. are usually required), and stir well. A copious yellowish- 
brown precipitate results, known as fulminating gold. The 
reaction is rather complex, but may be summed up thus : 

2AuCl 3 + 8NH 4 HO = Au(NH)NH 2 + AuNHCl + 5NH 4 C1 

Fulminating gold. + 8H 2 O. 

This precipitate if allowed to dry is very explosive, so 
that it must always be kept under water, and for this reason 
should be well washed by decantation, not on the filter. 
The first wash- water should be kept for the recovery of any 
trace of gold which it may contain, and the final wash- water 
need not be completely poured off. When washing is complete, 
add to the precipitate a solution of potassium cyanide of a 
strength of about 8 oz. per imperial gallon (50 gr. per 
litre), until it is just dissolved, and a clear pale yellow liquid 
will result. Sometimes a little undissolved matter from the 
impurities in the cyanide will be noticed, but this may be 

Q 



226 ELECTROPLATING 

disregarded. The solution is now boiled for a short time or 
until there is no smell of ammonia, and then diluted with 
distilled water to a bulk of one gallon. 

FORMULA III. 
Gold chloride crystals 1 

(AuCl 3 .HCUH 2 0) ' | ' 1 oz ' 



Weigh out the above quantities exactly, and place each 
in a Bohemian glass flask or beaker (say of 8 fl. oz. or 250 
c.c. capacity). To the potassium cyanide add 5 c.c. of 
distilled water. Heat both flasks by placing in a bath of 
boiling water, so that the temperature does not rise above 
100 C. The gold salt will gradually melt into a thick 
spongy liquid. The cyanide also will dissolve but may 
require the addition of a little more distilled water the 
solution should, however, be kept as concentrated as possible. 
When the contents of both flasks are perfectly liquid but 
not before add the chloride of gold very cautiously in small 
quantities at a time to the cyanide solution and shake 
thoroughly after each addition, still keeping the flasks hot. 
The chemical reaction is rather violent, but is quite safe if 
the additions are made slowly. When the last few drops of 
gold chloride have been added to the cyanide, the liquid will 
show distinct signs of crystallization, and on putting aside 
to cool the whole mass will crystallize in large colourless 
tablets. 

Under the above conditions of concentration potassium 
auri-cyamd.6 is formed, the composition of the crystals being 
2KAu(CN) 4 .3H 2 O (see p. 218). All that is necessary is to 
dissolve this salt in distilled water to any dilution required, 
and a very fine gilding solution results. 

This method is unusual and the constitution of the salt 
in aqueous solution is uncertain, but we have used a solution 
made in this way on several occasions in commercial practice, 



DEPOSITION OF GOLD 227 

and for "bright" gilding (p. 228) an excellent fine yellow 
colour is produced. 

All the above solutions may be worked either cold or hot 
according to the colour required and the class of work done. 
It may be stated generally that cold solutions give a lighter 
tone to the colour of the deposit than hot solutions. It 
need hardly be mentioned that the latter conduct electricity 
much more readily than the former. 

It will have been noted that in giving details of the 
composition of gilding solutions no recommendation has 
been made as to the addition of free cyanide. This is so 
because, in the opinion of the authors after considerable 
experience and observation, the proportion of free cyanide in 
these solutions should be kept as low as possible. All that 
is required is sufficient to keep the anode surfaces clean in 
actual working, and it is surprising how little is needed for 
this purpose. And in the making up of any cyanide solution 
it invariably happens in redissolving a precipitate in potassium 
cyanide (whatever the precipitate may be) that a little more 
than is actually required for dissolving is added, since it 
would necessitate extreme care and special precautions to 
gauge exactly the point at which the last particles of the 
precipitate disappear. 

Moreover the operation of gilding as usually practised is 
the imparting of a mere film of the metal as a protective or 
ornamental covering, not deposition by weight ; consequently 
the operation is short and the anode is scarcely ever 
immersed in the solution sufficiently long to become coated 
with the results of the decomposition taking place at its 
surface, as would be the case in a corresponding silver 
solution with a deficiency of cyanide. This, however, is 
only a comparatively minor reason for the omission of free 
cyanide. The most important is that in a large majority of 
cases the electrogilder is called upon to gild articles which 
have had their surfaces previously carefully prepared by 
burnishing or polishing; particularly is this the case with 
standard silver or electro-silver-plated goods. The operation 



228 ELECTROPLATING 

is usually termed " bright" gilding. The surface bearing as 
high a polish as it is capable of, must be given a thin film of 
gold without in the slightest measure deadening or dulling 
the surface. Now if the solution used contains a very slight 
excess of free cyanide, then unless the work is carried out with 
extreme rapidity, the surface is slightly acted upon and 
stained before the gold can be deposited, and as a con- 
sequence the brilliancy is lost and repolishing and some- 
times regilding is necessitated. The same remarks largely 
apply to other delicate surfaces of silver, such as those finely 
matted or grained, which are required to show the same 
appearance when gilt. This point is much more noticeable 
in hot solutions than in those worked cold, the former 
naturally being more active chemically. It often happens 
therefore that a solution for bright gilding which works 
unsatisfactorily when warmed will give quite good results 
if allowed to cool and worked only when cold. If accidentally 
a little too much cyanide has been added to any solution, the 
ill effects can often be overcome by giving the liquid a 
prolonged boiling, say for five or six hours. This treatment 
results in the partial decomposition of the free cyanide 
present and so assists in restoring correct conditions. The 
same treatment should be resorted to if the solution has 
acquired any organic matter. 

For the electro-deposition of gold where an appreciable 
weight of the metal is required the solution conditions are 
quite different. In this class of work free cyanide is not 
merely allowable but necessary, and the surfaces -upon 
which the deposit is to be made do not usually require such 
delicacy of treatment as " bright " work. The proportion of 
free cyanide generally employed is about one-fourth of the 
amount used to dissolve the gold precipitate in making up the 
solution. The quantity of free cyanide in solution can be 
tested for by the method recommended under silver de- 
position (p. 211), except in the case of very old solutions 
where the colour is often so dark as to make it difficult to 
detect the end of the silver nitrate reaction. In such cases, 



DEPOSITION OF GOLD 229 

however, if the solution is unsatisfactory it is better to make 
a new bath, recovering the gold in the old one as directed 
later. 

In many plating establishments it is customary to keep 
two separate solutions for the two classes of work described 
in the foregoing, and this plan will be found very advan- 
tageous, since then the best conditions of solution are obtain- 
able for each class. 

Anodes. Anodes in all cases should be of fine gold, 
and if it is not desired to have a large amount of gold in 
stock they should be rolled to as thin a degree as is 
reasonable, so that an anode surface may be obtained at 
least in some measure commensurate with the surface to be 
gilt. Some operators and text books recommend platinum 
as anodes, but there is no advantage obtainable in this 
way, and so long as this metal is at or about its present 
market price it is out of the question commercially. If for 
any reason gold is not available, a piece of J-in. or f-in. sheet 
carbon is the best substitute. 

Management of Solutions. Gold solutions are not 
particularly difficult to keep in order if proper care is 
observed to prevent the introduction of foreign matter. As the 
anode is only very slowly dissolved in the solution, and in 
the case of solutions for bright gilding scarcely at all owing 
to the absence of free cyanide, regular additions of dissolved 
gold must be made to keep up the strength of the bath. 
This may most conveniently be done by keeping at hand a 
supply of gold chloride either in the form of crystals or as a 
concentrated solution. A quantity, corresponding to about 
J oz. Troy of metallic gold to each gallon (3-42 gr. per 
litre) of the solution requiring the addition, is then converted 
into a strong solution of the double cyanide of gold and potas- 
sium by either of the two methods already described. In this 
way additions may be made without materially adding to the 
bulk of the liquid in use. It will be found of great advantage 
after every such addition to boil the solution for a short time 



2 3 o ELECTROPLATING 

and then filter it. These supplies to the solution should be 
made at regular intervals according to the quantity of work 
passing through it. The most reliable indication of the need 
for a fresh addition of gold to a solution is found in the 
colour of the deposit. The characteristic rich yellow tint of 
fine gilding is lost and the deposit is either of a pale brass 
colour or of a reddish copper colour according to the 
current conditions. 

Special treatment of Articles preparatory to 
Gilding. Gold can be deposited on most metals directly 
without any intermediate coating of another metal; the 
general preparatory treatment discussed in Chap. VIII. is 
therefore usually adopted for preparation for electrogilding. 
A few special points, however, deserve mention. In the 
preparation of surfaces for the classes of gilding variously 
known as " dead," " frosted," " satin," " matte," and " grain," 
sand-blasting is now very largely employed and a great 
diversity of effects may be thus produced. In all cases of 
the electro-deposition of metals the surface of the deposit to 
a large extent partakes of the same characteristics as the 
surface of the metal being plated. Consequently whenever 
it is desired to have a finished surface on an electro-deposit 
of a certain character, the surface to be plated should always 
be given some treatment which will give to it this character- 
istic at least to some extent. Some very pleasing effects of 
this nature may be given to gilded articles by using various 
grades of powdered pumice in the sand-blasting apparatus 
at pressures varying from 3 Ibs. to 5 Ibs. per square inch. In 
many classes of work very lovely soft tints may be obtained 
in the gilding by the ordinary preliminary treatment followed 
by treatment on the blasting apparatus with a very fine 
grade of pumice at the lower pressure referred to. 

Where the sand-blasting apparatus is not available frosted 
or " satin -finish " surfaces may be produced on silver or 
copper goods by using strong hard- wire scratch-brushes such 
as are supplied by makers for this purpose. These brushes 
should revolve at a speed rather higher than the normal. 



DEPOSITION OF GOLD 231 

Similar effects can also be produced by holding a block of 
wood firmly on an ordinary " chock " scratch-brush at a point 
just before it meets the article to be brushed; the bristles 
thus " spring " forcibly and suddenly on to the article and so 
impart to it the desired surface. 

In gilding copper and alloys rich in copper where a light 
rich yellow tint is required it is very often advantageous to 
give the article a slight coating of silver prior to gilding. 

At the present time for trade purposes mainly for the 
cheaper classes of work a large amount of gilding is done 
at a very low rate. The usual method of procedure is to 
give the article a preliminary film of copper from the alka- 
line bath, and then rapidly to pass it through the gilding 
solution to " colour up." A much better method for this 
class of work is to deposit the preliminary film from a 
brassing solution (see Chap. XVII.) worked with a very small 
current, either cold or only lukewarm. Under these condi- 
tions the deposit from such a brassing solution as recom- 
mended has a colour closely approaching 18-carat gold, and 
a very brief immersion in the gilding solution will impart 
quite a rich gold colour. 

Reference has previously been made to the gilding of 
articles, chiefly silver or electro-silver plate, which have been 
given highly polished surfaces. Such goods must obviously 
be very carefully handled in preparatory treatment. They 
should be well washed with a clean sponge in very hot 
water, then passed through a boiling solution of caustic 
potash (about 6 oz. per gallon) and rinsed in cold water. 
The manner in which the clean cold water runs off the 
surface is an infallible indication to the operator as to 
whether the surface is free from grease or soapy matter ; if 
not, the treatment must be repeated until water flows off the 
surface quite evenly. 

'All the particular types or classes of electrogilding 
described under the following terms are obtained by prelimin- 
ary treatment of the surfaces to be gilt ; namely, (a) Bright 
gilding, (#) Dead gilding, (c) Frosted, or " satin-finish " 



232 ELECTROPLATING 

gilding, (d) Grained gilding. With reference to these trade 
terms therefore little need be added to the foregoing 
details. With regard to bright gilding, however, which was 
described in discussing the question of free cyanide in gilding 
solutions, it should be emphasized that the highest possible 
polish be previously given to the article, or the gilt finish is 
not satisfactory. It may further be observed that only com- 
paratively thin films of gold can be deposited on these 
surfaces if the deposit is required to retain all the brilliancy 
of the original polish. As the gilding increases in thickness 
it acquires gradually a dull appearance unless special pre- 
cautions are used, and will in such a case need repolishing. 

Grained surfaces are sometimes produced by treating 
with the finest flour emery. For watch mechanisms and 
similar classes of work, Roseleur published a method of 
graining in use largely in Switzerland and France which is 
of considerable interest. In brief outline this method is, 
after rendering the surface perfectly smooth and cleansing in 
the usual manner, to treat the articles with a mixture of 
finely divided silver powder, potassium bitartrate and common 
salt in about the following proportions : 

Finely divided silver .... 5 parts by weight 
Potassium bitartrate .... 40 
Common salt 100 

The silver powder may be obtained by hanging strips of 
copper in a dilute solution of silver nitrate, so throwing down 
the silver as a metallic precipitate, which must be carefully 
washed and dried. The three ingredients are thoroughly 
mixed together and made into a thin paste with water. This 
paste is carefully and equally brushed over the entire surface 
to be gilt with a strong bristle brush, imparting the while a 
brisk and firm circular motion either to the article or to the 
brush or to both. The coarseness of the grain may be 
influenced by varying the proportions of tartar and salt in the 
mixture an excess of the salt producing a larger grain. 



DEPOSITION OF GOLD 233 

Electric Current Conditions in Gilding. Require- 
ments in electrogilding vary so greatly that it is difficult to lay 
down definite rules as to either voltage or current density to 
be employed. The former, however, should never be allowed to 
fall below 3 volts, and for irregular surfaces and large articles 
of hollow ware 4 volts will give more satisfactory results. 

In ordinary gilding operations by far the most reliable 
guide in the determination of correct current conditions is 
the colour of the deposited gold. This should be closely 
observed and the current regulated so as to produce con- 
tinuously throughout deposition a deposit of a deep yellow or 
light yellowish-brown colour, having of course the fine grain 
or pearly texture of electro-deposited metal. Any deeper 
shade of colour, such as a distinct brown (which is very liable 
to be produced), will prove unsatisfactory after final scratch- 
brushing. 

Gilding Insides of Hollow Vessels. This is a very 
usual requirement in electrogilding, particularly " bright " 
gilding. The article to be gilt inside is filled with the solu- 
tion and connected in some convenient fashion to the negative 
pole of the dynamo or battery, and a small sheet gold 
anode is hung in the centre of the liquid connected to the 
positive pole. For this class of work it will be found most 
convenient, however, to use a long and narrow piece of thin 
sheet gold as anode and to bind it firmly round a piece of 
hard wood about f or f inch in diameter and from 8 to 12 
inches long, according to the usual depth of the work to be 
gilt. The gold sheet need not be as long as the wooden rod, 
but it is advisable that it extend so far along the rod that 
when immersed in the gilding solution the copper connecting 
wire is not also immersed. The anode and rod should now, 
for at least three or four inches of their length, be covered 
tightly with two or three thicknesses of fine chamois leather 
or swansdown of good quality. This arrangement serves a 
double purpose. In the first place it prevents a possible 
short-circuiting of the current owing to the anode touching 
the bottom or sides of the article during gilding, and secondly 



234 ELECTROPLATING 

it enables the operator by the thorough saturation of this 
leather covering to draw the solution round the edges of the 
article, particularly irregular edges, lips of cream jugs, etc. 
This idea is of course adaptable (and often convenient) to 
other branches of electro -deposition as well as gilding, and is 
known in the trade as a " doctor." 

Colour-Gilding. No electro-deposited metal hitherto 
known is, at any rate so far as colour is concerned, so 
extremely sensitive to the slightest change in either current 
or temperature conditions or composition of electrolyte as 
gold. A few simple experiments in gilding with only the 
conditions of temperature varied will exemplify this and 
incidentally reveal and suggest to artistic workers some 
considerable possibilities in metal colouring. 

This colour sensitiveness of electro-deposited gold has 
given rise to a branch of the industry (perhaps more largely 
practised in the United States than in England) known as 
colour-gilding. 

The principal colours aimed at in this class of work are 
known as red, green, yelloic, and rose-colour, but a number 
of different shades under each of these descriptions are 
obtainable. 

As has just been observed, varying conditions of tempera, 
ture and current will readily produce varying tints of colour 
in the deposited metal. In actual practice, however, the 
colours enumerated above are usually obtained by very 
slight variation in the composition of the solution employed ; 
though the beginner in the art will find it a very 'great 
advantage to thoroughly familiarize himself with the changes 
obtainable by the regulation of external conditions before 
going on to the actual use of the solutions shortly to be 
described. 

The basis of all solutions for colour-gilding is the double 
cyanide of gold and potassium made up according to either of 
the formulae of pp. 223 to 226. It will, however, be 
usually found advantageous to dilute the solutions thus made 
by adding an equal bulk of water or more in order to reduce 



DEPOSITION OF GOLD 235 

the gold content per gallon to about one-half or one-third of 
that recommended for ordinary gilding, the different tints of 
colour being as a rule more readily obtained from weaker 
solutions, i.e. those containing not more than 10 to 14 dwts. 
per imp. gallon (= 8-33 to 11-66 dwts. per U.S. gallon, or 
say 3^ to 4J gr. per litre). Indeed some operators prefer 
baths containing as low a proportion of metallic gold as 4 
dwts. per gallon. The deciding factor in the matter is, how- 
ever, the depth of colour aimed at ; if dark or deep tones are 
required, the metallic gold content should never be less than 
10 or 12 dwts. per imperial gallon to obtain the best possible 
results. 

The modifications of the ordinary gilding solution just 
referred to, usually employed for the various classes of 
colour-gilding, are obtained by the addition of very small 
proportions of other metals, mainly silver, copper, arsenic, 
and occasionally lead. A large number of different formulae 
will be found scattered through the literature of electro- 
deposition, but the following will be found to yield excellent 
results with a little practice and proper attention to 
detail. 

1. Eed-gilding. 

One imperial or 1J U.S. gallon of ordinary gilding solution 

containing 10 dwts. metallic gold. 
200 grains of pure copper acetate (crystallized). 
The copper acetate should be finely powdered and made 
into a thin smooth paste by the addition of distilled 
water. A weak solution of potassium cyanide must 
now be added very carefully and slowly until the 
copper salt is just dissolved. Add the resulting 
liquid (after filtering to remove impurities) to the 
gilding solution and boil the whole for 15 to 20 
minutes. 

In working this solution, which should be done at a 
temperature of about 70 C., it is most essential for the 
operator to realize that it is rarely necessary to make any 
greater addition of copper salt to the solution than is 



236 ELECTROPLATING 

recommended above ; and in all further additions to the 
bath the above proportions of copper and gold must be 
adhered to. It must be remembered that gold is the more 
electro-negative element present, and as such has a decided 
tendency to deposit first. After the first addition therefore 
more copper should never be added without a proportionate 
amount of gold in order to correct this tendency. 

This latter point will further suggest the necessity of 
using a current slightly stronger than for ordinary electro- 
gilding. This indeed is necessary in all colour-gilding 
operations where the effects are sought to be obtained by 
adding to the bath solutions of more electro-positive 
metals. 

2. Green~gilding. 

One imperial or 14 U.S. gallon of ordinary gilding solution 

containing 10 dwts. metallic gold. 
150 grains pure recrystallized silver nitrate. 
50 grains caustic soda (quality not less than 85 per cent. 

NaOH). 

The silver nitrate is dissolved in a sufficiency of distilled 
water and a weak solution of potassium cyanide added 
until the cyanide of silver precipitate which at first 
forms is completely dissolved. The resulting solution 
is then added to the gilding solution and the whole 
thoroughly stirred. Finally, add the caustic soda 
(first dissolved in a little water) and boil the resulting 
solution for twenty minutes or so. 

This solution, worked at a temperature of about 70 C., 
yields a rich green-coloured gold of a rather dark shade. If 
a lighter shade is required, a rather larger proportion of 
silver must be added. It is better, however, to try the bath 
first with the above proportions and not to add any greater 
amount of silver until found necessary. 

For green gilding some authorities recommend the 
addition of arsenic, usually in the form of arsenious oxide, 
As 2 O, (more correctly arsenious anhydride). This should be 
dissolved in a strong solution of caustic soda and only added 



DEPOSITION OF GOLD 237 

to the bath in very small proportions, with or without the 
simultaneous addition of silver. Some very pleasing shades 
of green gold are obtainable by these means, but arsenic 
alone as the added ingredient is not so reliable as silver, and 
in any case as small a proportion as possible to obtain the 
desired effect should be employed. It is very liable to 
spoil the gilding solution completely if by any means the 
bath acquires an excess. 

Arsenical gold baths give the best results if a slightly 
weaker current is employed than would be the case in normal 
gilding operations. 

3. Yellow -gilding. 

This colour is obviously the effect obtained from the 
ordinary gilding solution. As the term is applied in schemes 
of colour-gilding, however, a very light tone of yellow, some- 
times called Eoman gold, is usually meant. This, where 
required to contrast with green or red gold in the schemes 
of gilding presently to be described, is not always easy to 
obtain. The normal colour of electrogilding is, or should 
be, a rich, rather dark shade of yellow, and it is consequently 
a little too dark to contrast properly with the red or even 
green tones obtained as above. 

In this class of colour-gilding, however, no additions 
which can be made to the bath itself, with the exception 
perhaps of a very small amount of caustic soda, will prove 
so satisfactory as a proper manipulation of external conditions, 
i.e. temperature, voltage, and current density. 

The best results are obtained from solutions containing 
not more than 8 to 10 dwts. metallic gold per imp. gallon 
(2f to 3J gr. per litre). If the solution is newly made by 
either of the chemical methods before described, an addition 
of from 25 to 50 grains of caustic soda per gallon (0-36 to 
0-72 gr. per litre) should be made. The best working 
temperature will be found to be not more than 60 C. with 
an E.M.F. of 2'5 volts, though both this factor and that of 
current density is largely dependent upon the class of work 
done. If the articles have deep recesses, a greater E.M.F. 



238 ELECTROPLATING 

is necessary. Exact conditions can only be determined by 
actual experiment. 

Newly-made solutions give as a rule the best results 
in light yellow tones, since baths usually yield darker 
deposits as organic matter and other impurities are acquired 
in process of working. 

Rose-coloured gold. The varied tones of colour which 
may be described under this general heading are usually 
obtained by the addition of both silver and copper to the 
gilding solution. 

The proportions already detailed under the respective 
descriptions of red and green gilding are suitable for de- 
veloping this colour, but it is obvious that many varieties 
of tone may be obtained by varying these proportions. 

An exceedingly rich effect which might be classed under 
the title rose-coloured gold is obtainable by first giving the 
article a very thin, almost infinitesimal, deposit of copper 
in a copper solution composed of copper sulphate and alum 
(see Chap. XL, p. 250). It is then thinly gilded in the 
yellow gilding solution and again treated in the copper vat, 
and finally shaded off in a normal gilding solution, using a 
fairly strong current. 

In finishing coloured gilding pleasing effects are often 
obtained, particularly on ornamented surfaces having high 
reliefs, by very gently rubbing the raised portions with 
finely powdered borax or pure anhydrous sodium carbonate. 
This should be done by hand or a very soft swansdown 
dolly, and great care must be taken not to scratch the 
surfaces. 

A sand-blasting apparatus such as is described in Chap. 
VII. is an invaluable adjunct to colour-gilding. Indeed for 
many effects needed to meet the requirements of modern 
art it is absolutely essential, and very careful note should 
be made of the recommendations in the section treating 
on that subject as to the use and applications of sand- 
blasting. 



DEPOSITION OF GOLD 239 

"Parcel" and "Partial" Gilding. The use of these 
two terms in trade circles, often as if they were synonym- 
ous, has given rise to some confusion as to their exact 
meaning and application. According to the best usage and 
the highest authorities, however, the former term parcel 
gilding should be confined strictly to the art of gilding 
one article in a variety of colours, i.e. relieving the various 
characteristics of the surface of a chased or embossed article 
in red, green, or yellow gold according to any colour 
scheme devised by an artist or by the operator himself. 

The term partial gilding on the other hand should be 
applied only to the part-gilding of an article where for 
example one part of a surface is required to be finished 
in copper or silver and the remaining part (often chased, 
embossed, or engraved portions) gilt. 

These two branches of the art of gilding afford con- 
siderable scope for the exercise of mechanical ingenuity 
and artistic skill. 

Both classes of work are done by means of " stopping- 
off" varnishes prepared according to one or other of the 
directions given below. 

Asphaltum stopping-off varnish. Dissolve a sufficiency 
of asphalt together with a little mastic (resin from the 
mastic tree) in oil of turpentine until the liquid is of the con- 
sistency of thin cream. Apply with a camel's-hair brush. 

Copal varnish. Take sufficient good quick-drying copal 
varnish and add to it ultramarine, or chrome yellow, 
with thorough incorporation until a thin paste is obtained. 
This also is applied with a camel's-hair brush, and care 
must be taken that it is thoroughly hard and dry before 
immersion in the plating solution. 

Common Brunswick black mixed with a little fine 
asphaltum powder is also favoured by some operators. 

Suppose an ornamented silver shield is required to 
be gilt, and finished to show a groundwork of fine yellow 
or green gold and all raised or embossed parts, say leaves, 
flowers, etc., coloured with red gold. The operator will 



240 ELECTROPLATING 

first gild the shield over its entire surface in a solution 
giving the required yellow or green colour of the ground- 
work (in any colour scheme the lightest shade is given 
first). It is then taken from the solution, carefully washed 
and dried out, and with a fine camel's-hair brush every 
part of the shield which, when finished, is to show the 
yellow (or green) colour is carefully covered with the 
particular stopping-off varnish chosen. This is the part 
of the operation needing the greatest skill, and some con- 
siderable practice is necessary to become efficient. The 
article is then exposed to a moderate dry heat for as long 
a time as may be necessary thoroughly to dry and harden 
the varnish. When this is accomplished it is washed with 
warm water or sprayed and rinsed through a moderately hot 
solution of caustic potash. Any stains which may happen 
to appear on the surface should be removed by rubbing 
gently with a clean rag or piece of linen dipped in potas- 
sium cyanide solution. It is then finally rinsed and 
immersed in the red-gilding solution and the deposit con- 
tinued from this solution until a sufficient depth of colour 
is obtained. 

After the gilding is completed the varnish is removed by 
means of a soft brush thoroughly saturated with benzene or 
best turpentine. If the varnish is very refractory, as some- 
times happens in cases where the baking or drying operation 
has been carried to extremes, it may be quickly and 
thoroughly removed by pouring over the surface pure 
concentrated sulphuric acid. Obviously great care is 
required in doing this, but the method is very effective. 

The Assay of Gold in Gilding Solutions. As already 
observed earlier in the present chapter, the exact assay of 
gold is a matter of skilled practice, and where absolute 
accuracy is required it is not advisable for the electrogilder 
to attempt this himself unless he has considerable knowledge 
of analytical chemistry. For all ordinary workshop purposes, 
however, the following method may with a little practice 
be carried out by an intelligent worker and will be found to 



DEPOSITION OF GOLD 241 

give results quite sufficiently accurate. The principle of 
the method is based on the precipitation of the gold in 
a finely divided metallic condition by means of ferrous 
sulphate solution. It is absolutely necessary, however, for 
obtaining this precipitate that the whole of the cyanide 
contents of the solution should be decomposed, and this is 
done by boiling with hydrochloric acid. The details of the 
method are as follows. 

Take a measured portion of the solution to be tested, say 
2 British fluid ounces (one-tenth of an imperial pint) in a 
12-oz. beaker and add not less than twice its bulk of strong 
hydrochloric acid. Boil the resulting liquid until there is 
no smell of cyanogen gas (the familiar odour of potassium 
cyanide itself). In the case of strong solutions a greater 
amount of acid is sometimes required. This part of the 
operation should be performed in a fume cupboard or well- 
ventilated place. Now add an excess of a clear solution of 
ferrous sulphate and allow the beaker to stand about twelve 
hours in a warm place. Under such conditions the gold is 
completely precipitated as a fine powder. The solution is 
then filtered and the gold powder washed on the filter with 
hot water, the filter and its contents are carefully dried and 
transferred to a weighed crucible. The crucible is then 
placed over a small bunsen flame and heated until the filter 
paper is burnt to a white ash. After cooling in a desiccator 
it is reweighed, and the difference in weight indicates the 
amount of metallic gold in the sample tested. 

Recovery of Gold from old Solutions. A similar 
procedure to the foregoing will be found the best method for 
recovering gold from old or spoilt solutions, as the metal is 
obtained in a form suitable for redissolving in aqua regia to 
make a new solution. 

An alternative method is to evaporate the solution to 
dryness and thoroughly mix the residue with litharge (lead 
oxide) in rather more than an equal bulk. The mixture is 
then fused, and the whole of the gold will be absorbed by the 
lead which will collect in button form at the bottom of the 



242 ELECTROPLATING 

crucible. The lead button is then dissolved in warm dilute 
nitric acid and thus separated from the gold which remains 
undissolved in the solution in a finely divided metallic 
condition. 

Stripping Gold Deposits from old Work, etc. 
This is a problem presenting some little difficulty owing to 
the fact that any mixture which will dissolve gold will also 
keenly attack the basis metal of the article. Many different 
methods have been suggested, but by far the best is the 
electrolytic method. 

This is carried out by making the article the anode in a 
solution of potassium cyanide containing about half a pound 
of cyanide per gallon. A strip of thick gas carbon forms a 
good cathode, and a voltage of not less than 4 or 4J volts 
should be employed. 

Even by this method there is considerable risk of the 
basis metal being attacked as soon as any part of the gold 
coating is dissolved, but if the article is given a gentle motion 
in the solution the gold is acted upon almost uniformly and 
consequently the operation can be stopped immediately the 
gold is dissolved and any further action prevented. 

Simple Immersion Processes for Gilding. Owing 
to the greatly superior advantages of electrogilding by 
separate current, simple immersion processes have now a 
very limited application, and only a brief reference need be 
made to the subject. A difficulty inherent to nearly all 
published processes for immersion gilding is that the deposits 
obtained are so often patchy and irregular and readily show 
stains, particularly if the articles treated have any consider- 
able surface. As would naturally be expected, the best re- 
sults are obtained if the articles have been first given a thin 
soating of silver. A surface of fine silver only is thus pre- 
sented to the action of the gilding bath, and the chemical 
exchange of metals is equal at all points. 

One of the best simple immersion solutions is a modifica- 
tion of that recommended by Langbein, viz. 



DEPOSITION OF GOLD 243 

Chloride of gold 1 part by weight 

Pure caustic potash 3 parts 

Crystallized sodium phosphate . . 5 

Potassium cyanide 16 ,, 

Water . . . 100 

The chloride of gold is dissolved in a little distilled water 
and the potassium cyanide, previously made into a strong 
solution in water, is added. The caustic potash and sodium 
phosphate are then dissolved in the remainder of the water 
required to complete the bulk of solution, and added to the 
cyanide solution. 

The resulting bath is boiled for a short time and is used 
at practically a boiling point temperature. 

The same precautions with regard to the preparation of 
surfaces must be observed in simple immersion gilding as 
for the separate -current process. 



CHAPTER XI 

THE DEPOSITION OF COPPEK 

UNDOUBTEDLY the most extensive commercial application of 
the art of the electro-deposition of copper lies in electrolytic 
refining operations, a constantly increasing proportion of the 
world's output of refined copper being produced by electro- 
deposition. As the electrolytic refining of metals does not, 
however, come within the scope of this work no attempt will 
be made here to discuss this section of the subject, which 
certainly demands at least a complete volume for adequate 
treatment. 

Of other applications of the electro-deposition of copper 
the more important are electrotypy ; the production of tubes, 
wire and sheet copper; and the coating of other metals, 
mainly iron, zinc, and alloys of the baser metals, with copper, 
for either protective or ornamental purposes. Of these again 
only the last-named can be regarded, strictly speaking, as 
electroplating; but as the main lines of research and pro- 
gress in the history of the deposition of copper have arisen 
chiefly in connection with the development of the former 
industrial applications, they deserve at least a brief account 
in the following pages. 

Properties of Copper. Copper is a lustrous metal of 
a peculiar reddish-brown colour. It is extremely tough and 
can be readily drawn into wire or hammered out into thin 
leaf. In its pure state it is an exceptionally ductile and 
malleable metal, but a very small percentage of some im- 
purities considerably impairs these qualities. 

Electro-deposited copper, newly liberated from an 



THE DEPOSITION OF COPPER 245 

electrolyte under correct current conditions, has a most 
pleasing and characteristic salmon-pink colour. 

Copper is not very susceptible to the action of dry air 
at ordinary temperatures, but in a moist atmosphere it is 
readily attacked, and if much carbon dioxide (CO 2 ) is present 
the surface becomes coated with a greenish coloured stain 
which is a basic carbonate of copper somewhat troublesome 
to remove. Heated in air or oxygen, black copper oxide is 
formed. 

Next to silver, copper is the best conductor of electricity 
and is undoubtedly the most efficient metal to use for current 
distribution in electroplating outfits. 

Nitric acid, either dilute or concentrated, dissolves 
copper very readily, but hydrochloric acid and dilute 
sulphuric acid attack the metal but slowly. Concentrated 
sulphuric acid is without action on copper if cold, but on 
heating, copper sulphate is formed with liberation of sulphur 
dioxide (S0 2 ), thus : 

Cu + 2H 2 SO 4 = CuS0 4 + SO 2 + 2H 2 O. 

Compounds of Copper. Copper forms two series of 
compounds, originating from two oxides, cupric oxide CuO, 
and cuprous oxide Cu. 2 O, respectively. The latter are colour- 
less, but the former in their usual condition, which is 
hydrated, are either blue or green. 

The most common salts of copper are the sulphate, 
chloride, and nitrate. Of these the first named is by far 
the most important in electro-deposition, since it is rarely 
that either metallic copper or any of its salts other than 
the sulphate is used, in the first instance at any rate, for 
making up electrolytic solutions of copper. 

Copper sulphate, often known as blue vitriol or bluestone, 
is produced in large quantities as a bye product in smelting 
operations and other chemical industries. In its usual form, 
crystallized out from aqueous solutions, it occurs in character- 
istic blue triclinic crystals having the formula CuS0 4 .5HoO. 
Its solubility in water is as follows : 



246 ELECTROPLATING 

Temperature. Degrees centigrade. 
Parts of CuSO 5HO) 1Q o 20 o 30 o 50 o 7Q o 9Q o 10Q o 

of water | 36 ' 95 42 ' 31 48 ' 81 65 ' 83 94 ' 60 156 ' 44 203 ' 32 

It is practically insoluble in alcohol. 

If crystallized copper sulphate is heated to 100 C., water 
is expelled and a bluish- white powder is obtained containing 
only one molecule of water, CuSOi.H 2 0. On continuing 
the application of heat up to 200-260 more water is 
driven off, but it is very difficult to obtain the salt wholly 
anhydrous. 

Commercial copper sulphate, particularly the recrystal- 
lized salt, is generally of a high degree of purity 98 to 99 
per cent. Its usual impurity is iron, of which small traces 
are often found in the trade varieties. The following is one 
of the best methods of testing for this impurity : 

Dissolve 4 grams of the salt, powdered in 100 c.c. of 
distilled water. Add 5 c.c. of pure nitric acid warm for five 
minutes, and then add ammonium hydrate in excess until a 
clear deep-blue liquid is obtained. Keep warm on a hot 
plate for about twenty minutes, then filter through a white 
filter paper, and wash the filter with dilute ammonia until 
the blue solution is entirely removed. If iron is present, the 
paper will show a reddish stain of ferric hydroxide. 

Copper nitrate is formed by dissolving copper in dilute 
nitric acid and allowing to crystallize out. This salt is 
extremely deliquescent and very readily soluble in water. 
Its formula is Cu(NO 3 ) 2 .3H 2 O. 

Cupric chloride, CuCl 2 .2H 2 O, is formed when copper is 
dissolved in aqua regia or by dissolving cupric oxide in hydro- 
chloric acid. It is a deliquescent salt, easily soluble in 
water. The trade varieties usually contain traces of copper 
sulphate and iron salts. 

Cuprous chloride, Cu 2 Cl 2 , may be prepared by boiling a 
solution of cupric chloride in hydrochloric acid along with 
copper turnings or foil ; the nascent hydrogen thus liberated 
reduces the cupric salt to the cuprous. Cuprous chloride is 
insoluble in water so that when the liquid is poured into 



THE DEPOSITION OF COPPER 247 

water, the salt is precipitated as a white crystalline powder. 
It dissolves readily in ammonia and in alkaline chlorides. 

This salt is at present little used in electroplating opera- 
tions, but proposals have often been made for its use, for 
reasons of greater current efficiency. According to the 
electrolytic theory of valency it will be clear that, theoreti- 
cally, double the amount of copper should be deposited from 
electrolytes of the cupwits salts than from those of the 
cupr/c compounds ; consequently if it is found possible to use 
the former salts, a very great saving of current should be 
effected. 

The great obstacle has been their very unstable character 
and the consequent difficulty of obtaining a suitable electro- 
lyte. It has recently * been found, however, that a saturated 
solution of cuprous chloride in solutions containing about 25 
per cent, of sodium chloride together with about 5 per cent, 
of free hydrochloric acid yields results showing a current 
efficiency of 90 per cent., the conductivity of the solution 
being stated to be equal to that of the ordinary copper sul- 
phate solution generally used. 

Solutions for Deposition. Solutions for the electro- 
deposition of copper are divided into two classes, " acid 
baths " and " alkaline baths." 

The former class presents by far the greater number of 
advantages in respect of simplicity, ease of working and high 
conductivity, but is unfortunately entirely unsuitable for use 
in plating the more electro-positive metals, zinc, iron, tin, etc., 
owing to the ease with which these latter displace copper from 
most of its compounds. Whenever, therefore, these metals 
or their alloys have to be coppered, the alkaline solutions 
must be chosen. For electrotypy and the solid deposition 
of copper in the production of tubes, sheet, wire, etc., as also 
for coating brass and similar metals, the acid baths are 
invariably used. 

Acid copper solutions. In their simplest form these 

* Thompson and Hamilton, Trans. Amer. Electro-Chemical Soc. t 
May, 1910. 



248 ELECTROPLATING 

solutions are copper sulphate dissolved in water together with 
a slight excess of sulphuric acid; and such solutions of a 
strength of from 1J Ibs. to 1^ Ibs. of copper salt per 
imperial gallon (1J to 1 Ibs. per U.S. gallon) yield excellent 
deposits of copper. 

The usual formula is as follows : 

Copper sulphate . . . . If Ibs. 875 gr. 

Sulphuric acid 4 to 8 oz. 125 to 250 gr. 

Water $ l imp ' gall> ' <5 litrpq 

' (orlJU.S.,, 

In modern practice, however, some modifications of these 
baths have been introduced which deserve attention in detail, 
the object being to obtain increased conductivity of solution 
and a finer quality of deposit. 

Many years ago Sir J. W. Swan drew attention to the 
fact that exceedingly minute additions of glue or gelatine to 
some copper depositing solutions exercised an important 
modifying influence on both the conductivity of the solutions 
and the character of the deposit. In the case of solutions of 
copper nitrate, for example, which under ordinary circum- 
stances do not give at all a satisfactory deposit of copper, 
the addition of a very small proportion of glue made it pos- 
sible to obtain a beautifully smooth, reguline, and coherent 
deposit of copper at a fairly Jiigli rate of deposition. 

Since that time marryBperators have made use, to a 
greater or lesser extent, of what&re now generally known as 
" addition agents " not merely to copper solutions but to 
those of other metals, as has already been indicated. In this 
connection, however, electrolytes of copper have been more 
extensively experimented with than have other metals, as 
indeed is natural in view of the extensive applications of 
copper depositing. 

Before dealing with the various re-agents suggested or 
actually used, it should be explained that in the present 
state of our scientific knowledge of the exact nature of the 
chemical and electro- chemical actions occurring during 



THE DEPOSITION OF COPPER 249 

electrolysis it is impossible to explain satisfactorily the 
reason of many effects observable in practice. But there 
seems good reason to believe that many substances in 
electrolytic solutions play a part very analogous to that 
familiar in chemistry as catalysis due to catalytic agents, i.e. 
substances which take part in or modify a chemical action 
without themselves entering into combination or being 
changed in composition. 

In some recent researches it has been suggested that 
these addition substances act as colloids, which, given 
favourable conditions, move to the cathode, and materially 
affect the character of the metallic deposit by cutting down 
the size of the crystals of the precipitated metal, and in this 
way allow of the use of greater current densities without as 
a result giving rise to rough or nodular deposits. 

It is of the greatest importance, however, to realize that 
these actions depend not only on the particular addition 
agent used but on the chemical constitution of the electrolyte. 
For example, Miiller and Bahntje * found that " in acidified 
copper sulphate solutions, starch, and gum arabic, did not 
move to the cathode and did not cut down the size of the 
copper crystals when the solution was slightly acid, but did 
both these things when the solution was made more acid." 

It has indeed been observed in regard to eledfi^jjes of 
other metals that addition substances were much more 
effective in solutions which contained an excess of free acid. 

These addition agents are by no means confined to 
organic compounds like glue, gelatine, or starch, but include 
a number of inorganic compounds, particularly salts_of the 
more extremely electro-positive metals, such as tbfltelkaline 
earths and aluminium and tin. Salts of the last 4jj|fl? framed 
have often been used in acid coppering baths. 

Since this subject is at present in a very incomplete state 

of development, much investigation remaining to be made, it 

is obviously impossible to lay down here any specific formulae 

as the best for all purposes ; the choice of an midition 

* Zeit. Ekktrochemie, 12. 320 (1906). 



250 ELECTROPLATING 

re-agent must be dependent upon local conditions and par- 
ticular requirements. Of a very large number of substances 
recommended for addition to acid copper baths the following 
should be named as the most generally used. 

Organic compounds. Benzoic acid, tannic acid, gelatine, 
glucose or dextrine and hydroxylamine. 

Inorganic compounds. Alum (the double sulphate of 
aluminium and potassium), sodium chloride, ammonium 
chloride, and aluminium sulphate. 

According to our experience the latter class the in- 
organic salts are to be preferred to the former. There 
seems little doubt that gelatine alone, though under favour- 
able conditions allowing the use of higher current densities 
in electrolytes, has a tendency to render the deposit brittle. 

Both alum and aluminium sulphate give very good 
results. The following formula, which has recently been 
strongly recommended by an American writer, is an example 
of several of this class 



Copper sulphate crystals CuSO 4 .5H 2 O . If Ibs. 



1kg. 



Sulphuric acid 3 oz. 

Alum 

Water f ^T^"' 5 litres 

(orl U.D. 

A report of a fairly exhaustive research into this question 
of addition agents to copper sulphate solutions, by a Chinese 
graduate (Ching Yu Wan) of Columbia University, U.S.A., 
has recently been published,""" and the results are extremely 
interesting as bearing on the question of obtaining pure 
deposits from impure solutions. According to this investi- 
gator, the most successful addition agent of a large number 
tried particularly in solutions containing up to ft per cent, of 
arsenic was a combination of an organic and inorganic com- 
pound in the shape of gelatine and common salt. The 

* (Abstract) Metallurgical and Chemical Engineering, June, 1911, 
vol. ix. No. 6, pp. 318-19. 



THE DEPOSITION OF COPPER 251 

results showed that a deposit of the highest purity and greatest 
ductility was obtained by the addition of from 0-01 to 0-02 
per cent, gelatine and 0-02 to 0-03 per cent, of sodium 
chloride. It must be noted, however, that these experiments 
were conducted in electrolytes containing arsenic, which 
substance itself may act as an addition agent, and influence 
the deposit though not itself liberated. 

Of very great importance also is the amount of free sul- 
phuric acid allowable in acid copper solutions. The effect of 
free acid is to increase appreciably the conductivity of the 
solution and at the same time to facilitate the dissolving of 
the copper anode, thus maintaining the strength of the 
bath. 

Considerable diversity of opinion and of practice exists in 
regard to the question of the most suitable proportion of free 
acid to use, but the determining factor is really the particular 
purpose of the electrolyte, whether to be used for protective 
coatings, for solid deposition, or for refining operations. 

Cowper-Coles * for solid deposition of copper has obtained 
excellent results from the following solution : 

Oz. per Percentage 

imp. gall. by weight. 

Copper sulphate CuS0 4 .5H 2 O . 32 ... 14-87 
Sulphuric acid H. 2 SO 4 .... 12-6 . . . 10-77 
Water 74-3G 

But such a proportion of free acid is rather too high for 
electrotypy, or for ordinary plating operations. 

For the latter it may be taken as a fairly safe generaliza- 
tion that the proportion of free sulphuric acid should not 
exceed 8 oz. per gallon (50 gr. per litre), and many expe- 
rienced operators prefer slightly less than this proportion, 
particularly if an inorganic addition agent be used, but to a 
large extent this point depends also on the current density 
employed and will be discussed again later. 

Alkaline Copper Solutions. The basis of practically all 

* Journ. Inst. of Eke. Engineers, vol. xxix., January, 1900, p. 276. 



252 ELECTROPLATING 

alkaline copper baths in commercial use now is the double 
cyanide of copper and potassium a solution very analogous 
to that used for the deposition of silver. Some few writers 
recommend in preference the less poisonous tartrate bath 
made usually by dissolving a copper salt in a strong solution 
of potassium sodium tartrate together with an excess of 
caustic soda. But such a bath is inferior in many respects 
to the cyanide solution. 

The simplest method of making the latter is to dissolve 
copper carbonate or copper acetate in a strong solution of 
potassium, cyanide in such a proportion as to obtain a 
metallic content of not less than 2 oz. per imperial gallon 
(1^ oz. per U.S. gallon, or 12J gr. per litre). 

These salts of copper (the carbonate and acetate) are, 
however, relatively rather expensive, so that in general 
workshop practice the solution is made, starting from me- 
tallic copper, or copper sulphate, which latter is much the 
cheapest copper compound available. 

To prepare the cyanide solution from metallic copper, 
dissolve 3 to 4 oz. of grain copper in warm nitric acid 
(1 part acid, 1 part water). Dilute the solution to about 1 
imperial pint or more by adding water. Make up now a 
strong solution of sodium carbonate and add this to the 
copper solution, stirring meanwhile, until no further pre- 
cipitation occurs. The precipitate is copper carbonate ; wash 
this with warm water two or three times ; and finally add 
to it a strong solution of potassium cyanide (4 oz. per pint 
or 20 gr. per 100 c.c.) until the precipitate is completely dis- 
solved. Note the quantity of cyanide solution used and add 
10 per cent, more as free cyanide. Boil the resulting solution 
for a few minutes and make up the bulk to one gallon by 
adding water. This method is a very old one and is largely 
used in the older plating establishments, with the addition 
usually of ammonia or ammonium carbonate. 

A more convenient method, however, is to prepare the 
solution from copper sulphate. The following formula will 
yield excellent results 



THE DEPOSITION OF COPPER 253 

Copper sulphate (CuS0 4 .5H 2 0) . . 16 oz. \ 500 gr. 

Ammonia, 0-880 Q.S. 

560 gr. 
62-5 to 
93-75 gr. 

5 litres 



Potassium cyanide 95 per cent. . . 18 oz. 
Potassium bisulphite . . . . 2 to 3 \ 



Water 



imp. gall. 



Lor 1J U.S. , 

Dissolve the copper sulphate (powdered) in about one 
quart of water, and when completely dissolved add ammonia 
until the bluish-white precipitate, which at first is observed, 
completely redissolves, and an intense deep-blue solution 
results. The effect of the addition of ammonia to copper 
sulphate is first of all to throw down a basic sulphate of 
copper ; then as further ammonia is added this dissolves, and 
the deep-blue solution obtained is known as an aqueous solu- 
tion of cuprammonium sulphate (CuSO 4 .4NH 3 .ILO). The 
potassium cyanide which meanwhile should have been dis- 
solved in about 1J pints of water is now slowly added to 
the copper solution obtained as above, and towards the end 
of the addition it will be noted that the deep-blue colour 
changes to a purple, and then the liquid quickly becomes 
clear and colourless. If the potassium cyanide is of a 
weaker strength than above specified, more will be required, 
but in any event the best guide as to the quantity of cyanide to 
use is to note the point of the complete discharge of the blue 
coloration which marks the formation of the double cyanide 
of copper and potassium. Further additions beyond this 
point are for free cyanide, and should not much exceed 20 
per cent, of the quantity used to obtain the double salt. 
The potassium bisulphite dissolved in a small quantity of 
water is then added, the solution boiled for a few minutes, 
and the liquid made up to one gallon with water. 

The addition of the potassium salt is made to improve the 
conductivity of the bath, the double cyanide solution alone 
being relatively rather a poor conductor. Several other salts 
have been recommended in this connection, notably po- 
tassium carbonate, but inasmuch as the bath while in use 



,254 ELECTROPLATING 

gradually acquires a considerable proportion of this salt 
through decomposition and contact with the atmosphere, 
it is inadvisable to make any such addition when preparing 
the solution. 

Another formula which yields a solution giving a very 
tine deposit of copper, and which we have often used for 
ornamental copper coatings on zinc and similar metals or 
alloys, is one of several originally introduced by Roseleur. 
As given below, however, it is slightly modified : 



Copper acetate 6 oz. 

Anhydrous sodium carbonate . . 4 ,, 

Sodium bisulphite 4 ,, 

Potassium cyanide, 95 per cent. . 8 

( 1 imp. gall. 
lor 1J U.S.,, 



Water 



187 gr. 
125 
125 
250 

5 litres 



To prepare the bath, make up the copper acetate into a 
paste by adding a little water as required. Dissolve the 
sodium carbonate in about one pint or a little more of 
water and add to the copper compound. Stir the resulting 
mixture vigorously. The acetate is thus converted into the 
carbonate of copper. Now add the sodium bisulphite dis- 
solved in a further pint of water, and finally the potassium 
cyanide also dissolved in a sufficiency of water. The re- 
sulting liquid should, and if pure materials have been used 
will, be practically clear and colourless. It must now be 
boiled for half an hour or so, made up to correct bulk by the 
addition of water, and is then ready for use. 

This bath may be used either hot or cold, but is pre- 
ferably worked at a temperature of from 60 to 70 C. 

Of other alkaline solutions for coppering which have 
been suggested the only ones which need be mentioned 
here are the tartrates to which reference has already been 
made. 

The two following are representative solutions of this 
class. 



THE DEPOSITION OF COPPER 255 



Formula (1) (Weil) 

Copper sulphate 7| oz. 

Potassium-sodium tartrate ... 36 

Caustic soda . . . 17 



Water 



( 1 imp. gall, 
lor U U.S. 



225 gr. 
1125 
530 

5 litres 



The copper salt is dissolved in a sufficiency of water, 
say one pint, and added slowly to the remainder of the water 
in which the tartrate and caustic soda are jointly dissolved. 
If any undissolved substance remains in solution after 
vigorous stirring it should be filtered off. 

Formula (2) (Eisner) 

Potassium bitartrate ..... 8 oz. 1 250 gr. 

Potassium carbonate ..... 1 5 31-25 gr. 

( 1 imp. gall. j 

Water ....... .S | 51ltres 



Copper carbonate ........ Q.S. 

The potassium bitartrate is dissolved in the whole of the 
water by boiling, and freshly precipitated wet copper car- 
bonate stirred into the solution to as great an extent as the 
liquid will dissolve. The addition of the small proportion 
of potassium carbonate ensures the alkalinity of the bath. 

Neither of the foregoing baths are, however, so reliable 
as the cyanide ones previously given. 

It may be of interest also to mention that Dr. F. W. 
Kern, whose nickel fluosilicate bath is referred to in the 
following chapter, has more recently patented (Amer. pat. 
946.903, Jan. 1910) an exactly similar solution for the 
deposition of copper, the approximate formula being : 

Copper fluosilicate, 10 parts ....... N 

Ammonium fluoride and aluminium fluosilicate, , 

5 parts each ........... ^ 

Water, 100 parts .......... , 

In the case of copper, however, the patentee prefers to 
add a small proportion of gelatine. 



256 ELECTROPLATING 

An important point with regard to cyanide coppering 
solutions is the proportion of free cyanide necessary or 
advisable. The action occurring in these baths, according to 
Hittorf, is, at the cathode the liberation of potassium (K) 
and the deposition of copper as a secondary action, and at 
the anode ithe separation of the complex radicle Cu(CN). (J ; 
dissociation of the double cyanides occurring thus : 

KCu(CN), = K + Cu(CN) a (compare silver). 

The potassium ion attacks the surrounding molecule of 
double salt and liberates copper, thus 

K + KCu(CN), = 2KCN + Cu (deposited). 

The anion Cu(CN);, is of course liberated at the surface of 
the anode, which is of sheet copper, and the cyanogen radicle 
(ON) seeks to combine with the metal to form copper 
cyanide (CuCN). Consequently as each molecule of copper 
is deposited at the cathode an equivalent of copper cyanide 
forms at the anode. 

Copper cyanide, however, like the corresponding silver 
salt, is insoluble in water, and even in potassium cyanide is 
soluble with greater difficulty than silver cyanide. Hence the 
necessity, even to a greater extent than in silver baths, for 
the presence of free cyanide. On the other hand, it must 
be borne in mind that cyanide copper baths are usually 
worked warm, 70 to 80 C., and under these circumstances 
the single cyanide is more soluble than in cold solutions. 

In workshop practice, therefore, a proportion of 20 to 
25 per cent, of free cyanide is generally sufficient, and it 
will be found advisable in the case of a new solution to 
commence with 10 to 15 per cent, as instructed, and add a 
little more from time to time as the bath is worked and as 
found necessary. 

A large excess of free cyanide is very harmful, par- 
ticularly in coating zinc and iron and steel goods. Further, 
more, in the case of coppering from the cyanide bath it is 
not so essential as in silver plating that the proportion of 



THE DEPOSITION OF COPPER 257 

free cyanide be high enough to keep the anode surface 
absolutely free from the film of single cyanide which forms, 
inasmuch as the time of immersion is comparatively very 
brief, the purpose being, usually at any rate, to give a 
preliminary coating only. The bath therefore has plenty 
of time to effect solution of the anode slime by diffusion. 

It will be found necessary from time to time to make 
further additions of copper to the bath, since under the above 
circumstances the solution is not sufficiently replenished by 
solution of the anode. Such additions are best made in the form 
of copper carbonate a salt which can be either made in the 
workshop or obtained commercially of a high degree of purity. 
Similar additions should also be made in cases where baths 
contain excess cyanide ; a small quantity placed in a muslin 
bag and suspended in the vat (stirring the latter frequently) 
will speedily restore such a liquid to correct conditions. 

It may here be advisable to remark that in cases where 
the operator has had little experience in chemical manipula- 
tions he will find it of distinct advantage to make up new 
solutions by means of copper carbonate purchased from 
reputable manufacturers, the only possible objection being, 
as has been mentioned, the increased cost. 

In such cases the following formula may b^adopted : 



Copper carbonate 5 oz. 

Cyanide of potassium, 95 per cent. .8 ,, 



156 gr. 

250 , 



XT7 , C 1 imp. gall. ... 

Water iarliU.8 51ltres 

Dissolve the cyanide in two pints of water and slowly 
add the copper compound, stirring until completely dissolved, 
then add remaining quantity of water. 

Anodes. Whether for acid or alkaline baths anodes 
should be of pure sheet copper of a thickness of about 
0-03 in. and of sizes proportionate to the vat. They should be 
annealed at a dull red heat before using, and thoroughly 
cleansed and scoured before immersion in the solution. In 
acid coppering under correct conditions the anodes will 



258 ELECTROPLATING 

work clear throughout, but in alkaline coppering this is 
rarely the case, and it is advisable to remove them 
occasionally for cleansing, the slime formed in cyanide 
solutions being very refractory and tending to interpose 
considerable resistance to the current. 

Electrical Conditions. For the alkaline bath the 
difference of potential between electrodes measured at the 
terminals of the vat should be about 4 volts. It is difficult 
to give any figures for current density, as this depends 
largely on the class of work being dealt with, and as the 
purpose of alkaline coppering is in most cases to give 
merely a preliminary film or coating it is also hardly 
necessary. In acid coppering, on the other hand, the 
question of current density as well as E.M.F. is of great 
importance. The latter is usually 1 to 1^ volts, but the 
former factor varies enormously and depends not only on 
the nature of the work being done but also on the con- 
stitution and temperature of the electrolyte, which likewise 
affects to some extent the E.M.F. 

In the determination of the correct current conditions for 
the electro-deposition of copper from the acid bath, the 
following general principle must be foremost in mind, viz. 
as in all other electrical operations, current density is de- 
pendent upon the E.M.F. and the resistance of the circuit. 
For the same C.D. (current density) a decreased resistance 
will mean or require a less E.M.F. (see Chapter III.). 

Consequently the alteration of any factor in the con- 
ditions of electrolysis which will affect the conductivity, or, 
what is the same thing, the resistance of the electrolyte, will 
mean a change in the values of both E.M.R and C.D. 
required. 

Such alterations are caused mainly as follows : 

(1) By increase of temperature of the solution. 

(2) By the addition of substances to the electrolyte to 
increase conductivity. 

(3) By the agitation of either cathodes or electrolytes. 

(4) By increase of the proportion of free acid. 



THE DEPOSITION OF COPPER 259 

All these factors, either in combination or separately, have 
the ultimate effect of allowing a larger current to pass at a 
lower voltage. In addition, it is most important to bear in 
mind that solutions having a comparatively small proportion 
of metal content will only permit of the use of low current 
densities to obtain satisfactory deposits. 

This question has been the subject of research by several 
experimenters, notably von Hiibl, whose investigations have 
been of great value to subsequent workers. His results, 
obtained from solutions of copper sulphate alone or with 
free sulphuric acid only, indicate that for baths of approxi- 
mately the composition of that on p. 248, the maximum C.D. 
allowable is from 15 to 20 amperes per square foot of 
cathode surface, the electrolyte being at normal temperature 
and in gentle motion. 

By means of increase of temperature, addition agents, or 
agitation of cathodes or electrolyte, however, these values may 
be very considerably exceeded, as also within certain well- 
defined limits by increase of free acid. It is not, however, 
advantageous to go beyond the figure already advised in this 
direction. Of the other factors tabulated above, the second 
and third are those most usually taken advantage of. 

Solutions containing suitable addition agents yield ex- 
cellent deposits at current densities of from 25 to 30 amperes 
per square foot and even slightly higher. Values much 
above these figures can only, however, be employed in solutions 
subjected to violent agitation. The most interesting recent 
experiments in this direction have been those of Mr. Cowper- 
Coles,* who by means of rapidly rotating cathodes obtained 
smooth reguline deposits of copper in the production of 
copper tubes, etc., with current densities as high as 170 
amperes per square foot. 

General Remarks on Coppering. The electro-de- 
position of copper is probably the least difficult of all 
branches of electroplating, but several important difficulties 
often arise owing to the nature of the basis metals usually 
* Journ. Institute of Electrical Engineers, vol. xxix. p. 265. 



260 ELECTROPLATING 

dealt with. Iron castings, for example, often give the 
operator considerable trouble in coppering (as also brassing) 
owing to their porous nature, by " spotting- out," as it is 
termed, after plating. No matter how carefully these have 
been prepared in the first instance before plating, or how 
thoroughly rinsed and dried out afterwards, small round 
spots or patches appear at intervals along the surface on 
standing, and in the case of articles being given a first 
coating in an alkaline bath, and subsequently transferred to 
an acid bath for heavier deposit, these spots considerably 
interfere with the protective value of the deposit. Many 
suggestions have been made for dealing with this trouble, 
but one of the simplest and generally a very reliable one is 
given by Langbein, who recommends after bringing the 
articles from the cyanide bath their immersion for from 
three to five minutes in a very dilute solution of acetic acid 
(1 part acid 50 parts water), afterwards rinsing in clean 
running water, dipping again for a few minutes in lime 
water, and finally rinsing and drying off. It is also advisable 
in dealing with this class of work wherever possible to 
resort to the sandblast instead of to acid dips and pickles for 
preliminary cleansing. 

Castings of antimony, lead, tin, or zinc, and alloys of 
these metals are also liable to this trouble and should be 
given similar treatment. Emphasis must also be laid upon 
the necessity for a strong and perfect coating of copper to 
be given to these goods in the alkaline baths before they are 
transferred to the acid bath, which is usually necessary to 
obtain a sufficient thickness of deposit for protective or orna- 
mental purposes. If these articles are immersed in acid 
copper baths, with a coating from the alkaline bath of an 
imperfect character, they will often be irretrievably injured. 

The Assay of Copper Solutions. A number of 
methods of estimating the content of metallic copper in 
plating solutions have at various times been published, and 
it is not easy to decide which is most suitable for electro- 
platers' requirements. For obtaining rapid and at the same 



THE DEPOSITION OF COPPER 261 

time accurate results we prefer, however, the volumetric 
method known as the " iodide " a method very largely used 
in works' laboratories in metallurgical practice. This method, 
while rather more complex than some others, is much more 
accurate when other metals are likely to be present, and is, 
therefore, adapted for the estimation of copper in solutions 
for depositing copper alloys such as brass, bronze, German 
silver, etc. An experienced works' chemist of the authors' 
acquaintance writes to us, " From long experience I can 
recommend the Iodide as an excellent method. The outlay 
of apparatus is small ; the end point with care can be judged 
to one drop ; and with a little experience duplicate assays 
should not differ by more than 0-1 per cent." The only common 
metals which interfere are iron and bismuth, and these are 
not likely to be present in ordinary coppering solutions. 

The following is a practical description of this method 
theoretical considerations being omitted for acid copper 
solutions; cyanide solutions are given a preliminary treat- 
ment, as will be explained later. 

Measure out by means of a pipette 20 c.c. of the 
solution to be tested, and deliver into a tall beaker. Add to 
this a cold saturated solution of sodium carbonate until the 
copper is just completely precipitated the beaker should be 
covered as much as possible during this process as the 
effervescence is rather violent. Stir the solution vigorously 
and allow to stand until the precipitate settles, so that the 
liquid may be tested by adding a further few drops of sodium 
carbonate solution. Now add just sufficient acetic acid to 
redissolve the copper salt (a small excess does not matter). 
Weigh out next about ten times as much powdered potassium 
iodide as copper believed to be present in the sample; in 
most cases this proportion will be about 4 to 5 grams of 
potassium iodide. Add this salt slowly and carefully to the 
solution in the beaker, again keeping the beaker covered to 
avoid any possible loss. When effervescence has ceased, 
wash down the sides and rim of the beaker with a spray of 
distilled water. The solution, which is brown in colour, is 



262 ELECTROPLATING 

now ready for titration, and for this purpose two solutions 
are required. 

(1) Sodium thiosulphate (hyposulphite) standard solution, 
containing 39'5 grams of the pure salt per litre. This solu- 
tion may be prepared in the workshop, or bought ready 
standardized. If the former, it must be first standardized 
by testing it, according to the method now being described 
against a known weight of pure copper in solution. For 
platers' requirements it is more convenient to buy the 
solution prepared as required. 50 c.c. of this solution are 
equal to 0*505 gram copper. 

(2) Starch solution. This is required as an indicator of 
the end of the reaction. Prepare by boiling a pint of 
distilled water and stirring into it 1 or 2 grams of powdered 
starch previously made into a thin paste with a little cold water. 

To carry out the estimation : Fill a 50 c.c. burette (see 
p. 177) with the thiosulphate solution, and carefully run 
the latter into the copper solution in the beaker with 
agitation of the latter until the brown colour fades to a 
yellow and the bleaching action of the thiosulphate is only 
faint by contrast. Now add about 15 c.c. of the starch 
solution to the beaker content and mix well. Again care- 
fully run in the standard solution from the burette until the 
violet colour which the starch produces begins to fade ; 
proceed now very cautiously, one drop at a time, shaking 
vigorously; the colour will slowly fade until one drop 
bleaches it to a cream shade. This is the end point. Bead 
off the figure on the burette, marking quantity of solution 
used, then add just one drop more if this causes a decided 
lightening of colour where it fell, the titration was not quite 
complete and the last reading would be correct. 
Example of three experiments : 

Burette readings 43, 42-8, 431 

mean taken as 43. 
50 c.c. = 0-505 Cu 



.43 c.c. = X 43 = 0-434 gram. 



THE DEPOSITION OF COPPER 263 

This figure 0-434 gram is the weight of copper in 20 
c.c. of solution. To obtain the weight in avoirdupois 
ounces per gallon, multiply by 8. Thus the above solution 
contained 0-434 x 8 = 3-47 ounces of copper per gallon. 

Cyanide solutions of Copper. These can be assayed by the 
same method as above described, but the whole of the 
cyanide must first be decomposed by boiling with excess of 
sulphuric acid. The addition of sulphuric acid must be 
made until the precipitate of copper cyanide which first 
forms is completely dissolved. The boiling of the liquid must 
be continued until the bulk is reduced to about its original 
measure, and the assay then carried out according to the 
directions in the previous paragraph. The decomposition of 
the cyanide solution must be carried out in a draught 
cupboard or in the open air, as the poisonous hydrocyanic 
acid gas is freely evolved^. 

Estimation of Free Acid in Copper Baths. The 
simplest method for workshop purposes is to neutralize the 
acid by means of a standard alkali solution. This may be 
carried out by preparing, or purchasing, a standard solution 
of pure sodium carbonate, containing 10-6 grams of Na^COj 
per litre. Take 25 c.c. of the copper solution, dilute with 
an equal quantity of water and place in a flask or beaker. 
Now charge a burette with the standard sodium carbonate 
solution and add this slowly to the copper solution, stirring 
constantly. Continue the addition until a faint permanent 
precipitate ensues, and read off the figure on burette. Re- 
peat the experiment two or three times until a good agree- 
ment between readings is obtained. The principle of the 
method is very simple. The reaction between the alkali and 
acid is thus expressed 

Na 2 CO 3 + H 2 S0 4 = Na 2 S0 4 + CO 2 + H 2 
Molecular weights 106 + 98 

106 parts of sodium carbonate will, therefore, exactly neu- 
tralize 98 parts of sulphuric acid, and consequently 1 c.c. 
of the standard soda solution is equivalent to 0-0098 gram 



264 ELECTROPLATING 

H 2 SO 4 . The end of the reaction, showing when the whole 
of the sulphuric acid is neutralized, is determined by the 
appearance of a faint green precipitate, which indicates that 
the copper is now being precipitated as copper carbonate. 
The first sign of a permanent turbidity, therefore, makes the 
point at which the burette reading must be taken. 

For the approximate estimations, which are often all that 
is necessary in electroplating practice in the deposition of 
copper, it will be sufficiently accurate to calculate the pro- 
portion of acid present on the basis that 

1 c.c. soda solution = O'Ol gram sulphuric acid 
or 100 =1 

Free Cyanide in Copper Solutions. The estima- 
tion of free cyanide in copper solutions is carried out exactly 
in the manner described at length in the section dealing 
with a similar estimation in silver solutions (see p. 211). 

ELECTROTYFY. 

The art of electrotypy is that of the reproduction of exact 
copies of objects of art, woodcuts, medallions, or even 
natural objects by means of electro-deposition of a metal, 
usually copper. 

The present chapter, therefore, is a suitable place for 
a brief description of an art which is closely akin to that 
of the electroplater, and which indeed the electroplater is 
often called upon to pursue to a greater or lesser degree. 
Exigencies of space will, however, preclude anything further 
than a general outline of the simpler processes in use. 

Electrotypy is made possible by reason of the peculiarity 
possessed by electro-deposited metal of following exactly 
every line or indentation, no matter how fine, in the object 
upon which it is deposited. Consequently if this coating, 
after reaching a sufficient thickness to make it feasible, is 
removed, its reverse will be a perfect reproduction of the 
surface from which it has been taken. 

The first essential, therefore, is the preparation of the 



THE DEPOSITION OF COPPER 265 

object to receive the deposit. Where this is a metal, the 
only requirement is to give, by means of moistened black- 
lead or extremely thin oil or similar material, a slight film 
which will prevent that perfect adhesion of the deposit which 
is the aim of the electroplater but obviously not of the 
electrotyper. Usually, however, moulds must be taken in 
non -metallic substances of such a nature as to be capable 
of taking a perfectly fine and accurate impression of the 
object to be copied. Such an impression is of course a 
reverse of the actual surface, and the deposit therefore, 
being taken off this is a true copy of the original. 

By far the most generally useful material for this purpose 
is gutta-percha, alone or mixed with other substances, such 
as marine glue, lard, or tallow. The main advantages of 
gutta-percha as a moulding material are that it is, by 
moderate heating, easily rendered soft and pliable, and 
yet on cooling becomes sufficiently hard to withstand sub- 
sequent treatment, while at the same time it possesses a 
degree of elasticity which enables it to be used for copying 
surfaces in high relief. 

The methods adopted in moulding depend entirely on the 
nature of the object to be copied. In the case of simple flat 
work the original may be placed on a flat board, the gutta- 
percha softened in hot water, placed on the centre of the 
object, and pressed carefully into every recess, working from 
the centre outwards (so preventing accumulation of small 
air-bubbles) until the surface is perfectly covered. 

Usually, however, the work is more intricate and delicate, 
requiring much more careful and skilful handling, particularly 
in cases where the object is thin and easily bruised. For 
such classes of work a preliminary operation technically 
known as "making the block" is necessary. The "block" 
consists of two slabs of gutta-percha, one having the article 
to be copied firmly embedded in it with the surface to be 
copied uppermost, the other bearing just a faint impression 
or outline of that surface. These when together are sur- 
rounded with a strong iron ring, the depth of which is about 



266 ELECTROPLATING 

1 inch less than the total thickness of the "block" itself. 
This procedure enables the operator to apply a much greater 
pressure exactly where required, so ensuring a clear and 
well-defined impression. 

The whole process of preparation of moulds is, therefore, 
divided into three stages : 

1. Making the block. 

2. Taking the impression. 

3. Preparing the mould for the depositing vat. 

1. Making the Hock. First soften sufficiently large slabs 
of gutta-percha by placing in hot water, or warming in a 
vessel immersed in hot water. When soft, the operator must 
be careful not to handle it except with hands thoroughly 
moistened with soapy water. The same remark indeed applies 
to anything which the soft gutta-percha is to touch. If 
the article to be copied has raised portions with correspond- 
ing hollows below, the latter must be filled up with the 
moulding material until the back is quite level with the outer 
edge. Now take one slab of gutta-percha 1J to 2 inches in 
thickness and of an area a little in excess of that of the 
model. Lay the latter as above prepared on this and press 
until the lowest edge is just level with the gutta-percha 
surface. When feasible, loops are sometimes soldered to the 
back of the model in order to give it a firm " grip " to the 
block. 

The block, after being surrounded by an iron rim deep 
enough to stand a little above the gutta-percha itself, must 
now be set aside to cool, and when hard, any portions of 
the outer edge which stand higher than the model must be 
pared off. 

It will be obvious that the original is now so placed as 
to stand any pressure which may be applied in making the 
mould proper. 

Next brush the block over with soapy water and take a 
second slab of softened gutta-percha of similar size and area 
to the first, and press gently on to the first surface. This 
block will of course be kept within bounds by the iron rim. 



THE DEPOSITION OF COPPER 267 

Again set aside to cool. In this way the second or upper 
slab containing a faint outline of the model is obtained. 
This must be removed for the second operation. The com- 
pleted block is now rea^y. 

2. Taking the impression. The next operation is to take the 
impression. Briefly this is accomplished by pressing a small 
quantity of prepared gutta-percha into every part of the 
surface of the model. 

Take a sufficiency of softened gutta-percha equal in area 
to " block " and about 1 inch thick. Knead thoroughly to 
remove any hard or foreign matter which may be present 
in the material and until a smooth surface results. Lay 
this out on a wet flat stone and brush over lightly with fine 
" electrotype " plumbago. Any air-bubbles or broken surface 
can now be seen and must be remedied. Again thoroughly 
brush with plumbago until the surface has a fine polished 
appearance. Take now the material thus prepared, hold it 
by the edges with the plumbago surface downwards, allow 
to " sag " and lower it gradually on to the model. In this 
way the soft material touches the article in the centre first 
and is then allowed gently to cover the whole surface. Now 
replace the top section of " block " and convey the whole to the 
" press." For large work a toggle press is usually employed, 
but for smaller articles an ordinary letter-press will be found 
quite satisfactory. 

The block, containing between its upper and lower 
sections the original model in perfect contact with soft 
pliable gutta-percha, is now subjected to a moderately firm 
pressure in such a press. After two or three minutes re- 
lease the pressure for a short time to allow any imprisoned 
air to escape. Then screw up to full pressure and leave 
until the mould is perfectly cold and hard. When this is 
so take out of the press, and by means of a mallet knock off 
the iron frame, thus releasing the two sections and allowing 
the mould proper to be taken away. The latter is now ready 
for wiring and rendering conductive. 

When both sides of an article are to be copied as in 



268 ELECTROPLATING 

statuary, for example, moulding composition must be 
applied to the bottom section and the object embedded half- 
way, the dividing line being made very exact. The upper 
half is then similarly treated and the process continued as 
above described. 

3. Preparation of mould for depositing vat. The methods of 
preparation of non-conducting surfaces to receive an electro- 
deposit have already been detailed in Chapter VIII. For 
electrotype moulds in gutta-percha, fine plumbago or mixture 
of plumbago with finely divided tin or silver powder is 
generally employed. The substance used is brushed over 
the entire surface thoroughly and systematically until every 
portion is covered. Prior to this treatment, however, the 
mould must be wired for immersion in the depositing vat. 
Methods of wiring are innumerable and but few helpful 
details can be given, the matter depending entirely on the 
ingenuity of the operator. Copper wire is used, and it is 
attached by warming it slightly and pressing superficially 
into the surface of the mould, holding until cold. Or in 
cases where the mould is fairly heavy, attachments are made 
by piercing the block with a hot wire and passing copper 
wire to and fro through the block, the wire showing at the back 
being covered with a thin strip of gutta-percha to prevent 
deposits taking place. It is obviously advisable to make as 
many such attachments as possible, particularly at remote 
portions of the surface, in order to assist in the rapid coating 
of the mould with copper on first immersion in the bath. 
When the wiring is complete, the plumbago or conducting 
material is brushed well round the points of contact and 
the whole surface polished until it appears perfectly uniform 
and completely coated. 

It is now ready for immersion in the depositing . vat, the 
deposit being allowed to proceed until a sufficient thickness 
of metal is obtained. The deposit can be readily re- 
moved from the mould by gently warming with a blow- 
pipe. 

Other moulding methods and compositions. For the 



THE DEPOSITION OF COPPER 269 

ordinary requirements of the electroplater who may oc- 
casionally be called upon to execute small electrotypes, the 
foregoing details will, it is hoped, be sufficient. For more 
elaborate work other moulding materials are often necessary. 
In the case of surfaces much undercut, for example, gutta- 
percha is not sufficiently elastic, and for these specially 
elastic materials are used, the most commonly employed 
being a mixture of glue and treacle. Plaster of Paris, bees- 
wax, mixtures of ordinary white paraffin wax and bess-wax 
are also in use as moulding materials, and finally must be 
mentioned, fusible metal, an alloy of bismuth, lead, tin, and 
cadmium. This with suitable proportions of its ingredients 
melts at a lower temperature than boiling water, a very good 
composition being as follows : 

Bismuth 50 per cent., lead 25 per cent., with 121 p er 
cent, each of tin and cadmium. 

This alloy melts at a temperature of about 60 C. 

For fuller details of these compositions and methods of 
moulding the reader is referred to books dealing entirely 
with the subject of electrotyping. It is impossible to treat 
these adequately in the space of the present volume. 



CHAPTER XII 

THE DEPOSITION OF NICKEL 

ALTHOUGH as early as 1843 Prof. Boettger, a German 
chemist, and one of the pioneers of electro-metallurgy, called 
attention to the beautiful results obtainable in the electro- 
deposition of nickel, and indeed suggested for the purpose 
the very solution now most extensively used, it was not until 
about 1870 that this branch of electroplating began to take 
any place of consequence in the industrial arts. Several 
reasons contributed to this delay, the principal, probably, 
being the difficulty prior to about 1872 or 1873 in obtaining suf- 
ficiently pure metal, and its comparatively high price. Since 
1875, however, the progress of nickelplating both in Europe 
and America has been phenomenal, and to-day from the 
point of view of extent of application and labour employed, 
it is the largest single section of the electroplating industry. 

This popularity is well deserved. Electro-deposited 
nickel is not only very pleasing in appearance, whether 
polished or left dull, but forms an extremely hard and 
durable protective coating to other metals which are not so 
impervious to the action of atmospheric and other influences 
as nickel itself is. 

Properties of Nickel. Nickel is a fine lustrous silver- 
white metal having a steel-gray tinge. It is very hard, 
capable of taking a high polish and is fairly malleable 
and ductile. Its melting point is very near to that of iron, 
to which metal it is closely related chemically. Nickel is 
not readily attacked by the atmosphere even at high 



THE DEPOSITION OF NICKEL 271 

temperatures. It is slowly soluble in hydrochloric acid or 
dilute sulphuric acid. Concentrated sulphuric acid dissolves 
it rather more quickly, but it is most readily soluble in dilute 
nitric acid. A rather singular feature of nickel is its suscepti- 
bility to organic acids. Most of the better known of these acids, 
such as citric, acetic, tartaric, slowly dissolve the metal, 
particularly in its electro-deposited condition. One of the 
present writers has several times successfully used strong 
solutions of citric acid for stripping nickel deposits, when 
time has been no object and it was desired to preserve the 
basis metal as much as possible from attack by the " strip." 
Nickel, as electro-deposited, under normal conditions is 
extremely hard, so much so as to render its subsequent 
polishing very difficult unless the coating has been made on 
a perfectly smooth surface. It is, further, very brittle, 
though in this respect considerably varying degrees are 
obtainable under different conditions of current and electro- 
lyte. The liberation of hydrogen during the electro-deposition 
of nickel affects its mechanical properties to a most important 
extent, and in extreme cases absolutely prevents the forma- 
tion of either adherent or coherent deposits. 

Compounds of Nickel. The principal salts of nickel 
of interest to the electroplater are the carbonate, chloride, 
oxalate, acetate, citrate, and sulphate. Solutions of all these 
either alone or in combination with other substances have 
been used or suggested for the electro-deposition of nickel. 
In addition to these, suggestions have been made for the 
use of some of the lesser-known organic compounds of 
nickel, as also recently the double fluorides of nickel and the 
alkali or alkaline earth metals. 

Of these, the sulphate, either single or double (with 
ammonium), is by far the most extensively used, but it 
should be observed that excellent results in the electro- 
deposition of nickel are by no means confined to the sulphate 
solutions. 

Although three oxides of nickel are known having the 
respective formulae, NiO, Ni 2 O 3 , and Ni 3 4 , generally only 



272 ELECTROPLATING 

one series of salts is formed corresponding to the first - 
named oxide. They nearly all possess in the hydratecl 
condition a characteristic green colour a peculiarity which 
enables them to be easily recognized. 

Solutions for Deposition. The solution most widely 
known, and probably at present most generally used, for 
nickel-plating is a simple solution of the double sulphate of 
nickel and ammonium in water, in the following proportions 
approximately : 



Nickel ammonium sulphate . . . 1 Ib. 



Water 



( 1 imp. gall, 
tor U U.S. 



500 gr. 

5 litres 



To prepare this solution it is generally recommended to 
dissolve the salt in a portion of the water heated nearly to 
boiling point, and when complete solution is effected, to make 
up the bulk by adding the necessary quantity of cold water. 
The great difficulty with this solution, however, of the strength 
above recommended is its constant tendency to crystallize 
out, due to the fact that these proportions correspond 
practically to the point of saturation. We prefer, therefore, 
to dissolve the salt in cold water as' follows. Prepare the 
vat in which plating operations are to be carried out 
by thoroughly cleansing and rinsing. It is of the utmost 
importance that the vat itself shall be perfectly clean. 
Measure into the vat the required quantity of water, pre- 
ferably distilled or filtered rain-water ; the level of the liquid 
should be at least four inches from the top edge of the vat. 
Prepare now a number of muslin bags or perforated stone- 
ware vessels and divide the nickel salt into equal portions 
in these ; hang them at intervals in the vat so that the salts 
are just immersed and stir the solution occasionally. In 
this way the water will absorb the crystals at a normal 
temperature and the danger of subsequent crystallizing out 
will be averted. This is also a good plan to adopt when 
making addition to the vat during working. 

When the solution is made, it should be tested for 



THE DEPOSITION OF NICKEL 273 

acidity or alkalinity by means of litmus papers. Blue 
litmus is reddened by acids, and red litmus turns blue when 
immersed in an alkaline solution. Usually the double sulphate 
solution will be found neutral. In commencing to work a 
new solution it is advisable first to pass the current through 
the vat for a short time by means of nickel sheets acting as 
both anodes and cathodes, and again test the solution with 
the litmus paper ; if the reaction is still neutral or, as will 
often be found, slightly alkaline, add a very few drops of 
sulphuric acid and test again, repeating the operation most 
carefully until the bath is found to be very slightly acid. In 
this condition the best results are obtained. 

The bath should be worked at a temperature of 20 to 
25" C. (Normal temperature = 15 C. = 59 F.) 

The reactions which occur during the electrolysis of the 
double sulphate bath are somewhat complicated and demand 
careful consideration. It is usually regarded that dissociation 
occurs thus : 

NiS0 4 into Ni + SO 4 
(NH 4 ),S0 4 2NH 4 + SO 4 

In dilate solutions probably this is so, but according to 
modern research there seems good reason to conclude that in 
concentrated solutions the reaction is rather different, and the 
ammonium ion only is supposed to be the cation, the rest 
of the compound forming a complex anion, thus : 

NiS0 4 (NH 4 ),S0 4 = 2NH 4 + Ni(SO 4 ). 2 

The possibility is therefore that in many nickelplating 
solutions both the above conditions obtain. 

Now, when electrolysis takes place, one or both of two 
actions may occur at the anode either separately or simul- 
taneously. 

(1) The anions may be discharged, 

or (2) New ions may be formed by combination with the 
anode metal. 

If the first occurs, then the anion S0 4 or the complex 

T 



274 ELECTROPLATING 

anion Ni(S0 4 )o combines with the water of the solution, 
thus 

(a) 2S0 4 + 2H 2 = 2H 2 S0 4 + O, 
(If) 2Ni(SO 4 ), + 2H,0 = 2NiSO 4 + 2H 2 S0 4 + 0, 
i.e. sulphuric acid is formed with the liberation of oxygen. 

If the second occurs, then direct union takes place be- 
tween the nickel of the anode and S0 4 or Ni(S0 4 ) tJ , thus 

(a) S0 4 + Ni = NiS0 4 
(b) Ni(S0 4 ), -f Ni = 2NiS0 4 

At the cathode, on the other hand, the reactions which 
may occur are 

Either (1) The discharge of the cations Ni and 2NH 4 re- 
spectively, 

or (2) The discharge of the cations 2NH 4 with the con- 
sequent liberation of metallic nickel as a secondary reaction 
with the undissociated molecules of nickel ammonium sul- 
phate, thus 

2NH 4 + NiS0 4 (NH 4 ) 2 S0 4 = 2(NH 4 ),S0 4 + Ni 

The first of these results in a deposit of metallic nickel 
with simultaneous liberation of 2NH 4 which breaks up into 
2NH 3 and H 2 (ammonia and hydrogen gas). The alternative 
reaction also gives a deposit of metallic nickel with, however, 
the formation of ammonium sulphate. 

A study of these reactions, which necessarily are but 
briefly outlined above, then reveals the fact that the con- 
stitution of the nickel solution during or after electrolysis will 
depend other conditions of temperature and current being 
normal upon the solubility of the anode, in other words on 
the extent to which it neutralizes the anions. 

Suppose for the sake of argument and taking the older 
view of the dissociation reactions that the whole of the 
latter combines with the metal of the anode, then the net 
results of electrolysis would be : 

At cathode. At anode. 



Ni 



HJ 



2(NH 4 )HO + H.J I S0 4 + Ni = NiS0 4 . 



THE DEPOSITION OF NICKEL 275 

The bath would gradually become alkaline owing to the 
liberated ammonia, and at the same time would acquire an 
increased content of nickel in the form of nickel sulphate. 
Experience in practical working has shown that this is the 
case to some extent. 

Rarely, if ever, is the anode, however, so completely 
soluble in the solution by electrolysis as would be required 
to make the above equations exactly true. Consequently 
the alternative must also be taken into review, viz. : 

At mtluxle. At anode. 

Ni } (SO 4 + Ni =NiS0 4 

2(NH 4 )HO + H a j JS0 4 + H,0 = H,SO 4 + 0. 

In the working of these baths, therefore, it is usually 
found that the increase in alkalinity, if any, is very gradual 
a considerable proportion of the liberated ammonia at the 
cathode being neutralized by a corresponding formation of 
sulphuric acid at the anode. 

To secure the highest possible efficiency in the working 
of these baths, then, it is essential that periodically the 
composition of the electrolyte be ascertained in the manner 
to be explained later, so that any irregularities of consti- 
tution may be rectified and the chemical equilibrium of the 
solution maintained, by the addition either of sulphuric acid, 
if the bath is found alkaline, of ammonia if too acid, of 
single nickel sulphate if found deficient in metallic content, 
or of water if too dense. 

It is a significant fact, and one which may be taken to 
bear out the foregoing theoretical conclusions, that almost 
invariably an analysis of nickelplating solutions which have 
been in actual use for any appreciable length of time reveals 
the existence in the solution of a certain proportion of single 
nickel sulphate along with the double sulphate of nickel and 
ammonium, even in cases where the operator in charge has 
rigorously excluded any addition to the vat other than the 
double sulphate only. 

The following is a typical result, the analysis being made 



276 ELECTROPLATING 

after six years' use of a solution originally made up of the 
double sulphate of nickel and ammonium, and replenished 
only by this salt : 

The analysis * showed a metallic content of 2 78 oz. of 
nickel per gall., and an ammonia content of 0-474 gram per 
100 c.c.. 

This result when calculated out corresponds to the 
following : 

Content of double nickel salts (nickel ) n ork 

ammonium sulphate) ..... J 8-80 oz. per gall. 

Content of single nickel salts (nickel sul- ) c Kt 

phate) 5 6 

or of double nickel salts, 57 per cent. 
single 43 

The great drawback, however, to a solution made origi- 
nally from the double sulphate of nickel and ammonium 
alone, is its relatively poor conductivity and consequent 
slowness of working. It is this disadvantage which, in 
recent years particularly, has turned the attention of inves- 
tigators to the question of making additions to this bath 
with a view to decreasing its resistance and even also to 
the substitution of other possible compounds for use as the 
basis for nickel baths. 

With regard to the former point it may be remarked 
that several recent writers on electroplating have passed 
rather severe strictures on some published formulas for 
plating solutions on the score of complexity. In many 
cases this criticism is justifiable, but it must be quite as 
emphatically asserted that complexity in the composition 
of plating baths is by no means necessarily an evil. Indeed, 
experience in practical working has repeatedly demonstrated 
that the characteristics of many metallic deposits can be 
profoundly modified, often to their advantage by the addi- 
tion of various substances to the electrolyte which appear, 

* Metal Industry, vol. iv,, No, 6 (1912), p. 236. 



THE DEPOSITION OF NICKEL 277 

from a purely theoretical point of view, to be totally un- 
necessary. A classical illustration of this point is found in 
the addition of carbon bisulphide and similar compounds to 
silverplating solutions. Theoretically, so far as present 
knowledge is concerned, this would appear to be a quite 
unjustifiable complication, without the slightest probability of 
obtaining by means of it the effect which is now so familiar 
to electroplaters. 

With reference to nickel, while a simple solution of the 
double sulphate of nickel and ammonium in water yields 
very good results, yet there is no doubt that certain additions 
and modifications of this solution can be made which result 
in improving both the character of the deposit and the con- 
ductivity of the bath. 

Before discussing some of the principal substances 
recommended in this connection, however, it will be advis- 
able to deal with the question of the use of nickel sulphate 
or single nickel salt as this substance is sometimes termed. 
This is a subject which at various times has aroused much 
controversy amongst nickelplaters, some operators strongly 
advocating its use as an addition to the bath, others just 
as strongly opposing it. There is little doubt, however, that 
for most classes of work and under ordinary conditions of 
temperature the addition of small proportions of nickel 
sulphate is of distinct advantage. This appears to be due 
largely to the fact that the single salt is proportionately 
much more soluble than the double salt, consequently by 
its use a greater content of metallic nickel can be given to 
the vat with the effect of appreciably increasing its con- 
ductivity. 

The following comparison of the molecular composition 
and solubilities of the two compounds will be of interest 
and assistance to the reader. 

NicM Ammonium Sulphate as usually obtained in com- 
merce has a composition corresponding to the formula 
NiS0 4 (NH 4 ) 2 SO 4 .6H,0. It is obtained by dissolving pure 
nickel in dilute sulphuric acid, and adding a molecular 



278 ELECTROPLATING 

proportion of ammonium sulphate to the concentrated acid 
solution. 

According to Link its solubility is as follows : 

Temperature in degrees Centigrade. 

Parts of NiS0 4 . (NH 4 ) 2 S0 4 \3J 16 20 30 40 50 68 85 
soluble in 100 parts of water/1-8 5-8 5'9 8-3 11-5 14-4 18-8 28-G 

Nickel Sulphate is obtained by dissolving metallic nickel, 
nickel hydroxide, or nickel 'carbonate, in dilute sulphuric 
acid. If crystallized out in excess of acid it has the formula 
NiSO 4 .6H. 2 O. The crystals from an aqueous solution have 
the composition NiS0 4 .7H 2 O. When heated, nickel sulphate 
crystals lose the greater part of their water of crystallization. 
At 100 C. only one molecule of water is retained, and above 
280 C. this is expelled, leaving the yellowish anhydrous 
NiS0 4 . 

According to Tobler the solubility of this salt is as 
follows : 

Temperature in degrees Centigrade. 

Parts of NiS0 4 soluble) 2 16 23 41 50 60 70- 
in 100 parts of water/ 30-4 37'4 41-0 49-1 52-0 57'2 61'9 

A glance at these figures will reveal the greatly superior 
solubility of the latter salt over the former. Obviously also 
the percentage of metallic nickel present in the single salt 
is much higher than in the double. The single sulphate 
alone t however, is absolutely useless for nickelplating. It 
can only be employed successfully either in conjunction 
with the double salt or with other substances, as will be 
explained later. 

A bath containing the single sulphate as an addition, 
which has been found by the authors to give excellent results, 
is made up as follows : 
Double sulphate of nickel and ammonium . 12 oz. 375 gr. 

( 93-75 to 
Single nickel sulphate ...... 3 to 4 < | -^ 



This bath should be prepared in the manner previously 
directed, and worked at a temperature of about 20 C. 



THE DEPOSITION OF NICKEL 279 

With regard now to the addition of other substances, 
usually termed " conducting salts," to the double sulphate 
nickel bath, a truly bewildering variety of compounds have 
been recommended. These include inter alia, ammonium 
chloride, ammonium sulphate, common salt, potassium or 
sodium phosphates, magnesium sulphate, potassium car- 
bonate, sodium bi-carbonate, calcium acetate, calcium 
chloride, and ammonium tartrate. 

In addition, many operators have recommended giving a 
slight degree of acidity to the bath by means of weak 
organic acids, e.g. benzoic acid, boric acid, citric acid, etc. ; 
in several instances claiming thereby not only an increased 
conductivity of solution but an improved character of deposit. 

A typical example of a solution containing one or more 
of these conducting salts is the following, which is recom- 
mended by an American expert, and quoted here as a fair 
example of a very large number of such formulae which 
might be given. 

Double sulphate of nickel and ammonium . 8 oz. i 300 gr. 

Single nickel sulphate 2 , 75 gr. 

Ammonium chloride 1 37*5 ,, 

Sodium chloride (common salt) . . . 3 ,, ! 112-5 ,, 

Boric acid 2 ,, 75 ,, 



Water J :~T cf 5 litres 

I or 1 U.S. | 

Such a bath obviously invites criticism on the ground of 
complexity, and certainly the ammonium chloride may be 
omitted without making any observable difference to the 
results. Nevertheless it is indisputable that this, and many 
similar solutions, yield remarkably good results in practice. 
They are good conductors, can be worked rapidly without 
giving off hydrogen to anything like the extent of a normal 
double sulphate solution, and yield a coherent and adherent 
deposit of nickel of a good colour. 

After considerable observation of the results obtainable 
from the use of various conducting salts or additions which 



280 ELECTROPLATING 

have been recommended for use in nickel baths, and also 
after a number of experiments which need not be detailed 
here, the conclusion we have arrived at is that it is in- 
advisable at the present stage of investigation in this direction 
to make any dogmatic statement as to the superiority of any 
one formula over another, the results from various experi- 
ments being almost indistinguishable. 

We have, however, obtained uniformly good results from 
solutions containing potassium chloride, a substance which, 
so far as we are aware, has not hitherto been noted in this 
connection. The corresponding sodium compound (common 
salt) has of course been extensively recommended and used 
by nickelplaters, but we prefer the potassium salt, not merely 
because its effect is fully equal, if not superior, to that from 
common salt, but also because of its distinct advantages, 
from an electrochemical point of view, of conductivity. 

The following is the bath we have used for general 
work : 

Double sulphate of nickel and ammonium . 10 oz. i 312 gr. 
Single nickel sulphate 4 ,, 125 ,, 

Potassium chloride 1 to 1J j ,p.p o , , 



Water. . ' {oA^sf^ i 5 Utres 

It cannot be too strongly emphasized, however, that this 
proportion of potassium or sodium chloride must not be 
exceeded. A great deal of trouble has arisen in recent years 
from an injudicious and often extravagant use of salt in 
nickel solutions, and it should be remarked that many 
operators, while using such additions for nickel-plating 
copper, brass, etc., prefer to omit them altogether for iron 
and steel. 

The following solutions form a representative selection 
from a large number of authorities, and are given here in 
order that the reader may be familiarized with some of the 
many possible combinations which have been or are used 



THE DEPOSITION OF NICKEL 281 

for nickelplating either for general work or particular pur- 
poses as noted. 

Solution I. (Weston) 
Double sulphate of nickel and ammonium . 10 ozs. j 375 gr. 

C 112-5 to 
Boric acid .......... 3 to 5 -j -, Q7 

(| Ib7 gr. 

^litres 



With regard to this solution Langbein observes that " it 
cannot be recommended because the bath works faultlessly 
for a short time only; all kinds of disturbing phenomena 
make their appearance, the deposit being no longer white 
but blackish, and the bath soon failing entirely." He him- 
self recommends the following, which also contains boric 
acid. 

Solution II. (Langbein) 

Double sulphate of nickel and ammonium . G oz. | 225 gr. 
Pure nickel carbonate ........ a 18'7 

,, Boric acid .......... 3 ,, ! 112-5 gr. 

Water .......... [ *** 8 ^ \ 5 litres 

( or 1 U.b. ,, 

Dissolve the nickel ammonium sulphate in water, and 
when solution is complete add the boric acid. Heat the 
liquid to boiling point, and then add the nickel carbonate. 
Allow the whole to boil a few minutes, cool and filter. 

Wahl, on the other hand, supports Weston 's claim that 
his bath gives an improved character of deposit, and allows 
more rapid working. 

Solution III. (Desmur) 

Double sulphate of nickel and ammonium . 11 oz. 343 gr. 
Bicarbonate of soda ........ 1^ ,, 39 ,, 

1 imp. gall. ,.. 
Water ......... , , i 5 litres 



C 1 imp. gall, 
lor U U.S. 



Watt, in quoting this solution, recommends it for small 
work, mounts, etc. The bicarbonate of soda must be added 



282 ELECTROPLATING 

in small portions, waiting after each addition until the effer- 
vescence has ceased. 

In our experience equally good results can be obtained 
by substituting potassium sulphate for the sodium salt. 

With regard now to solutions other than the double sul- 
phate of nickel and ammonium with or without additions, it 
has been already observed that single nickel sulphate has a 
much higher degree of solubility than the double salt. Many 
attempts, therefore, have been made to utilize this compound 
as a chief agent in nickel solutions, and of recent years these 
have been increasingly successful. As has been also stated, 
however, a solution of nickel sulphate alone is of no use for 
nickel plating. This salt can only be employed conjointly 
with " conducting salts." A large number of the special 
nickel salts sold under registered or trade names are com- 
pounds of this order, i.e. nickel sulphate crystallized out 
along with added conducting salts. The latter chiefly con- 
sist of the sulphates and chlorides of the alkali and alkaline 
earth metals. 

A type of nickeling solution often recommended which 
may be considered as coming under the foregoing generaliza- 
tion is that made by dissolving single nickel sulphate in 
water and adding varying proportions of ammonium sulphate. 
It is, however, obvious that such a bath is simply another 
form of the double sulphate bath, and attempts to obtain a 
solution of high nickel content by dissolving these substances 
separately and then combining them ends in obtaining a 
liquid from which the double sulphates quickly crystallize 
out, or in cases in which a strong solution of ammonium 
sulphate has been used, in the operator finding green 
crystals of the double sulphates at the bottom of the vat. 
This latter action is due to the peculiar property possessed 
by nickel ammonium sulphate of insolubility in a strong 
solution of ammonium sulphate a property often made use 
of in the recovery of nickel salts from old or spoilt solutions, 
as will be referred to later. 

The more successful solutions of nickel sulphate are those 



THE DEPOSITION OF NICKEL 283 

which contain, as conducting salts, potassium or magnesium 
sulphates, generally in molecular proportions. 
The following are examples : 

Nickel sulphate (single nickel salt) . . 2 Ibs. j 1 kg. 
Magnesium sulphate ....... 1 lb. j 0-5 



Water ......... 5 litres 

or 



Langbein quotes the two following formulae, which are 
interesting as illustrative of the use of organic compounds 
with nickel sulphates : 



(1) Nickel sulphate 7 oz. 

Neutral ammonium tartrate . . 5 

Tannin 15 grains 

C 1 imp. gall. 



Water 



218 gr. 
156 
0-97 

5 litres 



(2) Nickel sulphate . 


. . . . 7 oz. 


Tartaric acid 


. . . 4 


Caustic potash 




Water 


1 imp. gall. 




1 ' (orlJU.S. 



or 11 U.S. _ _ 

218 gr. 
125 
23-4 

5 litres 

Of solutions made from nickel compounds other than the 
sulphate the most successful are those of organic salts of 
this metal, notably the oxalate. Good deposits of nickel can 
be obtained from the double oxalate of nickel and ammonium , 
NiC 2 O 4 . (NH 4 ) 2 C 2 O 4 . This compound, however, has the dis- 
advantage from a commercial point of view that it is a more 
expensive salt without affording any commensurate advan- 
tage. The same remark applies to the double cyanide of 
nickel and potassium which has been recommended by 
Gore and other writers. With regard to this latter solution 
it must also be pointed out that cyanide of nickel is much 
less soluble in potassium cyanide than the corresponding 
silver salt, and the solution is a very troublesome one to 
make. 

The following solution by Potts containing nickel acetate 



284 ELECTROPLATING 

yields very good results and is strongly recommended by 
Wahl : 



Nickel acetate . 
Calcium acetate 


4^ oz. 
3i 


140 gr. 
109 


Acetic acid . . 
Water . 


. . 1 British Fl. oz. 


28-4 c.c. 
5 litres 



- or 

Dr. F. W. Kern of Columbia University, U.S.A., has 
recently (Dec. 1909, Amer. Patent 942,729) patented a 
solution of the fluosilicate of nickel with the addition of either 
an alkaline fluoride alone, or an alkaline fluoride and a 
soluble fluosilicate, preferably aluminium fluosilicate. The 
bath he recommends is as follows : 

Fluosilicate of nickel .... 10 parts by weight 
Ammonium fluoride .... 5 ,, ,, ,, 
Aluminium fluosilicate ... 5 ,, ,, 
Water 100 

Small quantities of ammonium fluoride should be added 
from time to time to prevent the separation of silica. 

According to another writer * the corresponding boric 
compound (nickel fluo-borate) can also be employed for 
nickel deposition. 

Anodes. The subject of anodes in nickelplating is an 
exceedingly important one, and a good deal of attention has 
been at various times devoted to it. The first factor to be 
considered is undoubtedly that of the degree of purity. The 
great improvements which the last two decades have wit- 
nessed in the metallurgy of nickel have rendered it quite 
possible and even common to obtain the metal commercially 
of a purity of 98 to 99 per cent. The most common impuri- 
ties consist of iron, cobalt, copper, arsenic, carbon, sulphur, 
antimony, and bismuth, but none of these, except perhaps 
the first two and carbon, are present in commercially pure 
nickel but in mere traces. The metals iron and cobalt 

* Trans. Amer. Electro-Own. Soc., vol. xviii. (1909), p. 464. 



THE DEPOSITION OF NICKEL 285 

are so closely akin to nickel both in their chemical and 
electrochemical as well as in their physical properties that 
they may be disregarded. Great care, however, must be 
observed to secure anodes free from copper. This latter 
metal, being very readily dissolved and more electro-negative 
than nickel, finds its way quickly into the bath and is more 
easily deposited than the nickel, greatly to the detriment of 
the colour of the deposit. 

With regard to the form in which the metal should be 
made into anodes, whether cast or rolled sheets, much dis- 
cussion has arisen, but the great majority of practical 
operators prefer the former; and if occasionally the latter 
are used, they are always considerably in the minority of the 
total number employed in the vat. The chief advantage 
possessed by cast over rolled anodes is that the casting, being 
appreciably more porous in texture than a rolled sheet is much 
more easily dissolved by the anodic product of electrolytic 
action. 

In neutral solutions, such as nickel baths usually are, it 
will be readily understood that the anode metal can only be 
dissolved into the solution by virtue of its combination with 
the particular product of electrolysis liberated at its surface. 
When this latter then is close grained and smooth, as is the 
case in rolled sheets, its physical characteristics do not tend 
to facilitate combination, but rather to resist attack by the 
liberated ions. In the case of a porous casting, on the other 
hand, these ions finding their way into the pores of the metal 
have a relatively far greater surface to act upon, and in the 
aggregate combine with and so dissolve a much larger propor- 
tion of metal. 

One disadvantage urged against the use of cast anodes is 
that they disintegrate rapidly and fall to pieces more 
quickly than rolled, thus forming a greater proportion of 
scrap. It must be borne in mind, however, against this, that 
if, when rolled sheets are used, the solution is not supplied 
with metal to an equivalent extent as in the case of cast 
anodes, then the liquid must be periodically renewed by 



2 86 



ELECTROPLATING 




fresh additions of nickel salts to a greater degree than other- 
wise, and the slight loss in remelting scrap is often more 
than balanced by the cost of this additional nickel salt. 

Anodes are now usually made with projecting lugs per- 
forated as in Fig. 60, so that they can be readily connected 
_ by means of hooks to the anode 

conducting rods. Watt makes 
the very good suggestion that the 
connecting hooks when passed 
through the hole in the lugs be 
soldered in order to obviate the 
possibility of an imperfect connec- 
tion. When working rich solutions 
it will be observed that their ten- 
dency to crystallize out familiar 
to all nickelplaters often leads to 
the formation of small growths of 
crystals on the part of the lug of 
the anode immediately above the 
surface of the liquid. These crystals 
once formed easily grow and extend to the hole in which the 
connecting hook is inserted and consequently materially 
interfere with the contact of a loosely hung anode. Solder- 
ing of course effectually prevents any interference of this 
kind and ensures a continuous sound electrical connection. 
The importance of this is obvious. 

Management of Solutions. Nickelplating solutions 
are not necessarily difficult to manage or keep in good work- 
ing order, provided one or two essential points are thoroughly 
grasped and understood. 

The first is the necessity, upon which emphasis has 
previously been placed, for the solution to be kept neutral 
or at most only slightly acid. The latter condition is the 
more advisable inasmuch as a little free acid assists in the 
effective solution of the anode and consequently in keeping 
up the metallic content of the bath. Too great acidity, how- 
ever, is fatal, since in this case hydrogen is most readily 



FK;. 60. Nickel Anode. 



THE DEPOSITION OF NICKEL 287 

liberated at the cathode surface and occluded by the deposited 
nickel, with the result that the deposit becomes neither 
adherent nor coherent, and may even be observed to " curl 
up" or "peel" during the process of deposition. On the 
other hand, if the bath is allowed to become alkaline, the 
deposit is usually of a bad colour and often the conductivity 
becomes impaired. Tests should be made frequently with 
litmus paper, and in the case of decided acidity one or two 
muslin bags containing nickel carbonate should be hung at 
intervals just under the surface of the solution. This salt is 
insoluble in water but quite soluble in acids, and will quickly 
neutralize the excess acid. This is best done at night. If 
the bath is alkaline, sulphuric acid should be added carefully 
with constant stirring until the point of neutrality or just 
beyond it is reached. 

The second essential in good management is to take steps 
to ensure that the metallic content of the bath is kept con- 
stant. This is accomplished in two ways, first by using a 
larger anode than cathode surface during deposition, and 
secondly by periodic additions of nickel salts. It rarely 
happens even in the best-managed solutions that as much 
metal passes into the bath from the anode as is deposited 
upon the cathode, owing largely tp the fact that free acid is 
not allowable ; still, much can be done by using cast anodes 
and arranging them so that their superficial area is always 
slightly in excess of that of the cathodes. When additions 
of nickel salts are found to be necessary in the case of a 
solution of the double sulphate of nickel and ammonia, single 
nickel sulphate should always be used. 

A third point which deserves more attention than usually 
appears to be given to it is the temperature of the solution. 
For normal and general working this should be kept as 
nearly as possible to 20 or 21 C. (68 Fahr.). This tem- 
perature is sufficiently high to prevent crystallizing out of 
the dissolved salts and yet not high enough to tend, as hot 
solutions usually do, to the too ready liberation of hydrogen. 
In well-fitted and managed nickelplating shops arrangements 



288 ELECTROPLATING 

consisting of steam or hot-water pipes are made so that the 
temperature of the vat rooms is kept at or about the point 
named. 

Electrical Conditions. It is generally known that 
nickelplating demands a comparatively high voltage, but a 
mistake often committed by inexperienced operators is to 
use one much higher than necessary. It is usually ad- 
visable at the moment of immersion of articles in a bath 
to apply a voltage up to about 5 volts until the cathode 
surface is completely covered with a film of the metal, but 
after that the voltage between the vat terminals should be 
reduced to 3 volts, or even slightly less, if the solution used 
is at all acid. 

The current density allowable depends almost entirely 
on the character of the electrolyte. For solutions of the 
double sulphate alone, with stationary cathodes, the value 
must not exceed about 5 amperes per square foot. With 
agitating arrangements or moving cathode rods a higher 
value may be adopted. With solutions of the single sulphate 
and conducting salts, however, double this current often 
more may be used. Exact figures cannot be given owing 
to the many variations which may be possible owing to 
local conditions and class of work. 

Special Treatment of Articles for Nickelplating. 
Owing to the extreme hardness of electro-deposited nickel 
and the consequent difficulty of polishing it, it is absolutely 
necessary, in all cases where a bright deposit is required, 
that the surface before plating shall receive as high a 
polish as it is capable of. For this reason the processes 
preparatory to immersion in the nickel bath vary some- 
what from those adopted for most other classes of electro- 
plating. The principal variation, as will be fairly obvious, 
is that strong dipping acids and coarse scouring or scratch- 
brushing must be avoided. As the function of the former 
is to remove oxides and scale from metallic surfaces, and 
the latter operation is to clear off stains or tarnish, it will be 



THE DEPOSITION OF NICKEL 289 

evident that if the polishing of articles is thoroughly done 
and they are carried through the plating operation without 
delay, these two processes are largely rendered unnecessary. 
It is, however, advisable, after the ordinary routine of cleans- 
ing from the films of grease, etc., which usually remain on 
polished goods, to scour lightly with soda-lime, fine whiting, or 
precipitated chalk for the reasons that the cleansing operation 
itself occasionally leaves stains on most surfaces, and that 
the adhesion of the deposited coating is rendered more 
reliable by the extremely slight deadness which even the 
gentlest scouring treatment will leave. 

Of the particular metals usually dealt with for nickel- 
plating those which call for special consideration are Britan- 
nia metal, lead or zinc alloys, and iron and steel goods. 

Dealing with the former, Watt remarks that " lead, tin, 
and Britannia metal are not suited for nickelplating, and 
should never be allowed to enter the nickel bath." The fact 
remains, however, that a very large amount of Britannia 
metal has been successfully nickelplated, and though to 
some extent this class of work has been superseded by 
silverplated goods, owing to the greatly reduced prices of 
the last-named which recent years have witnessed, yet it is 
still carried out for certain requirements, and wonderfully 
good results obtained. 

Several methods have been recommended for the treat- 
ment of these alloys, but the most successful results are 
obtained by giving the surfaces a preliminary coating of 
brass from the solution recommended on page 350. The 
articles are first given a high polish by means of dollies 
with lime and rouge composition, then rinsed through a 
strong caustic potash boil and immediately transferred to 
the brassing solution. From this, when the entire surface 
has received a sound coating of brass, they are taken 
quickly, rinsed through clean water, then through a second 
wash-water very slightly acidulated with sulphuric acid, and 
immersed in the nickel bath. 

An alternative method of treatment which results in the 



igo ELECTROPLATING 

articles retaining a high degree of polish is known as " dry 
cleaning." The bright polished surfaces in this method 
instead of being subjected to the action of caustic alkalies 
are thoroughly brushed first with soda-lime, then with the 
finest whiting or precipitated chalk. A perfectly dry brush 
is used, and care is taken not to seriously scratch the bright 
surfaces. The articles are then brassed, and subsequently 
nickeled in the ordinary way. 

Iron and steel goods, particularly in the best classes of 
work where thick deposits are required, are also very often 
coppered or brassed in the cyanide baths before nickeling, 
but this is by no means invariably necessary. A strongly 
adherent deposit of nickel can be given to iron or steel 
direct, and it is doubtful if any real advantage accrues in 
the case of preliminary coppering or brassing, except per- 
haps in the treatment of cast iron which is often extremely 
porous, and consequently gives considerable trouble to the 
nickelplater. It will be found in this case that a thin 
deposit of Irass given prior to immersion in the nickel bath 
will ensure almost perfect adhesion of the nickel deposit. 
In connexion with iron and steel it must here be pointed 
out that thin deposits of nickel are almost useless. The 
deposited metal is always slightly porous, and in a very 
short time, particularly in an atmosphere at all moist, the 
basis metal is gradually attacked through the pores of a thin 
coating and begins to rust. This action once begun speedily 
ruins the appearance of the article. 

Thick deposits resist the atmosphere to a degree far 
greater than in proportion to their thickness, and as the 
preparation involved is in either case the same, it is false 
economy to stint the deposit, seeing that the increased cost 
of a stronger deposit is so greatly disproportionate to the 
advantages gained. 

It may be advisable to point out with regard to both 
zinc, tin, and lead alloys and iron or steel goods that owing 
to the strongly electro-positive nature of all these metals 
relatively to nickel, a fairly high initial voltage must be used 



THE DEPOSITION OF NICKEL 291 

in order to overcome the back E.M.F. which is set up, if 
nickeled direct without intermediate coatings. If the 
average distance between anodes and cathodes is not more 
than 6 to 8 inches, a pressure of not less than 5 volts will 
be found satisfactory, though in the case of zinc, which of 
course is the most electro-positive of all, some operators 
prefer to " strike " with 6 or 7 volts. 

In dealing with copper or brass goods these high 
voltages are not in the least necessary. 

In all cases the goods immersed should be completely 
covered with a film of nickel of a clean white colour in 
from two to three minutes from immersion, and when once 
deposition has begun it must not under any circumstances 
be interrupted until the required weight of metal is de- 
posited. 

When goods of very unequal size are being dealt with 
and passed through the same nickeling bath, it is some- 
times an advantage to " strike " in a separate bath, at a high 
pressure and current density, and then transfer to the bath 
proper, which in the meantime may be working with other 
goods. If this plan is adopted, however, the transfer must 
be effected very quickly or the subsequent deposit will 
strip. 

Stripping of Old Nickel Deposits. The stripping of 
old coatings of nickel from articles which are required to be 
replated is a matter of some little difficulty, as any liquid 
which can ordinarily be used for this purpose will also 
attack the basis metal. Eeference has already been made 
to the stripping of nickel coatings by long immersion in 
organic acids, but this is far too tedious a method for ordinary 
trade requirements. The formula most generally adopted 
for stripping nickel is as follows : 

Concentrated sulphuric acid . . 2 parts by weight 

nitric acid . . . 1 part ,, 

Water . 1 



292 ELECTROPLATING 

The sulphuric acid is added slowly and carefully to the 
water, and when the mixture has cooled down the nitric 
acid is poured in. Some operators prefer to omit the water 
and use a simple mixture of nitric and sulphuric acid in the 
above proportion, but the action is much slower. In either 
case the operation must be closely watched and the article 
taken out of the liquid immediately the coating is com- 
pletely removed. 

The Assay of Nickelplatmg Solutions. Although 
not of such primary importance as in the case of silver- 
plating, it is yet greatly advantageous, and certainly con- 
ducive to greater efficiency, that periodically at least 
approximate estimates should be made of the amount of 
metallic nickel contained in nickelplating solutions, and for 
this purpose it cannot be too strongly emphasized that the 
hydrometer, which is the instrument apparently most com- 
monly relied upon for such tests, is absolutely useless. 
Worse than useless indeed, for it is misleading. An hy- 
drometer is simply an instrument for determining the spe- 
cific gravity of a liquid as compared with water and nothing 
more and the specific gravity (or weight compared with 
water) is of course influenced by the ivhole of the sub- 
stances contained in the particular liquid. The addition of, 
say, sulphuric acid or indeed any soluble substance will 
obviously influence the specific gravity reading just as well 
as the addition of nickel salts will do so. Consequently a 
particular reading on a hydrometer scale can convey no 
reliable idea of the really important factor, viz. the weight of 
metallic nickel in solution. 

Several methods are available* for this purpose, but 
probably the most accurate as well as the most convenient 
for electroplaters to adopt is that known as the "cyano- 
metric method," used largely for the estimation of nickel in 
steel, etc. 

To electroplaters, familiar with the chemical reactions of 

the double cyanides, this method will be readily intelligible, 

* See Metal Industry, vol. iv., April, 1912 ; May, 1912 ; June, 1912. 



THE DEPOSITION OF NICKEL 293 

as it is based on the formation of a double cyanide of 
nickel and potassium by means of a standard cyanide 
solution of known strength titrated into the nickel solution 
to be tested. 

The following details of the method have been carefully 
worked out with a view to the special requirements of 
nickelplaters. 

Prepare first standard solutions of silver nitrate, and of 
potassium cyanide, exactly as directed in Chapter IX. for the 
assay of commercial cyanide of potassium. 

The silver nitrate solution is that known as decinormal 
and will contain exactly 17 grams of AgNO 3 per litre. 

The exact strength of the cyanide solution will of course 
not be known unless the sample used has been previously 
assayed. This, however, is not necessary as it can be 
standardized by means of the silver solution. If the sample 
used is absolutely pure, the strength of KCN will be 
13 grams per litre ; as this is extremely improbable, it must 
be tested against the silver standard and its exact strength 
determined. It is usual in such a case to determine by 
experiment the numerical " factor," multiplication by which 
will bring the figures obtained in subsequent burette readings 
to that which would have been obtained had the solution been 
of absolutely accurate strength. An illustration will make 
this clear. Suppose as the result of the mean of several 
readings we find that 50 c.c. of potassium cyanide 
solution are equivalent to 48 c.c. of standard silver (i.e. the 
cyanide is 96 per cent. KCN) ; then since 

50 corresponds to 48, 
1 if = 0-96 = required factor. 

The multiplication of the cyanide readings by this figure will 
therefore bring them up to the equivalent of the silver 
standard, or which is the same thing, to the readings which 
would be given by KCN of 100 per cent, purity. 

Having now the standard solutions prepared and 
labelled, the nickel assay should be carried out as follows : 



294 ELECTROPLATING 

Take by means of a pipette 10 c.c. of the nickel 
solution, place in a beaker, add 20 or 30 c.c. distilled water, 
10 c.c. of 0-880 ammonia, and 5 c.c. of a 10 per cent, 
solution of potassium iodide (the reason of this addition will 
appear later). 

Fill two separate burettes with the standard silver and 
cyanide solutions respectively. See that the burettes are 
filled exactly to zero, and run into the nickel solution about 
2 c.c. of standard silver. This by combination with the 
potassium iodide, which thus acts as an indicator, causes 
the solution to become milky by the formation of silver 
iodide. Now add the standard cyanide solution carefully 
and slowly, constantly shaking the beaker until the nickel 
solution changes to a yellow colour and becomes perfectly 
clear. The nickel has now become converted entirely into 
the double cyanide of nickel and potassium. As, however, 
to attain this a little more cyanide than actually necessary 
has most probably been used, again run in drop by drop 
standard silver unless and until one drop causes a permanent 
milkiness after thorough agitation. Now take the readings 
of both burettes, and correct the volume of cyanide by 
multiplying by the factor previously determined. Then 
deduct the volume of silver solution used from the corrected 
volume of cyanide, thus : 

Say, corrected volume of cyanide ... 40 c.c. 
volume of silver 4 

Nett cyanide equivalent to nickel . 36 c.c. 
The equation representing the reaction is 

NiS0 4 + 4KCN =-- Ni(CN)., . 2KCN + K.,S0 4 
59 4(65) 

.-. 59 parts nickel require 260 parts of potassium cyanide. 
Each c.c. of standard cyanide contains 0-013 gram KCN. 
.*. 1 c.c. standard cyanide = 0-00295 gram nickel. 

An approximate value sufficiently accurate for practical 



THE DEPOSITION OF NICKEL 295 

workshop requirements is, in cases where the amount of 
sample tested is as above, 10 c.c. Then 

Each c.c. standard cyanide solution is equivalent to 4J oz. 
metallic nickel per 100 imperial or 120 U.S. gallons. 

General Remarks on Nickelplating. The neces- 
sity for absolute cleanliness in nickelplating operations 
must be very strongly insisted upon. A very short 
experience in this branch of electroplating will suffice 
to convince the operator of this, at least in regard to 
preparation of work for the vat. In silverplating, brassing, 
or gilding where cyanide solutions are invariably used, if by 
any chance a slight film of grease should remain on a 
prepared surface, the action of the strong alkaline cyanide 
itself is often sufficient to remove it and enable a sound 
deposit to take place. In nickelplating, however, where 
neutral solutions are most generally used no such safeguard 
exists, and the slightest touch with the tip of the finger is 
often sufficient to prevent perfect adhesion. But this 
necessity for cleanliness applies not only to the work 
entering the vat but to the solution itself. Floating particles 
of dirt or grit are often the cause of serious trouble and are 
particularly liable to be introduced owing to imperfect 
rinsing of goods from scouring operations. 

Great care should also be taken to avoid the introduction, 
inadvertently, of caustic potash or cyanide solutions, which 
are often apt to linger in the crevices and recesses of 
hollow-ware articles. Cyanide, particularly if used in the 
preliminary processes, should be thoroughly rinsed away by 
passing goods through clean running wash- waters and care- 
fully draining. 

One of the commonest troubles of nickelplaters is the 
" pitting," as it is termed, of nickel deposits. Instead of 
the fine, smooth and even deposit which, under correct 
current conditions, should be produced, the surface presents 
in these cases an appearance simulating a number of pin- 
holes. This trouble can be caused by floating particles in 



296 ELECTROPLATING 

the solution, but it is far more often due to the evolution 
of hydrogen while the deposit is proceeding. The principal 
conditions tending towards this are, (1) too low a content 
of metallic nickel in the vat, (2) too high a percentage of 
free acid, or (3) too strong a current. In either case the 
remedy is obvious, and the plater should exercise constant 
observation of the vats while working so as to note any 
excessive evolution of gas at the electrodes. 

Solutions should be thoroughly stirred every evening 
and water added to make up for loss due to evaporation. 
Otherwise it is almost impossible to secure that constant 
condition of the electrolyte which enables the operator to 
adjust current conditions correctly from day to day. 

Recovery of Nickel from Old Solutions. It is rarely 
worth the trouble and expense to attempt to recover nickel 
from old solutions in the metallic form. But as it is a 
comparatively simple process to precipitate nickel ammo- 
nium sulphate from such solutions, it is often worth 
while, when a bath has become unsuitable from any cause 
for deposition, to do this and so obtain from the old bath 
a supply of nickel compound which can be used to make 
up a new solution. The principle of the method depends 
on the insolubility of nickel ammonium sulphate in ammonium 
sulphate. As the latter salt is very cheap the cost of the 
process is sufficiently low to make it profitable. 

It is advisable in the first place to concentrate the 
solution as much as possible by applying heat to evaporate 
excess water. When this is done the liquid will begin to 
show signs of precipitating nickel salts ; at this point add 
a considerable excess of ammonium sulphate and stir 
vigorously for some time. Allow the liquid now to stand 
a few hours, then syphon off the clear liquor. Make 
now a saturated solution of ammonium sulphate, and by 
means of this wash the precipitate obtained in the vat 
several times. The precipitate finally remaining will be 
nickel ammonium sulphate of a high degree of purity. 

It can then be utilized for making up a new bath or, 
if preferred, for strengthening other solutions. 



CHAPTER XIII 

THE DEPOSITION OF IRON AND COBALT 

IRON and cobalt, the latter particularly, are both closely 
akin in their chemical and electro-chemical properties to 
nickel. In nature the three metals are usually associated 
together, and a close study of one will assist considerably 
in the understanding of all three. The reader who is in- 
terested in the electro-deposition of either iron or cobalt 
should therefore carefully read the chapter on nickel in 
conjunction with what follows. 

The Electro-Deposition of Iron. 

Up to the present time the principal commercial appli- 
cation of the electro-deposition of iron has been to give a 
coating of this metal to the surfaces of engraved copper 
plates or types used for printing purposes ; the effect being 
to obtain a considerably harder surface and consequently 
to greatly increase their wearing qualities. The process 
has often been termed " steeling," but as the deposit 
usually obtained is almost pure iron this term is a misnomer. 

During recent years the deposition of nickel has been 
strongly recommended and largely used in place of iron for 
this purpose. But the latter metal has at least one advan- 
tage over nickel in that it can be readily removed by a 
short immersion in dilute sulphuric acid, when necessary 
to replate after wear. Nickel, on the other hand, is very 
difficult to remove without risk of injury to the delicate 
lines of the surface engraving. 



298 ELECTROPLATING 

A further application of the electro-deposition of iron is 
now, however, slowly coming into prominence, i.e. what 
has been termed the solid deposition of iron a process 
corresponding to copper electrotypy, with the difference 
usually that the iron reproduction is used as a die for 
stamping or pressing an ornamental pattern on to other 
metallic surfaces of a softer nature. An example of this, 
which may be quoted, consists in taking a copy in reverse 
of a piece of flat chasing or ornamentation in low relief, 
executed in a metal like copper or even Britannia metal 
which is very easy to work. This object, prepared like 
the metallic mould of an electrotype, is made the cathode 
in an electrolyte of iron salts until a solid deposit of 
sufficient thickness is obtained. This deposit is removed 
from the original surface, and is then practically an iron 
die possessing in its face a pattern which can be stamped 
or pressed on any required surface. The process is not 
difficult, but demands some little care and, as will be seen 
later, is very tedious. 

Properties of Iron. Pure iron is white and lustrous, 
capable of taking a brilliant polish. It is unacted upon by 
dry air, but in moist air a thin film of oxide forms on its sur- 
face which rapidly develops into a coating of rust. 

Dilute hydrochloric acid and dilute sulphuric acid dis- 
solve iron most readily with rapid evolution of hydrogen. 
Very dilute nitric acid dissolves the metal with the formation 
of the ierrous salt, whereas stronger nitric acid gives the 
feme salt. Concentrated nitric acid (sp. gr. 1'45), on the 
other hand, does not dissolve this metal. 

Iron forms three oxides, Ferrous oxide, FeO, 
Ferric oxide, Fe./) 3 , 
Ferroso-ferric oxide, Fe 3 4 . 

Two series of salts are formed, corresponding to the two 
first-named oxides. Of these the ferrous compounds are the 
best known and are the only ones of general use to the 
electroplater, though some operators, including Watt, have 



THE DEPOSITION OF IRON 299 

claimed that they have obtained good results from some 
ferric compounds. 

Iron Solutions and Conditions of Deposition. 

One of the earliest solutions used for iron deposition is that 
recommended by Varrentrapp,"" consisting of a solution of 
ferrous sulphate in water of a strength of about 1 Ib. per 
gallon, to which is added a nearly equal quantity of ammonium 
chloride. This latter substance may be omitted, however, 
without materially affecting the deposit. The principal 
difficulty with this solution, as with similar ones, is that on 
exposure to the air the ferrous salt becomes oxidized and an 
insoluble basic salt is formed which separates out as a green 
powder and ultimately interferes considerably with the action 
of the bath. In this respect the double sulphate of iron and 
ammonium gives better results. It is of the utmost import- 
ance that iron solutions be kept neutral, or, like the corre- 
sponding nickel solutions, very slightly acid. 

In addition, however, to the ammonium compound, other 
double sulphates of iron can be used with equally good 
results, notably the double sulphate of iron and magnesium, 
and of iron and potassium or sodium respectively. 

A solution recommended by Klein is made by dissolving 
as much ferrous sulphate in water as the bulk used will 
dissolve, and adding an equal quantity of a solution of 
magnesium sulphate of similar strength. If the solution 
when complete gives an acid reaction with litmus, it must 
be neutralized by means of magnesium carbonate, preferably 
added by suspending the salt in the solution in a perforated 
tray or muslin bag. 

Another solution given by the same experimentalist is 
formed from freshly precipitated ferrous carbonate dissolved 
in dilute sulphuric acid. 

To prepare the bath, make a strong solution of ferrous 
sulphate in freshly boiled water. Add to this a solution 
of ammonium carbonate until no further precipitate is 

* Diiigler's Polytech. Journal, 187, 152. 



300 ELECTROPLATING 

produced. Wash this precipitate several times by decantation 
and then add dilute sulphuric acid (1 part of acid to 2 parts 
of water) until this precipitate is exactly redissolved. Great 
care must be exercised not to add an excess of acid. The 
solution should be made as strong as possible. 

Klein recommends that in working the above solution a 
very large anode surface should be used in order to guard 
against the bath becoming acid during working. Obviously 
a large anode surface will tend to supply iron to take up any 
free acid which may be produced during electrolysis. 

Another solution which yields good results and is very 
simple, is made by dissolving 1 Ib. of ferrous ammonium 
sulphate in one imperial gallon of water (or 100 grams in 1 
litre). The close resemblance of this bath chemically to 
that used for nickel deposition will be noted. It is of the 
utmost importance that the bath be exactly neutral. 

The main difficulty encountered in the working of these 
and other solutions for the deposition of iron lies in the ease 
with which ferrous compounds absorb oxygen either from the 
atmosphere or as the result of electrolytic action, and so 
form ferric compounds (mainly ferric hydroxide). Such 
compounds are insoluble in aqueous solutions, though they 
readily dissolve in excess acids. Solutions containing an 
excess of acid, however, liberate hydrogen on electrolysis far 
too readily to yield sound deposits of iron. 

A few years ago some exceedingly interesting investi- 
gations on the production of pure iron by electrolysis were 
undertaken by Professors Hicks and O'Shea of the University 
of Sheffield. By the kindness of Professor O'Shea we are 
enabled to give the following abstract of the results of their 
experiments, which should be of considerable assistance to 
workers in this branch of electro-deposition. 

The object of the research thus undertaken was to pro- 
duce iron free from foreign substances, especially carbon 
and sulphur. This had not previously been accomplished 
although Koberts-Austen obtained a sample containing as 
low as 0-007 per cent, of each of these two substances, 



THE DEPOSITION OF IRON 301 

whilst Arnold had also obtained electrolytic iron containing 
0-15 per cent, sulphur and O011 per cent, carbon. 

As these previous results had been obtained in both cases 
from solutions containing ferrous sulphate, and as it was 
conjectured that the presence of sulphur in the deposit was 
due to this compound, it was decided to use a salt abso- 
lutely free from sulphates or sulphuric acid. Absolutely 
pure ferrous chloride was first chosen as the electrolyte, but as 
in various ways this salt alone was found unsuitable for the 
production of continuous or heavy deposits (as is indeed 
usual in the case of single salts), the double ferrous ammo- 
nium chloride FeCLj . 2NH 4 C1 was the compound alternatively 
used. It was prepared by dissolving equivalent proportions 
of crystallized ferrous chloride (FeCl. 2 . 4H 2 O) and ammonium 
chloride in water. The latter salt was repeatedly recrystal- 
lized from water until it gave no trace of sulphates after 
standing for 24 hours subsequent to the addition of barium 
chloride. 

It is interesting to note, however, that these investigators 
found that even when this salt was used a brown precipitate 
was liable to form and cause great difficulty by settling on 
the cathode, but of further interest is their statement that 
{< The formation of this precipitate is due to the presence of 
ferric compounds in the solution, and if care is taken to 
reduce the ferric compounds before using the solution the 
formation of the ferric hydroxide practically ceases. When- 
ever it was necessary then to add fresh material to the 
electrolytic cell, the solution was shaken with reduced iron 
powder and quickly filtered before being used so that no 
ferric compounds were introduced into the cell ; under these 
circumstances the electrolyte remained perfectly clear and 
even after continuous working for three weeks only a small 
deposit of ferric hydroxide had collected at the bottom of the 
cell." 

The strength of the solution used was 5 to 6 grams of 
FeCl. 2 2NH 4 Cl per 100 c.c. equivalent to 1-2 to 1-4 grams of 
Fe (approximately 2 oz. per gallon). To maintain the 



302 ELECTROPLATING 

strength of solution, periodic additions of ferrous chloride and 
ammonium chloride were made. It is not desirable to allow 
the iron content to fall too low, for then it would appear that 
the ammonium chloride is decomposed in such quantities 
that the iron remaining in solution is precipitated as ferrous 
hydroxide. 

With regard to current density these investigators state 
that too great a current density causes the deposit to strip 
from the plate and with the above solution 0-15 to 0-17 
amperes per 100 sq. cm. was found to give the best results. 
It is advisable, however, to strike with a density of 0*2 amp. 
per 100 sq. cm. until the cathode is completely coated and 
then reduce it to the above value. The potential difference 
at the electrodes was kept at about 0'7 volt. 

Under the foregoing conditions of electrolyte and current, 
a pure coherent deposit of iron was obtained. The only 
remaining difficulty was the formation of microscopic gas 
bubbles which adhered to the cathode at intervals and pro- 
tected it from the electrolyte. This difficulty is a very 
familiar one to all who have attempted to produce thick 
deposits of either nickel or iron. These workers overcame 
the trouble to some extent by arranging an automatic glass 
scraper which periodically moved up and down over the 
surface of the cathode. 

In order to secure the electrolyte from contamination by 
any impurity of the anode, the latter was enclosed in a 
porous cell containing a 1 per cent, solution of FeCL . NH 4 C1. 
This anodic solution was charged every 12 hours. 

The deposit obtained was of a dense and closely ad- 
herent character and silver-grey in colour. It was very 
brittle but did not possess any great degree of hardness. 
This latter characteristic is contrary to the experience of 
Roberts- Austen and others who refer to the great hardness 
of electrolytically deposited iron. Prof. Arnold, however, who 
examined a number of specimens produced as above, reported 
that " it cannot be correctly called hard, as when mounted 
upon a steel backing it can be pared with sharp scissors and ifc 



THE DEPOSITION OF IRON 303 

files easily." The same expert explains the brittleness of the 
metal as being due " to its deposition in fine needles at right 
angles to the plane of the cathode." 

Successful results in solid iron deposition have recently 
been obtained by substituting calcium chloride for ammonium 
chloride as used in the above experiments, and working the 
bath hot. 

Anodes. Anodes for the electro-deposition of iron 
should always be of the best Swedish charcoal iron. After 
working for some little time in any electrolyte they will 
become covered to a greater or less extent with black slime 
most probably carbon. They should, therefore, be periodi- 
cally cleaned by taking out of the solution and scouring with 
fine sand, afterwards rinsing in clean water. The area of 
the anodes should be greater than that of the cathodes. 

General Remarks on Iron Deposition. No great 
difficulty will be found in the management and working of 
iron solutions if care is used in making up the bath so long 
as the operator realizes the necessity of keeping the electro- 
lyte as near the neutral point as possible and will see that 
it contains a sufficiency of dissolved metal. The most im- 
portant and at the same time the most usual fault is 
the liberation of hydrogen. This must not be allowed or 
the deposit will be speedily rendered useless. It is for this 
reason that the current density used must be kept low ; 
consequently deposition proceeds very slowly, and when 
thick deposits are required the progress seems very tedious. 
A current supply from accumulators is under these cir- 
cumstances very advantageous and indeed almost essential, 
for the reasons that deposition may be continued day and 
night, and both E.M.F. and current density exactly adjusted 
and kept constant at correct values. 

For preparation of work the same directions apply as 
given for nickel. 

Stripping of Old Deposits. As indicated earlier, 
deposits of iron are most readily removed by immersion in 



304 ELECTROPLATING 

dilute sulphuric acid (1 of acid to 9 of water). This liquid 
does not attack basis metals of copper or brass, and is, 
therefore, usually the most suitable to employ. 

The Deposition of Cobalt. 

This subject has been hitherto more a matter of laboratory 
experiment than of workshop practice, probably by reason 
of the comparatively high price of the metal, together with 
the fact that to the ordinary observer it is practically in- 
distinguishable from- nickel when electro-deposited, and 
offers only a few advantages over the latter metal. It is, 
however, in one or two respects, notably in resisting organic 
acids, superior to nickel, and if the present price could be 
reduced, there is great probability that it would enter into 
commercial use in the electroplating industry for special 
purposes. It is, for instance, much more suitable for a 
protective coating to cooking utensils than is nickel, and 
Langbein has suggested its use instead of iron or nickel for 
facing copper plates. This is quite a feasible suggestion, 
as a cobalt deposit is extremely hard, and yet more 
readily removable than nickel when a new coating is 
required. 

The deposit from a good cobalt solution under correct 
current conditions is harder than that of any other metal 
ordinarily deposited in the arts with the one exception of 
platinum, and it is obviously, therefore, suited to imparting 
a protective coating to the softer metals and alloys, a 
coating which at the same time is capable of taking a most 
brilliant polish. 

Properties of Cobalt. Cobalt closely resembles nickel 
in colour and general properties, but it is slightly harder, 
and when polished, though brilliantly white, it possesses a 
bluish cast. It is malleable and ductile, the latter par- 
ticularly when heated. Its most valuable property, from an 
electroplating point of view, in addition to its colour and 
hardness, is that it is practically unaffected by atmospheric 



THE DEPOSITION OF COBALT 305 

action. It is slowly dissolved by both sulphuric and hydro- 
chloric acids, but more readily by nitric acid. 

Compounds of Cobalt. Three oxides of this metal 
exist, corresponding to the formulae CoO, Co 2 O :! , and Co,O 4 
(note similarity to iron), and give rise to a varied series of 
compounds. The most soluble, however, are those formed 
from the first-named, i.e. cob&lious salts. 

Salts of cobalt can be distinguished, when in the hydrated 
condition, from nickel by their colour, which is usually pink 
of a distinctly characteristic shade. The only salts of 
interest to the electroplater are the chloride and the 
sulphate. 

Cobaltous chloride, when crystallized out from hydrochloric 
acid containing the metal or its oxides, deposits itself in 
dark-red prisms having the composition CoCl 2 . 6H.X3. When 
exposed to the action of sulphuric acid or some similar 
dehydrating agent, it loses 4 molecules of water and its 
colour changes to rose-red. Heated to about 100 C., the 
salt is converted to violet-blue crystals CoCL 2 . HO 2 , and 
loses its last molecule of water at 120 C. The salt in this 
condition is blue, but rapidly turns pink on exposure to 
the air. 

Cobaltous sulphate has the formula CoS0 4 . 7H 2 0, and 
crystallizes out from sulphuric acid in dark-red crystals. 
One of the principal characteristics of this salt is its property 
of forming double compounds with the alkaline sulphates, 
ammonium, potassium, and sodium. The most common of 
these double salts is potassium cobalt sulphate, CoSO 4 K 2 SO 4 . 
6H 2 O a salt which in conjunction with a little ammonium 
sulphate can be used for the electro-deposition of cobalt. 
Cobalt sulphate is not quite so soluble in water as the 
corresponding nickel salt. 

Solutions for Deposition. One of the best solutions 
for the electro-deposition of cobalt up to the present is 
undoubtedly that invented by Professor Sylvanus Thompson 
in the year 1887, though very good results can also be 

x 



306 



ELECTROPLATING 



obtained from some other formulae, particulars of which will 
presently be given. 

The main factor in Professor Thompson's patent for 
cobalt-plating solutions is the use of magnesium salts, and 
in describing the patent several different methods of making 
up the bath are quoted. The most usual method is to mix 
together one volume of a saturated solution of cobalt 
sulphate, and 20 volumes of a similar solution of magnesium 
sulphate, but the following alternative suggestions are given 
by the inventor : 

Take of 

(1) Double sulphate of cobalt and am- ( 

t . .I ID. oUvJ ^r. 
momum 3 

Magnesium sulphate a > I ^50 ,, 



Ammonium sulphate 
Citric acid 



Water 



. 1 oz. 

' 1 imp. gall, 
or U U.S. , 



Water 



250 
31-2 

5 litres 



250 gr. 
125 
125 , 



5 litres 



(2) Cobalt sulphate ....... J Ib. 

Magnesium sulphate ..... J ,, 

Ammonium sulphate ..... J 

$ 1 imp. gall. 
' (or U U.S. 

The similarity of the above solutions in principle to 
some of those detailed in the chapter on nickel will be noted. 

All the above give better results when worked warm than 
cold ; the patentee himself suggests a temperature ol about 
35 C. 

A bath which yields very good results, though scarcely as 
good a conductor as Thompson's baths, is the following ; 



Double sulphate of cobalt and ammonium . 6 ozs. 
Boric acid ........... 1J 



187 gr. 
46-8 gr. 

5 litres 



This solution is a modified form of one originally suggested 
by Langbein. 



THE DEPOSITION OF COBALT 307 

The simplest possible cobalting solution is made up by 
dissolving 1 Ib. of the readily obtainable double sulphate of 
potassium and cobalt referred to previously in one imperial 
gallon (or 100 grs. per litre) of water. Such a bath is 
improved by the addition of a small quantity, say 1 oz. per 
gallon, of sodium hypophosphite. This salt, it may be 
remarked incidentally, appears to be a very useful addition 
to cobalt solutions generally. 

Anodes. It is most essential in cobalt-plating that the 
anodes be the purest obtainable. The colour of cobalt 
deposits seems to be peculiarly susceptible to changes of 
conditions of the electrolyte, and is often greatly modified by 
the presence of impurities from the anode or indeed from 
any other source. The common impurities are iron, nickel, 
and arsenic, and occasionally bismuth, but the metallurgy of 
cobalt has undergone considerable improvements during 
recent years, and it is possible now to obtain cobalt anodes 
of a very high degree of purity. 

Since cobalt is rather more soluble than nickel in such 
electrolytes as are outlined above, it is not so essential that 
cast anodes should be used. They may, therefore, be either 
cast or of rolled sheet as found most convenient to procure. 
It is important, however, to anneal and thoroughly cleanse 
them before immersion in the vat. 

Current Conditions. The question of correct con- 
ditions in cobalt deposition is very important. The stumbling- 
block which the beginner will almost invariably find is that 
of obtaining a dark- coloured faulty deposit, through using 
too high a current density. In this respect, as in many 
others, it is very similar to iron, and the same values apply 
to both metals, i.e. about 1J amperes per square foot. For 
the first few seconds of immersion, a little higher current 
may be applied, but it must be quickly reduced. 

It appears to us to be probable that by the use of 
some suitable additive agent in the electrolytes, a higher 
value might be made allowable greatly to the advantage 



308 ELECTROPLATING 

of the process but this point requires further investiga- 
tion. 

The voltage required depends largely on the temperature 
of the bath as also on the class of work done, but should not 
much exceed 2 volts, particularly if the solutions are used 
warm. 

Stripping Cobalt Deposits. Old deposits of cobalt are 
more conveniently removed than nickel owing to the greater 
solubility of the former metal in dilute sulphuric acid. For 
copper and copper alloys which have been cobalt-plated the 
best treatment, therefore, is to immerse in a solution of 
dilute sulphuric acid (1 acid, 8 to 10 water). This solution 
has little or no effect on the basis metal. 

In the case of basis metals like iron or zinc, the process 
must, however, be carefully watched and the article taken 
out of the stripping liquid immediately the deposit is 
removed, since such metals are very readily attacked by the 
acid. 



CHAPTER XIV 

DEPOSITION OF ZINC AND CADMIUM 

THESE metals closely resemble each other both in physical 
and chemical properties, and are usually found associated 
in nature. Of the two, zinc is at present much the more 
important and the cheaper. Cadmium, however, possesses 
certain very useful qualities which are 'gradually bringing it 
into greater prominence in the arts, and the subject of its 
electro-deposition will consequently assume some degree of 
importance. Greater prominence, however, must necessarily 
be given in the present chapter to zinc. 

The Deposition of Zinc. 

Zinc has for a long period been largely used for impart- 
ing a protective coating to iron and steel, but most generally 
this has been carried out by means of the process techni- 
cally termed " hot-galvanizing." 

This process consists essentially of a simple immersion 
in molten zinc a thin coating of the metal in consequence 
adhering to the immersed article if properly cleansed and 
prepared. The term " galvanizing " applied to such a 
method is, however, obviously a misnomer, since this term 
implies electrical agency or the use of an electric current, 
which is not the case. 

Up to recent years this process for zinc deposition has 
practically held the field and even now is largely employed, 



310 ELECTROPLATING 

but electro-deposition methods are now prominently to the 
fore and their use is increasing since, as compared with the 
former and older method, they possess several important 
advantages, which may here be enumerated. 

These are 1. That from suitable electrolytes a perfectly 
adherent and coherent coating of a fair degree of thickness 
can be built up; whereas in hot galvanizing only a com- 
paratively thin coating can be acquired. 

2. The physical quality of the deposited metal is much 
more completely under the control of the operator, and 
Philip * has found that the same weight of zinc per unit 
of surface of iron has a greater protective action against 
certain tests when deposited electrolytically than when de- 
posited by the ordinary hot galvanizing process. 

3. The physical and mechanical properties of the bast's 
metal are much less liable to be detrimentally influenced 
when the zinc deposit is given in an aqueous electrolyte 
than when in a hot bath of molten zinc. An illustration of 
the vital importance of this point is found in the case of 
hardened and tempered steel articles which by careful 
manipulation have been given certain qualities required for 
special trade purposes. These properties may conceivably 
be entirely destroyed by the alterations in temperature which 
immersion in molten zinc would necessitate. 

Other advantages, such as greater smoothness of deposit, 
and less liability to loss of metal in dross and waste, have 
also been claimed for the electrolytic process. 

It should also be remembered that, as in most cases of 
metal obtained by electrolysis, electro-zinc deposits have a 
high degree of purity, certainly much higher than many 
grades of commercial zinc possess, and consequently are not 
so liable" to the disintegrating action which impure zinc 
undergoes in the presence of weak acids, alkalies, or even 
water itself (see below). 

Properties of Zinc. Zinc is a bluish-white metal 
closely resembling tin. It is moderately hard and fairly 
* Watt and Philip, Electroplating and Electro-refining, pp. G33, 634. 



THE DEPOSITION OF ZINC 311 

malleable and ductile. It exhibits the latter properties to 
its greatest extent when heated to from 100 to 150 C. 
At a little over 200 C., however, it becomes extremely 
brittle and may be powdered. Zinc is slowly attacked by 
the atmosphere, and according to Davies * it is attacked and 
slowly dissolved by water. The susceptibility of zinc to the 
action of acids largely depends on its degree of purity. 
Pure zinc is only very slowly dissolved by dilute sulphuric 
acid, while if only a small percentage of impurity is present it 
is rapidly dissolved with copious evolution of hydrogen gas. 
The reason for this lies in the fact that the usual impurities 
present, such as lead, tin, iron, and carbon, are more electro- 
negative than zinc itself, and form galvanic couples, over the 
entire surface acted upon by the acid, in which the zinc is 
electropositive to each of the other metals present. A minia- 
ture primary battery is, therefore, set up, arid by electro- 
chemical action, zinc dissolves and hydrogen is evolved from 
the negative elements. The surface of the zinc is thus con- 
tinually being exposed to this action, which continues until the 
metal is completely dissolved. With pure zinc, on the other 
hand, the film of hydrogen formed by the combination of the 
metal with the SO 4 radicle remains on the surface of the zinc, 
and prevents further action by the acid.f 

Zinc is also very soluble, under similar conditions, in 
hydrochloric acid/and also in strong solutions of the alkalies, 
e.g. potassium or sodium hydroxide. In this case also 
hydrogen is evolved and an hydroxide of the metal formed 
which is soluble in excess of the alkali solution. 

The common impurities of commercial zinc are iron, 
lead, cadmium, carbon, and traces of antimony and arsenic. 

Compounds of Zinc. Two oxides of zinc are known, 
the monoxide ZnO, and the peroxide Zn0. 2 . The former is 
the most stable and gives rise to all the commoner zinc salts. 
Of the latter the most important in electro-deposition are the 
chloride and sulphate. 

* Journ. Soc. Chem. Ind., vol. 18 (1899), page 102. 

t Eoscoe and Schorlemmer, Treatise on Chemistry, vol. ii. p. 641. 



3 T 2 ELECTROPL AT I NG 

Zinc chloride (ZnCL,) is a white soft waxlike substance 
usually obtainable in the form of cakes or sticks. It is very 
deliquescent, and soluble both in water and alcohol. When 
dissolved in its own weight of water a clear solution results. 
Dilute solutions of zinc chloride are often opalescent, but 
may be rendered clear by the addition of HC1. The usual 
impurities of trade varieties of this salt are iron, zinc sulphate, 
and traces of the heavy metals as well as arsenic. 

Zinc chloride forms double compounds with the 
corresponding ammonium salt, ZnCl 2 .2(NH 4 )Cl and 
ZnCl 2 .3(NH 4 )Cl. Those double salts have been suggested 
and often used for zinc deposition. 

Zinc sulphate, ZnSO 4 .7H 2 0, commonly known as white 
vitriol or zinc vitriol, is usually obtained as colourless needle- 
like crystals, similar to Epsom salts (magnesium sulphate). 
It is readily soluble in rather less than its own weight of water, 
but insoluble in alcohol (compare the chloride). It is obtained 
on a very large scale commercially by roasting ores con- 
taining zinc sulphide (ZnS) in air, thus oxidizing the sulphide 
to sulphate, afterwards dissolving the latter salt out in water ? 
evaporating, and allowing to crystallize. As usually placed 
on the market it has a high degree of purity ; the usual 
impurities are arsenic and iron. 

Zinc sulphate forms a series of double salts with the 
alkali sulphates having the same general formulae, e.g. 
ZnSO 4 .K2SO 4 .6H 2 O, the double sulphate of zinc and potas- 
sium. Both this salt and the corresponding magnesium 
compound have been largely used for the electro-deposition 
of zinc. 

Solutions for Deposition. A very large number of 
solutions have at various times been tried and used for 
electro- zincing, but though different workers have obtained 
rather variable results, the general consensus of opinion 
amongst practical operators is that those of the sulphate, 
alone or with other salts, give for general purposes the most 
reliable results obtained up to the present, with the minimum 
of trouble in working. 



THE DEPOSITION OF ZINC 313 

Philip* summarizes the result of a series of investi- 
gations which he has made into the question of suitable 
electrolytes for the deposition of sound and adhesive coat- 
ings of zinc upon iron as follows : 

11 Aqueous solutions of zinc sulphate, and of this salt 
mixed with about molecular proportions of sodium sulphate, 
potassium sulphate, ammonium sulphate, aluminium sul- 
phate, and magnesium sulphate, all gave electrolytes from 
which good and adherent deposits of metallic zinc could be 
obtained by electrolysis, but on the whole a solution of zinc 
sulphate and magnesium sulphate in molecular proportions, 
and containing about 30 ounces (avoir.) of zinc sulphate 
per gallon was the solution which yielded the most satis- 
factory results. Zinc deposited from this solution did not 
contain more than a very small trace of magnesium, and it 
is quite possible that the amount detected (0*028 part per 
cent.) may have been due to the small traces of magnesium 
salt dissolved in the electrolyte adhering to the deposited 
metal." 

More recently attempts have been made to improve zinc 
baths by the use of substances as addition-agents, and very 
promising results are being obtained in this direction. 
Notable instances which may be cited are the addition of 
ferrous sulphate (patented by Cowper-Coles) and aluminium 
sulphate, which appears to be largely used in American 
and Continental practice ; also organic additions such as 
glucose or grape sugar, and a class of substances known 
as glucosides, which as additions to zinc baths are patented 
by Classen (U. S. Pat. 809,492, 1906), an example being 
licorice root. 

A point upon which great emphasis must be laid is that 
good results in zinc deposition cannot be obtained from 
solutions which are weak in metallic content. It may be 
taken as a fairly safe generalization that whatever bath be 
used the proportion of metal should not be less than from 4 

* Watt and Philip, Electroplating and Electro-refining of Metals, 
p. 631. 



3 i4 ELECTROPLATING 

to 5 oz. per gallon (25 to 31 gr. per litre). With such or 
a greater strength, current densities of a fairly high value 
(25 to 30 amps, per sq. foot) can be used, and a greatly 
superior quality of deposit obtained than with the lower 
current densities necessitated by poorer solutions. The 
reason for this rather peculiar feature of zinc deposition is 
generally supposed to be due to the extremely electro-positive 
nature of the metal; hydrogen being more easily liberated, the 
proportion of gas to metal is abnormally high with low currents. 

In giving details of the composition of specific baths for 
zinc deposition it will be convenient to adopt the following 
classification, (a) neutral or slightly acid baths, (b) alkaline 
baths. 

(a) Of the former class the sulphate solutions are by far 
the most important, and these will first be described. 

Solution I. (Bichter) 



Zinc sulphate (ZnSO 4 . 7H 2 O) . 50 oz. 



VI7 L 

Water 



f 



1-56 kg. 
5 litres 



or II US 

This solution, as will be noted, is exceptionally rich in 
metal, and should be worked with a current density of not 
less than 25 to 30 amperes per sq. foot. With low currents 
there is a tendency to liberate hydrogen, and render the 
deposit loose and powdery. It is a particularly suitable 
bath for large wrought- or cast-iron work, also for iron or 
steel wire. It is, however, of great importance that the 
anode surface immersed shall be fully equal to if not greater 
in area than the cathode. The temperature of the solution 
also is an important feature in obtaining successful results. 
In any case this should not be below 30 C., and it is ad- 
visable to work at 50 C., or even more. 

Solution II. (Philip) 

Zinc sulphate (ZnSO 4 . 7H 2 0) ..... 30 oz. j 937 gr. 
Magnesium sulphate (MgSO 4 . 7H,O) . . 25 ] 780 



Water 



1 imp. gall, 
or H U.S. 



5 litres 



THE DEPOSITION OF ZINC 315 

This bath, which is typical of a number of other similar 
zinc solutions used in modern commercial practice is really 
a simple aqueous solution of the double sulphate of zinc 
and magnesium, and similar results are obtainable from the 
corresponding potassium compound. It is best worked warm 
at a temperature of from 50 to 70 C. 

Solution III. 

Zinc sulphate 2 Ibs. 1 kg. 

Aluminium sulphate . . . 1J oz. 46-9 gr. 

, TT C 1 imp. gall. 

Water )o 1 1 US 5 litres 

V. ~) ' " I 

4 ozs. (or 125 gr.) of alum may be substituted for aluminium 
sulphate in this solution with practically the same effect, and 
periodical additions of either of these substances should be 
made to the bath as experience indicates, the purpose of 
these salts being to allow currents of a higher density to be 
used in working. 

The influence of aluminium sulphate on zinc baths has 
been already referred to, and it may be of interest to remark 
here that an explanation of the phenomenon offered by a 
recent writer * is that the aluminium salt (A1 2 (SO 4 ) :! ) dis- 
sociates in solution into aluminium hydroxide and sulphuric 
acid. Under these circumstances the former acts as a colloid, 
which moves to the cathode, and influences the size of the 
deposited crystals in the same manner as starch or gum 
arable in an acid copper bath (see page 249). 

Solution IV. (Cowper-Coles patent) 



Zinc sulphate .... 40 ounces 
Ferrous sulphate ... 5 

Water . . \ \TJ?'^' 



1-25 kg. 
156 gr. 

5 litres 

The inventor states that the ferrous sulphate gradually 
becoming oxidized to ferric sulphate by the action of the 
atmosphere takes up acid from the bath, and so tends to 

* Schlotter, Galvanostegie, vol. i. 38-51 (1910). 



316 ELECTROPLATING 

keep it neutral. This solution is used with lead anodes, 
which are insoluble, and the strength of the electrolyte is 
kept up by continually pumping the liquid through scrubbers 
of coke charged with zinc dust or zinc oxide. By this 
method also the ferric salt is once more reduced to ferrous 
sulphate by contact with the zinc dust, and the solution 
consequently maintained at the correct constitution. 

Mr. Cowper-Coles considers that the presence of ferrous 
sulphate tends to prevent the formation of powdery deposits 
which Mr. Arnold Philip * thinks are probably caused by 
the formation of an oxide or hydrate of zinc. It appears to 
us, however, that the action of this salt is very analogous to 
that of aluminium sulphate (see p. 315), and its influence 
on the deposit may, in all likelihood, be similar. 

Other solutions for the deposition of zinc of the same type 
as the above, which have been suggested are, the double 
chloride of zinc and ammonia, the double chloride of zinc 
and sodium or potassium, and one of equal molecular pro- 
portions of zinc chloride and aluminium chloride, but none 
of these present advantages over the sulphate baths. 

(b) Of distinctly alkaline baths for zinc deposition only 
one calls for detailed description, viz. the cyanide bath. 

This bath appears to have been originally introduced by 
Watt, who obtained a patent for it in 1855. It can be made 
either chemically or electrolytically, but the inventor pre- 
ferred the latter method, and carried it out as follows. 

Two hundred ounces of potassium cyanide were dis- 
solved in 20 gallons of water, and to this solution were added 
80 ounces by means of liquid ammonia. The solution was 
thoroughly stirred and nitrated and then electrolysed by 
means of large zinc anodes and small copper cathodes the 
latter enclosed in ferrous cells. Electrolysis was continued 
until the bath had gained a metallic content of about 60 ounces. 

Watt also recommended the addition of 80 ounces of 

* Watt and Philip, Electroplating and Electro-refining of Metals, 
p. 636. 



THE DEPOSITION OF ZINC 317 

potassium carbonate, but the solution works quite well 
without such addition. 

A solution very similar in composition and working 
qualities to the above is made up chemically as follows : 

Zinc sulphate ...... 15 ounces ! 468 gr. 

Potassium cyanide ....... Q.S. 

Ammonium carbonate ... 5 ounces | 156 gr. 

( 1 imp. gall. 



Water 



5 litres 



A strong solution of potassium cyanide is made up con- 
taining 1 Ib. per imp. gallon (100 gr. per litre), and added to 
the zinc salt, which has been previously dissolved in half a 
gallon (2Jr litres) of water, until the white precipitate which at 
first forms is redissolved. The solution must be constantly 
stirred during the process to ensure complete conversion of 
the zinc salt to the double cyanide, and about 10 per cent. 
more cyanide added to form free cyanide. Add then the 
ammonium carbonate dissolved in a little water and, if 
necessary, make up the bulk of the liquid to 1 gallon or 5 
litres, by adding water. 

The cyanide solutions work very well and give good 
results, particularly for small work and thin deposits, but 
they are not suitable for thick deposits, and for larger work 
they are very costly. 

One other alkaline bath may be given brief mention, viz. 
zinc hydroxide (Zn(HO) 2 ), dissolved in excess of caustic 
potash. It is formed very simply by dissolving in water 
sufficient zinc sulphate or chloride, to give a strength of 3 
ounces of zinc per gallon of resulting solution, and adding 
a strong solution of caustic potash until the precipitate 
which first forms is redissolved. 

During the last few years, a number of patents for 
solutions for zinc deposition have been taken out both in 
Europe and America. Very few of these, however, possess 
any features of interest or novelty ; most are based on ad- 
ditions to the sulphate bath, such as sodium sulphate, sodium 



318 ELECTROPLATING 

chloride, and salts of ammonium, aluminium, etc. The one 
possessing greatest novelty is that of Dr. Kern, who has 
patented a fluosilicate bath analogous in composition to 
those already described for copper and nickel. The formula 
recommended is 

Zinc fluosilicate 12 parts by weight 

Aluminium fluosilicate . . . 10 ,, 
Ammonium fluoride .... 5 ,, ,, 
Water 100 

with the addition of small proportions of grape sugar. 

Anodes. Except in cases where the supply of metal 
into the electrolyte is regulated by special methods, as in 
the Cowper-Coles process to be described later, anodes for 
zinc-plating should be of the purest zinc obtainable, and it 
will usually be found advantageous to procure them in the 
form of cast plates, f -inch thick or more, so that their current- 
carrying capacity is high. Lead is the commonest impurity 
of zinc, and it is very difficult to procure the latter " lead- 
free." Fortunately, however, this impurity is not important, 
and there is now no difficulty in getting metal of 98 to 99 
per cent, purity, so that other metals present are only in 
very low proportion. 

Current Conditions. The voltage required in zinc 
deposition varies somewhat according to the composition and 
temperature of the electrolyte, zinc sulphate baths requiring 
rather a higher value than some others. In most cases, 
however, from 4 to 6 volts will be found satisfactory. 

The current densities in general use range from 25 to as 
high as 45 amperes per square foot. The sulphate baths, as 
a rule, give excellent results with C.D.'s of approximately 
30 amperes per square foot. 

Management of Zinc Solutions. The most important 
point in the control of the electrolytic deposition of zinc is to 
keep up the strength of the bath in metallic content. In 
stagnant solutions this is a matter of some little difficulty, as 



THE DEPOSITION OF ZINC 319 

any appreciable degree of free acid is not allowable. Cowper- 
Coles, in connection with the solution No. IV. described on 
p. 315, has devised and patented the method there detailed 
of overcoming this difficulty, viz. by continually pumping the 
electrolyte from the vat during electrolysis and forcing it 
through coke scrubbers containing a plentiful supply of zinc 
oxide or zinc dust. The solution is thus not only kept fully 
charged with metallic zinc, but, for the same reason, pre- 
vented from becoming acid. The pumping arrangement is 
so devised that the level of the solution inside the vat is 
kept practically constant, but as the electrolyte is denuded of 
its metal at the cathode it is taken off, pumped through the 
zinc dust, and enters at the other end of the bath. Philip, 
whose investigations on the subject of zinc deposition have 
already been referred to, points out in a discussion of the 
Cowper-Coles process, that the zinc solution could under 
similar conditions be kepti saturated by pumping it through 
scrubbers containing zinc and copper or zinc and carbon in 
intimate contact the electric couple thus formed setting up 
local action and neutralizing the acid present with solution 
of zinc. Methods of this description are not patented. 

The main advantage claimed for the Cowper-Coles pro- 
cess is that the use of zinc dust is considerably cheaper than 
an equivalent of zinc in the form of any of its salts. It is a 
matter of some doubt, however, whether on the basis of 
present-day prices this claim could be substantiated to any 
great extent. 

It may be pointed out that the zinc bath can also be kept 
neutral by suspending zinc carbonate at various points in 
contact with the liquid, particularly if agitating arrangements 
are employed. The salt can readily be prepared in the 
workshop by first dissolving zinc in sulphuric or hydrochloric 
acids, and precipitating as carbonate by adding a strong 
solution of washing-soda crystals. 

In attempting to obtain thick deposits of zinc con- 
siderable difficulty is often experienced through the tendency 
which seems to be inherent in all these solutions to deposit 



320 ELECTROPLATING 

the metal in a spongy tree-like condition, particularly on the 
edges or extruding points of the cathode. The best method 
apparently available at present to overcome this trouble is to 
use an organic addition agent such as grape sugar. A 
fruitful field of investigation, however, lies open in this 
direction. 

Special Treatment of Articles for Electro-zincing. 

For zinc deposition the electroplater is often, indeed 
usually, called upon to deal with one of the most difficult 
and troublesome basis metals known to platers, i.e. cast iron. 
The porous nature of such surfaces combined with the 
difficulty often encountered of removing scales and oxide 
render the problem of preparation no easy one to solve. 
Electrolytic cleansing and pickling are now usually resorted 
to as described on p. 155, the sodium sulphate bath being 
very useful, with arrangements for reversing currents. 

Probably, however, the best results are obtained by 
combining these methods with sand-blasting ; the sand blast 
should be used immediately prior to immersion in the de- 
positing vat. 

Philip in the treatment of high- carbon steel wire adopted 
the expedient of cleansing by running it as anode through 
a preliminary vat of zinc sulphate solution immediately 
before it entered the depositing vat proper. Cowper-Coles 
describes * a method, based on the same principle, in which 
the articles are immersed in the zincing vat in the ordinary 
manner, but for the first 2 J minutes they are made" anodes 
instead of cathodes, the current being reversed; after that 
period the direction of the current is again changed, and the 
deposit takes place in normal fashion ; the adhesion of the 
zinc coating was found to be considerably better than in 
the case of plates treated in the ordinary manner. 

Testing Zinc Deposits. Several methods have been 
designed for testing the quality and thickness of zinc 

* Electrician, vol. xliv., 1900, p. 434. 



THE DEPOSITION OF ZINC 321 

deposits, but as a general rule these tests are only relative, 
and are thus of value mainly as a means of roughly com- 
paring the thickness of a number of different specimens of 
zinc-plated iron articles. The best known of these "tests," 
and probably the most generally convenient for workshop 
practice, is that suggested by Sir W. H. Preece. This test 
has been slightly modified by Mr. Arnold Philip, and the 
following description is that given by this authority. " The 
zinc-coated iron is immersed in a saturated solution of copper 
sulphate at a temperature of 15 C. for one minute, then 
immediately removed, and placed under a rapidly running 
stream of water from a tap in which it is well shaken. In 
this way is removed any of the loose flocculent deposit of 
copper which has been formed on the surface of the zinc by 
zinc displacing the copper from the copper sulphate solution, 
but if the zinc has been so far removed as to expose the 
surface of the underlying iron to the action of the copper 
solution a much more coherent deposit of bright copper is 
formed on the iron which is not removed by shaking under the 
water stream. The number of successive times, therefore, 
that a zinc-coated piece of iron will withstand this treatment 
is a measure of the thickness and regularity of the zinc 
coating." 

The copper sulphate solution must only be used for one 
immersion and then thrown away, as of course it becomes 
contaminated with zinc. In the case of steel goods it 
should be noted that the copper deposited on such surfaces 
when revealed to the action of the solution can sometimes 
easily be removed by rubbing with the finger no steps 
should, therefore, be taken to remove the deposit of copper 
other than shaking under running water. If the whole of 
the zinc is not removed, the copper is easily washed away 
by this treatment. 

Mr. Philip has found that the protective effect of the zinc 
depends upon how it has been applied, and states that the 
same iveight of zinc per unit of surface has a greater 
protective action against the Preece test when deposited 



322 ELECTROPLATING 

electrolytically than when deposited by the ordinary " hot- 
galvanizing" process.* 

The Deposition of Cadmium. 

This subject is at present of academic rather than of 
practical interest, very few commercial applications having 
been found for the metal from an electroplating point of 
view. Cadmium possesses, however, some very useful pro- 
perties, and there is at any rate the probability that in the 
future its electro-deposition will find some useful applica- 
tion. 

It may be of interest to observe that a few years ago one 
of the present authors in a series of experiments dealing 
with the deposition of the principal white metals of commerce 
electroplated a number of small trays with a coating of each 
of the following metals, silver, nickel, cobalt, zinc, tin, lead, 
and cadmium, and exposed these for some months to 
ordinary atmospheric influence in various rooms. Several 
interesting results were obtained bearing on the action of the 
atmosphere on electro-deposited metals, but a point of great 
interest relating to cadmium was that, when polished, the 
deposit of this metal had a colour more nearly approaching 
that of silver than any of the others, and retained its polish 
much longer than silver without tarnishing or discolouring. 

The metal is rather high in price, but as it occurs fairly 
abundantly in nature this should be reduced if a steady 
demand arose. 

Properties of Cadmium. Cadmium resembles zinc 
very closely both in physical, mechanical, and chemical 
properties. It is a shade whiter in colour than zinc, but has 
a slightly bluish cast. It is very malleable and ductile at a 
normal temperature, but when heated becomes brittle. When 
polished it resembles tin, but takes a more brilliant polish 
than this metal and is somewhat denser. Cadmium is not 
* Watt and Philip, Electroplating and Electro-refining of Metals, 
p. 634. 



THE DEPOSITION OF CADMIUM 323 

attacked by air at ordinary temperatures and is only slowly 
dissolved by strong acids. For the purpose of preparing 
electrolytic solutions, it is most conveniently dissolved in 
dilute nitric acid (1 acid, 1 water). 

Compounds of Cadmium. The salts of cadmium are 
closely analogous to those of zinc. The principal ones are 
the nitrate, sulphate, chloride, and carbonate. The dis- 
tinguishing feature of cadmium is its formation in chemical 
reaction of a yellow sulphide insoluble in alkalies. It can 
thus be tested for in alkaline solution by the addition of 
ammonium sulphide or sulphuretted hydrogen gas, and 
distinguished from all other metals by this yellow precipitate. 
For making electrolytic solutions the nitrate is most com- 
monly employed as a starting-point ; formula 

Cd(N0 3 ) 2 .4H 2 0. 

Solutions for Deposition. The most successful solu 
tions for the electro-deposition of cadmium are those of the 
cyanides. Solutions of the sulphate, alone or in combination 
with other salts, have often been tried, and some operators 
have claimed good results therefrom, but for most classes of 
work the double cyanide of cadmium and potassium will be 
found most reliable. 

As far back as 1849, Russell and Woolrich obtained a 
patent for a cyanide solution for the deposition of cadmium, 
and the method they adopted for making the solution is as 
convenient a one as could be devised, viz. to prepare a solu- 
tion of cadmium nitrate either by dissolving the metal in 
dilute nitric acid or by dissolving the salt directly. Add to 
this a solution of sodium carbonate until no further precipitate 
is produced. Stir vigorously and wash the precipitate with 
warm water, allow to settle, and decant the clear liquid. 
The compound thus obtained is a normal carbonate of 
cadmium. Prepare now a strong solution of potassium 
cyanide (1 Ib. per imperial gallon, or 100 gr. per litre) 
and add this slowly with constant stirring until the whole 
of the cadmium salt is dissolved and a clear liquid results. 



3 2 4 ELECTROPLATING 

A further addition of about 10 per cent, must be made for 
free cyanide, and after boiling the solution is ready for use. 

The strength of the bath may be varied considerably, 
but it is not wise to attempt to work a cadmium solution 
weak in metallic content. The following proportions will be 
found satisfactory : 

Cadmium nitrate . . . . . . 1 Ib. | 500 gr. 

Sodium carbonate ....... Q.S. 

Potassium cyanide ...... Q.S. 



VK7 J. 

Water 



K Vi. 

5 litres 



If metallic cadmium is used 5J ounces (170 gr.) will be 
required to yield the above proportion of the nitrate. 

The bath may also very conveniently be formed electro - 
lytically in the manner described for silver (page 184). The 
electrolyte should be made up by dissolving 1J Ibs. of 95 per 
cent, potassium cyanide per imperial gallon of water (or 
125 gr. per litre). The anodes should be of a fair thickness, 
say \ or f of an inch, and it will be found convenient to use 
strong strips of lead as cathodes enclosed in porous jars also 
containing cyanide solution. 

The only objection to this method as in the case of silver 
is the difficulty of adjusting exactly the proportion of free 
cyanide a large excess must be avoided, since in this case 
there is a decided tendency to roughness of deposit. 

Current Conditions. The voltage usually advised for 
cadmium deposition is 3 to 4 volts, but good deposits can be 
obtained with lower values than these figures, particularly if 
the solution is used warm. It is, in fact, advisable to employ 
as low a voltage as possible, otherwise the deposit is liable to 
be rough and crystalline. 



CHAPTER XV 
THE DEPOSITION OF LEAD, TIN, AND ANTIMONY 

AT the present moment, and writing from an electroplating 
point of view only, the three metals dealt with in this 
chapter here, with the exception possibly of tin, are of 
comparatively little interest for the practical worker. 

It is quite possible and even probable, however, that the 
immediate future will witness an increase of the commercial 
possibilities of electroplating with these metals, and some 
little space should therefore be devoted to an outline of the 
principal methods of their deposition. 

Deposition of Lead. 

The electro-deposition of this metal has received consider- 
able attention in modern times from the refining point of 
view, several processes for the electrolytic refining of lead 
having been worked with more or less success. The greatest 
difficulty has been found in the choice of a suitable electro- 
lyte, owing to the peculiar and characteristic tendency of this 
metal to deposit in tree or fern-like crystals from simple 
solutions of its salts, a familiar illustration of which is found 
in the old experiment of growing a " tree " by the simple 
immersion of a strip of zinc in a strong solution of lead 
acetate. On electrolysis of lead solutions similar effects are 
obtained. 

Properties of Lead. Lead is a very soft metal of a 
bluish-white colour, and when freshly exposed to the atmo- 
sphere presents a bright metallic lustre. It speedily oxidizes, 



326 ELECTROPLATING 

however, to a slight extent, and is covered with a dull film 
after a short exposure in air. It can easily be rolled to 
extreme thinness, but it cannot be drawn into wire. If 
repeatedly melted, lead becomes hard and brittle, due, 
according to some authorities, to the formation of oxide. 
Lead containing also small percentages of impurities, notably 
antimony, zinc, bismuth, and arsenic, is decidedly brittle. 
The most important property of lead from the point of view 
of use as a deposited coating, is its power of withstanding 
water and most acids to an appreciably greater degree than 
most of the common metals. It is this latter property which 
is likely to lead to its adoption as a protective coating to 
some of the harder metals and alloys for particular purposes. 

Compounds of Lead. The most important of lead 
compounds is the monoxide (PbO) commonly known as 
" litharge," though as many as five different oxides are 
known. Of the salts of lead the best known are the chloride 
(Pb01 2 ), the nitrate (Pb(NO 3 ) 2 ), the carbonate (PbC0 3 ), and 
the sulphate (PbSO 4 ). Other salts which have been 
brought into prominence in electrolytic practice recently are 
the fluosilicide (PbSiF 6 ) and the perchlorate (Pb(01O 4 ). 2 .3H 2 O). 

Solutions for Deposition. One of the oldest published 
formulae for lead deposition is the following : 

Litharge (PbO) ... 5 parts by weight 
Caustic potash .... 50 ,, 
Water 1000 

The caustic potash is dissolved in the water, the solution 
raised to boiling point, and the powdered litharge added ; 
boiling is continued until a clear solution results. It is very 
difficult, if not impossible, however, to obtain a deposit of 
any appreciable thickness from this bath, though it is quite 
suitable for thin coatings. 

In our experience the best solution at present available 
where thick deposits are required is that used in the Betts 
process of lead refining by electrolysis. This solution 



DEPOSITION OF LEAD 327 

consists of an aqueous solution of lead fluosilicide with about 
10 per cent, of free hydrofluoric acid. Generally, however, 
a small percentage of glue or gelatine is added to prevent or 
reduce the tendency, which, even in this electrolyte, is 
evident, to the formation of " trees " on the cathode edges. 

For hydrofluoric acid, pyrogallic acid is occasionally 
substituted with beneficial results. 

The following formula has been found by one of our 
colleagues to yield an excellent deposit in continuous electro- 
lysis for upwards of 60 hours : 

Lead fluosilicide 8 oz. 250 gr, 



Pyrogallic acid 1 ,, 

Glue 1 

Water $ 1 imp. gall. 

Watei (or U U.S. 



31-2 
31-2 

5 litres 



The anode readily dissolves in the electrolyte and, when 
pure lead is employed, no slime is formed. 

Some very good results have recently also been obtained* 
from solutions of lead perchlorate in water. Such an electro- 
lyte is an extremely good conductor and yields a beautifully 
smooth coherent deposit. Mathers has carried out experi- 
ments with the bath, but finds that the best results are only 
obtained when a small proportion of peptone is added. 
These experiments, it may be remarked, simply bear out the 
experience of most investigators in this direction, that the 
use of some addition agent is absolutely necessary in lead 
baths to prevent treeing. 

The proportions of the bath recommended by Mathers 
are as follows : 



Lead perchlorate [Pb(C10 4 ). 2 3H.,O] . 1 Ib. 
Perchloric acid (HC10 4 ) i 



500 gr. 
250 , 



Peptone 0-05 per cent. 



Water 



\ Y^n'<f "' 
(or H U.S. 



5 litres 



'5 

* Transactions of Amer. Electro-chemical Society, vol. xvii. (1910), 
p. 261. 



328 ELECTROPLATING 

The effect of the peptone gradually wears off as the bath 
is worked, and further similar additions must be made 
about every four days. 

The constituents of the bath should be freshly prepared 
as required, and the following directions are taken from 
the paper to which reference was made above. 

Perchloric acid is formed from sodium perchlorate by 
treating with excess of concentrated hydrochloric acid. 
The mixture is filtered through asbestos, and the residue, 
which is sodium chloride (NaCl), is washed with a further 
small quantity of concentrated HC1. The filtrate consists 
of an aqueous solution of perchloric acid, hydrochloric acid, 
and a small proportion of sodium perchlorate. By heating 
to 135 C. the hydrochloric acid is volatilized, leaving an 
almost pure solution of perchloric acid. 

Lead perchlorate is formed by neutralizing this acid 
with litharge (lead monoxide). 

With the bath as above formed and with careful periodic 
additions of peptone, current densities up to 27 amperes 
per square foot can safely be used. 

Anodes. Anodes for lead-plating should be as pure as 
possible. Electro-negative impurities, which may easily be 
present, readily find their way into the electrolyte and are 
accordingly deposited, with material effects, on the quality 
of the deposit. In either of the two baths last described 
pure lead anodes are readily soluble, so that the metallic 
content of the solution is continually replenished,- and 
obviously the degree of purity of the latter is dependent 
entirely upon that of the metal of the anode itself. 

Nobili's Rings, or Electrochromy. A peculiar pheno- 
menon of some lead electrolytes is their tendency to deposit 
peroxides of lead on the anodes. These peroxide films, if 
produced under correct conditions and in an extreme degree 
of thinness, give most beautiful colour effects. Nobili was 
the first to observe this peculiarity, and the production of 
these effects is now known under his name. 



DEPOSITION OF TIN 329 

A good solution for the purpose is that proposed by 
Becquerel and made by dissolving litharge in a solution of 
caustic potash. 

Becquerel's formula is as follows : 



Litharge 10J oz. 

Caustic potash 14 ,, 



328 gr. 
436 

5 litres 



Water 1 imp. gall. 

The required weight of caustic potash is dissolved in 
water, the solution raised to boiling point, and the litharge 
added slowly with constant stirring. 

The articles to be treated are prepared exactly as if for 
plating and suspended in the solution from the anode rod, 
the cathode being a piece of platinum or copper wire. The 
films of colour are produced very quickly, being successively 
yellow, green, red, violet, and blue. The current must be 
low and adjusted according to the distance between the 
electrodes. Too high a current or too long immersion 
completely spoils the colour effects. 

Variations of the patterns formed by the colours, can be 
made by introducing cardboard discs with perforated designs, 
between the anode and cathode. 

Some little practice and experience is, however, necessary 
to obtain good results in this field. Each difference of 
shape or size in the article treated demands a variation in 
current conditions or time of immersion, and the correct 
values can only be determined by experiment. 

Deposition of Tin. 

Tin is largely used as a protective coating to iron and 
steel goods, but in the case of a large majority of such 
articles it is applied by the simple method of dipping the 
work, after previous cleansing, into a bath of molten metallic 
tin. This is both a simpler and cheaper method of deposit- 
ing tin than processes involving the electro-deposition of 
the metal from aqueous solutions. In spite of this, however, 
a good deal of electro-tinning is carried on in the Midlands ; 



330 ELECTROPLATING 

its application being mainly to small goods and to some 
extent to providing an intermediate coating to articles of iron 
and steel which are to be subsequently silvered or nickelled. 

Properties of Tin. Tin is a very lustrous white metal 
which is not acted upon by air, hence its suitability as 
a protective coating to more readily oxidizable or tarnishable 
metals. It is malleable and ductile, can be beaten out into 
leaf (tin -foil) or drawn into wire. If, however, it is heated to 
just over 200 C. it becomes curiously brittle and may be 
powdered. With regard to hardness it comes between zinc 
and lead, being harder than the latter metal but not quite so 
hard as the former. 

Tin is readily attacked by nitric acid of a specific gravity 
of 1-24, but the strongest pure nitric acid (sp. gr. 1-5) is with- 
out action upon it. It is slowly soluble in dilute nitric acid. 
For the requirement of the electroplater tin is usually best 
dissolved in strong hydrochloric acid; stannous chloride 
being formed with the liberation of hydrogen. From this 
salt as a starting-point most electro-tinning solutions are 
made. 

Solutions for Deposition. A large number of solu- 
tions have at various times been suggested for the electro- 
deposition of tin, and the choice of a solution depends largely 
upon the particular kind of work to be done and the condi- 
tions with regard to temperature of working and current 
available. Any of the baths given below will yield good 
results if made and used according to the directions outlined. 

Formula I. 

Metallic tin 2 oz. | 62-5 gr. 

(converted into stannous chloride by dissolving in hydro- 
chloric acid) 
Pure potassium hydroxide (caustic potash) . 4 oz. j 125 gr. 



Water 



1 imp. gall. 



5 litres 



or li U.S. 
Just sufficient acid should be used to dissolve the tin ; 



DEPOSITION OF TIN 331 

and the potassium hydroxide, previously dissolved in 2 quarts 
of water, is then added. A precipitate of stannous hydrate 
is first formed and then redissolved. If required, a further 
quantity of potash may be added to effect complete solution. 
The bulk is then made up to 1 gallon by a further addition 
of water as necessary, and boiled for a short time before use. 
Formula II. (Eoseleur) 



Stannous chloride . . . . 1 oz. j 31-2 gr. 
Pyrophosphate of soda . . 10 ,, 312 

TIT j. ( 5 i m P- I 

Wafcer lor 6 U.S. 



25 litres 



The pyrophosphate of soda is dissolved in the water and 
when solution is complete the tin salt is added. The best 
method of adding the latter is to enclose it in several muslin 
bags and hang these just under the surface of the liquid. 
Stannous chloride is soluble with difficulty in the pyrophos- 
phate solvent, and this is practically the only way to ensure 
its complete solution. 

This bath is decidedly one of the best, particularly for thin 
coatings of tin. The objection principally made with regard 
to it is its comparatively small content of metallic tin. It is 
this which renders it unsuitable for thick deposits ; but it is 
very largely used for electro-tinning where only thin films 
are needed. 

It is best worked warm and requires a voltage of about 
3 volts. 

Formula III. 



Stannous chloride . . . . J oz. 
Potassium cyanide . . . . 3| ,, 
Potassium carbonate . . . 30 ,, 

Water $ 2 * imp> galls< 

Watei or3U.S. 



15-6 gr. 
109 
937-5 gr. 

12 litres 



The bath is made up by dissolving the tin salt in sufficient 
water, then adding the potassium cyanide and finally the 
potassium carbonate, each previously dissolved in water. 



332 ELECTROPLATING 

Further additions of water are made to bring up the required 
bulk. 

The above solution is representative of several others in 
which potassium cyanide is used. They are not as a rule 
very good conductors, but with a fairly high voltage good 
deposits can be obtained. Their most suitable application lies 
in the treatment of articles which are to be tinned simply as 
a preliminary coating to some further deposit of another 
metal. 

Other solutions which deserve mention are those com- 
posed of the double chloride of tin and ammonium and the 
double oxalate of tin and ammonium. The latter of these 
compounds gives the best results. 

The following formula is based upon that given by 
Classen for the electrolytic separation of tin in electro- 
chemical analysis : 

Tin chloride (Crystallized salt) 4 oz. | 125 gr. 
Ammonium oxalate .... 9 I 280 ,, 
Oxalic acid \ 15-6 

Water \ Vi^'a 8 * 11 ' 5 lifcres 

(orlJU.S. 

Dissolve the tin salt in sufficient water and the ammo- 
nium oxalate and oxalic acid together in half a gallon (or 2J 
litres) of water. Add the latter to the tin solution with 
vigorous stirring. The white precipitate which first forms 
will redissolve, but the solution is rarely quite clear though 
sufficiently so for practical purposes. Add the remaining 
water required and boil the liquid for a short time. 

This solution yields good deposits and possesses the dis- 
tinct advantage that a tin anode dissolves comparatively 
freely in the electrolyte. 

Most tinning baths recommended, for example formulas 
I. and II., require periodic additions of tin salt to keep up the 
strength of the bath. 

The Management of Tinning Baths. When, as is 



DEPOSITION OF TIN 333 

largely the case m practice, electro-tinning simply means a 
thin coating sufficient to present a good appearance, there 
will be found little difficulty in working any of the foregoing 
solutions. If, however, deposits of any appreciable thick- 
ness are required, several difficulties arise. The deposit 
from ordinary baths has a very great tendency to become 
crystalline and brittle, and this is more decided, the longer 
the immersion. In this connection the influence of addition- 
agents has been largely studied during recent years, and, as 
is the case with lead, it appears almost essential to make 
some such addition to the bath to obtain good results. 

Glue (or gelatine) is a very successful agent for this 
purpose, an addition of Ol per cent, having a remarkable 
effect on the character of the deposit, and at the same time 
allowing the use of a higher current density. 

Other addition substances which have been recommended 
include glucose, saccharine, acetone, and the organic salts of 
aluminium or iron, but it must be noted that the effects of 
such agents are not permanent, and further additions must 
be made from time to time as found advisable. 

Tinning by Simple Immersion. The use of simple 
immersion processes of tinning is fairly widespread. Tin is 
a very useful metal as an ornamental coating to small iron 
or copper or brass articles such as hooks, eyes, pins, buttons, 
etc., and consequently a demand exists for a simple method 
of producing tin deposits on such articles. One of the most 
common solutions for this purpose, and a very good one, is 
prepared by dissolving cream of tartar in water, using as 
much of this salt as the quantity of water taken will dissolve ; 
add about -J- an ounce of stannous chloride to each gallon of 
the liquid and raise to boiling point. The articles to be 
treated should be contained in a tin sieve or the solution may 
be placed in a strong solid tin vessel and the articles agitated, 
as Langbein suggests, with a tin rod. 

Another very simple bath is that proposed by Eisner, 
which, with copper or brass goods, yields reliable results. 
It consists of J of an ounce each of sodium chloride and tin 



334 ELECTROPLATING 

chloride dissolved in 1 gallon of water (or 7'8 gr. of each per 
litre). This solution also is used hot. 

For iron articles a solution of tin chloride in alum is often 
employed. About 5 ounces of alum (ammonium alum is 
best) are dissolved in 1 gallon of water, and about J an ounce 
of tin salt added. 

In cases where a rather better class of deposit is required, 
articles for simple immersion tinning in the above or similar 
baths should be placed in contact with pieces of zinc. In 
this way a quicker action ensues owing to electro-chemical 
action, and a stronger and more durable deposit results. 

Articles for simple immersion tinning must of course be 
as thoroughly and systematically cleaned as for the separate 
current process. After treatment in the tinning bath they 
are generally dried and polished by shaking with sawdust in 
a tumbling barrel revolved either by hand or by power, as 
shown in Fig. 52. 



Deposition of Antimony. 

The deposition of antimony is rarely practised, but as 
this metal possesses a few properties which render it useful 
for certain purposes, and which might ultimately prove of 
value in the arts, a brief outline of the commonly known 
processes for its electrolytic deposition may be useful to the 
student. 

Properties. Antimony is a fine lustrous silver-white 
metal. It is hard and extremely brittle, and can readily be 
powdered. It is practically unaffected by exposure to air at 
ordinary temperatures. Under similar conditions also it is 
unaffected by dilute sulphuric acid. Nitric acid converts it 
into a white powder namely, oxide the exact composition 
of which varies according to the strength of the acid. Per- 
fectly pure antimony is somewhat difficult to dissolve, but 
the commercial variety is readily dissolved by hot hydro- 
chloric acid, also in the cold by aqua, regia. The common 



DEPOSITION OF ANTIMONY 335 

impurities of the metal are arsenic, iron, lead, copper, traces 
of silver and gold, also sulphur. 

The most common compound of antimony is the tri- 
chloride (SbCl 3 ), but other salts which have been used in its 
electro-deposition are the double tartrate of antimony and 
potassium (tartar emefi<c), the double chlorides of antimony 
and the alkalies, and the corresponding double fluorides. 

Solutions for Deposition. The best known solution 
for the deposition of antimony is the tartrate. It is made up 
very simply according to the following formula : 

Double tartrate of antimony and potassium . . 4 Ibs. 

Hyd.rocloric acid 2 

Water 1 

Water and hydrochloric acid are mixed in the above propor- 
tions and the antimony salt slowly added. 

This solution gives good results, but like most antimony 
baths only a comparatively low current density is allowable 
about 5 amperes per sq. foot. 

The following solution, due to Eoseleur, also yields a 
good deposit, but must be worked hot practically boiling. 



Antimony tersulphide . . . . i Ib. 
Sodium carbonate ..... 1 



Water 



[orliU.8.,, 



250 gr. 
500 

5 litres 



The sodium carbonate is dissolved in the water, the antimony 
salt added, and the whole boiled together for an hour or so. 
Below boiling point the solution tends to throw down a pre- 
cipitate ; hence the requirement that it should be used .hot. 

Deposited antimony obtained from the foregoing solutions 
is rather gray in colour, not so white as the ordinarily occur- 
ring metal. It will, however, take a high polish and retain 
its colour for a considerable time. 

A very peculiar phenomenon in the electro -deposition of 
antimony is the occurrence of explosive antimony. This was 
first noted and has been extensively studied by Gore. He 



336 ELECTROPLATING 

obtained from a solution of 1 part of antimony chloride and 5 
parts hydrochloric acid (and other similar solutions) a deposit 
of amorphous antimony which under some conditions 
changes to the crystalline variety, and develops an intense 
heat, sometimes to an explosive degree. The cause of this 
is said to be due to the presence of antimony chloride in the 
deposit itself. The phenomenon is referred to here as show- 
ing how unsuitable the chloride is for ordinary requirements 
in the deposition of antimony. 

It is interesting to note that while the bromide and iodide 
compounds of antimony have the same tendency as the 
chloride to give explosive deposits (though in less degree), 
the fluorides do not give such results. This point suggests 
the possibility of the employment of the fluorides in anti- 
mony deposition, particularly if an addition-agent was also 
employed. This, however, demands further investigation. 

Anodes. The anodes employed in antimony deposition 
should be of the pure metal, preferably cast. Some writers 
recommend platinum, but the use of this metal is inefficient 
and at the present time out of the question by reason of its 
cost. 

General Remarks. Antimony deposits require careful 
treatment after withdrawal from the vat. The deposited 
metal readily stains, and if scratch-brushed a fine wire brush 
should be used. It is better, however, to brush lightly over 
with fine whiting and water and then transfer to the polish- 
ing lathe for any further treatment. 

One application of this metal in electro-deposition which 
might well be further extended lies in the treatment of 
articles for metal colouring. The films of deposited antimony 
impart very pleasing tones to silver goods in cases where 
artistic decorative finishes are required. The first solution 
outlined is a very reliable one for this purpose and is not 
difficult to manage. Delicate differences of "tone " may be 
readily obtained by varying the time of immersion. 



CHAPTER XVI 

THE DEPOSITION OF PLATINUM AND 
PALLADIUM 

THE constant and great increase in the price of these metals 
during the last decade or so has strongly militated against 
the application of their electro-deposition in many directions 
in which but for their cost they could be very usefully 
employed. Particularly is this the case in giving ornamental 
and at the same time protective coatings to silver and silver 
alloys. Still the subject of the deposition of these metals is 
one of some importance owing to their peculiar properties of 
withstanding so completely many of the most powerful 
chemical reagents known. It is these properties, indeed, 
which have given rise to one of their most useful applications 
in industry, i.e. the manufacture of chemical apparatus. In 
this field also is found their greatest use from the point of 
view of their electro-deposition, particularly with platinum. 

The two metals are very closely akin in physical and in 
many chemical properties, and generally occur together in 
nature, pure palladium being often found in platinum ore. 
Platinum, however, is much the more important of the two. 

Deposition of Platinum. 

Properties of Platinum. The pure metal is tin- white 
in colour with a greyish cast. It is fairly soft, being similar 
in this respect to copper, though when electro- deposited 
from the phosphate solution described below, it appears 
hard, like nickel. Next to gold and silver it is the most 

z 



338 ELECTROPLATING 

malleable of metals. Its great power of resisting chemical 
reagents has already been referred to. In this respect it is 
superior to gold. No single acid will dissolve it, but like 
gold it is soluble in aqua reffia, giving rise when the solu- 
tion is crystallized to the formation of platinichloric acid 



Platinum as obtained in commerce is rarely if ever pure ; 
it contains up to 2 per cent, of iridium (a metal belonging to 
the same chemical group), and thus alloyed it is even more 
useful in the arts, being more impervious still to the action 
of acids. This peculiarity has led recently to the suggestion 
of the feasibility of depositing alloys of platinum and iridium. 

Compounds of Platinum. The principal compounds 
of platinum from the point of view of the electroplater are 
platinichloric acid, previously referred to, which is very 
soluble in water, platinic chloride (PtCl 4 ), potassium chloro- 
platinate, K 2 (PtCl 6 ), and the corresponding ammonium com- 
pound (NH 4 ) 2 (PtCl 6 ), usually known as ammonium platini- 
chloride. 

Solutions for Deposition. The solution in our expe- 
rience most generally reliable for the deposition of platinum 
for decorative purposes, where a comparatively thin coating 
is sufficient, is that introduced by Eoseleur and made up as 
follows : 



Metallic platinum 1 oz. 

Ammonium phosphate . . . .12 ozs. 
Sodium phosphate (NaJIPOJ . 4 Ibs. 

Water J 1 imp. gall. 
{or II U.S. 



31-2 gr. 
375 
2kg. 

5 litres 



The platinum must be dissolved in a sufficiency of aqua regia 
and evaporated until the solution can be crystallized out (see 
Chap. X. p. 223). The crystals must then be dissolved in 
distilled water, say one quart, meantime the ammonium and 
sodium salts should be dissolved, the former in one quart 
and the latter in two quarts of water. The ammonium 



DEPOSITION OF PLATINUM 339 

phosphate is now added to the platinum solution, a dense 
lemon-yellow precipitate being produced. This should be 
disregarded and the sodium salt added with constant stir- 
ring. A practically clear solution will result. This solu- 
tion must now be boiled to expel any free ammonia and to 
improve its working qualities. It is then ready for use. A 
further addition of water will be necessary, however, to make 
up for loss by evaporation. This bath, as most others for 
platinum deposition, is worked hot with a voltage of about 
4 volts. 

It will be found necessary from time to time to make up 
a new solution in the same way, as the bath becomes ex- 
hausted owing to the insolubility of platinum anodes. The 
authors have found this a better plan than making additions 
of platinum salt, the exhausted solution being boiled down to 
a small bulk and added to the new one. 

It may be of interest to observe that (about 15 years ago 
when the metal was considerably lower in price) one of the 
authors worked a similar solution to the above for some time 
for applying decorative coatings to silver articles. The 
deposited metal has an exceedingly fine artistic appearance 
a steel-gray colour the tone of which can be slightly 
varied by altering the distance between anode and cathode. 
On chased or embossed surfaces, particularly those in fairly 
high relief, some very pleasing effects were also obtained 
by partially gilding the raised portions after coating with 
platinum. The procedure adopted was, first, to coat the 
entire surface with a thin deposit of platinum, and then to 
" stop-off" the groundwork of the ornament and the plain 
surface with a varnish, such as is described on p. 239, so 
revealing only the portions to be gilt. The article was next 
rinsed in weak caustic potash, and rapidly passed through 
an alkaline copper solution (see p. 253), thus imparting an 
extremely thin film of copper. It was finally immersed in the 
ordinary gilding bath for a short time, dried out through hot 
water, the varnish removed by benzene, and scratch-brushed 
by means of a very fine German silver wire brush. This method 




340 ELECTROPLATING 

was found preferable to the converse process which is pos- 
sible, i.e. first coating the article entirely with gold, stopping 
off the raised portions and depositing the platinum over the 
gold on the revealed surface. The colour of the deposited 
platinum was not so good. 

Another solution which can be recommended to give 
good results is Bottger's formula, as quoted by Langbein. 
The platinum salt used in this instance is ammonium platinic 
chloride. The following directions are those given by Lang- 
bein (slightly modified). 

Dissolve 15 oz. of citric acid in | imp. gallon (or 0-6 
U.S. gallon) of water. Add caustic soda to this until the 
acid is quite neutralized ; raise the resulting liquid to boiling 
point, and add with constant stirring 2 oz. of ammonium 
platinic chloride. Continue heating until solution is com- 
plete, dilute to 1 imp. (or 1-2 U.S.) gallon, and add oz. of 
ammonium chloride. This bath also is worked hot and 
yields a deposit similar in character to Eoseleur's bath. 

Some interesting experiments have recently * been 
carried out by McCaughey and Patten with solutions of 
potassium chlorplatinate for platinum deposition. A simple 
solution of this salt in water yields its metal to more electro- 
positive elements by simple immersion. Copper, for example, 
readily becomes coated with a loosely adhering film of 
metal by immersion in such a solution. This constitutes a 
difficulty in using this bath for electro-deposition, a difficulty 
which, however, where thick deposits are required,- may be 
overcome by giving the article a thin preliminary coating of 
gold. 

The investigators above referred to obtained some en- 
couraging results in the electro-deposition of platinum from 
an electrolyte made up by dissolving potassium chlor- 
platinate in water and adding a considerable proportion of 
citric acid. The solution which they found most successful 
was made up in the following proportions : 

* Trans. Amers Electr. Chem. Socy., vol. xv. (1909), p. 523 ; also 
vol. xvii. (1910), p. 275. 




DEPOSITION OF PLATINUM 341 

Potassium chlorplatinate . . 2 parts by weight 

Citric acid 10 

Water 100 

The corresponding ammonium salt may be substituted 
for the potassium compound with, in some respects, even 
better results. 

This bath is rather difficult to manage inasmuch as it 
appears to be necessary to keep up the strength of the 
solution to the above standard. Additions of the platinum 
compound must therefore be regularly made as the bath is 
worked, as also of citric acid from time to time. 

A very simple platinum solution, described by Langbein, 
is made by dissolving 1 oz. of platinic hydroxide in a solu- 
tion of 4 oz. of oxalic acid and diluting to one imperial 
gallon by the addition of water. This bath also must be 
replenished by additions of the oxalate, and it is recom- 
mended to use a little free oxalic acid. 

Langbein states that a deposit of any required thickness 
can be obtained from the foregoing solution, and that the 
metal obtained is sensibly harder than that from the alkaline 
baths. The working temperature should not exceed 70 C. 

Treatment of Articles for Deposition. Gold, silver, 
copper, German silver, or brass articles can be given a 
deposit of platinum direct from the phosphate bath, but 
iron should be previously coppered or gilt. The other baths 
mentioned have rather a tendency to deposit their metal, by 
simple immersion, on copper, and it is advisable, therefore, in 
using these solutions to give a preliminary coating of silver 
or gold. Gold is more suitable as being more electro-negative 
than silver, but if only thin films of platinum are deposited 
the colour is somewhat affected. 

Deposits of platinum of any appreciable thickness require 
scratch-brushing or scouring in order to bring up the colour. 
Fine German silver wire brushes should be used in the former 
case, and flour pumice powder or whiting in the latter. 

Simple Immersion Deposits of Platinum. Very 



342 ELECTROPLATING 

thin films for ornamental purposes are sometimes given to 
silver or silver-plated goods by simple immersion in a solu- 
tion of platinum, but such deposits have a decided tendency 
to be dark coloured, and not very adherent, though very 
useful for ornamental purposes such as the antique colouring 
of silver surfaces. A good solution of this kind is obtained 
by dissolving 5 dwts. (J Troy oz.) of platinum in sufficient 
aqua regia, evaporating the solution down to a syrupy con- 
sistency, then adding distilled water to make up one gallon 
of solution. This liquid gives the best results when used 
warm, and the length of immersion regulated as found 
necessary. A brief treatment is generally sufficient. 

Deposition of Palladium. 

Properties of Palladium. The colour of palladium is 
of a shade somewhat between silver and platinum. It is 
very ductile and malleable. It does not oxidize in the air at 
ordinary temperatures and, while possessing some of the 
properties of silver, it is distinctly superior to that metal in 
contact with the atmosphere as it is quite unattacked by 
sulphur compounds. Palladium dissolves readily in hot 
nitric acid, particularly if the metal is not quite pure. In 
the spongy form palladium is also soluble in hydrochloric 
acid, but in its compact form it is scarcely attacked either by 
hydrochloric or sulphuric acids. 

Compounds of Palladium. The principal salts of 
palladium are the chloride (PdCl 2 ), the nitrate [Pd(NO 3 )J, 
and the cyanide PdCN 2 . The chloride forms also a large 
number of double compounds of which the chief are those of 
the alkalies and ammonia, e.g. potassium palladiochloride 
K 2 PdCl 4 , ammonium palladiochloride (NH 4 ) 2 PdCl 4 . The 
cyanide also forms a double salt with the alkalicyanides, the 
potassium salt having the formula K 2 Pd(CN) 4 3H 2 O. 

Solutions for Deposition. The best known solution 
for the electro-deposition of palladium is that proposed 
originally by Bertrand, being a simple solution of the double 




fi 

A I 



DEPOSITION OF PALLADIUM 343 

chloride of palladium and ammonia in water together with 
an excess of ammonium chloride. The proportions usually 
taken are as follows : 

Ammonium palladiochloride . . 1 oz. ! 31-2 gr. 
Ammonium chloride !-? ! 46-8 

^litres 

The solution should be used very slightly warm with a 
voltage of from 4 to 5 volts. 

Of other solutions which have been suggested only the 
cyanide needs mention here. Gore and several other 
writers recommend this bath, though Langbein considers it 
inferior to the chloride solution above. It may be made by 
precipitating palladium cyanide from a solution of the 
chloride and after well washing the precipitate redissolving 
in potassium cyanide. The solution should contain not less 
than 2 oz. of the metal per gallon and very little free 
cyanide. 

Under these conditions we have found this solution to 
work fairly well in giving thin protective films to silver or 
silver plated goods. 

Anodes for general work should be of the metal itself, 
but Cowper-Coles, in using the chloride solution for coating 
reflectors, employs carbon anodes. 



CHAPTER XVII 

THE DEPOSITION OF BRASS AND OTHER ALLOYS 

THE subject of the deposition of alloys from electrolytic 
solutions is at once exceedingly interesting and complex. 
While the theoretical considerations involved are extremely 
complicated, the practical difficulties to be overcome are 
equally formidable. 

Most probably this accounts for the fact that of an 
enormous number of commercial alloys in everyday use in 
the arts, brass (a copper-zinc alloy) is the only one used to 
any considerable extent in the electroplating industry. 

Before proceeding to the discussion of the practical 
electro-deposition of brass, as. well as of one or two other 
alloys which deserve mention, it will be advisable to consider 
to some extent at least the chief theoretical principles which 
govern the deposition of metals from mixed electrolytes. 

It is a fact familiar to observant electroplaters that an 
electrolytic solution may contain a number of different 
metals and yet yield only one at the cathode as the result of 
the passage of a normal electric current. Several different 
explanations have been put forward to account for this very 
well-known phenomena. The simplest, most feasible, and the 
one now most generally adopted is that of Le Blanc. In his 
classical text-book on Electro-chemistry this authority lays 
down the following conception of electrolysis by a moderate 
current in complex solutions : " All of the ions in the solution 
taJce part in the conduction of the electric current, but only those 
ions the separation of which requires 1he least expenditure ofworJr 
or energy are deposited or separated at the electrodes* Thus it 



DEPOSITION OF BRASS 345 

may happen that ions which conduct scarcely a measurable 
part of the current play the most important part in the 
chemical decompositions at the electrodes, in so far as they 
are formed with sufficient rapidity." * 

Le Blanc uses the following illustration, which will assist 
in making the matter clear. " Suppose a fairly concentrated 
solution of a mixture of potassium, cadmium, copper, and 
silver salts be electrolysed with a moderate current between 
platinum electrodes. In conducting the current, potassium, 
cadmium, hydrogen, copper and silver ions migrate to the 
cathode. At the cathode from actual experiment it is known 
that the silver is first deposited. This deposition goes on until 
the number of silver ions remaining is no longer sufficient 
for the current density maintained, when the copper begins 
to separate in the same manner. Following copper, cad- 
mium, and finally hydrogen is deposited. These results are 
obtainable by actual experiments and are simply explained by 
the following statement. 

"Those ions separate first which give up their electric 
charges most easily. The other ions must wait their turn in 
the order of their ease of deposition." The ions most easily 
giving up their charges are, of course, the electro-negative ones. 

A careful consideration and study of the foregoing will 
convince the student of the supreme importance of the 
" electro -motive force " factor in all cases of mixed elec- 
trolytes. A specific E.M.F. between electrodes will maintain 
a definite current density, and on the latter will depend the 
weight of metal deposited, or in other words, the number of 
ions liberated. An increase in E.M.F. therefore implies an 
increased C.D. and vice versa. Reverting to the illustration 
quoted above, the deposition of silver will go on so long as 
there are sufficient silver ions for the particular current 
density maintained. When this ceases to be the case, then 
the copper ions are called into play to carry the current and 
later the cadmium and so on. 

* Le Blanc, Text-book of Electro-chemistry ', English translation, 
p. 303. 



346 ELECTROPLATING 

Now in an earlier chapter it has been explained that 
different metals require different values of E.M.F. to effect 
their liberation from electrolytes in the metallic form. Sup- 
pose, therefore, that the E.M.F. used in the above example 
was only sufficient for the liberation of silver, then directly 
the whole of the silver ions had been deposited the 
passage of the current would be stopped and electrolysis 
would cease. 

This principle is of great importance and plays a pre- 
eminent part in the applications of electrolysis to the 
separation of metals either for refining or for electro-chemical 
analysis ; and it must be regarded as of equal importance in 
the question of the deposition of alloys or mixed metals from 
electrolytes. A study of it will reveal the conditions 
necessary for the deposition of alloys. These are mainly as 
follows : 

Either (1), the particular solution used must be such 
that the compounds of the metals contained are as nearly 
as possible equal in the .values of their heats of formation 
this, it will be remembered, denotes the specific E.M.F. 
required for decomposition. In such a case the metals con- 
cerned require practically the same E.M.F., and so long as 
the ions of each are present in the correct proportion the 
tendency will be for them to be deposited simultaneously so 
long as this value of E.M.F. is maintained. 

Or (2), the current used, being of a sufficient E.M.F. to 
liberate the more electro-positive metal, is also of a density 
so high that the number of more electro-negative ions in the 
vicinity of the cathode is not sufficient to convey all the 
current from the solution to the cathode, and therefore 
the more electro-positive ions are called upon to take part in 
the process as well as the electro-negative. 

Both the above conditions obtain to a greater or lesser 
extent in the practical electro-deposition of alloys. 

This naturally leads us to lay down the dictum, which 
cannot be too strongly emphasized, that in all experiments in 
the electro-deposition of alloys and indeed in workshop 



DEPOSITION OF BRASS 347 

practice a voltmeter is almost essential to secure con- 
tinuously the best results. Obviously also when the current 
has once been regulated to secure the desired E.M.F., the 
conditions of supply must be such as to ensure that it shall 
be kept constant. 

In this connection it may be well to point out again the 
advantages of supply from accumulators rather than from the 
dynamo, particularly if the latter is at all liable to vary in 
voltage owing to variations of speed, a circumstance which 
is not unusual in factory driving. 

The point of first importance in the deposition of alloys 
is to obtain uniformity of composition in the deposit, and here 
is the greatest difficulty. In a large number of cases of 
binary alloys particularly it is comparatively easy to obtain a 
deposit of the two metals concerned, but to obtain a definitely 
ascertained proportion of the metals together over an appre- 
ciable period of time from one electrolyte is a very different 
matter. 

In discussing the question of brass, however, it may be 
urged that the colour is the main desirability, and the exact 
proportion of the two metals concerned, copper and zinc, is 
immaterial. For any deposits, however, beyond the merest 
film, uniformity of composition is essential to uniformity of 
colour, and the latter is therefore just as important in the 
case of brass as in that of other alloys where colour is not so 
material. Hence the necessity for a thorough grasp of the 
foregoing principles and their application. 

Properties of Brass. Brass, as is well known, is an 
alloy of copper and zinc. These two metals alloy in 
practically all proportions, but for industrial purposes the 
proportions most commonly used are from 60 to 70 per cent, 
copper and 30 to 40 per cent. zinc. Those alloys containing 
less zinc are usually the most malleable and ductile. Dutch 
metal, which is simply brass containing rather more copper 
than ordinarily, is exceedingly malleable and can be rolled to 
an extreme thinness in imitation of leaf gold. Brass of 
average composition is not so susceptible to the action of the 



348 ELECTROPL ATI NG 

atmosphere as is pure copper ; hence its suitability for pro- 
tective films, and also for intermediate coatings preliminary 
to deposits of silver, gold, or nickel. A brassing solution in 
thorough working order is always useful in general plating 
shops from this point of view, and it might with advantage 
be more extensively used than appears to be the case at 
present. 

With regard to colour, which is possibly the most impor- 
tant property of brass from the electroplater's standpoint, 
the characteristic pure yellow colour of the alloy is shown 
most uniformly in alloys of from 60 to 70 per cent, copper 
and 30 to 40 per cent, zinc., and it is the object of brass- 
plating usually to obtain a deposit of as near this composition 
as possible. In the manufacture of copper-zinc alloys, con- 
siderable modifications of texture and of colour are obtain- 
able by the addition of very small percentages of some other 
metals, and there is good reason to believe that similar 
modifications can be obtained in electrolytic deposits of brass. 
This aspect of the subject, however, requires and deserves 
careful investigation and research, since little can be said on 
the point at present. 

Solutions for Deposition. The only practical solu- 
tions in use at present for the deposition of brass are the 
cyanides. Many attempts have been made to devise an acid 
bath for use in this direction, but without avail. The chemical 
and electro-chemical properties of the two metals concerned 
are so widely different as to render it unlikely that a simple 
mixture of solutions of their simple salts only can be made 
to yield a satisfactory joint deposit. This will be fairly 
evident on reference to the relative position of the elements 
in the electro-chemical series. The double cyanides of these 
metals are, however, so stable in composition, so much less 
easily decomposed chemically than the simple salts, and 
possess heats of formation so nearly equal, that they are ob- 
viously the most likely compounds to use for joint deposition 
of the metals. 

The preparation of the solution is carried out in a way 



DEPOSITION OF BRASS 349 

very similar to the cyanide coppering solution, but before 
detailing the composition of the plating bath, one or two 
theoretical points should be noticed. 

(1) Copper in cyanide solutions acts as a univalent 
element, zinc on the other hand is bivalent ; consequently the 
proportion of the two metals deposited by the same current 
are as 63'5 (the chemical equivalent of univalent Cu) and 
32-5 (the chemical equivalent of Zn). If, therefore, equal 
proportions of the two metals in double cyanide solutions 
were mixed together and electrolyzed, we should expect, 
under correct conditions of E.M.F., a mixed deposit of the 
composition, 63-5 Cu : 32-5 Zn, which, it will be noted, is an 
ordinary commercial brass. Moreover, in view of the above, 
it is obvious that in order to get such a result it would seem 
to be necessary that equal proportions of the two metals 
should be present. This is borne out by practical experience, 
and while admittedly it is possible by manipulation of current 
conditions and temperature to obtain a good brass deposit 
from solutions containing less zinc, it is very much more 
difficult. This point must be borne in mind, since some text- 
books and writers recommend the preparation of a brassing 
solution from the commercial metal itself with approximately 
the composition 2 of Cu, 1 of Zn. Such a plan, it will be 
clear, is not favourable to the best results. 

(2) The chemical constitution of the alkaline double 
cyanides formed by the two metals zinc and copper respec- 
tively, is not quite analogous. The double cyanide of zinc and 
potassium has a composition corresponding to the formula 
K>Zn(CN) 4 , while that of copper and potassium, on the other 
hand, in aqueous solution is practically KCu(CN) 2 . In pre- 
paring a solution, therefore, of the mixed cyanides it will be 
obvious that the zinc salt will require a much larger propor- 
tion of potassium cyanide (approximately double) than a 
corresponding weight of .copper. This point should be borne 
well in mind, not only in making up a new solution for 
electro-brassing, but also in replenishing an old one the fact 
being, as will be deduced, that the electrolyte has a constant 



35 ELECTROPLATING 

tendency to dissolve a greater proportion of copper than zinc 
from the anode. 

In view of the foregoing, therefore, it is strongly recom- 
mended to make up brassing solutions from zinc and copper 
or their compounds separately, and not from metallic brass. 

One of the best and most widely used electro-brassing 
baths is the following : 



Copper sulphate ..... J Ib. 
Zinc ..... 



250 gr. 
250 



Ammonia (0-880) ..... Q.S. 
Potassium cyanide ..... Q.S. 



Water 



or 



5 litres 



Powder the copper salt in a mortar and dissolve together 
with the zinc salt in about a quart of warm water. To this 
solution add liquid ammonia until the precipitate which first 
forms is completely redissolved and the solution assumes a 
deep blue colour (see page 253). Now make up a solution of 
potassium cyanide by weighing out 2 Ibs. and dissolving it 
in 1 quart of water (or 800 grams per litre) ; add this to the 
mixed ammoniacal solution of zinc and copper until the 
blue colour is completely discharged and a clear, almost 
colourless, solution results. Note the quantity of cyanide 
solution required to do this, and add about 10 per cent, 
additional to form free cyanide. Make up the bulk of the 
liquid to 1 imp. gallon (or 5 litres) by adding water. 

It will be noted that this solution is exactly analogous 
to that recommended for alkaline coppering on page 253. 

The bath should be worked at a temperature of about 
20 C., i.e. the normal temperature of the workshop. If 
worked hot, the colour is usually rather too red. Solutions 
intended to be worked hot should not be so rich in metal 
content as the above. 

Another solution, similar in principle to the above, is 
that invented by Norris and Johnson (1852), which is 
composed according to specification as follows : 



DEPOSITION OF BRASS 351 



Copper cyanide 2 oz. 

Zinc cyanide 1 

Ammonium carbonate . . . 1 Ib. 
Potassium cyanide . . . . 1 ,, 
Water $ 1 imp. gall. 
(orUU.8.,, 



62-5 gr. 
31-2 

0-5 kg. 

0-5 

5 litres 



Dissolve the cyanide and ammonium carbonate in a 
sufficiency of water and add the zinc and copper com- 
pounds,* stirring until completely dissolved; make up the 
bulk to 1 gallon (or 5 litres with the above metric amounts) 
and work at a temperature of about 70 to 80 C. 

In modern practice, however, the solution has been 
considerably modified, the proportion of potassium cyanide 
given above being too large in comparison with the small 
amounts of copper and zinc cyanides. Better results are 
obtained by using 4 ounces of each instead of 2 and 1 respec- 
tively. 

This bath gives excellent results, but requires careful 
management. 

Some operators prefer to use a bath containing a small 
proportion of potassium or sodium carbonate, claiming 
thereby an increased conductivity of solution. Such a bath 
can be readily prepared as follows : Take of 

Copper sulphate . . . . 6 oz. 
Zinc .... 6 

Dissolve in water separately and add to each a strong 
solution of sodium carbonate until no further precipitation 
occurs. Stir vigorously and allow to settle, then pour off 
the clear liquid as far as possible and mix the two precipi- 
tates, which are copper and zinc carbonates, together. 
Now add a sufficient quantity of a strong solution of potas- 
sium cyanide (2 Ibs. per gallon) to completely dissolve these 
precipitates and a further proportion of about 10 per cent. 

* These can be bought or prepared in the workshop by precipitating 
a solution of copper and zinc sulphates respectively by means of potas- 
sium cyanide. 



352 ELECTROPLATING 

to form free cyanide. The reaction between the two 
carbonates and potassium cyanide results in the formation 
of a sufficient amount of potassium carbonate in solution 
without making any specific addition of this salt. (See 
discussion on analogous point in Chap. XI. p. 254.) 

A solution deserving of mention, though of rather com- 
plex constitution, is that recommended by Eoseleur, viz. : 

Copper carbonate 2 oz. ] 62-5 gr. 

Zinc 2 62-5 



Crystallized sodium carbonate . 3 
bisulphate . 3 

Potassium cyanide 8 ,, 

Arsenious acid 15 grains 



93-75 

93-75 

250 

1-07 

Water ] ViTa 6 " ' 5 litres 

(or II U.S. 

The weights of ingredients as given above are slightly 
modified from Eoseleur's figures in accordance with what 
we have found from experience to be advisable. 

The solution is best made by mixing the copper and 
zinc carbonates together with a little water so as to give 
the consistency of thick cream. Dissolve separately 
the sodium carbonate and bisulphite in about 1 imperial 
pint of water each, and add them slowly with constant 
stirring in the order named to the copper-zinc compound. 
Considerable effervescence ensues owing to the liberation of 
of CO 2 , so that the operation should Be carried out in a 
deep vessel. Now add the potassium cyanide which has 
been dissolved in about a quart of water, and stir until the 
solution becomes practically clear and colourless. If the 
cyanide used is of a low percentage, more than the above 
amount may be necessary. Finally add the arsenious acid 
(white arsenic) dissolved in a sufficiency of hot water in which 
a little KCN has been dissolved, and make up the bulk of 
solution to 1 gallon by adding water. 

It is advisable to boil the solution for a short time before 
using. In actual working it may be used either hot or 



DEPOSITION OF BRASS 353 

cold, but the colour is rather too coppery at a high tem- 
perature. 

The addition of arsenious acid to this bath is of interest, 
since this substance has been rather extensively used in 
brassing solutions for the purpose of obtaining brighter 
deposits. Like carbon bisulphide in silver solutions, how- 
ever, arsenic should be used in very small quantities and with 
judgment. There is no doubt that the character and colour 
of the deposits are appreciably influenced thereby, but any 
accumulation of it will ruin the working qualities of the bath, 
and render the deposit useless for all ordinary requirements. 

General Remarks on Brassing Solutions. Experi- 
ence has shown that deposits of metal obtained from 
brassing solutions in colour particularly are very readily 
influenced by very small and apparently insignificant 
additions to the bath. It has furthermore been observed 
that the addition of certain substances has the effect of 
materially increasing the conductivity of the electrolyte. The 
attention both of experimentalists and of practical workers 
has accordingly been given to these points to a considerable 
degree, and many modifications of the ordinary cyanide bath 
have been proposed. Some of these, such as the addition of 
sodium carbonate to improve conductivity and arsenious 
acid for colour, have received mention already. Other 
recommendations include the addition of sodium bisul- 
phite, and small proportions of the organic salts of iron, e.g. 
ferrous acetate or oxalate. These latter are useful addition 
agents to brassing solutions, but care must be taken to have 
plenty of free cyanide present, or there is a possibility of 
complex chemical reactions occurring which may precipi- 
tate some of the zinc. 

Some very experienc6d operators regard the presence of 
a large excess of ammonia as advantageous in these solutions, 
particularly when thick deposits are required, and there is 
little doubt that this is the case, since by its means solution 
of the anode is facilitated, giving consequently a more uni- 
form composition of the bath. 

2 A 



354 ELECTROPLATING 

Anodes. Though the use of copper and zinc anodes 
alternately in brassing baths is sometimes adopted, it will be 
found most generally advisable to use rolled brass only, and 
the anode surface immersed should always be in excess of 
the superficial area of the articles being plated. 

Current Conditions. The voltage required for brass- 
ing solutions is usually from 4 to 6 volts. Exact figures for 
either this or current density cannot be given, since these 
depend on local conditions of composition of solution, tem- 
perature, and class of work. The operator should determine 
by experiment what readings give the best results for the 
particular work upon which he is engaged, and endeavour 
to keep these values constant. 

Management of Solutions. To obtain consistently 
good results from an electro-brassing bath is not a very easy 
matter, particularly in giving thick deposits. It is always 
advisable to note the appearance of the anode and prevent 
the formation of any oxide or slime on its surface by the 
addition of ammonia, or free cyanide, or both, to the solu- 
tion. Increasing the proportion of free cyanide tends to 
produce a greater proportion of copper in the deposit, but 
this can be remedied by the addition of water which tends 
to facilitate the deposition of zinc. Considerable variations 
in the composition and therefore colour of a brass deposit 
may be obtained by varying the temperature, but for most 
workshop purposes cold solutions are much more con- 
venient; the temperature, however, should, if possible, be 
kept constant, and any necessary alterations made by vary- 
ing other conditions of working, viz. composition of solution 
or conditions of current. If the bath is not working satis- 
factorily, and the current conditions and free cyanide content 
appear correct, the operator must determine whether the 
metallic content of the bath is at fault. This may be done 
by trying the effect of the addition of either copper or zinc 
cyanide or, more scientifically, by estimating the amount of 
each metal present by the method described below. The 
fault will usually be thus located. 



DEPOSITION OF BRASS 355 

Some interesting researches on the subject of the electro- 
deposition of brass from cyanide solutions have been under- 
taken by Field,"'' whose principal conclusions may be briefly 
summarized thus : 

(1) Conditions which tend to raise the E.M.F. increase 
the percentage of zinc in the deposit. Such conditions are : 
(a) Dilution of solution ; (b) increase of temperature. 

(2) Anodes are freely soluble with warm agitated solu- 
tions even in the presence of only small amounts of free 
cyanide. 

(3) The effect of free cyanide is to (a) increase the per- 
centage of copper in deposits ; (b) increase the evolution of 
hydrogen ; and (c) induce abnormal anode efficiencies. 

It is further concluded that free cyanide does not impart 
conductance to a solution in the same way that acid affects 
a copper sulphate solution, but simply makes the anode 
products dissolve more readily. 

Deposits of brass may be made directly upon all metals 
and alloys without intermediary coatings. Indeed, brass is 
almost equally, if not quite, as useful as copper as an inter- 
mediate coating itself prior to deposition of other metals. 
Watt recommends the use of a warm solution for brassing 
lead and pewter, the former particularly a strong current 
should also be used at the moment of immersion in order to 
coat rapidly every part of the surface being plated. As in 
the case of coppering, the greatest trouble to the operator 
is usually given by cast-iron, and a similar treatment should 
be adopted as recommended for coppering (see page 260). 

Estimation of Metallic Content of Brassing Solu- 
tions. The estimation of the copper content of a brassing 
bath is best carried out by means of the method already 
fully described in Chap. IX., page 261. The presence of 
zinc does not interfere. The estimation may be made on a 
separate sample of solution or on the copper precipitated 
from the sample taken for the zinc estimation as described 
below. 

* Trans, of the Faraday Society, vol. v., Sept., 1909, pp. 172-196. 



356 ELECTROPLATING 

For the following excellent method of estimating zinc we 
are indebted to our friend Mr. F. Ibbotson, B.Sc. 

Measure by means of a pipette an exact amount, from 
25 to 50 c.c. of the solution, and transfer to a large 
beaker. Add to this hydrochloric acid, stirring until the 
whole of the cyanide is decomposed, and the solution is 
distinctly acid (test with litmus paper). Now add first 
4 or 5 c.c. of sulphurous acid, then ammonium thiocyanate 
solution until no further precipitate is produced. (This 
precipitate contains the whole of the copper and may, by 
redissolving in nitric acid, be used for copper estimation, 
as mentioned above.) Transfer the whole solution con- 
taining the precipitate to a graduated flask holding 300 c.c. 
Carefully add distilled water until the 300 c.c. mark is 
reached. 

Now filter off through a dry filter paper, and measure out 
250 c.c. exactly of the filtrate. This will contain fths of 
the zinc. This solution must now be rendered exactly 
neutral or very slightly acid. The best method is to add 
ammonia until the liquid is just alkaline (test by litmus), 
then add hydrochloric acid drop by drop until the neutral 
point is reached or the character made slightly acid. Weigh 
out now an amount of ammonium phosphate of between ten 
and twenty times that of the weight of zinc supposed to be 
present it is usually possible to form an idea of the zinc 
present between such limits and add this to the zinc 
solution with continuous stirring preferably on a warm 
plate. The resulting precipitate which contains all the zinc 
as zinc ammonium phosphate is at first very flocculent,>but 
soon becomes dense and crystalline, and easily settles. 
Filter, and transfer the precipitate to a weighed crucible. 
Strongly heat now over a Bunsen burner until the salt is 
white throughout (test by pricking with a pointed glass rod). 
Allow to cool in a desiccator and weigh. Deduct, of course, 
weight of crucible, and the result is the amount of zinc as 
pyrophosphate (Zn 2 P 2 7 ). This salt contains 42-55 per cent, 
of zinc, so that by multiplying the result by 0-4255, the exact 



DEPOSITION OF BRASS 357 

weight of metallic zinc in fths of the sample is ascertained. 
If, say 30 c.c. of solution was originally taken, we have 
obtained the weight of zinc in 25 c.c. To ascertain the 
weight per gallon this figure must be multiplied by 181'5 
(4540 c.c. = 1 gallon). 

The technology of the electro-deposition of alloys other 
than brass is at present in a very imperfect condition, and 
this part of the subject is consequently of laboratory rather 
than of workshop interest. The following are a few of the 
principal alloys which have been suggested for electro- 
deposition, but none have yet assumed any commercial 
importance. 

Copper Alloys. (1) Bronze (copper-tin). The solution 
generally considered best for this alloy is the oxalate, made 
up by dissolving separately 4 oz. of copper sulphate, and 
2 oz. tin bichloride (Sn01 2 ). To each solution add an excess 
of ammonium oxalate solution until the precipitates which 
at first form are redissolved. Add a little free oxalic acid 
to both and mix together, making up the bulk to one im- 
perial gallon by the addition of water. The solution should 
be boiled before use. 

(2) German silver (copper-nickel-zinc). The usual pro- 
portions of this alloy are from 15 to 20 per cent, nickel 
55 to 60 per cent, copper, and 25 to 30 per cent. zinc. A 
mixture of the double cyanides of each of these metals 
with potassium in about these proportions forms probably 
the best solution for deposition. 

The alloy is, however, rarely if ever used, though Watt 
recommends it for coating revolvers, dental instruments, 
scabbards, etc. 

Nickel Alloys. In addition to German silver referred 
to above, several alloys of nickel have been suggested for 
electro-deposition of which the following are the principal. 

(1) Nickel and Iron. Solution recommended is a mixture 
in any proportion desired of the double sulphates of these 



358 ELECTROPLATING 

metals and ammonium. The bath must be exactly neutral, 
or very slightly acid. 

(2) Nickel and Cobalt. This alloy has been suggested by 
AYeiss, who recommends the following as a suitable solution 



Nickel ammonium sulphate . . 8 oz. 
Cobalt ammonium sulphate . . 2 
Ammonium sulphate .... 3* ,, 



250 gr. 
62-5 
93'7 ,, 

Water ...... \ ' 5 litres 

(orlJU.S. 

(3) Nickel and Zinc. Alloys of these two metals have 
also been proposed, the electrolyte being a mixture of the 
two sulphates, with nickel sulphate in greater proportion, 
and a little ammonium sulphate. 

Silver Alloys. A number of silver alloys have been 
proposed at various times for electro-deposition, many of 
which have been patented. The principal are silver and 
platinum, silver and zinc, silver and cadmium, silver and 
tin. In each case the cyanide solution is suggested. 

Tin Alloys. A recent proposal of some interest is to 
deposit an alloy of tin and lead from a solution based on 
the JBetts formula, to which reference has been made on 
page 326. 



CHAPTEK XVIII 
FINISHING PROCESSES 

THE finishing of electroplated surfaces is a subject of 
considerable importance to electroplaters, though in many 
branches of the industry it is considered and carried on as 
a separate trade. It is, however, not possible within the 
limits of the space here available to give a detailed description 
of all the methods in vogue, and only a general, though it is 
hoped useful, outline will be attempted. 

The subject may be divided into two distinct types, 
(1) hand-finishing, (2) machine-finishing. The former is 
mainly confined to the silver and gold-plating industries ; 
the latter is used in all branches of the art of electro- 
plating. 

1. Hand-finishing. This term, though formerly pos- 
sessing a wider significance, is now practically confined to 
the operations of " burnishing " and " handing." 

Burnishing essentially consists in imparting a fine 
smoothness and brilliant lustre to a surface by means of a 
perfectly smooth tool of a very hard nature usually either 
steel or bloodstone held firmly in the hand and pressed over 
every portion with an even pressure. Some illustrations of 
the various shapes of these tools are given in Fig. 61, 
and in Fig. 62 is illustrated the correct method of holding 
them. 

A large number of different patterns and sizes of these 
burnishing tools are required owing to the variety of the 
surfaces to which they are applied. Some considerable 
experience is necessary in the operation in order to obtain 



3 6 



ELECTROPLATING 



the absolute evenness of surface necessary for brilliance and 

perfection of finish. 
The effect of burnish- 
ing is really to lay 
down or make quite 
flat and smooth the 
surface of metal ope- 
rated upon, and as a 
result light is reflected 
from every point of 
such a surface quite 
evenly and regularly, 
so conveying to the 
eye a fine lustre or 
mirror - like appear- 
ance. All electro-de- 
posits of metal are 
more or less uneven 
on their upper surface 
owing to the fact that 
the deposit does not 
cover the article like a 
sheet of rolled metal, 
but is _ liberated from 
the solution in in- 
finitesimally small 
grains. Viewed 
through a powerful 
microscope such a de- 
posit, particularly if of 
appreciable thickness, 
has an appearance 
which may not inaptly 
be described as that of 
a number of tiny hills 

congregated close together with a number of equally tiny 

valleys lying between. 




FINISHING PROCESSES 



361 




FIG. 62. Method of holding 
Burnisher. 



Burnishing, therefore to follow out the illustration is a 
process of laying down the hills side by side until they 
exactly fill up the valleys and 
the character of the surface is 
changed into that of a plain. 

The applications of burnish- 
ing lie mainly in the electro- 
silver-plating and gilding in- 
dustries, though similar pro- 
cesses are often used in the 
brass and art metal trades. 
It is a method of finishing 
particularly suited to the pro- 
duction of artistic effects, since certain portions of the surface 
can be burnished and others left dull, the Hne of demarcation 
being sharp and well defined, as is necessary in embossed 
work. 

Before burnishing, all surfaces should be lightly but 
thoroughly scoured with very fine sand or whiting moistened 
with soapy water, then rinsed in warm water and dried with 
a soft linen cloth. During the process of burnishing the 
tool is dipped regularly into a solution made by dissolving 
common yellow soap in hot water, or stale beer, the latter 
liquid being preferred by many workers for gilt surfaces. 
For brass, dilute vinegar is usually employed. 

" Handing " is a process almost peculiar to the finishing 
of silver and gold surfaces either plated or solid. Even the 
most efficient burnishing leaves a silver or gold surface with, 
to some extent, a scratchy appearance ; handing consists in 
carefully polishing such surfaces with rouge and water by 
means of the palm of the hand or the fingers until all such 
scratches are eradicated, and in the case of silver the perfect 
black lustre so characteristic of well-finished silver surfaces 
is obtained. In the case of gold or gilt work a similar 
brilliance of polish is obtained but a specially prepared rouge 
must be employed. When every trace of burnish marks or 
scratches has been thus removed, the article is thoroughly 



362 ELECTROPLATING 

washed with soap and a sponge in very hot water until 
entirely cleansed from rouge, then finally dried with a linen 
cloth and wiped up with chamois leather. 

2. Machine-finishing. Machine-finishing is carried 
out by means of a lathe such as described in Chap. VII., 
Fig. 44, fitted with buffs, dollies, or mops. The essential 
difference between this method and that of burnishing may 
be fairly illustrated from the analogy already made between 
a surface of electro-deposited metal and a number of hills 
and valleys. While burnishing levels the surface by laying 
down the hills, machine-finishing secures the same effect by 
removing the tops of the hills, or, in other words, rasing 
them to the level of the valleys. It will be obvious, there- 
fore, that these methods invariably result in some loss of 
metal. In many cases this is not a matter of much concern, 
but in others, particularly where the precious metals are 
concerned, it is. On the other hand, machine methods 
are much quicker and in very many classes of work much 
more suitable than burnishing by hand. Nickel, iron, 
and cobalt deposits, for example, are too hard for the 
latter process, and must therefore be finished by machine. 
During recent years also, partly for the sake of economy 
and partly to obtain a fine finish (showing no traces of 
burnish marks) with the minimum of handing, machine- 
finishing has become very popular for silver-plated work, 
the general methods pursued being very similar to those 
recommended for nickel-plated goods. The articles before 
plating are given a fine smooth surfa.ce and high polish, 
and after plating are taken direct to the finishing lathes 
and polished. 

The polishing materials employed in machine-finishing 
are mainly Sheffield or Vienna lime, whiting, Tripoli and 
crocus compositions, and fine rouge. These are applied by 
means of felt buffs, fibre brushes, and calico and swans- 
down mops or dollies attached to the lathe spindles and 
run at a speed of approximately 2000 revolutions per 
minute. Nickel deposits are usually finished by Sheffield 



FINISHING PROCESSES 363 

lime or compositions largely containing this or a similar 
substance. Calico mops are used for this purpose, and the 
composition is applied in small quantities at a time to the 
face of the mop as it revolves ; the article is held gently but 
firmly so that each part is subjected to the action of the 
polishing agent. 

Silver-plated work is generally first treated by means of 
a soft felt buff with Sheffield lime mixed with a very small 
quantity of oil. When the operator has gone over the 
entire surface in this way very little pressure being 
needed the buff is taken off the 'spindle and a calico mop 
substituted. To the face of this mop a slight touch of oil 
is applied together with a little of the prepared lime, and 
the article held to its surface so that every portion is 
treated. 

A slightly bright but greasy polish results. The calico 
mop is now changed for one of swansdown, which is 
treated with a simple mixture of rouge made into a thin 
cream by the addition of water. This produces the final 
brilliant black polish, though in the best classes of work it 
is usual to follow this by the handing treatment previously 
described. 

Copper deposits when required bright are finished by 
a similar, though rather simpler, process to the above. 
Generally, however, such deposits are coloured or given 
artistic light or shade effects by one or other of the pro- 
cesses described in the subsequent chapter. 

Deposits of iron, zinc, tin, or lead are not usually 
given any finishing treatment after deposition further than 
sand-blasting, scouring, or scratch-brushing. 

It should be remarked that a large number of special 
polishing compositions are now on the market of excellent 
quality which may be purchased from manufacturers 
making a speciality of these materials, and should be used 
according to the directions issued with them. 

A particularly important point in the machine-finishing 
of articles like spoons and forks is the care of the edges. 



364 ELECTROPL ATING 

Unless the operator is both experienced and careful a con- 
stant tendency arises, in finishing, to apply too much friction 
to the edges or to any sharp points such as the ends of 
spoon-bowls, etc. The fault can easily be avoided by care 
in applying the felt buffs or mops to the surface of the 
article, working first from the centres and carefully grading 
the pressure so that the edges are scarcely touched. 

It is necessary also to mention that slight losses occur in 
polishing by means of handing. It will be observed that 
after rouge has been applied to a silver surface by the hand 
the latter is blackened owing most probably to a slight 
indirect chemical exchange of the rouge (iron oxide) and 
metallic silver. 

The use of the Sand-blast in Finishing. As well as 
being often an important factor in preparatory processes, 
sand-blasting is a very useful occasional adjunct in finishing 
electroplated goods. 

The principal methods of its application are outlined in 
the following : 

Belief effects on silver or silver-plated goods. Use the sand- 
blasting apparatus at a pressure of from 8 to 10 Ibs. per 
square inch with powdered pumice in the case of silver- 
plated goods before plating. Scratch-brush after plating on 
a fine brush, then dip rapidly through a hot dilute solution 
of potassium sulphide (see also p. 367) until the surface 
assumes a deep bluish-black colour due to the formation of 
a film of silver sulphide. Then by means of a calico mop 
or dolly and fine Trent sand gently polish off the colour 
from all raised or embossed portions of the surface. By 
careful regulation and variation of conditions very pleasing 
effects can thus be produced. 

Gold or gilt surfaces. Great care must be taken in treating 
these surfaces by the sand-blast or they will be completely 
spoiled. In the case of gilt work the colour of the article 
when taken from the bath should be rather darker than 
the final colour required. Scratch-brush gently on a soft 
brush, then subject the surface to the action of the 



FINISHING PROCESSES 365 

sand-blasting apparatus at a pressure not exceeding 3 Ibs. per 
square inch with No. 120 pumice powder. The operation 
should only occupy a few seconds (unless a large surface is 
treated), and the article is then thoroughly washed in hot 
water with a sponge to clear away all powder lingering 
in recesses. It is then finally wiped over with chamois 
leather. 

Nickel-plated tvork. In this class of work the use of the 
sand-blast is mainly to obtain partial effects alternately 
bright and dull to suit the style of the article. These can 
be readily obtained in the manner described in Chap. VIII., 
page 163. 

Use of Scratch-brush in Finishing. Deposits of 
gold, silver, copper, zinc, and some other metals are some- 
times finished by means of the scratch-brush only, without 
the use of any of the ordinary polishing appliances and 
compositions. It is obvious that a " finish " imparted in 
this way will not compare in brilliance of polish with that 
obtained, say, with felt buffs and mops or by burnishing. 
Nevertheless the effects obtained are more suitable for 
certain classes of work, and they can be widely varied by 
using different types of brushes. It will be found, for 
example, that scratch-brushes of German-silver wire are 
particularly suitable for finishing gilt work which is required 
to have a " dull-bright " effect. In this case a very fine 
crimped wire is used. For silver and copper deposits also 
similar brushes are now being used. Indeed, German-silver 
wire is preferred by many operators recently instead of 
brass, since thinner wire can be employed to give an equal 
" resistivity," as it may be termed, to the pressure of the 
brusher, with the result often of marked improvement in 
the surface treated. 



CHAPTEE XIX 
METAL-COLOURING AND BRONZING 

THE terms "metal-colouring" and "bronzing" possess now 
a wide significance. Broadly speaking they have become 
almost synonymous and apply to the whole art of the 
decoration of metallic surfaces, whether by chemical or 
mechanical methods. 

Such a subject cannot be treated adequately within the 
limits of a brief chapter, but it seemed desirable, as the 
electroplater is often called upon to do certain classes of 
work of this kind, to outline a few of the methods in general 
use, particularly those corresponding to the ordinary require- 
ments of a plating shop. 

Preparation of surf aces for colouring. The general methods 
of Chap. VIII. for the treatment of metals prior to electro- 
plating are adopted usually for preparation for metal- 
colouring ; little need, therefore, be said on this point. It 
seems, however, to be necessary to emphasize its importance. 
Imperfect cleansing, pickling, or dipping can only result in 
disappointment, for their effects are, inequalities of colour- 
ing, failure of the colouring chemicals to act correctly, and 
general patchiness of the final surface. , 

The general methods of metal-colouring may be classed 
under two headings : (1) Chemical (including electro- 
chemical), and (2) Mechanical. 

I. METAL- COLOURING BY CHEMICAL METHODS. The 
principles involved in these methods are (a) to form, on the 
surface of the particular metal treated, by the agency of 
heat or some chemical compound, a salt or oxide which 



METAL-COLOURING AND BRONZING 367 

possesses some distinctive colour or colours. The formation 
may be quite a simple one, such as that of silver sulphide 
on silver surfaces by means of the action of a sulphur 
compound ; or a complicated one, due to the application 
of a mixture of a number of different compounds, oxides, 
carbonates, sulphides, or chlorides. Variations of colour are 
also produced by varying the thickness of the film. 

Or (b) to give by electro-chemical methods, I.e. electro- 
deposition, a film or coating of some metal or compound, 
which possesses a desirable colour. The former are the 
generally adopted methods and will, therefore, be given 
greater prominence here. 

Colouring of Silver. The production of colour effects 
on silver is generally known as oxidizing; the term, how- 
ever, is quite misleading, as silver oxide rarely forms the 
colouring film or any part of it except to a very slight 
extent. Sulphur is the chief agent employed in this con- 
nection and compounds containing this reagent in some 
form or other are in very general use, the most popular 
being potassium sulphide (liver of sulphur). A simple 
solution of this substance in water is very effective, but other 
substances are often added to improve either the appearance 
or adhesive properties of the film of silver sulphide formed. 
The following is an excellent solution : 



Potassium sulphide . . . 1 oz. 
Ammonium carbonate . . 2 , 



31-2 gr. 
62-5 



It is better to dissolve the ammonium carbonate in part 
of the water separately and add to the sulphide solution 
when the latter is dissolved. The resulting solution should 
be worked hot and the time of immersion of the article 
regulated according to the depth of colour required. A few 
seconds', or at most half a minute's, immersion is usually 
sufficient to produce a deep bluish black colour, which is 
very adhesive and will stand scratch -brushing. 



368 ELECTROPLATING 

For lighter shades of colouring barium sulphide may be 
substituted for potassium sulphide, the colour produced 
varying according to temperature and time of immersion 
from a light golden shade to brownisJ^Jblaek.-* >The solution 
should contain about 1 oz. of barium sulphide to each 
imperial gallon of water. 

Another useful agent in the colouring of silver, particularly 
in the production of antique effects, is platinum chloride. 
This salt is soluble in both alcohol and water, and solutions 
of each kind have been used, usually in the proportion of about 
a quarter of an ounce per imperial gallon. The solution should 
be used hot, and the article immersed until the surface is 
uniformly attacked. In the case of alcoholic solutions the 
liquid is generally applied by means of a camel's-hair 
brush; the alcohol quickly evaporates and leaves behind 
a slight filmy grey or greyish black deposit which will stand 
scratch-brushing lightly, and gives a very pleasing antique 
effect. The shade of colour may be considerably varied by 
altering the strength or working temperature of the solution. 

A hot solution of antimony chloride in water is also used 
for a similar effect. Usually from 1 to 2 oz. per imperial 
gallon is the strength employed, and articles are immersed 
as long as is found necessary for the desired colour. This 
solution is often used for the colouring of silver toilet ware, 
particularly in conjunction with a sand-blast apparatus as 
explained later. 

The artistic effects obtained in the colouring of silver 
depend to a large extent on the after-treatment of the 
surface. It is rarely that an article coloured in the sulphide 
solution, for example, is left exactly as it appears after 
immersion and scratch-brushing; it is generally treated to 
obtain light and shade effects according to the type of the 
ornamentation of the surface. 

Such treatment, known as " relieving," consists as a rule 
in carefully polishing or rubbing off by means of a calico 
mop or soft brush or the hand with fine whiting or pumice 
powder the oxidizing colour from the raised or embossed 



METAL-COLOURING AND BRONZING 369 

portions of the article, thus producing shades of almost any 
degree of lightness to contrast with the dark or black coloured 
groundwork. Surfaces so treated are often given a further 
treatment by sand-blasting with fine whiting or pumice 
powder at a very low pressure. 

Silver toilet ware and other goods of a similar character 
are first oxidized either in the potassium sulphide or antimony 
chloride solution, then relieved according to the taste of the 
operator, and finally sand-blasted with fine pumice powder 
at a pressure not exceeding 3 Ib. per square inch. 

Colouring of Copper. This metal is probably the most 
important to be dealt with in a survey of the subject of 
metal- colouring inasmuch as many artistic effects are given 
to other metals and alloys by first imparting to them a 
coating of copper by electro-deposition and afterwards colour- 
ing this deposit. Copper also readily responds to the actions 
of many simple chemical reagents which result in the 
formation of films of salts of the metal of very pleasing 
artistic appearance. 

The following are the principal solutions and methods in 
use: 

(1) Ammonium sulphide . 1 to 2 British fluid oz. 
Water 1 imp. gall. 

This solution, while very simple, is one of the most 
useful for obtaining shades varying from light brown to 
black. The depth of colour varies according to the time of 
immersion and temperature. Some operators prefer to use 
the solution warm, but the colour is under more complete 
control if the bath is cold. The uniformity of colour 
obtained is entirely dependent on the composition of the 
surface metal, and consequently more successful results are 
often obtained on freshly electro-deposited copper surfaces 
than on solid copper articles, unless of course the latter are 
given a slight film of metal from a copper depositing bath. 
When the required depth of colour is obtained the article 
should be well rinsed in clean water, lightly scratch-brushed, 
relieved if so desired by means of fine sand, rinsed again and 

2B 



370 ELECTROPLATING 

dried and finally thoroughly brushed, with a little beeswax 
softened by immersion in turpentine over the whole surface, 
by means of a soft bristle brush a plate brush of good 
quality will do very well. 

Coppered goods treated in this way possess a very 
pleasing surface which is improved if the article is periodically 
brushed over with a very slight film of oil or beeswax as 
above. 

(2) Another solution of very similar character to the above 
is composed of 



Potassium sulphide . . J oz. 
Water 1 imp. gall. 



31-2 gr. 
5 litres 



with the addition of a few drops of strong ammonia. 

This bath which is generally used warm gives a varied 
brown tone on copper, often known as Japanese bronze, the 
variation of colour depending on the temperature and length 
of immersion. A few seconds' immersion is usually suffi- 
cient. The articles may be finished as directed under (1), 
or simply scratch-brushed, lightly dried and lacquered (see 
later). 

Solutions of the sulphate or nitrate of copper in water are 
often used in the colouring of copper or copper plated 
articles. Such solutions also give varying tones of brown, 
tending with longer immersion and on heating to black. 
The following solution is an example : 

(3) Copper nitrate .... 4 Ibs. '2kg. 



Water 



5 litres 



. . . 

(or 1J U.S. 

This liquid should be used warm. If a deep black tone 
on copper is required the article should be immersed several 
times, allowed to dry without rinsing, then heated in a 
lacquering stove or over a Bunsen flame gently, and after- 
wards well brushed with a soft brush. 

A fine antique effect is imparted to copper by the 
following : 



METAL-COLOURING AND BRONZING 371 



(4) Copper nitrate 20 oz. 

Hydrochloric acid . . . . 1 Ib. 



Water . 



( 1 imp. gall. 
' JorlJlLS. 



625 gr. 
500 

5 litres 



This solution may be used warm or cold. The effect is 
more quickly and rather more uniformly obtained if warmed, 
but the operation must be carefully observed so as to obtain 
the exact tone desired. The article should be scratch- 
brushed after immersion, relieved if desired, then thoroughly 
brushed over with a waxed brush in the manner previously 
directed or, if preferred, lacquered. Copper, coloured in the 
above or similar solutions, darkens on exposure to the 
atmosphere, hence the necessity for treatment with oil, wax, 
or lacquer. 

Green colours on copper are generally obtained by means 
of solutions of metallic carbonates or chlorides together with 
acetic acid. 

The following are typical solutions : 

(5) Copper carbonate J Ib. 

Ammonium chloride -*- ,, 

Cream of tartar 2 oz. 

Vinegar or dilute acetic acid . . 1 imp. pint, 



(6) Ammonium carbonate . . J Ib. 

Sodium chloride . . . . 2 oz. 

Copper acetate . . . . 3 ,, 

Cream of tartar . . . . 2 

Water 1 quart 



200 gr. 
50 
75 
50 

1 litre 



The above solutions are used for the darker shades of 
green (patina). The following yields a lighter shade : 



(7) Ammonium chloride . . . 4 oz. 
Potassium oxalate . . . . 1 

( 1 imp. gall. 
Water < -, i ^ v 


125 gr. 
31-2 

5 litres 


(or U U.S. 





372 ELECTROPLATING 

Langbein recommends a solution of similar constituents 
dissolved in vinegar. 

In using the foregoing or similar solutions for the pro- 
duction of a green patina, the article should be painted with 
the liquid (or if feasible immersed) as uniformly as possible 
and ivithout rinsing set aside to dry ; while drying ' it should 
be continually touched with the brush to prevent one part 
being more deeply affected than another. The operation is 
then repeated after the lapse of some hours if possible 
twenty-four hours should be allowed, so as to enable the 
action to complete itself as fully as possible the coating is 
again allowed to dry with similar treatment, then if necessary 
treated a third or even fourth time and finally finished off 
with a soft waxed brush as previously directed. 

It is a matter of some importance not to allow the coating 
of colouring liquid to dry quickly the slower the better, and 
some operators therefore add a small amount of glycerine to 
the bath to retard its action in this respect. 

Langbein advises the exposure of articles treated to pro- 
duce a patina, to an atmosphere of carbonic acid gas (C0 2 ), 
by placing them, after brushing over with the solution used, 
in a hermetically closed box in which are arranged one or two 
dishes containing a few pieces of marble (calcium carbonate) 
together with very dilute sulphuric acid, carbon dioxide being 
thereby evolved in a moist atmosphere, thus facilitating the 
formation of a patina. 

A number of pleasing shades of colour can be impacted to 
solid copper goods by heating them either clean or coated 
with some oxidizing substance. A paste prepared by mixing 
equal parts of finely divided plumbago and the finest jeweller's 
rouge with alcohol yields good results in this connection 
Even without such a coating, however, copper heated over a 
clear spirit flame assumes a number of shades of colour, 
varying according to conditions, and due to the oxidizing 
influence of the atmosphere. The colours obtained in this 
way are often improved by dipping the work for a few minutes 
in a hot caustic potash boil. It is then dried, and either 



METAL-COLOURING AND BRONZING 373 

lacquered or thoroughly brushed with a waxed brush. If an 
oxidizing paste, such as described, is employed the article 
should be coated as evenly as possible by brushing the paste 
over it until each part of the surface is uniformly covered 
and it should then be placed in an oven or exposed to an 
even heat. The temperature must be regulated according to 
the colour required. High temperatures must be employed 
for the darker shades and the operation continued longer 
than for light colours. The paste is afterwards removed by 
vigorous brushing, and the surface finished off by rubbing 
lightly with a sponge dipped in alcohol and finally with a 
waxed brush. 

Colours produced in this way are usually very pleasing 
and will resist subsequent atmospheric action. 

Colouring by heat as a method of treating copper is, how- 
ever, obviously confined to solid copper articles and is not 
available for copper-plated work. For the latter class the 
methods previously outlined are most suitable. 

It may be also remarked here that the commoner metals 
such as zinc, tin, and lead, and their alloys, are usually 
coloured by first coating with copper electrically and after- 
wards treating by one or other of the reagents named in 
the foregoing paragraphs. 

Colouring of Brass. The direct colouring of brass 
presents considerably greater difficulty as a rule than that of 
copper. As will be readily understood, a slight variation in 
the composition of the alloy gives rise to modifications of the 
particular chemical actions of the colouring baths used, and 
consequently to differences in the shades of colour produced. 
It is, therefore, often found that a process which produces 
a certain shade of colouring on one class of goods will give a 
decidedly different shade on another. Wherever special or 
very exact tones are required it will usually be found the 
best practice to give the article in question a coating of 
electro-deposited copper and use this as a basis for the 
subsequent colouring. This, however, is only necessary in 
partic ular cases ; for many classes of brass goods the 



374 ELECTROPLATING 

colouring can be imparted directly, small variations of shade 
not being important. 

The following are amongst the most generally useful 
solutions for brass colouring. 

Tones varying from a light straw colour to brown may 
be imparted by the use of an alkaline solution made up by 
mixing copper carbonate with caustic soda of a strength 
corresponding to about 4 oz. of copper salt per imp. gallon. 
The copper carbonate may be bought ready prepared or 
made by dissolving metallic copper in dilute nitric acid and 
precipitating the copper as carbonate by means of sodium 
carbonate. 

The following is a reliable formula : 

Copper carbonate . . . . J lb. j 125 gr. 
Caustic soda 1| ! 750 ,, 



Water 



C 1 imp. gal 
(orlJU.8.,, 



5 litres 



The caustic soda should be first dissolved in the water and 
the copper salt slowly added with vigorous stirring. The 
liquid should be used hot and the time of immersion varied 
according to the depth of colour required ; a very light brown 
colour is first produced passing by longer immersion into a 
dark greenish shade. 

For dark-brown shades on brass, solutions containing 
arsenic or antimony sulphide (sometimes both) are often 
used. A solution typical of many recommended by various 
operators is made up by dissolving antimony sulphide in a 
hot solution of caustic soda thus : 



Antimony sulphide 
Caustic soda . 


. . . . i oz. 

4 


15-6 gr. 
125 


Water .... 


( 1 imp. gall. 


-L^JtS jl 

5 litres 




(orUU.S. 





Immerse the article to be coloured in this solution for a 
few seconds, then lightly scratch-brush, rinse, and re-immerse, 



METAL-COLOURING AND BRONZING 375 

repeating the operation until the colour is sufficiently deep, 
then finally scratch-brush with a very soft dry brush. 

Such solutions as the foregoing and other similar contain- 
ing arsenic often give very pleasing tones of colour, but work 
best when freshly prepared. 

Blue colours on brass. The following solution is very 
widely used for colouring brass : 

Sodium hyposulphite . . . 8 oz. | 250 gr. 
Lead acetate ...... 4 125 



The sodium salt is first dissolved in a portion of the water, 
the lead acetate in the remainder, and the two solutions then 
mixed. The resulting solution is used either boiling or 
very nearly so. A light steely-blue colour results on first 
immersion, the tone slowly deepening as the action 
continues. 

The reactions of this solution on brass are supposed to 
be due to the slow decomposition of the lead hyposulphite 
(formed on the mixture of the solutions) into lead sulphide, 
which reacts upon the brass surface immersed so producing 
the various colourations. 

Some operators prefer to use a solution of double the 
strength given in the above formula. 

Blue-black or black colours on brass are usually obtained 
by using strong ammoniacal solutions of copper. The follow- 
ing is a good solution : 

Copper carbonate . . . 1 Ib. 
Strong ammonia .... 1 imp. gall. 

The copper salt is dissolved in the ammonia, the well-known 
deep blue solution of ammoniuret of copper resulting. To 
this is added Ib. of sodium carbonate dissolved in 1 quart 
of hot water. 

The article is immersed in this solution for a few seconds 
or until the colour is sufficiently deep, then rinsed in clean 



376 ELECTROPLATING 

water and immersed for a short time in a boiling solution of 
caustic potash, re-washed, dried, and lacquered. 

Hiorns recommends a rather simpler method than the 
last, viz. : Take 10 oz. copper nitrate, dissolve in 20 oz. of 
water, and add ammonia until the precipitate which at first 
forms is just redissolved. 

The solution should be used hot, and appears to give 
better results after some little use, but care must be taken 
not to have any excess of ammonia present, since free 
ammonia would tend to dissolve the coloured film. 

Colouring of Iron and Steel. Brown colours on iron 
are obtained by covering with a paste consisting of antimony 
chloride and olive oil in equal parts and slightly heating. 
The paste should remain on overnight, then be rubbed off 
with a soft cloth, and the article again coated with a fresh 
layer of paste and placed in a warm place for a further 12 
hours. The work is then brushed with a stiff brush until 
the paste is completely removed and afterwards finished off 
with a soft waxed brush. 

Before applying the paste the work must be thoroughly 
cleaned and given a final dip in a pickle of dilute nitric acid. 

Blue-black colours on iron are produced by immersion 
in a hot solution of sodium thiosulphate of the following 
strength approximately : 

Sodium thiosulphate ... 4 oz. 
Water 1 imp. gall. 

Pleasing shades of gray are given to iron and steel goods 
by immersion in acid solutions of salts of antimony or arsenic. 
A typical solution is made by dissolving 2 oz. of arsenious 
oxide in a sufficiency of strong hydrochloric acid and diluting 
the liquid to 1 gallon. Such solutions are used hot. 

Iron and steel articles are very often coloured by means 
of heat treatment. A very well-known example of this 
treatment is the Bower-Barff process, which consists essen- 
tially in imparting to the surface of iron a protective film of 
the black oxide of iron (Fe ; .O 4 ) by means of heating to a red 



METAL-COLOURING AND BRONZING 377 

heat in superheated steam. This method, however, obviously 
demands special apparatus. 

In addition to coatings of black oxide produced in this 
way, steel goods may be readily coloured by heating in air 
at various temperatures. The following Table * gives details 
of the colours obtained on steel containing 0-89 per cent, of 
carbon under different temperature conditions. 

TABLE XIV. 

Colours obtained at certain temperatures on steel containing O89 per cent. 

carbon. 

Degrees Centigrade. Colours. 

235 Straw 

250 Brown 

273 Purple 

296 Blue 

336 Blue-grey 

381 to 417 . . . . Blue-black 

Metal-colouring by Electro-chemical Methods. 
Under this heading will be briefly described those processes 
which depend upon electro-deposition by separate current. 

Deposits of arsenic either alone or in conjunction with 
other substances are very often used in this connection. 

Arsenic has a grayish-white colour but in its deposition 
electrolytically various shades may be obtained according to 
the composition and temperature of the solution and the 
current conditions employed. 

The following will be found a very useful solution : 



Sodium arsenate (Na 3 AsO 4 . 12H 2 0) . . J Ib. 
Potassium cyanide .... 6 oz. (approx.) 

Water . 



orliU.S. 



250 gr. 

187 

5 litres 



Sodium arsenate is dissolved in half the water, cyanide 
in the remainder, and the two solutions mixed together and 
boiled. 

The bath is worked hot by means of carbon anodes, and 
an E.M.F. of from 3 to 4 volts is employed. 

* J. 0. Arnold, Jour. Iron and Steel Institute, 1910, No. 1. 



378 ELECTROPLATING 

Another solution of arsenic from which a black pulveru- 
lent deposit is obtained, which, however, adheres very well, 
is made up by dissolving 4 ozs. of arsenious oxide (As 2 O :) ) in 
8 ozs. of hydrochloric acid, and diluting to one imp. gallon 
by the addition of water. This solution is also used hot 
with carbon anodes. A current of low voltage is advisable 
(from I to 1 volt). 

Antimony is also often employed in the metal-colouring 
art to produce light grey shades of colour. Methods of 
depositing this metal by separate current have already been 
described in Chapter XV. 

Black-nickeling. This is probably the most popular 
of the processes of metal- colouring which may be classed 
under separate current methods. 

From a suitable solution a very pleasing dead-black 
colour is produced on almost any basis metal in from twenty 
minutes to an hour. 

The solution used is i practically an ordinary nickel-plating 
solution to which varying proportions of ammonium thio- 
cyanate (NH 4 CNS) has been added; together, in many 
cases, with small proportions of zinc and copper sulphates. 

The following formula has been strongly recommended, 
and has the advantage of being rather simpler than many 
which appear to be in use : 

Double sulphate of nickel and ammonium . 9 oz. ' 285 gr. 
Ammonium thiocyanate ....... 2^V 78 

Zinc sulphate .......... 1 3-1-2 



Water .......... / '' 5 litres 

\or 1J U.S. ! 

It is very important that the solution should be neutral. 

The method of working the bath is much the same as an 
ordinary nickel-plating. Nickel anodes are used, but the 
current must have a much lower voltage than in normal 
nickel-deposition, generally about ^ a volt is sufficiently 
high. If a higher pressure is used, there is a distinct ten- 
dency to whiteness in the colour. Such is the case 



METAL-COLOURING AND BRONZING 379 

sometimes even at the voltage recommended ; but in this 
event a little more ammonium thiocyanate should be added, 
and from time to time also a little zinc sulphate. 

In the preliminary treatment of metal for this process 
the sand-blasting apparatus is a very useful adjunct. By 
means of Trent sand or a medium grade of powdered 
pumice a fine matte may be given to the surface of the 
metal which results in the production, after treatment in 
the black-nickeling bath, of a beautiful satin -like black 
finish. 

To preserve the appearance of black-nickeled goods they 
should always be given a coating of clear lacquer, immedi- 
ately after drying out from the bath. 

General Remarks on Metal-colouring. The ope- 
rations of sand-blasting and scratch-brushing are both of very 
great importance in the art of metal-colouring, inasmuch 
as both the preliminary and final treatment of the surface of 
the article considerably influence the character of the ulti- 
mate finish produced. The art of sand-blasting has already 
been rather fully discussed in the sections dealing more par- 
ticularly with electroplating, and the metal-colourer will 
find a study of those references of advantage. It is also, 
however, of equal importance to realize the possibilities that 
lie in scratch-brushing. Indeed some pleasing finishes can 
be imparted to copper and brass by this means without the 
use of any chemical reagent whatever. On the latter metal 
particularly a very popular finish is produced by brushing 
with applications of fine sand or powdered pumice stone, 
using as a lubricant either water or a very thin light oil. An 
appreciable variety can be obtained in such methods by 
using various grades of brushes, from those of very fine 
wire (45 or 47 B.W.G.) up to strong frosting brushes. 

For the treatment of chemically coloured surfaces the 
scratch-brush is indispensable in the preliminary operations 
and after colouring will be found more generally useful 
than any other process particularly in the case of goods 
intended for subsequent lacquering as most coloured metals 



380 ELECTROPLATING 

are. When used with judgment very delicate shades of tone 
are thus produced, but it is obvious that some experience and 
practice are essential. 

A further matter upon which it is necessary to lay con- 
siderable stress has reference to the colouring of electroplated 
work. Articles which are intended for subsequent colouring, 
particularly chemical colouring, should always be given a 
very substantial coating of the deposited metal. The reason 
for this is that the chemical action of the colouring bath is 
usually that of converting the metal upon which it is re- 
acting into some compound, such as chloride, carbonate, 
sulphide, etc., and if this metal is only a film or very thin 
coating the action quickly penetrates it and in further ope- 
rations the metal below is exposed. In the treatment of 
a zinc article for example, which has been given a coating 
of electro-deposited copper, and subsequently coloured by 
rne&is of ammonium sulphide or a similar solution, then 
relieved on i the scratch-brush or calico-mop ; it is quite pos- 
sible for the copper coating if only thin to be entirely con- 
verted, on the more exposed parts of the surface, to copper 
sulphide, with the consequence that in the relieving ope- 
ration it is readily brushed off, leaving the zinc surface quite 
unprotected. 

Lacquering. As mentioned earlier in this chapter most 
metals after colouring are given a coating of lacquer as a 
final treatment; the purpose being to preserve the colour 
and finish exactly as it leaves the colouring operations, 
and to prevent the action of the atmosphere from affecting 
the appearance when such articles are in use. Lacquers are 
made in immense variety at the present time, and are pre- 
pared by reputable manufacturers with great skill. Many 
different compositions are used, but essentially lacquers con- 
sist of solutions of shellac, seed lac, or celluloid, and similar 
substances in pure alcohol, acetone or amyl acetate or 
mixtures of these. Except when required coloured for 
special purposes, they should be perfectly clear and of a 
thin consistency. 



METAL-COLOURING AND BRONZING 381 

Lacquers are now made suitable for either hot or cold 
application. Cold lacquers are generally applied by means 
of a fine quality camel's-hair brush and then allowed to dry 
cold, but lacquers for use in this way must be specially 
prepared and used according to the directions of the 
manufacturers. 

For ordinary lacquering the work should be first warmed 
to about 60 to 65 C., then dipped into the lacquer, or, if 
more suitable, brushed over with it quickly and in uniform 
direction. The article is then suspended in an oven or 
stove specially fitted for such purposes, heated either by 
gas, steam, or electricity, but in such a manner that the 
interior is kept perfectly dry. The temperature of the stove 
is varied to some extent according to the nature of the 
lacquer, but is generally from 100 to 120 C., and the 
process is continued until the coating of lacquer is perfectly 
dry and hard. j 

If gas is used for heating, precautions must be taken that 
no naked flame is brought near to the lacquer since nearly 
all such liquids are very inflammable. 

II. MECHANICAL METHODS OF METAL-COLOURING can be 
given little description here. They include the use of pig- 
ments of various kinds; the application of specially pre- 
pared bronze powders, and Dutch-metal or gold leaf; also 
of varnishes or coloured lacquers, and other kindred pro- 
cesses. 

The most common of the operations under this heading 
are those involving the use of bronze powders and coloured 
lacquers. The latter particularly are now to be obtained in 
great variety and of excellent quality ; they should be applied 
according to the instructions issued by manufacturers. 



APPENDICES 



1. THE ASSAY OF SILVER. VOLHARD'S METHOD. 

THE principle of this method of silver assaying depends upon the 
fact that when a solution of ammonium thiocyanate is added to silver 
nitrate a white insoluble precipitate is produced consisting of silver 
thiocyanate. If before this addition a small quantity of a ferric salt has 
been added to the silver solution, then at the instant when the whole 
of the silver is precipitated, the characteristic blood-red ferric thio- 
cyanate forms, so that the end of the silver reaction is easily perceived. 

A solution of ammonium thiocyanate known as deci-normal (con- 
taining 7*6 grams per litre) must first be prepared by weighing out 
8 grams of the crystallised salt and dissolving in one litre of distilled 
water. This solution must now be standardised as follows:* Take 
25 c.c. of a deci-normal solution of silver nitrate (16*966 grams of 
AgN0 3 per litre), transfer to a small flask and add 3 or 4 c.c. of a 
solution of ferric sulphate. This salt is made by dissolving a little 
ferrous sulphate (a few crystals) in water to which has been added half 
its volume of strong nitric acid, and boiling the mixture to expel all 
nitrous fumes. The thiocyanate solution is then carefully run in from 
a burette until a permanent red coloration appears. The experiment 
must be repeated several times until a close agreement of the various 
burette readings is obtained. From the volume used the exact strength 
of the thiocyanate solution is calculated, and therefore the amount of 
distilled water which must be added to make the solution the strength 
required, viz. 7*6 grams per litre. 

Now 1 c.c. of the thiocyanate solution contains 0*0076 gram of the 
salt and is equivalent to 0*010766 gram of silver. The chemical 
reaction is shown in the following equation : 

AgN0 3 + (NH 4 )CNS = AgCNS + NH 4 N0 3 
* See Newth's Manual of Chemical Analysis (Longmans), p. 165. 



384 APPENDICES 

To carry out an assay dissolve the metal in nitric acid diluted with an 
equal bulk of water, and make up to a definite volume. Thoroughly 
shake and take a suitable proportion according to the amount of silver 
which the whole is supposed to contain. 

The actual estimation is carried out exactly as directed above for 
standardising the thiocyanate solution, the ferric salt being added to 
the solution to be assayed before addition of the standard solution. 
Several readings should be taken until three successive ones are found 
to be in close agreement. The burette reading multiplied by 0*010766 
(the weight of silver equivalent to 1 c.c. of thiocyanate) gives the 
weight of silver contained in the portion taken for assay. 

Where standard silver and similar alloys have to be regularly 
assayed, and the approximate composition is therefore known, this 
method is particularly useful ; the solution in which the sample is 
dissolved in such cases is diluted to a strength roughly corresponding 
to that of the standard thiocyanate solution. 

The method is one of extreme accuracy in experienced hands, but 
some considerable practice is necessary to get the best results. 



2. THE DETERMINATION OF WEIGHT OF DEPOSIT ON SILVEK- 
PLATED ARTICLES. 

This question is one which, during recent years, has assumed 
considerable commercial importance, due to the growing practice on 
the part of large buyers of such goods to specify the minimum weight 
of deposit which shall be given to each article. In many cases a 
guarantee is required from the manufacturer that such a weight 
actually obtains on the finished article when delivered. It is con- 
sequently often necessary to make determinations of the deposit on a 
sample article taken from the bulk, e.g. a spoon or fork. 

Such determinations are often made in workshop practice by 
weighing a plated article carefully, then stripping the silver deposit 
by immersion in the stripping liquid described on page 213, then 
reweighing and ascertaining the difference, which is taken to represent 
the silver deposit. This method, however, is never quite accurate, 
under the most favourable conditions, as it is practically impossible 
to prevent a slight solution of the basis metal. The best practice 
is, therefore, to strip the silver deposit completely and then assay 
the stripping liquid to determine its resulting silver content. 

A good method is to make up, in a vessel large enough to contain 
the article to be tested, a stripping liquid consisting of powdered 



APPENDICES 385 

potassium nitrate and strong sulphuric acid in the proportion of 
^ oz. of the salt to 1 pint of acid. The containing vessel is then 
placed in a bath of hot water, and the article completely immersed 
until every trace of silver is removed. On cooling, the liquid should 
be considerably diluted by adding to a larger volume of water, and 
the whole bulk made up to an exactly measured quantity by further 
addition of water as necessary. If the resulting volume is not too 
large to be reasonably handled, the whole may now be assayed by 
Volhard's method above described or by that advocated on page 
210. If, on the other hand, the volume is very great some small 
but definite proportion, say j^th or Jjytli is taken, after thorough 
mixing, and assayed, the result being multiplied to give the exact 
weight of the total silver contents. 

3. To CALCULATE THICKNESS OF ELECTRO -DEPOSITS. 

When the electro -chemical-equivalent and the specific gravity of 
any metal are known (see page 393), the thickness of the metal deposited 
per hour with a given current density may readily be calculated, from 
which the thickness per hour for any current spread over a suitable 
area may be deduced. 

Example. Let us assume a current density of one ampere per 
square inch, and calculate the thickness of silver thus deposited per 
hour. 

From page 63. 

Weight of silver deposited by one ampere in one hour = 4*0245 
grams. Assume this deposit to take place on one square inch area. 

Let t = thickness of deposit in inches ; 
then volume of deposit = area x thickness. 
= lxlx cub. ins. 
= t cub. ins. 

But 1 cub. in. = 16-38 c.c., and 1 c.c. of silver weighs 10-5 grams 
(see Appendix 10). 

/. 1 cub. in. of silver weighs 10-5 x 16*38 grams, and t cub. ins. 
of silver weigh 10-5 x 16-38 x t grams. 

But under the conditions assumed 4*0245 grams are deposited 

.-. 10-5 x 16-38 x t = 4*0245 



Hence, with a current density of one ampere per square inch, the 

-2 c 



3^6 APPENDICES 

thickness per hour = 0*0234 inch, and it follows that if I is the 
current and A the area deposited upon, the current density would be 

y, and the thickness would be . ._ . = inch. 

0-0234 x I 

Similar calculations may be made for other metals. 



4. To ASCERTAIN THE CAPACITY OF A PLATING VAT IN GALLONS. 

For rough estimations a fairly accurate method is to multiply the 
length, width, and depth together so obtaining the volume in cubic 
feet and to further multiply the result by 6, thus : 

Find the capacity of a vat measuring 6 feet in length x 2i feet in 
width x 2 feet in depth. 

6 x 2| x 2 = 30 cubic feet. 
30 "x 6 = 187 allons. 



More exact results are obtained by ascertaining the measurement 
of the vat in inches, multiplying the three factors, length, width, and 
depth together, and dividing the result by 277'27. 

Thus, find the capacity of a vat measuring 6 feet 3 inches in 
length, 32 inches in width, and 21 inches in depth. 

75 x 32 x 21 = 50,400 cubic inches. 
50,400 -i- 277-27 - 181J gallons. 

5. TESTING POLARITY OF SUPPLY AND DIRECTION OF CURRENT. 

The terminals of a dynamo are frequently marked + (positive) and 
(negative), while the poles of primary and secondary cells may 
generally be distinguished by inspection. 

In cases where no distinction can be made by inspection, one of 
the following tests may be applied : 

Test 1. Remove about two inches of the insulation from the ends 
of two pieces of thin insulated copper wire, and clean the exposed 
copper. 

Connect one end of each wire to the terminals of the source (if 
this be a dynamo it must be running), and dip the other ends into 
the coppering vat, or a little coppering solution in a bowl, taking care 
that the wires do not at any time come into contact. In a short time 
copper will be deposited on one of the wires ; this wire is connected to 
the negative terminal of the source. 

Test 2. Take the wires prepared and connected to the source as 



APPENDICES 387 

described above, and place the free ends about half an inch apart on 
a strip of pole-finding paper which has been damped with water. A 
red spot will appear on the paper under the wire connected to the 
negative terminal. 

A handy form of pole-finding paper is that known as Wilke's, 
which may be purchased in miniature books similar to litmus paper. 

To determine the direction in which a current is flowing in a 
given conductor, (1) arrange the latter, if possible, in the magnetic 
meridian (approximately north and south). (2) Hold a compass 
needle directly over or under the conductor, and observe the direction 
in which the N. pole of the needle is deflected. (3) Grasp the con- 
ductor and needle with the right hand so that the former is next 
the palm, and the N. pole of the latter towards the wrist, then the 
outstretched thumb pointing along the conductor indicates the direction 
of the current. 

6. DIRECTIONS FOR FIRST-AID IN CASES OF POISONING. 

Plating shop chemicals are for the most part virulent poisons. 
Cases of poisoning therefore by any of them are usually serious, and 
no time should be lost in summoning medical aid. Meantime, how- 
ever, the following information and simple outlines of treatment will 
be useful. 

The usual course adopted in ordinary cases of poisoning is to 
administer immediately an emetic such as detailed in the table at 
the end of this section. In cases, however, when the poison is an 
acid or strong alkali such as are found in plating shops, the proper 
course is to neutralize the poison according to directions below, and 
not to attempt to remove it by giving emetics. 

Poisoning by Hydrochloric, Sulphuric, or Nitric Acids. 

1. Neutralize the acid by giving any one of the following 

(a) Chalk or whiting (calcium carbonate). 
(&) Sodium or potassium carbonate dissolved in plenty of 
water. 

(c) Half to one ounce of magnesium carbonate in a glass of 

water. 

(d) Soap and water in large draughts. 

2. Afterwards give the patient milk and egg, or thick gruel. 
Olive oil (j pint in 1 pint of water) is also very useful in such 

cases. 



388 APPENDICES 

Poisoning by Oxalic Acid or by Salt of Lemons. 

Treatment as above, and after neutralizing administer a full dose 
of castor oil and give milk freely. 

Poisoning by Cyanides or Hydrocyanic Acid. 

1. Place the patient in the open air, and if the poison has only 
just been taken administer an emetic (if not, this ; may be omitted), 
then proceed to give a cold water douche. Let the water fall from a 
height on to the head and spine, or dash cold water on continuously. 

2. Artificial respiration may also be necessary, and the patient 
should be allowed to inhale ammonia by the nostrils. 

3. Administer any of the following stimulants : 

Sal volatile ; brandy ; hot coffee or tea. 

The following is a very useful draught in such cases if a chemist 
is at hand : 

Sulphate of iron 15 grains. 

Tincture of iron perchloride . . 20 minims. 

Dissolve in a wine-glassful of water, and add 1 to 2 drachms of 
magnesium carbonate previously made into a thin cream with water. 
Repeat if necessary. 

Poisoning by Caustic Alkalies (Caustic Potash, Caustic Soda, or 
Strong Ammonia'). 

1. Do not give emetics, but neutralize the alkali by administering 
any one of the following : 

(a) Vinegar well diluted with water. 
(6) Lemon juice in water, 
(c) Tartaric acid, drachm in i pint of water. 
Repeat as necessary. 

2. Afterwards give the patient either plenty of milk, or | pint of 
olive oil in 1 pint of water, or the white of an egg. 

3. Give stimulants, sal volatile, hot coffee or tea. 

Poisoning by Antimony or Arsenic Compounds. 

1. Incessant vomiting usually follows antimony or arsenic poison- 
ing, and this should be encouraged by giving tepid water. If vomiting 
does not occur, give an emetic. 

2. Strong tea should be given as often as vomiting occurs. 



APPENDICES 389 

3. Afterwards, milk or white of an egg, the former freely. 

4. In cases of collapse, give stimulants and apply hot-water bottles 
to extremities. 

Poisoning by Copper Salts. 

1. If vomiting does not occur, administer an emetic, but before 
doing so give large quantities of milk. 

2. Then an emetic. 

3. Afterwards, milk and egg, thick gruel, or barley water. 

Poisoning by Mercury or Mercury Salts. 

1. Give large quantities of white of egg mixed with milk or water, 
or both. 

2. Then an emetic. 

3. If much pain, give the following: 

Opium tincture 20 minims. 

Water 1 oz. 

4. Milk and eggs, gruel, or barley water. 

Poisoning by Silver Nitrate. 

1. First and immediately give : 

One ounce of common salt in a tumblerful of water, and 
repeat if deemed necessary. 

2. Then an emetic to remove the silver chloride formed by the 
above treatment. 

3. Give white of egg in water, freely. 

Poisoning by Zinc Salts. 

1. Do not give emetics, but large draughts of white of egg and 
milk. 

2. Good doses of sodium carbonate dissolved in warm water. 

3. Strong tea, and afterwards thick gruel or barley water. 

4. For acute pain give the opium tincture prescribed above. (See 
Mercury poisoning.) 

EMETICS. 

1. Mustard powder, 1 table-spoonful in a tumblerful of warm 
water. 

2. Common salt, 2 table-spoonfuls in a tumblerful of tepid water, 

3. Zinc sulphate, 30 grains in half a tumblerful of warm water. 



390 APPENDICES 

4. Ammonium carbonate, 30 grains in half a tumblerful of warm 
water. 

5. Powdered ipecacuanha, 30 grains in half a tumblerful of warm 
water. 

6. Copper sulphate, 5 to 10 grains in half a tumblerful of warm 
water. 

7. THE METRIC SYSTEM OF WEIGHTS AND MEASURES. 

On this system, the multiples and submultiples are arranged on a 
decimal basis. The multiples are designated by the Greek prefixes : 
deka = 10, hecto = 100, kilo - 1000. For the subdivisions Latin 
prefixes are employed : deci = T T o, centi = T Q, milli = TQ^O- 

LENGTH. The unit of length is the metre. The British standard, 
kept at the Board of Trade in London, is a bar of a platinum-indium 
alloy, the measurement being represented by the distance between 
two fine lines marked on the bar when the metal is at a temperature 
of C. 



1 kilometre = 1000 metres = 0-6214 mile. 


1 hectometre = 


100 


- 109-361 yards. 


1 dekametre = 


10 


= 32-8 feet. 


1 metre = 


1 


= 39-37 inches. 


1 decimetre = 


o-i 


= 3-937 


1 centimetre = 


o-oi 


- 0-3937 


1 millimetre = 


O'OOl 


= 0-0394 



MASS. The unit of mass, the gram, was derived from the metre, 
and represents very nearly the mass of one cubic centimetre of water 
at its temperature of maximum density, 4 C. A standard weight of 
1000 grams or 1 kilogram is now kept at the Board of Trade. 

1 kilogram = 1000 grams = 2-2046 Ibs. 

1 hectogram = 100 = 3'5274 ozs. (avoir.). 

1 dekagram = 10 = 154-3236 grains. 

Igram = 1 = 15-4324 

1 decigram = O'l = 1-5432 

1 centigram = 0-01 = 0-1543 

1 milligram = 0-001 = 0-0154 

VOLUME. The unit of volume, the litre, is derived from the unit 
of length. The litre is a cubic decimetre, or 1000 c.c. It is therefore 
also the volume of 1000 grams (1 kilogram) of distilled water at 4 C. 
A standard litre is also kept at the Board of Trade, London. 



APPENDICES 391 

1 kilolitre = 1000 litres = 220'4 imp. galls. 

1 hectolitre = 100 = 22-04 

1 dekalitre = 10 = 2-20 

1 litre = 1 = 1'76 imp. pints. 

1 decilitre = O'l = 3-52 Brit, fluid ozs. 

1 centilitre = O'Ol = 0-352 

* 1 millilitre = 0-001 = 16-894 minims. 

8. WEIGHTS AND MEASURES. 

Fluid Measure (British). 
60 minims = 1 fluid drachm. 

8 fluid drachms = 1 ,, ounce.f 
20 ounces = 1 imp. pint.J 
2 pints =1 quart. 

4 quarts = 1 gall. 

Avoirdupois Weight (British and U.S.A.). 
16 drachms = 1 ounce (437-5 grains) = 28-35 grams. 

16 ounces = 1 pound (7000 ). 

28 pounds = 1 quarter. 

4 quarters = 1 hundredweight (cwt.). 

20 hundredweights = 1 ton. 

Troy Weight (British and U.S.A.). 

24 grains 1 pennyweight (dwt.) = 1-555 grams. 

20 pennyweights = 1 ounce (480 grains) = 31-1 
12 ounces 1 pound (5760 grains). 

Apothecaries' Weight (British and U.S.A.). 
3 scruples = 1 drachm (60 grains). 
8 drachms = 1 ounce (480 ). 
12 ounces = 1 pound (57GO ). 



* Commonly known as a cubic centimetre (c.c.). 
t 1 British fluid oz. = volume of a weight of 437'5 grains (i.e. 1 oz. 
Av.) of water = 1-73 cub. in. 

1 U.S.A. fluid oz. volume of a weight of 455'6 grains of water 
= 1-8 cub. in. 

J 1 imperial pint = 20 fl. oz. = 567 c.c. 

1 U.S.A. = 16 fl. oz. = 473-15 c.c. 
1 imperial gallon = 277-274 cub. in. 
1 U.S.A. = 231 



392 APPENDICES 

9. USEFUL DATA. 

1 gallon of water weighs 10 Ibs. and occupies 0-1605 cubic feet. 
1 cubic foot of water contains 6*232 gallons. 
1 pint = 0-567 litres. 1 litre = 1-76 pints. 

1 imp. gall. = 4*54 litres. 
1 oz. per gallon = 6-25 grams per litre, 
lib. =100 

To convert Fahrenheit degrees (F.) to Centigrade degrees (C.), first 
subtract 32, then multiply by 5, and divide by 9. 

5(F.-32) 
0. = g 

To convert Centigrade degrees to Fahrenheit degrees, multiply by 
9, divide by 5, then add 32. 

F. = ^ + 32 
5 

Useful Factors. 

To convert grams into grains multiply by 15 - 432 

ozs. (avoir.) .... 0-03527 

kilograms into pounds 2-2046 

grains into grams 0*0648 

(avoir.) ozs. into grams .... 28-35 

(Troy) .... 31-10 

cubic centimetres into (British) tiuid ozs. 0*0352 

litres 35-2 

British fluid ozs. into cubic centimetres 28*42 

pints into litres 0*567 

,, metres into inches 39-37 

inches into metres 0*0254 

The following information will enable coins to be used as make- 
shift weights : 

One sovereign . weighs 123-274 grains, or approximately 5 dwts. (Troy). 

half-sovereign 61*637 2J 

five-shilling piece 436*363 1 oz. (avoir.). 

half-crown 218*181 

florin. . . 174-543 f 

shilling . . 87*2727 1 

sixpence . 43-6363 ^ 

threepenny piece ,,21*8181 -^ 

penny . . 145*83 J 

halfpenny . 87*5 



APPENDICES 



393 



10. SPECIFIC GRAVITIES OF METALS AT ORDINARY TEMPERATURES. 
(Water = 1.) 

Name. Sp. gr. 

Manganese . . . 7*40 
Mercury .... 13-55 

Nickel 8-80 

Palladium . . .11-40 
Platinum . . . .21-50 

Silver 10*50 

Tin 7-29 

Zinc 6-92 



Name. 

Aluminium . . 
Antimony . . , 
Cadmium . . 
Cobalt . . . 
Copper . . . 
Gold . . . 


SP. gr. 
. 2-60 
, . 6-62 
. . 8-64 
. . 8-70 
. . 8-95 
19-30 


Iron . . . 
Lead 


. . 7-86 
. 11-38 



11. SOLUBILITIES OF VARIOUS COMMON SUBSTANCES IN WATER AT 
ORDINARY TEMPERATURES. 



One part of 


is soluble in 


One part of 


is soluble in 


Citric acid . 


0'75 parts 


Boric acid 


30 pts. 


Ammonium carbonate . 


4 


Mercuric chloride . 


16 


chloride . 


3 


Potassium iodide . 


0-75 


phosphate . 


4 


nitrate . 


4 


Silver nitrate .... 


0-54 


Sodium chloride . 


2'8 


Copper sulphate . . . 


3-5 


phosphate 


6 


Ferrous ... 


09 


Zinc sulphate . . 


0-53 


Magnesium sulphate 


1 


Antimony tartate . 


17 


Lead acetate .... 


0-5 







394 



APPENDICES 



sp^OS'l 



uan^a 
[sp^os 



*ui 'bs 



o 



II 



ip 

gls^ 



00 ^ T^ JP JO CD 
t~ G^l 00 ^ O O O^ CO 

THCqt-O-^COtHrH 

CQiiOOOOOO 



o 



^ CO 00 00 
OCOtDrH 









I 



INDEX 



ACCUMULATORS, 85 

advantages of, 145 

capacity of, 90 

care and management of, 91 

charging of, 93 

working with dynamo, 143 

Acid copper solutions, 247 
Acid, definition of an, 10 
Addition agents, to brassing baths, 
353 

to copper baths, 248 

to lead baths, 327 

- to tin baths, 333 

to zinc baths, 313-315 

Alkaline copper solutions, 251 
Alloys, deposition of, 344, 357 

conditions in, 346 

theories of, 344 

Aluminium, plating of, 164 

preparation of, 163 

Ammeters, 130 
Ampere, definition of, 40 
Ampere-hour, 40 

meter for plating, 134 

Analysis of old silver solutions, 

193 

Anion, 24 
Anodes, 22 

efficiency, 72 

insoluble, 69, 70 

reaction at, 71 

soluble, 69, 70 
Antimony, anodes, 336 

deposition of, 334 

deposits in metal-colouring, 

336 
treatment of, 336 

explosive, 335 

impurities in, 335 

properties of, 334 



Antimony solutions for deposition, 

335 

Antique effects on copper, 370 
Armature, drum, 106-108 
Arsenic, deposition of, 377 
Atom, definition of, 3 
Atomic theory, 4 

BACK E.M.F., 48, 55 
Barrel, tumbling, 151 
Base, definition of a, 10 
Black colours on brass, 375 

nickeling, 378 
Blue colours on brass, 375 
Board of Trade unit, 50 
Brass, anodes, 354 

deposition of, 344 

current conditions for, 

354 

researches on, 355 

solution for, 348-352 
properties of, 347 

solutions, additions to, 353 

estimation of content, 

355 

management of, 354 

Bright gilding, 228, 230 

plating, 206, 208 

Britannia metal, nickelplating of, 
289 

silver-plating of, 203 

Buffing, 148 
Burnishing, 359 
tools, 360 

CADMIUM, deposition of, 322 

current conditions in, 

324 

solutions for, 323 

properties of, 322 



39 6 



INDEX 



Calorie, 54 

and joule, relation between, 

67 

Capacity of plating-vat, 386 
Cathode, efficiency, 72 

movement of, 119 

reactions at, 71 

Cation, 24 

Cells, arrangement of, 94 

care and management of, 84 

E.M.F. of, 94 

Cells, primary, 75 

bichromate, 80 

Bunsen, 82 

chromic acid, 80 

Daniell, 78 

Edison-Lalande, 83 

Fuller's bichromate, 81 

simple, 16, 75 

local action in, 77 

polarization in, 77 

Cells, secondary, 85 

advantages of, 145 

capacity, 90 

care of, 91 

charging of, 93 

uses of, 96 
Chemical effect of current, 15, 17, 

29 

equations, use of, 9 

symbols, 6 

work by a current, 54 

Circuit, electric, 30 

external, 31 

internal, 31 

Circuits, arrangement of, 56, 125 

parallel, 58 

series, 57 

Cleansing electrolytic, 155 

processes, 151 

Cobalt anodes, 307 

compounds of, 305 

deposition of, 304 

current conditions for, 

307 

solutions for, 305 

properties of, 304 

stripping of, 308 

Colour gilding, 234 
Colouring of brass, 373 

of copper, 369 

of iron and steel, 376 

of silver, 376 



Commutator, 107 
Compounds, definition of, 3 
Conductance, electrical, 41 

unit of, 43 

Conductivity, electrical, 43 

of electrolytes, 46 

Copal varnish, 239 
Copper anodes, 257 

assay of, 260 

compounds of, 245 

conductors, 394 
deposition of, 244 

solution for, 247 

electrical conditions in, 

258 

properties of, 244 
Coppering castings, 260 
Coulomb, definition of, 40 
Current, definition of, 40 
: density, 41 

direction of, 385 

measurement of, 130 

unit of, 40 

Cyanide of potassium, 173 

assay of, 176-180 

impurities in, 175 

preparation of, 173 

properties of, 173 

DEPOSITION of alloys, 357 

arsenic, 377 

antimony, 334 

brass, 344 

bronze, 357 

cadmium, 322 

cobalt, 304 

copper, 244 

gold, 217 

German silver, 357 

iron, 297 

lead, 325 

nickel, 270 
alloys, 357 

- silver, 172 

alloys, 358 

tin, 329 

alloys, 358 

zinc, 309 

Difference of potential, 32 
Direction of current, 386 
Double cyanide of silver and 

potassium reactions, 196 
Dynamo, 98 



INDEX 



397 



Dynamo, armature of, 103 

care and management of, 113 

commutator of, 107 

field magnet of, 101 

- plating, 110, 112 

used with accumulators, 143 

EFFICIENCY of anode and ca- 
thode, 72 

of plating solutions, 72 
Electric current, 30 

properties of, 29 

Electrical energy, 50, 114 

conversion of, 17, 53 

Electrical power, unit of, 51 

pressure, unit of, 47 

principles, 29 

work, unit of, 50 
Electro-chemical equivalent, 61, 

63 

Electro-chemical series, 20, 21 
Electro-chromy, 328 
Electro-deposition, quantitative, 

61 
Electromotive force, 33 

" back," 48, 55 

due to electrolysis, 54 

for electrolysis, 65, 68, 70 

generation of, 76, 104 
Electrolytes, conductivity of, 46 

resistivity of, 46 
Electrolysis, theory of, 22, 23 

laws of, 25 

Electrolytic cleansing, 155 
Electrotypy, 264 

moulds for, 265 

preparation of, 268 

Element, definition of an, 2 
Estimation of free acid in copper 

baths, 263 
cyanide in copper baths, 

264 
in gold baths, 228 

in silver baths, 

211, 212 

- of zinc, 356 

FARADAY, the, 64 

Faraday's laws of electrolysis, 25, 

61 

Filter paper, folding of, 210 
Finishing processes, 359 
silver, 363 



Finishing copper, 363 
Force, 1 

Free cyanide in copper solutions, 
252 

in gold solutions, 227, 

228 

in silver solutions, 

assay of, 211, 212 
Fulminating gold, 225 

GILDING, cheap, 231 
dead, 231 

electric current conditions 

for, 233 

frosted, 231 

grained, 232 

green, 236 
in colours, 234 

insides, 233 

preparation for, 230 

by simple immersion, 242 

.watch mechanisms, 232 

Glass, plating of, 166 

Gold anodes, 229 

assay, 218 

in gilding solutions, 

240 

chloride, 220 

compounds of, 218 

deposition of, 217 

deposits, colour of, 233 

properties of, 217 

-- recovery of, 241 
- tests for, 219 
solution, management of, 229 

preparation of, 221 

solution, preparation of, by 

electrolysis, 221 

preparation of, by chemi- 

cal methods, 222 
Green colour on copper, 371 
Gutta-percha moulds, 266 

HANDING, 361 

Heat of chemical combination, 69 

produced by current, 54 
Heating effect of current, 30 
Horse power, 51, 115 

and watt, relation be- 
tween, 53 

ION, 23 

Iron anodes, 303 



398 



INDEX 



Iron, deposition of, 297 

solutions for, 299 

properties of, 298 

pure, by electrolysis, 300 

solution, management of, 

303 
stripping of deposits of, 303 

JAPANESE bronze, 370 
Joule, the, 50 

and calorie, relation between, 

67 
Joule's law, 54 

KERN'S copper bath, 255 

nickel bath, 284 

zinc bath, 318 

LACQUERING, 380 
Lathes, polishing, 138 

scratch-brushing, 136 
Laws of electrolysis, 25, 61 
Lead anodes, 328 

compounds, 326 

deposition of, 325 
solutions for, 326, 327 

impurities in, 326 
properties of, 325 

refining, Betts' process of, 

326 

Lines of force, 99, 100 
Local action, 77 

MACHINE finishing, 362 
Magnetic effects of current, 30, 
100 

field, 99 

Matter, 1 

changes of, 1 

constitution of, 2 

Metal-colouring, 366 

by chemical methods, 

366 
- by electro - chemical 

methods, 377 
by mechanical methods, 

381 



preparation for, 366 



Mho, definition of, 43 
Molecule, definition of, 3 

NICKEL anodes, 284 
compounds of, 271 



Nickel, deposition of, 270 
-- solutions for, 272 
-- - reaction in, 273, 274 
- deposits, stripping of, 291 

- electro-deposited, 271 
fluosilicate, 284 

fluoborate, 284 

- plating Britannia metal, 289 

iron and steel, 290 
-- pitting in, 295 
--- treatment of articles for, 
288 

- - recovery from solutions, 296 

- solutions, analysis of, 275, 

276 
assay of, 292 

conducting salts in, 279 



Desmur's, 281 
- Kern's, 284 

Langbein's, 281 

- . - management of, 287 
-- Potts', 284 

- . - . Weston's, 281 
Nobili's rings, 328 
-- . solutions for, 329 
Non-metallic surfaces, prepara- 

tion of, 165 

OHM, definition of, 42 
Ohm's law, 38, 47 
Oxidizing copper, 372 

PALLADIUM anodes, 343 
compounds, 342 

deposition of, 342 
-- - solutions for, 342 
Parallel circuits, 58 
Parcel gilding, 239 
Partial gilding, 239 . 

- frosting, 163 
Patina, 372 

Plant, arrangement of, 141 

electroplating, 117 
Platinum, compounds of, 338 

deposition of, 337 
--- by simple immersion, 

341 

on silver, 339 
-- treatment of metals for, 

341 

- properties of, 337 

- solutions, 338, 340, 341 
Poisoning, first aid in, 387 



INDEX 



399 



Polishing lathes, 138 

materials, 362 
Potassium auricyanide, 226 

aurocyanide, 226 

Potential, 32 

difference of, 32 

rate of fall of, 33 
Power, electrical, 50 

Primary cells, 75 ; vide Cells, 

primary. 
Processes, cleansing, 151 

preparatory, 147 

scouring, 158 

Properties of a current, 29 

QUANTITATIVE deposition, 61 
Quantity of electricity, 40 
Quanti valence, 11 

RECOVERY of gold, 241 

Red-gilding, 235 

Relief effects on gold, 364 

on silver, 364 

Resistance, electrical, 38, 41 
frames, 123, 126 

laws of, 45 

unit of, 42 

Resistivities, table of, 44 
Restivity, electrical, 43 

of electrolytes, 46 

Rheostats, 123, 126 
Roman gold, 237 
Rose gold, 238 

SAND-BLASTING, 160 

apparatus for, 139 

iron and steel, 162 

nature of, 161 

silver, 162, 163 

table of, 163 
Salt, definition of a, 10 
Scratch-brushes and lathes, 136 
Scratch-brushing, 158, 365 
Scouring processes, 158 
Secondary cells, 85 

advantages of, 145 
capacity of, 90 

care of, 91 

charging of, 93 
Series circuits, 57 
Silver anodes, 199 
frame for, 201 



Silver, deposition, bright, 206 

on Britannia metal, 203 

electrical conditions for, 

202 

on iron and steel, 205 
simple immersion, 214, 

215 

special treatment for, 

202 

deposits, stripping of, 212 

in solutions, assay of, 209 

recovery of, 213 

solutions, management of, 

199 

reactions in, 200 
testing of, 212 

TANKS, cleansing, 135 
dipping, 135 

electrolytic cleansing, 135 

Tin, deposition of, 329 

simple immersion, 333 

solutions for, 330, 331 

solutions, additions to, 333 

management of, 332 

Touchstone, touch-needles, 219 
Tumbling barrel, 151 

USEFUL data, 392 
factors, 392 

VALENCY, 11, 12 
Valencies, table of, 13 
Vats, 117 

- agitators for, 120, 121 

connections for, 119 

framework for, 119 

Volt, definition of, 47 
Voltmeter, 130, 133 

WATT, the, 51 
Weight, atomic, 6 

equivalent, 11 

molecular, 9 

of deposit, calculation of, 385 

Weights and measures, metric, 

390 
imperial, 391 



. U.S.A., 391 

Wiring, 170 

Wood, plating on, 169 

Work, electrical, 50 



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