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 weight—is 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
0
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
N02
7:16
Nitrogen pentoxide .
28
80
N305
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 0 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 : — H2O (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, (NH4)2SO4. 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 NH4 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 2H20 represents two molecules of water,
FUNDAMENTAL CHEMICAL PRINCIPLES 9
2HC1, two molecules of hydrogen chloride, 2(NH4)2SO4, 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 O2,
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 C12 = 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 + H2S04 = ZnSO4 + H2.
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 + H2
Sulphuric acid (H2S04) H2S04 + Zn = ZnS04 + H2
Nitric acid (HNO3) 2HNO3 + 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 + H2SO4 = CuSO4 + H2O
(Silver oxide) (Nitric acid) (Silver nitrate) (Water)
Ag20 + 2HN03 = 2AgNO:5 + H2O
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 = NH4C1
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 (H2O =, 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 H2 -f- C12
= 2HC1. Similarly sodium replaces hydrogen in equal
atomic proportion, 2Na -f 2HC1 = 2NaCl + H2. 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.
i4 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 SO4, 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 + H2S04 = ZnSO4 + H2
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 H2 and SO4 by means of electrical action,
and the energy required
is produced by the com-
bination of the zinc with
the SO4 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 (CuS04), 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
SO4 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- SO4 being
higher than that of Cu + SO4, the zinc will dissolve and
form ZnSO4, and as a consequence metallic copper will be
liberated on the surface of the zinc immersed, thus —
2Zn + 2CuS04 = 2ZnSO4 + 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 = ZnCL2 + 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, CuSO4, 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 S04 respectively do not exist
merely as chemical individvals. " Cu " is the chemical
symbol for metallic copper. " SO4 " 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 S04 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 S04 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, SO4, chemically com-
bines with the metal of the anode and forms copper sulphate.
Cathode Ion Ion Anode
Before
After
Cu SO4
<- ->
Cu S04
SO4 4- Cu = CuSO4
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 S04, which being liberated at
the anode, combines with it to reform CuSO4, 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 S04 attacks the water present,
and liberates oxygen with the formation of sulphuric acid, thus
2S04 -f 2H20 = 2H2S04 + 03.
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, T2, T3, T4, T5, 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 S2 corresponding to the
stop-cock, and conductors MA2, A2B2, and B2C2D2 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 Ax or A2 and M are close to one another or con-
nected by pipes or wires of large area as shown, and likewise
points D! or D2 and N, so that virtually A2 and D2 are con-
nected to the terminals MN of the dynamo, while in the
FUNDAMENTAL ELECTRICAL PRINCIPLES 35
water circuit At and Dt 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 Gr 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 Sx 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 Tlf T2, 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 B2 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 S2 to D2, via C2. But the potential of the portion
MA2B2 is higher than that of S2C2D2, 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 MA2B2, the other to
any point on the conductor S2C2D2, 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 Ax to Blt 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 B1} 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
T2 and T3 is the same as that between stand-pipes T3 and T4.
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 B3F3G8H8Ia.
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 T4 than between T, and T., must be put down
to the extra resistance introduced by the restriction between
d./ly 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 MA2B2C2N, 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 A2B.,
to have a constant cross-sectional area, the fall of potential
is indicated by the full straight line EaA2, representing to
scale the potential at A2 with respect to zero or the earth's
potential ; similarly the heights to the full straight line re-
present the potentials at dlt d2, etc. If, therefore, the length
d^L — the length d2d-3, then the P.D. between di and d2 = the
P.D. between d» and cl3, 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 0° 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 0° 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
0° 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 0° 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(0hni8) =
./sq. cm.) X '(cms.)
106 X A(gq. cms.)
46 ELECTROPLATING
, N -D O" (ohms per in. /sq. in.) X '(ins.')
W EC'""»5> = A(sq.ins.)
/ 7\ T> ^(microhm per in./sq. in.)' X ?(ins.)
(«) -K(ohms) = -lfy, *
1U X A(Sq. 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
= I2E,
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 dl 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 </! d2, then the energy used up in the portion considered
in t seconds is IV£ or I2!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 I2R> 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 et 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.
56 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 Elf E2, 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 Blt E2, E3. 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 Elf B2, E:!, E4 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, E2, B:, represent the resistance of the
respective elements, then
E (the total resistance) = Ej + E3 + 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
E2
conductance is then ^- _j_ =g-, and similarly when E.> is also
K! 1*3
introduced the total conductance = ^ 4. =- 4. ^ , and so on
±V1 ±V.2 ±1.,
for any number. The total conductance is, however, the
reciprocal of their combined resistance, E, therefore
E = E! + E3 + R:
Example : — Let Ej = 4 ohms, E2 = 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
SO4. 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? = 4o7,916
= 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 = -jL1- — r— ,
coulombs
the electrical pressure required is —
%$£-*»***•
Another example which may be taken (almost a classical
one) is acidulated water, H2O.
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 NiSO4 229,400
Cupric chloride CuCl2 51,630
Cupric sulphate CuS04 182,600
Silver nitrate AgN03 28,700
Gold chloride AuCl3 22,800
Water H20 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 CuS04 is re-
formed by combination of SO4 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, ZnS04 .
Nickel „ NiS04 .
2-35
2-09
Sodium hydroxide, NaOH
Ammonium hydroxide,
NH4OH
1-69
1-74
„ chloride, NiCl2 .
Lead nitrate, Pb(N03)2
Silver „ AgN03 .
Nitric acid, HN03 . .
18-5
1-52
0-70
1-69
Cadmium nitrate, Cd (NO 3)2
Cobalt sulphate, CoS04 .
Sulphuric acid, H2S04 .
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 S04 -> Anode.
QUANTITATIVE ELECTRO-DEPOSITION 71
Cu is consequently deposited as metallic copper, and SO4
is left, which, however, is liberated in contact with a fresh
supply of metallic copper, and we get the re-formation of
CuSO4 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 S04 is liberated
in contact with the Pt anode. Now, however, it is evident
that no re-formation of CuSO4 can take place, and what
happens is that the S04 (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
S04 + H20 = HaS04 + 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, " H2 " and " S04 "
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 H2S04 = ZnS04 + H2
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 ; H2S04, dilute sulphuric
acid ; CuS04, 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 + H2SO4 = ZnS04 + EL
(2) In outer vessel H2 + CuSO4 = H2SO4 -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
(K2Cr2O7) 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 parts 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 ; H2S04, 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 = 3ZnS04 + 3H2.
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 —
K2Cr2O7 + 7H2SO4 + H2O = 2H2CrO4 -f K2S04 + 6H2SO4.
.^ (chromic acid) (potassium
^^ sulphate)
(3) 3H2 -f 2H2Cr04 = Cr2O, + 5H2O.
(fronv(l)) — • (chromium
<£— — trioxide)
(4) Cr203 + 3H2S04 = Cr2(S04):! + 3H20.
(5) K2S04 + Cr2(S04)3 = K2Cr2(S04)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
(K2Cr2(S04)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 H2SO4 = ZnSO4 -f H2.
(2) In porous pot H2 + 2HN03 = 2NO2 -f 2H2O.
(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 ;
H2S04, dilute sulphuric acid ; HN03, 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
84
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 = K2ZnO2 -f H,.
(caustic potash) (potassium zincate)
(2) H2 + 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 H2SO4 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 (PbO2), 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 (Pb304) and sulphuric acid, the mere mixing of which
forms a certain amount of PbO2 according to the following
equation : —
Pb;!O4
4 -f H20 = PbO, + 2PbS04
2H2O.
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 2PbS04
== 3Pb02 + 2H.,S04 + 2H2.
(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 PbS04 (lead sulphate),
PbO + H2S04 + H20 = PbS04 -f 2H20.
Electro-chemical treatment, by making them cathodes in
dilute sulphuric acid, reduces the PbSO4 to spongy lead, due
to the action of the hydrogen liberated, thus : —
PbS04 + H2 = Pb + H2SO4.
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 PbS04J
while the peroxide plate is converted into PbSO4, 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 + H2S04 + 0 = PbS04 + H.O.
At the positive plate
Pb02 + H2S04 + Ha - PbS04 + 2H20.
Chemical Changes during Charge. — During charging
oxygen is liberated at the positive and hydrogen at the nega-
tive plate. The PbSO4 on the plates is reconverted into
Pb02 and Pb respectively, water is used up and sulphuric
acid formed, the reactions being —
At the positive plate
PbS04 + 2H20 = Pb02
At the negative plate 4
PbS04 + H2 = 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\Nv.
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'°^,*;n746 = 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. thouSh rather exPen'
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 Tx, 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.
CM O t- O O
o t~ TH co co t—
tH CO OjD TH O5 GO
6 6 6 CM O GO
00 t-TH 05 TH 10
cb cb »b 6 cb t-
00 »0 CO Oq TH
TH »o O O CM O
2 .a iH CO CO CO CO CO
6 6 6 <M *b t-
iH
»O CO CO O5 -* GO
fl o CO CM Oi CO O5 TH
SS COiOt^COOt-
S 2 TH CM co
" *a
O TH GO O CO O
6 b- b- tH rH GO
00 O CO CM iH . _
g^
g_ ^ CO CO t- CM ?5'
ft CMOO5QO»OCO &
TH CO CO C5 GO CO
6 6 6 TH TH o
49
Q K^
^jiOOOOJCMTH p^il
«>j C30O«MTHCpCp H^
If S^g^S^
_fS rHTH ^
OCO C5 O O CO O
TH CO CO CO CO CJ5 «!
O CO TH CM rH
O TH O O O
S£" OtHCOpCOTH
6 6 6 tH CM GO
- _5> :_ *?
«j
Jc &?» Oicpt-t-kpo H'H'
c ao iOTHt-coost- §§
^i__^_^^. °°
° g, • THOOTHTHOT
<^ '~> O5 b- O CO
t- TH CO TH
s ! 8^S
0
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 Wx 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 Wlf 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 I2E joules per sec.
(see page 52), and obviously as I is the current we desire
to measure, the first factor (I2) 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
V2
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
i36
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 *
(H2S), 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 (AgNO3) 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 K4FeC6N6 . 3H2O, and
potassium carbonate (K2CO3). The reaction of these sub-
stances in fused mass results in the formation of potassium
cyanide (KCN), potassium cyanate (KCNO), carbonic acid
gas (CO2), 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 : —
K4Fe(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 : —
AgNO3 + KCN = AgCN + KN03.
This occurs in the proportion of their respective mole-
cular weights, viz.
AgN03(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 AgNO3 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(AgN03) 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.
AgN03 4- 2KCN = KAg(CN)o (a soluble compound) + KNO
170 2 (65)
relative weights
Therefore 170 AgN03 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 AgN03 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.
SdVyihe*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 2H20 = 2KHO + H2.
The products of electrolysis at the cathodes are therefore
potassium hydroxide or caustic potash (KHO) and hydrogen
gas (H2), 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 H2
* 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.2O.
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 AgNO3 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
HN03 (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
AgN03 + KCN = AgCN + KN03.
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 (KN03), 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 (Ag2COa). 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.
i92 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. + Na2CO, = Ag2CO;3 + 2NaN03
(washed away).
(2) Ag2C03 + 4KCN = 2KAg(CNJ2 + K2CO3
(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,C03 13-05 . . 11-49
sulphate, K8S04 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
0
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;! = BaCO3 +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 —
AgN03 + 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 xl = 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 (CS2) 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 CS2 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 CS2. 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 CS2 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 CS2, 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, Au013, 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)4.3H2O.
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..HC1.4H20. 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.
224 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 —
AuCl3 + 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 : —
2AuCl3 + 8NH4HO = Au(NH)NH2 + AuNHCl + 5NH4C1
Fulminating gold. + 8H2O.
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
(AuCl3.HCUH20) ' • | • ' 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)4.3H2O (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
23o 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,
As2O, (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 (CO2) 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 (S02), thus :—
Cu + 2H2SO4 = CuS04 + SO2 + 2H2O.
Compounds of Copper. — Copper forms two series of
compounds, originating from two oxides, cupric oxide CuO,
and cuprous oxide Cu.2O, 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 CuS04.5HoO.
Its solubility in water is as follows : —
246 ELECTROPLATING
Temperature. Degrees centigrade.
Parts of CuSO 5HO) 1Qo 20o 30o 50o 7Qo 9Qo 10Qo
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.H20. 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(NO3)2.3H2O.
Cupric chloride, CuCl2.2H2O, 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, Cu2Cl2, 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 CuSO4.5H2O . 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 CuS04.5H2O . 32 ... 14-87
Sulphuric acid H.2SO4 .... 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 (CuS04.5H20) . . 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 (CuSO4.4NH3.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 —
Na2CO3 + H2S04 = Na2S04 + CO2 + H20
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
H2SO4. 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 per
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, Ni2O3, and Ni304, 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 : —
NiS04 into Ni + SO4
(NH4),S04 „ 2NH4 + SO4
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 : —
NiS04(NH4),S04 = 2NH4 + Ni(SO4).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 S04 or the complex
T
274 ELECTROPLATING
anion Ni(S04)o combines with the water of the solution,
thus —
(a) 2S04 + 2H20 = 2H2S04 + O,
(If) 2Ni(SO4), + 2H,0 = 2NiSO4 + 2H2S04 + 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 S04 or Ni(S04)tJ, thus—
(a) S04 + Ni = NiS04
(b) Ni(S04), -f Ni = 2NiS04
At the cathode, on the other hand, the reactions which
may occur are —
Either (1) The discharge of the cations Ni and 2NH4 re-
spectively,
or (2) The discharge of the cations 2NH4 with the con-
sequent liberation of metallic nickel as a secondary reaction
with the undissociated molecules of nickel ammonium sul-
phate, thus —
2NH4 + NiS04(NH4)2S04 = 2(NH4),S04 + Ni
The first of these results in a deposit of metallic nickel
with simultaneous liberation of 2NH4 which breaks up into
2NH3 and H2 (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(NH4)HO + H.J I S04 + Ni = NiS04.
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 } (SO4 + Ni =NiS04
2(NH4)HO + Haj JS04 + H,0 = H,SO4 + 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
NiS04(NH4)2SO4.6H,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 NiS04 . (NH4)2S04 \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
NiSO4.6H.2O. The crystals from an aqueous solution have
the composition NiS04.7H2O. 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
NiS04.
According to Tobler the solubility of this salt is as
follows : —
Temperature in degrees Centigrade.
Parts of NiS04 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 ,
NiC2O4 . (NH4)2C2O4. 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
286
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 AgNO3 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 —
NiS04 + 4KCN =-- Ni(CN)., . 2KCN + K.,S04
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, Fe304.
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 . 2NH4C1 was the compound alternatively
used. It was prepared by dissolving equivalent proportions
of crystallized ferrous chloride (FeCl.2 . 4H2O) 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.22NH4Cl 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 . NH4C1.
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, Co2O:!, and Co,O4
(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 CoCl2 . 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 CoCL2 . HO2, 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 CoS04 . 7H20, 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, CoSO4K2SO4 .
6H2O — 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 SO4 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, ZnCl2.2(NH4)Cl and
ZnCl2.3(NH4)Cl. Those double salts have been suggested
and often used for zinc deposition.
Zinc sulphate, ZnSO4.7H20, 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.
ZnSO4.K2SO4.6H2O, 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.
3i4 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 (ZnSO4 . 7H2O) . 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 (ZnSO4 . 7H20) ..... 30 oz. j 937 gr.
Magnesium sulphate (MgSO4 . 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 11US 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 (A12(SO4):!) 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(N03)2.4H20.
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.
324 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
(Pb012), the nitrate (Pb(NO3)2), the carbonate (PbC03), and
the sulphate (PbSO4). Other salts which have been
brought into prominence in electrolytic practice recently are
the fluosilicide (PbSiF6) and the perchlorate (Pb(01O4).2.3H2O).
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(C104).23H.,O] . 1 Ib.
Perchloric acid (HC104) 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 imP- 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^'a8*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 (SbCl3), 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 (PtCl4), potassium chloro-
platinate, K2(PtCl6), and the corresponding ammonium com-
pound (NH4)2(PtCl6), 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 (PdCl2), the nitrate [Pd(NO3)J,
and the cyanide PdCN2. 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
K2PdCl4, ammonium palladiochloride (NH4)2PdCl4. The
cyanide also forms a double salt with the alkalicyanides, the
potassium salt having the formula K2Pd(CN)43H2O.
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 ] ViTa6" ' 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 CO2, 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 (Zn2P207). 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 (Sn012). 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
36°
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 (C02),
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;.O4) 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 (Na3AsO4 . 12H20) . . 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 (As2O:)) 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 (NH4CNS) 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
AgN03 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 : —
AgN03 + (NH4)CNS = AgCNS + NH4N03
* 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 . ._0 . = 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 = TTo, 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 0° 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 »0 ^ T^ 0 JP JO CD
t~ G^l 00 ^ O O O^ CO
THCqt-O-^COtHrH
CQi—iOOOOOO
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
396
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
PRINTED BT
W1LIIAM CLOWES AND SONS, LIMITED,
LONDON AND BECCLE8.
14 DAY USE
RETURN TO DESK FROM WHICH BORROWED
This book is due on the last date stamped below, or
on the date to which renewed.
Renewed books are subject to immediate recall.
UI»ar'58jZ
9 1959
REC'D LD
>-. . '
MAR 11 195
'
LIBRARY USE
MAY - 1 $58
6Nnv'58JT
6BS8
LD 21-50m-8,'57
(.C8481slO)476
General Library
University of California
Berkeley
YB 15194
/u
rsi
6:3
282167
• .
UNIVERSITY OF CALIFORNIA LIBRARY
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11