Full text of "India-rubber laboratory practice"

See other formats

INDIA-RUBBER LABORATORY
PRACTICE

MACMILLAN AND CO., LIMITED

LONDON . BOMBAY . CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
DALLAS . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD.

TORONTO

INDIA-RUBBER
LABORATORY PRACTICE

W. A. CASPARI

&Sc. (Viet.), PH.D. (Jena), F.I.C.

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1914

PREFACE

IN these pages an attempt is made to give the
specialized practical information — at least in broad out-
lines— required by chemists of sound general training
who may be called upon, in whatever capacity, to deal
with india-rubber and its accessories. The botany and
theoretical chemistry of india-rubber, with which it has
become almost common form to embellish all books on
the subject, are here barely mentioned. More reluctantly,
two important branches of science applied to india-rubber?
viz., the production of the raw material and the mechanical
testing of manufactured rubber, have also been passed
over, mainly because they are as yet in the mere
beginnings of giving rise to systematic laboratory practice.

There are two kinds of analytical text-book, the more
or less exhaustive and less or more critical compilation
from literature, and the strictly practical (and inevitably
somewhat biassed) manual based on personal experience.
Which of the two is the more valuable is a debatable
point ; and indeed they are rather complements than
competitors. The present work, at any rate, claims in all
modesty to belong to the second category, no analytical

vi PREFACE

method being described in it which has not been practised
and found satisfactory by the author, in the course of the
last ten years. If thus some information of value to the
rubber chemist fails to find a place, it may be hoped, on
the other hand, that the minimum of space is taken
up by matter which time and experiment have proved

otiose.

W. A. C.
LONDON, Dec. 1913.

CONTENTS

CHAPTER I

PAGE
CRUDE AND WASHED RUBBER 1

Wild and Plantation Rubbers. Sampling. Analysis.
Significance of Analytical Data.

CHAPTER II

MACHINERY AND APPARATUS 18

Washing Mills. Determination of Washing Loss. Mixing
Mills. Centrifuges. Laboratory Apparatus.

CHAPTER III

RUBBER DILUENTS 37

Factice. Reclaim. Bitumen and Pitch. Resins.
CHAPTER IV

SOLID COMPOUNDING MATERIALS 60

Accelerators. Fillers. Pigments.

CHAPTER V

MISCELLANEOUS ACCESSORIES 85

Solvents. Sulphur and Sulphur Chloride. Oils and
Waxes. Fabrics.

viii CONTENTS

CHAPTER VI

PAGE
SPECIFIC GRAVITIES 99

Liquids. Powders. India-Rubber.

CHAPTER VII

ANALYSIS OF MANUFACTURED RUBBER: ORGANIC . . . .110

Extractions. Combined Sulphur. Unvulcanized Goods.
Solutions. Ebonite. Reclaim in Rubber Goods.

CHAPTER VIII

ANALYSIS OF MANUFACTURED RUBBER : INORGANIC . . . 140

Isolation of Charge. Analysis of Charge. Special Deter- •
minations. Ash-Analysis.

CHAPTER IX

GUTTA-PERCHA AND BALATA 167

Crude and Manufactured Material. Analytical Methods.
APPENDIX

TABLES 181

INDEX 193

INDIA-RUBBER LABORATORY
PRACTICE

CHAPTEE I

CEUDE AND WASHED RUBBER

THE raw material which forms the basis of the'rubber
industry is a gum imported from the tropical regions of
Asia, Africa, and America. Most of it is obtained prim-
arily in the form of an aqueous emulsion or latex secreted
by the bark of certain trees and creepers ; from this latex
the rubber is won, as a tenacious, springy, self-adhesive
mass, by methods of coagulation which vary somewhat
widely with botanical origin, local custom, and other
factors. In a few instances (notably that of Guayule, a
Mexican sort), the rubber as such is directly extracted
from roots and other plant parts. Bather more than
half the total production of rubber originates from plants
growing wild, and this " wild " rubber, which is almost
the only kind produced in America and Africa, is always
more or less moist, dirty, and inhomogeneous. The

CHA1'.

2 ^ c JMr)IA:RU£$ER LABORATORY PRACTICE

imports from Asia nowadays consist for the greater part
of clean dry rubbers prepared by rational methods on
plantations organized and managed by Europeans in
Ceylon, the Malay Peninsula, the Malay Archipelago
islands, and India. At the same time, plantation rubber
is now also being produced in East and West Africa,
whilst Asia continues to export a certain amount of wild
rubber.

Wild rubbers arrive in a great variety of forms, such
as sheets ; blocks or slabs composed of sheets ; balls
ranging in size from a fives ball to a large gourd and
composed either of stratified concentric layers or of
wound cord; spindles, thimbles, spiral twists, etc.; and
scraps of irregular size and shape, which represent the
offal of larger pieces and may be either loose, or com-
pressed into blocks, or sewn into bags. Each sort of
rubber has a characteristic and more or less powerful
odour, which may be due to resinous constituents, de-
caying albuminoids, or foreign matter introduced in the
preparation. Wild rubbers are usually soiled with vege-
table refuse and earthy impurities and in addition contain
amounts of moisture which may rise to 50 per cent, or
over. In the manufacture of rubber goods, the crude
wild rubber is first of all subjected to processes of washing
and drying whence it emerges in the form of corrugated
sheets or crapes (also spelled crepes) which are light grey
when wet and dark grey or brown when dry. The
percentage diminution of weight undergone by crude
rubber as the result of this treatment is known as
washing loss ; it represents the moisture, dirt, and
(almost, but not completely) the soluble non-rubbers
originally present, and its amount, needless to say, is of
high commercial importance.

i CRUDE AND WASHED RUBBER 3

Plantation rubbers are produced on well-organized and
scientific lines, and leave little to be desired on the score
of purity. As a rule they are sufficiently free from
moisture, dirt, and albuminous matter to require no
washing in the factory. They appear on the European
markets in the form of sheets ; biscuits resembling tea-
cakes ; crapes ; or translucent blocks. The colour is
mostly light yellow and the odour sweetish and very
faint. Smoked sheet and crape have a dark red colour
and a pronounced smoky smell.

Chemically, crude rubber is composed of rubber proper
(rubber hydrocarbon), resin, moisture, dirt, albuminoids
or " protein," arid soluble organic substances of the nature
of carbohydrates and tannins. The four last-named
impurities are removed in the washing and drying
operations, and constitute washing loss. Speaking
generally, and leaving out of account physical and
vulcanizational properties, it suffices for purposes of tech-
nical analysis to determine total washing loss and, in the
thoroughly washed and dried material, the proportion of
resin to true rubber and the ash. At the same time,
occasions when more detailed analyses are desirable arise
frequently enough, notably in the case of abnormal or
unfamiliar rubbers.

Now crude rubber, especially when it takes the shape
of large compact masses, is such a very inhomogeneous
material that correct sampling is a matter of no small
difficulty. The only really accurate way, therefore, of
ascertaining washing loss is from the weights recorded in
the factory process itself, where batches of the order of a
hundredweight are worked up. But the problem which
commonly confronts the chemist is that of determining
washing loss upon a comparatively small sample. This

4 INDIA-RUBBER LABORATORY PRACTICE CHAP.

is done in the laboratory by means of a miniature washing
mill and drying oven, and is in itself an operation
admitting of high precision. If a definite hand-sample
of crude rubber should be submitted, then the whole of
the sample is thus dealt with, and the chemist's responsi-
bility is limited to carrying out the assay correctly.
When, on the other hand, a large quantity of rubber has
to be reported upon, much care and judgment must be
exercised in drawing as representative a sample as
possible. Generally speaking, rubbers in small pieces
can be far more accurately sampled than those which are
in large masses, because the former are usually com-
paratively dry, whereas the moisture content of the latter
is very unevenly distributed.

A ten to twenty pound lot of crude rubber should be
drawn for experimental washing, and should be imme-
diately wrapped in oilcloth or rubber sheeting pending
treatment, so that evaporation of moisture may be
reduced to a minimum. The following modes of proce-
dure, modified in such ways as the sampler's judgment
may suggest, are to be recommended for sampling from
bulk :-

If the consignment consists of more than one cases or
bales, it is best to take one sample from each. Should
there be many packages, the contents of which seem
much alike on inspection, a sample may be taken from
every second or third package. Each sample is assayed
separately for washing loss, and the arithmetical mean is
taken. Aliquot parts of each washed lot are then mixed
on the rollers to form a single sample on which the
further analysis can be conducted.

The whole contents of a package should be turned out
on the floor and inspected. If the rubber is in the form

i CRUDE AND WASHED RUBBER 5

of fairly regular small pieces, sampling is comparatively
easy : pieces of normal appearance are picked from
various places all over the heap until the required
quantity is made up. If the pieces are balls of medium
(and possibly varying) size, a representative collection,
amounting to more than the sample quantity, is first
made, and the sample is then reduced to convenient
weight by halving or quartering each ball.

In the case of small scrap, the bulk should be classified
by eye into a few dominant categories according to size
and shape, and specimens of each picked in due proportion.

Large balls, slabs and sheets are more difficult to
sample. Usually the outside is the dirtiest and driest
portion, moisture and with it the soluble constituents
being concentrated inwards ; often a white, dripping _wet
core is concealed by a dark dry exterior. There will not
be many large balls or ball segments in one package, and
the safest plan is to cut a proportional fragment from
each. The correct sample to take from a ball is a wedge
with its apex at the centre. The cutting is performed
with a long-bladed knife or toothless saw. A half-ball,
i.e., a hemispherical piece, may be sampled by cutting
out a parallel-sided slice through the middle.

With sheet and slab rubber also, it is best to cut
a portion from each slab or agglomeration of slabs that is
covered all over with a dark crust. A slice or rasher cut
diagonally from corner to corner of a rectangular slab re-
presents the best average ; but it does nearly as well, and
is less troublesome, to cut a rasher right through the
middle, at right angles to the longer side of the slab.
The bulk of each slice should be roughly proportioned to
that of the slab itself, and total weight may afterwards
be reduced, if necessary, by halving.

6 INDIA-RUBBER LABORATORY PRACTICE CHAP.

Details of the process of determining washing loss are
given in the next chapter (p. 21). Washing losses and
other analytical data for the chief types of crude rubber
are tabulated in the Appendix (p. 181).

When it is desired to make an analysis of crude rubber
before it has been washed, a suitable sample, which may
be much smaller than a washing sample, is dried in the
vacuum oven and is subsequently mixed to a homogeneous
sheet on the hot rollers. This method is by no means to
be recommended for crude rubber analysis generally, but
may often prove serviceable in the absence of a washing
mill, or when the rubber is not intended to be washed on
the large scale.

Plantation rubbers, in the nature of things, are mostly
quite uniform for a given consignment from one estate,
and seldom contain more than one or two per cent, of
moisture, dirt being practically absent. There is rarely
need, therefore, for extra care in sampling, or indeed
for washing. Unless obviously wet or dirty, plantation
rubbers may be regarded as ready for chemical examina-
tion without preliminary treatment.

Analysis of Crude or Washed Rubber.

1. Moisture. — The most satisfactory method of deter-
mining moisture consists in heating 5-10 gr. of rubber,
cut into snippets and laid on a flat porcelain dish, in a
laboratory vacuum-oven (see p. 23). Factory-dried
rubber requires about two hours, being already not far
from anhydrous ; damp material such as unwashed crude
rubber should be heated until the weight is constant, and
may require a day or more. Unvulcanized rubber of the
better sort, being less prone to oxidation than vulcanized,
may indeed be dried in an ordinary air-oven without

I CRUDE AND WASHED RUBBER 7

serious error, provided the operation be conducted ex-
peditiously. Flabby or resinous sorts, however, are best
dealt with in vacuo at a temperature of about 60°.
Drying over sulphuric acid in a vacuum-exsiccator is an
excellent method, but far too tedious, since it always
requires days and sometimes weeks.

2. Resin. — In rubber generally, whether vulcanized or
not, resin is determined by extraction with boiling ace-
tone. This is carried out by means of a Knofler extractor,
which acts intermittently on the Soxhlet principle whilst
surrounding the centre of extraction with acetone vapour
(cf. Fig. 13). For raw rubber analyses the inner tube
may with advantage be about 25 mm. wide, with a
siphoning height of about 8 cm. ; a filter- thimble may be
dispensed with. The rubber should be cut up into cubes
of about 2 mm., but very thin crapes may be extracted
without previous comminution, if only they are rolled up
loosely enough to provide circulation-space within the
spiral. Soft resinous rubbers are apt to run together in
the extractor and so prevent complete extraction. In
such cases the original snippets may be spread on a, sheet
of muslin which is then rolled into a spiral. Or again,
the snippets may be extracted as usual and the sintered
lump dried, cut up afresh, and re-extracted.

Of Brazilian and plantation rubbers not less than 10 gr.
should be weighed out ; of African sorts, Borneo, Guayule,
&c., 5 gr. will suffice. A duration of ten hours is ample
for complete extraction. The acetone solution is then
distilled nearly dry on a water-bath and heated for three
hours in an air-oven at 110°, by which time the weight of
the residue, unless there be an unusually large amount of
it, will be constant. Fluid resins of the Brazilian and
plantation types give off volatile matter at 110° and never

8 INDIA-RUBBER LABORATORY PRACTICE CHAP.

attain real constancy, but three hours is a useful con-
ventional drying period to apply to them.

With the majority of crude rubbers a certain amount
of carbohydrate matter (mainly sugars of the inosite class)
will be found in the extract. This impurity, though
freely soluble in water, is seldom completely got rid of in
washing the rubber, and may make up as much as 5 per
cent, of the acetone extract. It is only sparingly soluble
in acetone, and generally gathers into a white sub crystal-
line ring near the top of the extraction-flask ; a separation
from resin proper may easily be effected by means of
chloroform, but as a rule there is no harm in weighing
the carbohydrate as part of the resin.

3. Ash. — Of the roughly cut up material 5 gr. are
weighed in a flat porcelain dish and gently heated to
fusion and decomposition. It is well not to allow the
vapours to kindle ; if nevertheless they should take fire,
the flame should be extinguished by an improvised lid of
sheet metal or asbestos. Royal Berlin porcelain dishes
are the best to use, as being little liable to crack on heat-
ing. After the bulk of volatile matter has been driven off,
the dish is heated to redness in a muffle until all charred
matter has disappeared.

The ash of washed and plantation rubber ranges from
0-1 to 1-5 per cent., rising exceptionally to 4 or 5 per cent.
In most cases the ash is white or pale yellow and infu-
sible, and consists mainly of silica, lime, and magnesia.
A few rubbers give ashes so highly ferruginous as to be of
brick-red colour. The ash content of a rubber is signifi-
cant (1) as an item in the non-rubbers and (2) as a check
on the effectiveness of washing operations. Occasionally it
may prove useful, qualitatively and quantitatively, for
characterizing the species and botanical origin of a rubber,

I CRUDE AND WASHED RUBBER 9

4. Proteid and other Nitrogenous Matter. — Direct deter-
minations of protein in rubber cannot be made : indeed it
is not yet known what the nature of rubber protein is.
The usual custom is to determine total nitrogen and to
multiply by the conventional factor 6*25. Kjeldahl's
method of nitrogen assay is applied as follows : —

Two gr. of rubber are placed in a 100 c.c. long-necked
decomposing flask with 30 c.c. of concentrated sulphuric
acid and a small drop (about 1 gr.) of mercury. The
flask is loosely stoppered with a small funnel and heated
over the naked flame. At first there is copious evolution
of sulphur dioxide, and the flame must be cautiously
regulated. Later on, the dark liquid is heated to vigorous
ebullition. After five or six hours the liquid will usually
have become straw-coloured, beyond which point it is
needless to go. Should there be any difficulty in reach-
ing straw-colour, the cooled liquid may be treated with a
little permanganate and boiled up afresh. The residue is
now cautiously diluted and rinsed into a large flask ; one
or two gr. of sodium sulphide, an excess of caustic soda
solution, and a few scraps of zinc are added. The
ammonia is distilled, with the usual precautions, into 20
c.c. of N/5 sulphuric acid, which are subsequently
titrated back with N/5 alkali and methyl orange.

5. Insoluble Matter, i.e., matter insoluble in rubber sol-
vents. This includes all such mechanical impurities as
sand, clay, woody fibre, humus, &c., but not the whole
of the nitrogenous nor of the colouring matter. As a rule
it includes slightly less than the total ash, part of the
inorganic matter being present as salts of calcium and
other metals soluble in organic solvents. The determin-
ation of insoluble would be easy enough if it were not for
the extreme viscosity of rubber solutions and the presence

io INDIA-RUBBER LABORATORY PRACTICE CHAP

of part of the rubber in a pectous modification not directly
soluble in cold solvents. With some few low-grade rub-
bers, simple dissolution, filtration, and washing of the
residue are practicable within reasonable time. In general,
however, it is necessary to begin by applying heat treat-
ment to the rubber. The three following methods are
available : —

1. Where mixing rollers are at hand, a quantity of the
rubber is worked hot until plasticized, i.e., until it can be
drawn out in a uniform translucent sheet. The pectous
part of the rubber is thus rendered soluble, and the
resulting solutions are comparatively fluid. Two gr. of
the prepared material are swelled overnight in 100 c.c. of
toluene or solvent naphtha and dissolved by shaking.
Should the solution still be too viscous to be manageable,
it is now boiled under reflux condensation, by the aid of
an oil-bath, for several hours. The liquid, when sufficiently
deviscified, is allowed to settle and is filtered ; or, far
better, it is whirled in a centrifuge (see p. 27), when the
insoluble goes to the bottom in so compact a form that
the supernatant liquid can be simply poured off. The
insoluble is ^best received on a tared ashless filter ; for
washing, there is no neater method than suspension in
toluene vapour for an hour or so (see under " Gutta-
Percha," p. 172). The weighed filter is incinerated and
the ash is deducted. This gives organic insoluble, which
is the true objective of the assay.

2. C. Beadle and H. P. Stevens 1 heat one gr. of finely-
cut rubber in a test-tube with 5-10 c.c. of phenetol for
several hours, first at 100°, then at temperatures rising
to 140° or over. The thin solution is diluted with 100 c.c.
of benzene and allowed to settle. The residue may be

1 India-Rubber Journal, 43 (1912), p. 193.

CRUDE AND WASHED RUBBER

filtered off, or it may be washed repeatedly by decantation,
rinsed on to a tared dish, and dried.

3. In lieu of phenetol, petroleum may be used for
obtaining thin solutions from unworked rubber, the
procedure .being exactly the same as that applied to
vulcanized rubber (see p. 141). Two gr. are heated with
15 c.c, of petroleum (B.P. 200° upwards) in a small flask
until reduced to a thick, but homogeneous, solution.
Half an hour's heating at 200° generally suffices. After
dilution with 100 c.c. of any convenient solvent, the
insoluble is filtered off directly or after centrifugal treat-
ment, and 'dealt with as under 1. On the whole, this
method is the cheapest and most expeditious ; unlike the
first two methods, however, it provides no possibility of
simultaneously determining the rubber-substance which
has gone into solution.

The following figures, obtained by method 3, give some
idea of the amounts of insoluble that may be expected.
They represent percentages upon washed dry material : —

Rubber Sort.

Acetone
Extract.

Ash.

Total
In sol.

Ash of
Insol.

Organic
Insol.

Para

3 -04

0-26

0'50

O'lO

0'40

Sernamby
Manitoba

1-84
3-10

1-27
2-60

1-55
4-90

1-05
2-35

0-50
2-55

Gruayule

29-40

2*36

4-35

2-10

2'25

Gaboon Ball . . .

26-50

0-40

1-80

0-35

1-45

Borneo .... ....

8-92

0-37

0-50

o-io

0-40

Manihot Plantation ....
Malay Plantation Crape . .
Ceylon Plantation Biscuit .

8-25
3-30

2-87

1-93
0-30
0-40

4-10
1-10
1-80

1-10
0-25
0"25

3-00
0-85
1-55

6. Rubber Proper. — The most widely practised, and
certainly the least troublesome, method of arriving at
the percentage of true rubber hydrocarbon in unvulcanized

12 INDIA-RUBBER LABORATORY PRACTICE CHAP.

material is the simple process of subtracting the combined
percentages of moisture, ash, organic insoluble, protein,
and acetone extract from 100. For technical purposes it
commonly suffices to subtract resin and ash only ; but
this involves a possible error of one or two per cent., and
in the case of badly washed rubbers of several per cent.

In washed rubber the estimation of rubber hydro-
carbon by difference from the fully determined non-rubbers
is usually accurate to within one per cent., sufficiently,
that is, for all ordinary purposes. When, as may some-
times be the case, it is inconvenient to determine all the
non-rubbers, rubber proper may be determined directly,
either as such (the determination is then in principle the
same as that by difference) or in the form of a derivative.
Direct determination is also to be preferred, as a rule,
in the analysis of uncured doughs and rubber solutions.

Determinations in which the rubber is weighed as such
may be carried out (1) by precipitation or (2) by
evaporation.

1. A deviscified solution of one or two gr. of material
in rather less than 100 c.c. of toluene is prepared as
described under "Insoluble Matter'' (p. 10). When cool,
it is made up to 100 c.c. and allowed to settle completely ;
this may be accelerated by warming on a water-bath or
by the aid of a centrifugal machine. Of the clear liquid
50 c.c. are pipetted off, and dropped into 100 c.c. of warm
alcohol, which is kept in brisk motion the while. Eubber
hydrocarbon is thus precipitated as a clot. Should there
be present 10 or more parts of resin to 100 of rubber,
the clot is redissolved and reprecipitated ; if the proportion
of resin be very high, a further reprecipitation is advisable.
The clot is kneaded with several relays of warm alcohol,
drained as far as possible, transferred to a tared dish,

I CRUDE AND WASHED RUBBER 13

dried to constant weight in the vacuum oven, and
weighed.

2. Fifty c.c. of clear solution are pipetted off as above
into a tared wide-mouthed flask of 100 c.c. capacity. The
solvent is distilled off and the flask is cursorily dried in
the vacuum oven. Alcohol is then added and the flask is
set to boil for an hour under reflux, when the film of
rubber detaches itself from the glass and gives up its con-
tent of resin to the alcohol. This treatment with alcohol
is repeated three or four times, whereupon the liquid is
drained off and the flask is dried to constancy in the
vacuum oven and weighed.

Both these methods work better, as regards accuracy,
the less resin there is present. With highly resinous
rubbers it is preferable to begin by submitting the material
to acetone extraction. If the extraction has been exhaus-
tive, a single boiling out with alcohol will suffice in
method 2 ; it is less safe to dispense with alcohol alto-
gether, owing to the difficulty of driving off the last traces
of original solvent. In this and all similar analytical
processes it is well to weigh out original substance before
extraction rather than to start from a weighed quantity of
extracted material. Errors due to imperfect drying and
to oxidation are thus avoided, and calculation is simpli-
fied.

Eubber can also be determined on the orthodox analy-
tical principle of weighing the object of assay in the form
of a derivative, which in this case is an addition-product,
rubber tetrabromide. The rubber molecule, unfortunately,
cannot be trusted to combine quite neatly with just four
atoms of bromine per C]0H16 radical, and there are ques-
tions with regard to the behaviour under bromination of
proteins and resins, and the analytical decomposition of the

14 INDIA-RUBBER LABORATORY PRACTICE CHAP.

tetrabromide, which are nob fully cleared up. Hence in
point of accuracy determination as bromide has little or
no advantage over determination as isolated rubber.
Nevertheless, the tetrabromide method is not without its
usefulness in emergencies. It is carried out (T. Budde's l
method, modified by D. Spence and J. C. Galletly2) as
follows : —

About 0'2 gr. of material is weighed out, extracted with
acetone, covered in a wide-mouthed stoppered bottle with
50 c.c. of carbon tetrachloride, and allowed to swell over-
night. Fifty c.c. of a reagent composed of 6 c.c. of bromine
and 1 gr. of iodine in a litre of carbon tetrachloride are added
to the solution and left to react for six hours with occa-
sional shaking. The tetrabromide is precipitated by
adding 50 c.c. of alcohol with constant stirring and is
washed by decantation several times with alcohol. To
remove the last traces of free bromine and foreign
bromides, the drained precipitate is allowed to swell in
carbon disulphide, reprecipitated by 50 c.c: of petroleum
spirit, well washed with alcohol, and roughly dried at low
temperature. Instead of being weighed as such, the
tetrabromide is now transferred to a porcelain or metal
crucible, where it is mixed and covered with about 4 gr. of
a mixture of sodium carbonate and potassium nitrate
(2 : 1). The crucible is heated to redness, but to a point
just short of complete fusion. The contents are dissolved
in the minimum of water, acidified with a decided excess
of nitric acid, and well boiled. On cooling, the bromine
is determined by adding an excess of TV/10 silver nitrate
solution and titrating back with thiocyanate solution in
presence of iron alum. For converting the bromine thus

1 Gummi-Zeitung, 24 (1909), p. 4.

2 Le Caoutchouc et la Guttapercha, 8 (1911), p. 5513.

i CRUDE AND WASHED RUBBER 15

found into terms of rubber hydrocarbon, the factor 0'42
may be used.

General Remarks on Raw Rubber Analysis.

For the works control of crude wild rubbers it suffices
to assay each consignment, after washing, for ash and
resin. The ash-content is of interest with respect to the
efficiency of the washing process ; in special cases it may
be desirable to determine insoluble as well. The efficiency
of works drying ovens may be tested by determining
moisture from time to time. Even the stickiest rubber
can quite well be dried on the large scale to within O5 per
cent, of moisture, and this should be set as an upper limit.
Plantation rubbers are comparatively easy to control by
simple inspection, and call for chemical examination only
in exceptional cases. As for rubber " protein," that
portion of it which remains after washing is practically
inert and need not claim the regular attention of the
laboratory. •

The washing loss of crude rubber is of importance in
regard to the price of the material, but has nothing to do
with the quality of the rubber. For a given rubber sort it is,
within easy limits, constant and therefore to a certain
extent characteristic.

Resin-content varies between 1 per cent, and 30 per
cent., but in a given rubber sort is constant within fairly
close limits. South Americans as a class are mostly poor
in resin : in Hevea rubbers it rarely exceeds 4, in Manihot
rubbers 7 per cent. Plantation rubbers usually contain
only 2 — 3 per cent., but young trees are apt to yield rather
more resinous rubbers. From the manufacturing point of
view it is by no means to be assumed that a rubber is
better the less resin it contains : the wide variations in

16 INDTA-RUBBER LABORATORY PRACTICE CHAP.

the physical properties of the rubber hydrocarbon itself
are of far greater import. At the same time, it does
usually happen that rubbers associated with, say, 20
per cent, of resin are of poorer quality than rubbers
associated with, say, 2 per cent., but even this mild
generalization cannot be regarded as free from exceptions.
In any case, resin is always a non-rubber and always a
diluent, just as much as added pitch or wax would be.
Further, when present in a large amount, rubber-resin is
apt to exercise an unfavourable effect on vulcanization.

South American resins are generally liquid, dark in
colour, and of an empyreumatic odour. Plantation resins
are similar, but of a lighter colour. The majority of
African resins are yellow, transparent, and brittle, but still
capable of flowing slowly at ordinary temperatures. A
very few resins (Gambia, certain Congos, Rambong,
Penang, Java, &c.) are hard and pulverizable. Eesins
from Para and plantation rubbers are optically inactive,
whereas most others show a moderate dextro-rotation.1
A list of the resin-contents of various rubber sorts will be
found in the Appendix, p. 181.

Nitrogen in crude unwashed rubber may amount to as
much as 1 per cent. The usual practice of classifying
the whole of this as proteid nitrogen and multiplying by
6-25 is open to question, but for the present there is
nothing better to offer. A portion of the nitrogen
certainly exists as albuminoids soluble in water, which
are, or should be, completely removed by washing. It is
this impurity which by its putrefaction gives rise to the
evil odour emitted by some rubbers, and there is reason
to believe that decay of the associated albuminoids has a

1 F. W. Hinrichsen and J. Marcusson, Z. Angew. Chem., 23
(1910), p. 49.

I CRUDE AND WASHED RUBBER 17

deleterious effect on the rubber itself. Hence in general it
may be said that a crude rubber will keep better, the less
nitrogen it contains. The only practicable method of
determining soluble albuminoids is by the difference
between nitrogen in the crude and nitrogen in the
washed rubber, due allowance being made for washing
loss.

The remainder of the nitrogen, which in extreme cases
(notably in hard fine Para) may reach 0*4 per cent, of the
washed dry rubber, appears to be a sign of strength rather
than of weakness. In what form exactly it exists is not
known, but it would seem to be closely bound up with
that sometimes very considerable " pectous " portion of
the rubber which swells up to a gel in rubber solvents
without going into fluid solution.

The pectous rubber here referred to scarcely differs
chemically from soluble rubber and becomes itself soluble
when the material is subjected to hot rolling or other heat
treatment. The expressions " rubber proper " or " rubber
hydrocarbon " are to be taken throughout as connoting
that which ranks technically as such, without regard to
differential solubilities.

CHAPTER II

MACHINERY AND APPARATUS.

A LABORATORY in which the determination of washing
losses is carried on will require a small washing mill and
a vacuum drying oven. In addition, a small mixing mill
is an exceedingly useful adjunct for general purposes. This
and other machinery should be set up, preferably against
an outside wall, in a soundly-floored room separate from
the laboratory proper. The washing mill, moreover,
should be screened off by itself, on account of the inevit-
able splashing. The drive is best conveyed from an over-
head countershaft and may proceed from a small gas-
engine, or better from an electric motor, of 2 — 5 H.P.
Another plan, which is very convenient as regards control
but involves rather more floor-space, is to drive each
machine independently by an electric motor, copiously
geared down. Roller mills should be lighted either
from the top or from the side immediately opposite the
operator.

Washing of Crude Eubber. — The process consists in
squeezing the gum repeatedly through a pair of grooved
rollers in a constant shower of cold water. Small-scale
washing mills, which are practically identical in all but

CHA1\ II

MACHINERV AND APPARATUS

size with the full-scale machine, are supplied to order by
most of the rubber-engineering firms. For laboratory
work the rollers, of solid chilled iron, may advantageously
have diamond or spiral grooving more closely set and in

FIG. 1 (Scale 1 : 16).

shallower relief than on full-scale rollers. The floor-
space taken up by such a mill, apart from motive
machinery, may be from 2 J to 3 feet square ; the height
of the rollers from the ground should be about 3 feet.

20 INDIA-RUBBER LABORATORY PRACTICE CHAP.

A laboratory washing mill l with rollers 9 in. long by
4 1 in. diameter, is shown in Fig. 1 ; another, with rollers
12 in. by 6 in.,2 in Fig. 2. It is quite feasible to wash

FIG. 2 (Scale 1 : 15).

even very small batches of rubber on full-scale machines
of the smaller kind as used in factory and plantation
work.

1 By Messrs. D. Bridge & Co., Manchester.

2 By Messrs. J. Robinson & Co. , Salford.

ii MACHINERY AND APPARATUS 21

The weighed crude rubber is softened, if necessary, by
steeping in hot water and is cut into pieces small enough
to pass through the rollers without unduly straining
them. The mill is set in motion, and a continuous spray
of water is directed from above towards the space between
the rollers. By sending it repeatedly through the rather
widely spaced rollers, the whole of the batch is worked
into a coherent sheet of sodden crape. This crape is then
passed through again and again, the rollers being mean-
while screwed up gradually into closer contact, until at
length the wash-water flows off colourless and free from
suspended matter. The capacious pan which receives
the wash-water must be fitted with a grid at its outlet :
pieces of rubber which are carried away to this grid are
recovered and added to bulk. The washed crape is
eventually hung up to drain for an hour or two and is
then ready for drying. The difference in weight between
the original batch of crude rubber and the fully dried
washed crape, calculated as a percentage of the former,
represents washing loss.

Drying Ovens. — Washed rubber is dried in vacuum-
ovens which may conveniently be miniatures of the vacuum
driers in use on the large scale. A small drier of this
sort1 is shown in Fig. 3. The boiler-shaped drying
chamber is closed by an hermetically-fitting door and
contains three shelves, each of which is heated by means
of steam-coils. A wide vacuum-pipe leads out of the
chamber to a vertical condensing column, and this in
turn is connected to a power-driven air-pump. By the
arrangement of pipes shown in the figure, the shelves can
be heated up by direct steam before evacuation, -and there-
after by waste steam issuing from the air-pump. This
1 By Messrs. Emil Passburg, Berlin,

22 INDIA-RUBBER LABORATORY PRACTICE CHAP.

drier has rather over 20 sq. ft. of floor-space on the
shelving, and is therefore more especially suitable for
laboratories in which a good deal of rubber-washing is
continually going on.

Laboratory vacuum-ovens proper, of the type shown in
Fig. 4,1 are commonly horizontal jacketed copper cylinders,

STEAM

FIG. 3 (Scale 1 : 30).

capable of being heated either by steam or by gas-burners
acting on a jacket of water or other liquid. They are
obtainable in a wide range of sizes, from about three litres
capacity upwards. Jacket-heating is not so efficient as
shelf-heating, but the latter principle is not easy to apply
to small drying chambers. The larger vacuum-ovens are

1 By Messrs. A. Gallenkamp & Co., London.

ir MACHINERY AND APPARATUS 23

best evacuated by means of a Geryk air-pump actuated
by a small motor. For those of only a few litres capacity
a metal filter-pump answers very well, given an ample
and undisturbed head of water.

A small vacuum drying oven as in Fig. 4, is an
exceedingly useful, if not indispensable, piece of labora-

FIG. 4 (Scale 1 : 15).

tory apparatus for general analytical use in connection
with rubber. The oven is here shown heated by means
of a liquid boiling under reflux condensation; but low-
pressure steam, wherever available, is more convenient.
As regards air-ovens for ordinary laboratory purposes,
it need only be remarked that 110° and 60° are the most

INDIA-RUBBER LABORATORY PRACTICE CHAP.

generally useful temperatures, and that the best mode of
heating is by a directly impinging Bunsen burner governed
by an ordinary mercury thermo-regulator.

Drying ovens through which a slow current of carbon
dioxide or other indifferent gas is kept passing are capable
of rendering very good service for drying small quantities
of rubber or gutta-percha. Any small air-oven of which
the door and its frame are suffi-
ciently robust to be made gas-
tight by means of a rubber
washer may thus be utilized.
A makeshift, but quite efficient,
small drier of this kind may be
constructed from a laboratory
vacuum-still consisting of a metal
or porcelain pan and a glass
dome (Fig. 5). Inside the pan,
a disc of perforated brass or zinc
rests, one or two inches from
the bottom, upon a tripod. The
pan is heated by a burner admit-
ting of delicate regulation. A
thick rubber or asbestos washer
is interposed between pan and
dome, which are clamped together
by three small thumb-screws not
shown in the illustration.

Boiler Mills. — These serve (1) for comminuting rubber
and other materials, and (2) for plasticizing, mixing, and
sheeting unvulcanized rubber. Whilst similar in build
to washing mills, they have smooth-faced hollow rollers
capable of being water-cooled or steam-heated at will.
Miniature mixing mills are supplied by rubber engineers

II MACHINERY AND APPARATUS 25

under the same conditions as miniature washers. Fig.
6 shows a heavy laboratory mill l fitted up with all the
paraphernalia of a self-contained electrical drive, trans-
mitted through a worm gear ; the rollers are exceptionally
stout, viz., 9 in. long by 8 in. diameter. An essentially

.:: :.. .. ;. :.::.::.: :.- •:•

FIG. 6 (Scale 1 : 20).

similar mill,2 having rollers 9 in. by 4J in. and a friction-
clutch for belt drive, is shown in Fig. 7. Another
laboratory mill,3 characterized by slighter build and more
elongated rollers, viz., 14 J in. by 5^ in., is shown in Fig. 8.
In so far as they are used for mixing or sheeting, it
should be noted that for a given size of rollers there are
both an upper and a lower limit to the amount of material

1 By Messrs. Iddon Bros., Leyland.

2 By Messrs. D. Bridge & Co. , Manchester.

3 By Messrs. H, Berstorff, Hanover.

26 INDIA-RUBBER LABORATORY PRACTICE CHAP.

with which these mills can effectively deal. Boilers of the
above-mentioned order of magnitude would take a mini-
mum batch of about 50 gr. of rubber dough and a
maximum of about 500 gr.

WATER STEAM

FIG. 7 (Scale 1 : 16).

A relatively inexpensive hand roller mill on the very
small scale, which is often useful enough as a makeshift,
is the metallurgical apparatus shown in Fig. 9. Though

„ MACHINERY AND APPARATUS 27

inconvenient by reason of the superposed position of the
rollers, it serves very well for crumbing or sheeting rubber
previous to analysis, but for little else. The mill is made

WATER STEAM

Fig. 8 (Scale 1 :,;17)

in several sizes, with rollers from 2 in. by 1J in. to
4 in. by 2J in.

Centrifugal Machinery.— A laboratory in which much

INDIA-RUBBER LABORATORY PRACTICE CHAP.

general rubber work is done can ill dispense with a
centrifugal machine or " centrifuge." This piece of
apparatus is a necessity in certain phases of rubber

FIG. 9 (Scale 1 : 14).

analysis and is in general extremely useful in all sorts of
cases where solid matter in liquid suspension resists
filtration or does not readily settle. The centrifuge
should be of tolerable size, say of 18 — 20 cm. whirling

MACHINERY AND APPARATUS

ELEVATION.
FIG. 10 (Scale 1:8)

30 INDIA-RUBBER LABORATORY PRACTICE

radius, and should be driven at 1,500 — 2,000 revolutions
per minute. The most convenient drive is from an
electric motor of \ H.P. or so, which may either be
built-on or transmit directly through a belt. A stout
metal bowl enclosing the whirling parts is a sine qua
non. There will be four, six or eight whirling-arms
carrying glass vessels which should have a capacity of
at least 60 c.c. apiece. A solidly-built six-armed
centrifuge1 carrying cylindrical vessels is shown on
plan and elevation in Fig. 10. The machine is fitted

FIG. 11 (Scale 1:18).

with a speed-gauge, and the cylinder-holders rest upon
springs intended to counteract errors of balance.
Specially designed for rubber work is the six-armed
centrifuge 2 shown in Fig. 11, which, though of slight build,
does very good service. A special pear-shaped vessel
(Fig. 12) is used with this machine, weighing about
40 gr. and having a capacity of about 150 c.c. The

1 By Messrs. A. Gallenkamp & Co., London.

2 By the Vereinigte Fabriken fiir Laboratoriumsbedarf, Berlin.

ii MACHINERY AND APPARATUS 3i

main points to be observed in working with centrifuges
are to counterpoise opposite arms with great care, to
make sure of efficient lubrication, and to start the
machine very gradually. A free-wheel mechanism on
the spindle may be dispensed with if the drive is from
an electric motor without interposed gearing.

Laboratory Apparatus. — Machinery being housed in a
separate room, the rubber laboratory proper may be
similar to any other laboratory in which both inorganic
and organic analyses are performed. Generous draught -
cupboard accommodation will be re-
quired for the various fusions and
incinerations, evaporations of acid
liquids, etc. The processes of distilla-
tion and of boiling under reflux con-
densation on water-, sand-, and oil-
baths play a great part in rubber
analysis, and ample provision should
be made for them. A few observa-
tions on useful forms of apparatus, and
especially on extractors, will not be out FlG 12 (Scale i : 3).
of place.

The continuous extraction of all sorts of materials,
either with acetone or with carbon disulphide, stands
perhaps foremost among rubber laboratory operations.
The Soxhlet and the Knofler (Fig. 13) forms of extractor
both act on the intermittent-siphon principle ; in the
former, the liquid, though not actually cold, is well below
boiling-point ; in the latter it is surrounded by its own
vapour and is therefore as hot as it can be. No form of
extractor can be depended upon to do its work thoroughly
in which the substance is not submitted to complete
immersion in the extracting liquid. Soxhlet's extractor

32 INDIA-RUBBER LABORATORY PRACTICE CHAP.

is too well-known to need description. Knofler's, which
is the rubber extractor 'par excellence, consists of separate
inner and outer tubes ; to accommodate filter-thimbles of
60 x 25 mm., the most convenient dimensions are : —

Inner tube : — bore 28 mm., siphoning height BC
75 mm., height of wide part AC 90 mm.

FIG. 13 (Scale l:2i).

Outer tube: — bore 40 mm., height of wide part DE
150 mm.

Extractors are connected at the upper end to a reflux
condenser- and at the bottom to a wide-mouthed flask

ii MACHINERY AND APPARATUS 33

(Soxhlet flask), of which the most generally convenient
sizes are 100 and 150 c.c. Boiling-flasks of any other
size and shape may be used, but in that case the contents
should be transferred to a tared flask of the Soxhlet type
before evaporating, drying, and weighing the extract. The
connexions, if of cork, must be of the finest and densest
material procurable. A good pair of corks which are not
being used for the first time will yield about 1 mg. of
dissolved solids to acetone or carbon disulphide after a
ten hours' extraction. In ordinary technical rubber
work an error of this magnitude is of little account, so
that in general there is no harm in making use of cork
connexions, provided the corks be carefully selected and
subjected to preliminary extraction. Whenever it seems
desirable to eliminate this source of error, recourse must.
be had to ground-glass connexions. This will involve
not only special ground-in flasks but also special arrange-
ments at the condenser end. The condenser may be
connected up by a ground-glass joint, or else an inserted
condenser may be used and such a joint rendered un-
necessary. Either principle is applicable to the Soxhlet
or Knofler forms indifferently. Soxhlet's apparatus with
two ground-in connexions is shown in Fig. 14. A
Knofler extractor fitted with inside condensation is seen
in Fig. 15 ; here the elongated outer jacket serves as part
of the condenser, whilst the water-cooled condenser
proper (shown at the side) slides into it and rests loosely
on the mouth, or may, if preferred, be fixed by a not
quite air-tight cork ring, which will not come into contact
with condensed solvent.

For heating the Soxhlet flask a water-bath is not very
satisfactory on account of the condensed steam drippings
and the loss in weight which the flask may suffer during

INDIA-RUBBER LABORATORY PRACTICE CHAP.

lengthy extractions. An excellent, but costly, mode of
heating is by means of electric incandescent lamps. On
general grounds, a much simpler device, viz. a talc-bath,
may be recommended. This consists of a hemispherical
iron sand-bath, about 10 cm. across, charged not with
sand but with talc (French chalk) and heated by an

Fro. 14 (Scale 1 :5). FIG. 15 (Scale 1 :5).

ordinary Bun sen burner. Little heat is wasted, the
flasks are not scratched or chemically attacked, and if a
crack occurs the talc forms a kind of lute and prevents
a rapid outflow of solvent. In a busy laboratory an
ample number of condensers and heating-baths should be
permanently set up, preferably in a row against a wall,

ii MACHINERY AND APPARATUS 35

and a sufficiency of Soxhlet flasks should be kept in stock,
each of which is etched with a number and has its
approximate tare recorded on a tablet hung near the
balance.

Scarcely less important than acetone extraction is the
boiling of substances with alcoholic alkali under reflux
condensation. Cork connexions are inadmissible for this
operation if there is any chance of the cork being reached
by alkaline spray. A Soxhlet flask with ground-in
condenser, like the gutta-percha extractor illustrated
in Chap. IX., Fig. 226, is a very suitable form of
apparatus.

The indispensable muffle-furnace, in which incinerations
are performed and batches of crucibles ignited, should be
of a type which admits a fairly generous air-draught into
the muffle proper. The smallest practicable size of muffle
has an internal floor-area of about 6 x 4 in., but in general
a somewhat larger area will be found more convenient.
Incineration-dishes and small porcelain crucibles for
igniting barium sulphate should be marked with numbers
or letters and listed with their tare weights. For igniting
crucibles at specially high temperatures, small one-
crucible furnaces, consisting of double fire-clay cones
which are slipped over a roaring Bunsen burner, are
now reasonably cheap and do the work better than the
laborious blowpipe.

A short tube-furnace carrying a hard glass, porcelain,
or silica tube of about 12 ins. total and 6 ins. effective
length is a very useful laboratory adjunct for a variety
of purposes. Fig. 16 shows such a furnace constructed
entirely of fireclay and heated by five gas-burners. The
tube should be covered with asbestos to avoid direct
impact of the flames. Electrically heated tube-furnaces

36 INDIA-RUBBER LABORATORY PRACTICE CHAP, n

are pleasanter to work with but are much more expensive
and take longer to heat up and cool down.

In conclusion, it may be mentioned that weighing to
four places of decimals is usually a waste of time in a
rubber laboratory. Upon one gramme weighed out, a
milligramme error corresponds to an accuracy of 0*1 per
cent., and on the organic side no analytical method in

rubber work can
boast a greater
inherent precision
than this, to say
nothing of the vari-
ability of samples.
In inorganic deter-
minations it may ex-
ceptionally be neces-
sary to weigh to four
places when small
quantities of sub-
stance are in ques-
tion. At any rate, one should make perfectly sure of
the third place of decimals, and this certainty is best
attained by making use of a balance of the ordinary
analytical type capable of weighing to four places, with-
out troubling to adjust the tenths of a milligramme.
Eubber and other bulky solids should be weighed out on
a metal (aluminium or nickel) scoop provided with a
counterpoise.

FIG. 16 (Scale 1 : 6).

CHAPTER III
KUBBEB DILUENTS

IP it had never occurred to anyone to mix extraneous
substances (quite apart from sulphur, the vulcanizing
agent) with rubber, it is safe to say that the rubber
industry as we know it would be inconceivable : rubber
manufacture would have expanded perhaps in volume,
but hardly in scope, beyond its boundaries of pre-vulcaniz-
ation times. In unvulcanized rubber we have one
material, in soft-cured rubber a second, and in ebonite a
third ; but by admitting compounding ingredients we
gain innumerable new materials. Moreover, the lower-
ing of prices thus achieved has done much to widen the
market for rubber goods. Nowadays manufactured rubber,
taken in the lump, contains at least as much non-rubber
as rubber.

Anything which is oily, waxy, or resinous, any
homogeneous semi-solid or plastic mass free from water,
or any finely-divided powder can be incorporated with
raw rubber to form a smoothly uniform dough. The
process by which the great bulk of soft rubber articles is
manufactured consists in preparing a dough (by the aid
of hot rollers) of rubber, sulphur, and the requisite —

38 INDIA-RUBBER LABORATORY PRACTICE CHAP.

pulverulent and other — compounding materials, and
" curing "or " vulcanizing " this dough at temperatures of
125° — 170° for periods rarely exceeding three hours. The
proper compounding materials are selected with several
aims in view, e.g. : —

1. Imparting the desired physical properties, such as
hardness or softness, stiffness or pliability, smoothness,
strength, extensibility, and so forth ; further, dielectric
and insulatory properties, or resistance to chemical
attack.

2. Imparting the desired colour.

3. Weighting or lightening the product as required.

4. Modifying the first cost of the product.

5. Accelerating or retarding the duration of the
cure.

Technical and economic evolution has provided the
manufacturer with a fairly well defined repertory of sub-
stances suitable for compounding, many of which find
little or no use outside of the rubber industry. These
substances fall into two main groups : — diluents and
pulverulent solids.

Diluents, as the name implies, are coherent bodies,
plastic, liquid, or semi-solid, which, as distinct from dis-
continuous solids, serve to dilute the rubber proper : they
assist the rubber in its functions as a vehicle for the
pulverulent charge which it has to carry. In the nature
of things, diluents belong to the domain of organic
chemistry, whilst the materials constituting charge are
with few exceptions inorganic. The diluents in common
use are factice, reclaimed rubber, pitches, waxes, oils, and
resins. They are all considerably cheaper than rubber,
and those which can be incorporated in generous pro-
portions, viz., factice, reclaim, and bitumen, are added

in RUBBER DILUENTS 39

primarily with a view to cheapening the product.
Needless to say, each diluent exercises a specific
mechanical effect, both in the dough and in the finished
goods, which must be duly taken into account. In the
case of such diluents as waxes and oils, which are seldom
introduced in proportions exceeding one-tenth of the
rubber, specific effect rather than cheapness is [the
dominant consideration.

Factice.

Syn. Substitute, Vulcanized Oil (Ger. Faktis, Oelkaut-
schuk, Fr. Factice). — Before the advent of reclaimed
rubber, this was the only rubber diluent in a large way
known to commerce, nor is it at all likely to become
obsolete. Factice not only has in itself something of the
feel and appearance of rubber (whence the names " rubber
substitute" and " Para fran§ais "), but it can be added
in large proportions, even 1 : 1 or over, without greatly
affecting the elastic properties of the rubber. This is
owing to the curious mechanical consistency of factice,
which possesses compressile elasticity in a high degree,
though it is all but destitute of tensile strength. Hence
added factice, whilst somewhat diminishing the strength
of a rubber article, does not proportionately impair its
springiness. Moreover, factice, of which (free from
mineral oil and wax) the specific gravity varies between
0'98 and 1*02, is the only effective material with which
floating rubber goods, other than black ones, can to any
important extent be cheapened. White factice was dis-
covered in the early fifties of last century, brown factice
a little later ; both varieties were well established as
rubber diluents in the ensuing decade.

Factice is a product of the action of sulphur chloride

40 INDIA-RUBBER LABORATORY PRACTICE CHAP.

at moderate (80° — 100°) or of sulphur at somewhat high
(160° — 200°) temperatures upon raw or blown glyceride oils.
In many respects its mode of formation is analogous to
the vulcanization of rubber itself. What particular oil
serves as raw material depends largely on market prices
for the time being ; the best results, however, are obtained
with castor and rape oils. Sulphur chloride yields a light-
coloured product known as White Factice, whereas factice
made with sulphur alone has a dark colour and is dis-
tinctively called Brown Factice. The necessary proportion
of sulphur or chlorine of vulcanization in factices may
be brought within reasonable limits by previously blowing
the oil ; this is of especial importance with white factice,
in which high vulcanization is inconsistent with stability,,
and 6 — 7 per cent, of sulphur is the usual thing. Brown
factices, on the other hand, may contain from about 7 to
about 20 per cent, of combined sulphur; the soft low-
sulphur sorts can be made only from strongly blown oils
and are very different in character from the stiff high-
sulphur sorts, which are mostly made from raw oils.

Factices are solid bodies of the consistency of stiff jelly,
insoluble in rubber solvents but swelling up in them to
form very dilute gels. Whilst in general inert chemically,
they are eminently saponifiable. Aqueous alkalies under
drastic conditions and superheated steam decompose them
slowly ; alcoholic alkalies split them with great ease into
glycerine and fatty acids of which the alkali salts are
soluble in water. Brown factice is the kind chiefly used
in heat-cured rubber mixings, white factice in cold-cured ;
but moderate admixtures of white factice are quite
admissible in the heat-cure.

Factice contains normally (1) unvulcanized fatty oil,
(2) a little free sulphur, and (3) factice proper, including

in ItUBBER DILUENTS 41

sulphur and chlorine (if any). Many brown factices
also contain admixtures of paraffin wax or heavy petroleum
fractions. These latter are incorporated with the oil
before vulcanization, and have more than one advantage
from the factice-maker's standpoint. They are not to be
regarded by the rubber manufacturer as adulterations,
provided he be aware of their nature and amount. Brown
factice is made in compact slabs, 10 — 30 cm. thick, and
is usually delivered in this form. White factice is sent
out either in crushed amber-coloured lumps of irregular
size, or ground into white powder of a fluffy or crumby
consistency. It is important to note that the chemical
composition of compact factice may vary widely from
spot to spot. A package of slab or lump material should
never be analysed on a single small fragment. A sample
of at least 500 gr. should be drawn and thoroughly mixed
by grinding between cold rollers. The analysis of factice
is carried out as follows : — •

1. Extract. — Two gr. of ground factice are placed in a
filter-thimble plugged with fat-free cotton wool, and
extracted with acetone for 10 hours in a Soxhlet or
Knofler tube. The extract is dried at 110° and weighed ;
it comprises unvulcanized oil, unsaponifiable matter (if
any), and free sulphur. The presence of paraffin may be
detected in the acetone solution by its crystallising out,
that of mineral oil by its fluorescence. As for the un-
vulcanized glyceride oil, it is but sparingly soluble in
acetone and is often seen to settle out in heavy drops.
This substance is not really unchanged fatty oil, but
always contains one or two per cent, of combined sulphur.
Nevertheless, it is. an oil, as distinct from a gelatinous
solid, and therefore "unvulcanized oil" is not a misnomer.

2. For the determination of free mlphiir the three

42 INDIA-RUBBER LABORATORY PRACTICE CHAP.

methods given in Chapter VII (p. 116) for the case
of vulcanized rubber are available. The most expeditious
method consists in exhausting the acetone extract with
petroleum spirit saturated with sulphur, but it should be
resorted to only in presence of a fair amount of sulphur,
e.g., when sulphur crystals are plainly visible. The
method of Davis and Foucar gives trustworthy results
with large or small amounts of sulphur but breaks down
in presence of chlorine and is therefore out of court for
white factice. The oxidation method has the disadvantage
that the small proportion of combined sulphur existing
in factice-extracts undergoes partial oxidation, thus
increasing the apparent free sulphur. In brown factice
there is seldom more than 1 per cent, of free sulphur, and
mostly considerably less. Whenever the amount of free
sulphur is large, faulty manufacture is indicated. White
factice usually contains one or two, and sometimes several,
per cent, of free sulphur, which is always produced to
some extent in the reaction between sulphur chloride and
oil; its presence may also be due in part to " reversion "
of the manufactured factice.

3. Unsaponifiable Matter (i.e. hydrocarbons) is deter-
mined in the acetone extract from a separate 2 gr. lot of
factice. This extract is boiled for two hours with 40 c.c.
of N/l alcoholic potash. The further procedure depends
upon whether the unsaponifiable is solid (paraffin, ceresine)
or liquid (mineral oil, vaseline), which is easily ascertained
by cooling or watering a portion of the alcoholic solution.
If it is liquid, the alcohol is distilled off, the residue is
taken up with water and shaken with two or three
batches of ether, and the ethereal extract is evaporated,
dried at 110°, and weighed. Should the unsaponifiable
be solid, it will be necessary to extract with petroleum

in RUBBER DILUENTS 43

ether (B. P. 40° — 60°) ; in this case the alcoholic liquid is
not evaporated but diluted with an equal volume of
water and then shaken up with solvent. Aqueous or
weakly alcoholic alkaline solutions cannot readily be ex-
tracted with petroleum ether owing to the formation of
persistent emulsions.1 The petroleum ether solution is
separated and washed, first with a few c.c. of concen-
trated sulphuric acid, then with semi-alcoholic caustic
potash solution ; it is finally evaporated and dried as usual.

4. Free sulphur and unsaponifiable subtracted from
total acetone extract give unvulcanized oil. The residue
insoluble in acetone represents vulcanized oil or factice
proper. In order to determine combined sulphur and
chlorine, \ gr. (brown factice) or 1 gr. (white factice) of
this residue is treated by the nitrate fusion (see p. 128)
or potash-peroxide method. The latter is carried out
thus : —

The substance is gently heated in an iron bowl
(cf. p. 129) with 10 gr. of stick-potash and 10 c.c. of
alcohol. Factice goes into solution, whilst the solvent
slowly evaporates. • One c.c. of water is added and heat
is more vigorously applied, the bowl being frequently
stirred or shaken. The bulk of the organic matter is
allowed to fume off until the contents of the bowl form a
thick, partially charred magma which shows a tendency
to incandesce here and there. Sodium peroxide is now
cautiously sprinkled in from a spatula, with continual
agitation. As more and more peroxide is added, the melt
becomes more fluid and finally darkens owing to forma-
tion of ferrates. Care must be taken that every part of
the bowl receives its share of peroxide, so that the whole
contents are uniformly oxidized. The melt is then cooled

1 M. Honig and G. Spitz, Z. Angew. Chem., 4 (1891), p. 565.

44 INDIA-RUBBER LABORATORY PRACTICE CHAP.

and taken up with water. If chlorine is absent, the
solution is acidified with hydrochloric acid, boiled up,
and precipitated with barium chloride. In presence of
chlorine, the solution is halved, and chlorine and sulphur
are determined in the moieties acidified with nitric and
hydrochloric acid respectively.

Alternatively, total sulphur may be determined, by the
above or other method, in the original factice. Provided
that one or the other practice be adhered to, it is mainly
a matter of taste whether original or acetone-extracted
material be taken. In the latter case combined sulphur
(as a percentage of original factice) always comes out
slightly lower, at any rate in brown factices.

5. Moisture is seldom worth determining in brown
factice. White factice which is well advanced in decom-
position may contain several per cent. Moisture is best
determined by drying at 60°.

6. Free Acid, i.e. organic acid, is always present,
generally in quantities not exceeding 2 per cent, calculated
as oleic. It is quite harmless, or rather not less so than
unvulcanized oil. Free sulphuric acid may occur in de-
composed factice ; it is detected and determined by
shaking the ground material with hot water, filtering clear,
and boiling with barium chloride.

7. Ash in brown factice is mostly well under 1 per cent,
and has no particular significance. White factice often
receives an addition, in the manufacture, of a few per
cent, of lime or magnesia intended to neutralize any
hydrochloric acid which may be split off. The ash of
white factice should be not only determined but examined
qualitatively.

Some recent analyses of factice, in which all the per-
centages are calculated upon original substances : —

Ill

RUBBER DILUENTS
WHITE.

English.

Gei-man.

French.

Unvulc. Oil
Free Sulphur

10-3

0-8

7'7
0-3

13-8
2-1

Vulc. Oil (by difference)
Combined Sulphur(in extracted factice)
Chlorine ... « ....

86-2
6-7

7-8

89-4

7-2

7'7

81-3
6-4

7-2

Ash

1-0

2-6

2-8

BROWN, FREE FROM HYDROCARBONS.

English,
hard.

German,
soft.

German,
medium.

German,
hard.

French,
medium.

Unvulc. Oil
Free Sulphur . ...
Vulc. Oil (by difference)
Combined Sulphur (in
extracted factice) . .
Ash . .

21-7
0-3

77 1

13 1
0-9

22-7
02
77-0

7-1

o-i

17-2
0-1
82-5

10-2

o-i

9-6

o-i

902

16-3
O'l

23-6
1-5
74-3

98
0'6

BROWN, CONTAINING HYDROCARBONS.

English.

German
I.

German
II
(Para
frangais).

French
(Para
frangais).

Unvulc Oil

25-1

10 6

25 '0

23 '4

Free Sulphur
Paraffin Wax

2-5
20-1

1-6

0-5
213

0-4
25 •*>

Mineral Oil . .
Vulc. Oil (by difference) . .
Combined Sulphur (in ex-
tracted factice)
Ash

51-3

13-8
O'l

231
64-6

13-5
O'l

52-5

3-6

0*7

47-9

3-1
O'l

Other things being -equal, the less unvulcanized oil
there is in a factice, the better. This constituent has the

46 INDIA-RUBBER LABORATORY PRACTICE CHAP.

same effect in a rubber mixing as glyceride oil pure and
simple, which is credited with a tendency to shorten the
life of the goods. Mineral oil is less open to this objec-
tion, and paraffin not at all. In compounding a rubber
with factice containing either of these constituents, their
specific effects must be taken into account ; for instance,
when 30 parts of the favourite " Para fra^ais " type of
factice are added to 100 of rubber, there will be 8-9 parts
of paraffin present, which verges upon high proportions.
The ash of white factice should not overstep reasonable
limits, say four per cent., and should be magnesian rather
than calcareous : magnesia with insufficient hydrochloric
acid forms a non-deliquescent oxychloride, which lime
does not, and the presence of hygroscopic matter induces
slow decomposition in white factice on storage. On the
whole, the best white factices are those which, in the
ground state, are the driest and least coherent. Sulphur
and chlorine together should not exceed 20 per cent.

Brown factice, unlike white, offers a considerable range
of mechanical consistencies depending, when allowance is
made for the specific influences exercised by unvulcanized
and unsaponifiable, mainly on the amount of combined
sulphur. At the low-vulcanization end of the series we
have soft, sticky, gelatinous products ; at the opposite end
stiff, crisp, caseous ones. These properties are, naturally,
of high importance in the choice of a suitable factice for a
given mixing.

Eunning deliveries of factice should be assayed for
acetone extract, free sulphur, and (if present) unsaponifi-
able. For one and the same brand they should not vary
seriously in consistency and the percentage of extract
should remain fairly constant, say within 2 per cent,
above or below the mean. Except in a rough way, the

in RUBBER DILUENTS 47

mechanical properties of factices of different makes can-
not fairly be judged by analytical data alone. Thus two
specimens made from different or unequally blown oils
may show identical contents of acetone extract and com-
bined sulphur and yet behave very dissimilarly. In such
cases experimental mixings and vulcanizations must
decide. A knowledge of the free sulphur content is of
value chiefly with respect to compounding-calculations ;
an inordinate amount, however, e.g. more than 3 per
cent., may indicate faulty manufacture or " reversion"
in storage.

Little or nothing is to be gained by applying the
standard methods of fat-analysis to factice. Information
thus supposed to be obtained as to the raw material of
manufacture is largely illusory and in any case is of little
interest to the rubber manufacturer. The saponification
numbers of factices are much higher than those of oils,
and are constant only when saponification is carried out
by a strictly uniform method; they mostly lie in the
neighbourhood of 300 for white factice and between 180
and 250 for brown. The yield of fatty acids from factice
is a datum which plays some part in general rubber
analysis (see p. 124).

Rubber Waste and Reclaim.

For many decades past it has been the practice in rubber
works to use up vulcanized waste by grinding it to pow-
der on cold rollers and incorporating it with fresh rubber
for the manufacture of goods of inferior quality. As time
went on, chemical processes for removing tissue &c. from
the waste were devised, and it was further found that a
modest degree of plasticity could be imparted to some
kinds of waste by treatment with oils. Eubber waste,

48 INDIA-RUBBER LABORATORY PRACTICE CHAP.

however, remains rubber waste. It cp,n only be used in
conjunction with fresh rubber and always imparts a
certain lack of homogeneity to the resulting product.
It still plays its part inside factories, but is seldom
nowadays bought from outside as a compounding
material.

Quite another thing is reclaimed rubber, which is now
bought and sold in enormous quantities. It is prepared
from ground waste by treatment with water at high tem-
peratures (in the neighbourhood of 180°) in presence of
acid or, more usually, of caustic alkali. The material is
thereby rendered plastic, so that it can be sheeted like
unvulcanized dough ; though containing the original
combined sulphur intact, it can be revulcanized with
added sulphur like virgin rubber. Eeclaimed rubber is
of utility chiefly as a cheapening agent in admixture with
fresh rubber ; to considerable extent also, inferior rubber
goods are now made from reclaimed rubber alone, plus
filling materials.

Eeclaimed rubber, or reclaim for short (Ger. Begenerat,
Fr. Begenere), comes into trade in compact sheets 5 to
25 cm. thick, less often in tight rolls of very thin sheet,
and has the appearance and feel of stiff half -cured dough.
Black, red, white, and grey sorts are current. As may
be imagined, reclaim is an exceedingly variable sub-
stance, a wide range (especially as regards content of
mineral matter) being procurable in each colour.

The suitability of a reclaim for use in manufacture may
best be decided upon by a combination of vulcanization
experiments and chemical tests. From the analytical data
alone, however, a shrewd technologist will often derive
all the information he needs. Chemical examination of
current deliveries, to secure uniformity, is of especial

in RUBBER DILUENTS 49

importance with reclaim. The complete analysis is
identical with that of a complicated manufactured rubber
(see Chaps. VII and VIII) and is seldom worth carrying
out. As a rule, the following determinations are suffi-
cient : —

1. Specific Gravity. — A compact piece of 5-10 gr. weight
is cut off and weighed in air and water (see p. 105). The
specific gravity of a reclaim must be known in order to
calculate that of mixings in which it is included.

2. Acetone Extract. — Two gr. of reclaim, crumbed or
cut up fine, are extracted for ten hours in a Knofler tube,
and the extract is dried for 3 hours at 110° and weighed.
Acetone extracts usually run high, containing, as they do,
the mineral, fatty, or rosin oil which is almost invariably
added to reclaims to enhance their plasticity. Free sul-
phur rarely occurs except in' red reclaims : in the re-
claiming process red rubber turns grey and is restored to
colour by the addition of antimony red, whereby a little
free sulphur is imported.

3. Factice. — The acetone-extracted substance is dried,
turned into a small flask with ground-glass connexions,
and boiled for three hours under reflux with 50 c.c. of
N/5 alcoholic potash solution. The extract is freed
from alcohol and the residual reclaim is boiled out with
water. The combined aqueous extract is acidified and
shaken with ether, the ethereal solution being evaporated,
dried, and weighed (cf. p. 124). By multiplying the
result by I'l the weight of factice proper is found. In
order that the extraction of factice may be complete, it is
essential that the reclaim be as finely divided as possible.
Few reclaims, even among those manufactured by the
alkali process, fail to show one or two per cent, of matter
insoluble in acetone but soluble in alcoholic potash, which

So INDIA-RUBBER LABORATORY PRACTICE CHAP.

may or may not consist of real factice. The deliberate
incorporation of factice in reclaim is not an unknown
practice.

4. Ash. — Two gr. are incinerated in the usual way. It
is approximately correct to regard ash as equivalent to
the mineral matter present, though it mostly falls a little
short of the latter.

The percentages of extract plus factice plus ash sub-
tracted from 100 give roughly the percentage of rubber,
and this is the figure of merit by which the value of a
reclaim may best be judged. As a further empirical test of
quality, the consistency of the reclaim after acetone
extraction should be noted : the softer and stickier it is,
the more likely it is to mix well and yield rubber-like
products. Black reclaims of sp. gr. up to 1-1 are richest
in rubber (70 — 80 per cent.), but there a,re plenty of blacks of
higher sp. gr. with correspondingly smaller rubber contents.
Floating blacks sometimes contain a great deal (up to 30
per cent.) of factice. The carbon black occurring in many
black reclaims will masquerade as rubber unless specially
determined by the method on p. 155, but the amount is
usually trifling. Eed reclaims similarly cover a wide
range and may show anything from 40 to 70 per cent, of
rubber. White reclaims are always heavy and full of
mineral loading ; the rubber content mostly lies between
30 and 50 per cent.

Acetone extracts of black reclaim generally run from
10 to 30 per cent., those of reds and whites being, as a
rule, rather lower. Part of this extract is always added
oil, which, in addition to its plasticizing action, has the
desirable effect of restraining the tendency to extremely
rapid vulcanization which is characteristic of reclaimed
rubber.

in RUBBER DILUENTS 51

Eeclaims intended to be used for thin sheet goods must
contain the very minimum of mineral or metallic grit, and
in this case it is well to dissolve a gramme or two as on
p. 141 and submit the separated pulverulent matter to
minute inspection. Few reclaims, as is only natural, are
wholly free from coarse solid particles.

In a small way, reclaims made from unvulcanized
waste and intended for making rubber solutions are put
on the market. These should be examined by treatment
with 100 parts of benzene or solvent naphtha. Un-
dissolved gelatinous particles which are retained by a
sieve point to the presence of vulcanized rubber.

Bitumens and Pitches.

Natural bitumen, especially Trinidad and Syrian
asphaltum, has long been in use as a compounding
material, notably in cable-coverings, to which it is
added in order to diminish micro-porosity and increase
the insulation. It is only within the last few years,
however, that bitumen, in the form of Mineral Rubber
so-called, has come into any great vogue as a filler
for rubber goods generally. Mineral Eubber is essen-
tially a diluent and a cheapener, like f actice ; it mixes
readily with rubber and can quite well be added to the
extent of 50 per cent, of the rubber or even more. It
is a tough black non-fluid substance yielding to pressure
but capable of breaking with vitreous fracture, and is
prepared either from soft natural bitumens or from blown
petroleum residues, or from mixtures of both. There is
little, if any, chemical difference between the essential
constituents of the one and the other, and the distinction
is of small moment from the rubber point of view. Th'e
value of a mineral rubber depends on its behaviour when

E 2

52 INDIA-RUBBER LABORATORY PRACTICE CHAP.

incorporated in the mixing and vulcanized, which, in the
present state of knowledge, there is no certain means of
judging except by direct experiment. As a rough guide,
it may be said that bitumens of high softening-point
produce strong rubbers with a tendency to defective
resiliency, whilst low softening-points correspond to poor
tensile strength but comparatively good resiliency. Pro-
vided that a mineral rubber be an asphaltum or petroleum
product containing little or no ash, and that it be free
from water, mineral acid, and admixtures of coal-,
wood-, or stearine-pitch, the minuter questions of its
applicability must be settled by making experimental
mixings. Laboratory tests are useful mainly for control-
ling uniformity in running deliveries.

1. Asphaltene Content, d'C. — One gr. of substance is
treated in the cold with 50 c.c. of light petroleum spirit.
After soaking for some hours, the softened bitumen is
crushed with a glass rod or in a mortar and well shaken
up with the liquid. The thoroughly settled solution is
filtered, and the black pulverulent residue is washed by
decantatioii and on the filter with petroleum spirit.
The brown petrolene fraction now in solution is of a greasy
or treacly consistency and often contains appreciable
quantities of paraffin wax. The residue on the filter is
dissolved in benzene or carbon disulphide, w7hen mineral
matter and pulverulent carbon, if present, remain on the
filter and may be weighed ; the clear solution is evapo-
rated in a tared flask, dried at 110°, and weighed.
Asphaltene thus isolated is a shiny black brittle substance
which does not soften until temperatures well above 150°
are reached. Mineral rubbers usually contain about 30
per cent, of asphaltene. Carbon, if present in any con-
siderable amount, points to an admixture of coal-tar

in RUBBER DILUENTS 53

pitch. This latter is also indicated by strongly fluor-
escent petroleum spirit solutions, since bituminous petrol-
enes have but a feeble fluorescence.

2. Moisture. — A qualitative test may be made by
dissolving 5 gr. of substance in 100 c.c. of benzene which
has been rendered anhydrous by means of sodium or by
distilling off the first runnings. The solution is distilled
through a dry condenser ; if a marked turbidity be
observed in the first drops of distillate, the sample must
be regarded as objectionably moist.

3. Softening-Point. — This is a property which can be
defined only on somewhat arbitrary and conventional
lines. Softening-points, to be comparable, must be deter-
mined by a strictly uniform method. That of G. Kramer
and C. Sarnow l (modified by L. Barta2), which is much
in use for asphalts, may be carried out as follows :—

Pieces of glass tube of 6 mm. bore are cut and ground
true at the ends so as to be exactly 5 cm. long. Such a
tube is filled with melted and well stirred bitumen by
pouring or by suction, care being taken to avoid enclosures
of air. After cooling, the surplus of bitumen is shaved
off flush with the ends of the tube. The filled tube is
connected by means of rubber — glass to glass — with a
similar but longer empty tube, and exactly 5 gr. or 0'37 c.c.
of mercury are poured in ; the whole is then fixed in a
wide test-tube acting as air-bath, which itself stands in a
bath of molten paraffin. Heat is applied at the rate of
2° per minute, and the temperature at which the mercury
forces its way through the bitumen and sinks to the
bottom of the test-tube is taken as the softening-point.
The thermometer should stand in the air-bath, close to

Chem. Ind., 26, (1903), p. 55.
Petroleum, 7, (1911), p. 158.

INDIA-RUBBER LABORATORY PRACTICE CHAP.

the bitumen tube. Tested by this method, mineral
rubbers show softening-points ranging from 100° to 150°.

When bitumen is compounded with rubber and sulphur,
and vulcanized, it undergoes chemical changes, part of it
combining with sulphur and becoming insoluble in rubber
solvents, like the vulcanized rubber itself. The subjoined
experimental results give some idea of the behaviour of
bitumen towards solvents after vulcanization, which is of
especial interest with regard to the analysis of rubber goods.
The bitumen used for these experiments was one of the
best known brands of the " mineral rubber " class. Each
mixing was vulcanized at 138° for varying periods and then
analysed in the usual way. The figures, all but the last
row, stand for percentages upon original material.

From the figures on p. 55 it will be seen that the bitumen
has undergone partition, in round numbers, as follows : —

Acetone-Soluble

Soluble in Carbon Bisulphide
Insoluble . . ....

17
25

14
16

14
10

100

It will further be perceived that the degrees of sulphur
(coefficients of vulcanization, cf. p. 126), which in the
present cases are referred not to pure rubber but to a mix-
ture of rubber and insoluble bitumen, come out abnormally
high, especially when a large proportion of bitumen has
been added to the mixing. That is, bitumen, no less than
rubber, is a vulcanizable material. Both from the manu-
facturing and the analytical points of view, then, one has

111

RUBBER DILUENTS

o oo

3 s d

CO CO CO

s s a

5 3 S

S88

g£S

CO O CO

»0

1— 1

CO 0 CO

J-

,_,

CO 10 CO

«p

£J

CO 1O CO

CO — i •*

„

CO CO CO

05 t- 10

t>-

i— i

Ol CO O

OS (M l~»

»0

01 ^ o

,„

OS 05 05

•«

(M ^ 0

-* o o

G<l (M CO

§5

-* 1- (M

01 G^ 00

*•

•^ CO CO

O1 CO 00

-S £^

• s •

J_l

•

*§* • • J '

•^ • O

C6 O

^
1

I : : -2 :

•S ' g 1^

If 1* s i

ll » 11

-§5 | S^S

0
13

esidual Bi

ombined S

£
s

P5 Pm S

56 INDIA-RUBBER LABORATORY PRACTICE CHAP.

to reckon with the absorption of sulphur by bitumen and
the partial production of a new substance, insoluble in
carbon disulphide and containing combined sulphur,
distinct from the original bitumen.

Coal-Tar Pitch is an old-established ingredient of
inferior black mixings, in which it acts more or less as a
substitute for bitumen. The association of coal-tar pitch
with overshoe rubbers, especially, has become almost
classical. Coal-tar pitch cannot be added to rubber in
any very large proportion owing to its odour and its
content of finely-divided carbon. This carbon is charac-
teristic of coal-tar pitch and is usually present to the
extent of about 30 per cent., but as much as 40 per cent,
is not uncommon. To determine carbon, 1 gr. of pitch is
dissolved in 200 c.c. of boiling benzene, and the super-
natant solution, after settling, is poured through a tared
Gooch crucible. The residue is boiled up with fresh
benzene at least twice. Finally, the residue is rinsed into
the filter, washed, dried, and weighed. Coal-tar pitch
should contain less than 1 per cent, of mineral matter.
Softening-points are determined as in the case of
bitumen.

Resins.

Since all rubbers are themselves more or less resinous,
it would seem a very natural thing to dilute rubber
with added resin. Apart from the fact, however, that
there is no great profusion of suitable resins avail-
able, admixtures of resin are relatively far more detri-
mental to the valuable mechanical properties and
durability of vulcanized rubber than admixtures of factice
or bitumen. One advantage which resin undoubtedly
possesses is that it does not interfere with the pigmenta-
tion of the mixing. Ordinary Hosin (colophony) is used

in RUBBER DILUENTS 57

to some small extent in soft rubbers. The comparatively
high-priced hard resins, Shellac, Copal, Acroides, Sandarac,
Dammar, etc., find a limited application as compounding
materials for ebonite (cf. p. 137). In place of resin
proper, it is a very common practice to make use of the
various rubber-containing resins treated of in the following
section.

Rubber- Containing Resins. — The gums classifiable as
rubber can be arranged in order of their resin-content
until a lower limit of about two parts of true rubber to
one of resin is reached. Nature then leaves a gap, and
the series recommences with gums in which resin is the
principal constituent, at a ratio of about one of rubber to
two of resin. Eubber-containing resins of this sort are
produced in tolerably large quantities in Borneo, Sumatra,
and the Malay Peninsula, and are employed not only in
vulcanized and unvulcanized rubber compositions, but
also extensively outside the rubber industry, e.g., for
cements, chewing-gum, etc. West Africa, also, exports
so-called flake and paste rubbers which belong essentially
to this category. The Asiatic sorts are known by a
variety of names, Jelutong, Palembang, Besk, Pontianak,
Dead Borneo, etc. Qualitatively these gums are much
alike; moreover, their nomenclature is by no means
sharply defined. In the raw state they contain widely-
fluctuating amounts of moisture, according to the degree
to which they have been dried naturally or artificially :
fresh Jelutong may be more than half water, and has the
appearance of cream cheese. Mechanical impurities are
mostly low in amount. The ratio of rubber to resin,
again, is subject to variation; on the average it runs
about 1 : 3. Jelutong occasionally shows 1 : 2, whilst at
the other end Dead Borneo shows ratios more like 1 : 5 or

INDIA-RUBBER LABORATORY PRACTICE CHAP.

1 : 6. The subjoined analyses, without pretending to
represent types or averages, may be cited by way of
example : —

Jelutong I.

Jelutong II.

Ponti-
anak.

Dead
Borneo.

Moisture 52*5

16'5

2'7

19-1

Dirt j 1-0
Resin 35'1
Rubber (by difference) . 1 T4

0-8
65-6
17-1

0-4

72-7
24-2

1-6

68-2
11-1

100-0

The pure resins isolated from these gums are hard and
subcrystalline, and are thus very different from the resins
accompanying ordinary rubber. The rubber-like hydro-
carbons contained in them are usually weak and soft.
The gums themselves possess, or should possess if not too
much oxidized, a peculiar tackiness and ropiness which
are not the least valuable of their properties. Rubber-
containing resins are not to be confused with low-grade
gutta-perchas containing resin and hydrocarbon in similar
proportions, some of which are of a moist, caseous,
Jelutong-like consistency. After removal of the resin by
solvents, there is no difficulty in deciding whether the
residual hydrocarbon is rubber or gutta-percha.

The analytical data to be determined are moisture,
resin, and dirt, rubber being estimated by difference.
From large lots, samples are best taken after the material
has been washed, or at least sheeted, and dried. When a
hand-sample is analysed, moisture is first determined on
a carefully-averaged portion of about 50 gr., or still
better on the whole sample, and the further determinations
are carried out on portions of the dried mass. Moisture

in RUBBER DILUENTS 59

is determined as usual by heating to constancy at 95° in
the vacuum-oven or at 110° in a current of indifferent gas.
Accuracy in moisture determinations is of importance,
because the percentage of rubber, which is itself small, is
arrived at by difference ; for this reason, also, the deter-
mination of moisture in technically dried material should
never be omitted.

Eesin and dirt are best determined after dehydration,
but no great harm is done by taking original material if
it contains only a few per cent, of moisture. Kesin is
determined by extracting 1 — 2 gr. in a Soxhlet tube with
acetone. For this purpose the gum is preferably rolled
flat on a sheet of extracted filter-paper, which is then
wrapped into a spiral ; or the material may be extracted
in the form of snippets, the shrunken mass being
eventually cut up again and re-extracted. Dirt may be
determined by dissolving 2 — 5 gr. in benzene and
filtering on to a Gooch crucible or tared filter. Eubber-
containing resins also lend themselves very well to the
determination of dirt by Pontio's method (see p. 172),
according to which 1 gr. of substance is exposed in a
tared filter to the dissolving action of toluene vapour.
Mineral matter may be determined as usual by inci-
neration.

CHAPTEE IV

SOLID COMPOUNDING MATERIALS

THE general effect of pulverulent fillers in vulcanized
rubber is to increase the specific gravity, tensile strength
(within limits), compressile strength, and resistance to
wear, and to diminish the extensibility and resiliency. In
addition, each material exerts its specific effect, which
may be chemical, pigmentary, or mechanical. Under the
latter head the decisive factors are fineness of subdivision,
as affecting the texture of the product; hardness, as
affecting its resistance to attrition; and specific gravity,
inasmuch as the elastic properties of a compounded
rubber depend rather on the volume than on the weight
of rubber present. The special uses, moreover, to which
a rubber article is destined must be taken into account in
the choice of filling materials (resistance to acids, alkalies,
high-pressure steam ; hygienic considerations, etc.).
Further, all fillers (with the exception of vermilion and
cadmium yellow) are in a high degree cheapening agents,
and in mixings which are heavily loaded the difference in
price between one filler and another is a matter of some
importance. The problems which have to be faced by a
technologist in making up mixings are thus exceedingly

CHAP, iv SOLID COMPOUNDING MATERIALS 61

complex, and cannot be properly solved unless he knows
exactly, chemically and otherwise, what he has before
him in the way of compounding materials. Conversely,
the chemist who has to report on such should have sound
notions on the suitably of a given material for a given
mixing. A few indications in this sense have been given
in the preceding chapter, and more will be given in the
sequel ; but this subject, besides being mainly technolo-
gical, would require a treatise to itself if dealt with in
detail. All this apart, the duties of a rubber laboratory
are to see that the compounding materials are what they
purport to be, that they are free from impurities which
are noxious from the rubber-manufacturing standpoint,
and that successive deliveries do not vary in such a way
as to upset the manufacture. In a rubber works having
its own laboratory, every consignment of compounding
material should be sampled by the chemists themselves
and tested, before being allowed into the mixings. This
may seem a tedious and irksome proceeding, and it may
well turn out that a material is reported as up to the
mark hundreds of times in succession ; nevertheless so
many costly goods can be rendered unsaleable by a single
package of untested material happening to be not
up to the mark, that the rule should be adhered to
inexorably.

Pulverulent compounding materials may be divided
according to their function into three classes : — accelera-
tors of vulcanization, fillers pure and simple, and
pigments. There is some overlapping between these
classes, and in any case all the substances in question are
fillers, with or without other distinctive properties ; the
division, however, is a convenient one. The more
important members of each class will be passed in review

62 INDIA-RUBBER LABORATORY PRACTICE CHAP.

below. The following general considerations on testing
may be noted : —

All materials should be as finely divided as possible.
Precise data as to the bearing of degree of fineness on
the resulting product are as yet lacking ; but it may be
taken for granted that for most goods, and certainly for
thin sheet, a finer powder is preferable to a coarser. Fine-
ness can usually be gauged with sufficient accuracy by
inspection, or by rubbing between finger and thumb ; if
greater accuracy be desired, experimental elutriations
may be carried out. Lumps of clotted powder are
undesirable, though not positively noxious ; on the other
hand, coarse mechanical impurities such as fragments of
wood or metal are in the highest degree objectionable.
These latter cannot always be detected by means of
laboratory samples, and great vigilance is therefore
necessary on the part of those responsible for storing
materials and weighing out mixings. The whole contents
of a package in which such contaminations are known
or suspected should be sent through a sieve before being
put to use. With substances of a composite nature
(Antimony Bed, Lithopone, Black Hypo, etc.) inequality
of composition within a given package is another possi-
bility which it is well to bear in mind.

Chemical impurities which are in all cases undesirable
or even fatal are moisture, free mineral acid, and copper.
Pulverulent matter in equilibrium with air is seldom
quite free from hygroscopic moisture and will contain
more, the lighter and finer the powder ; but this generally
amounts to less than 1 per cent, and may be disregarded.
On the other hand, in ill-dried powders very considerable
percentages of moisture are sometimes encountered.
Determinations are carried out by drying from 2 to

iv SOLID .COMPOUNDING MATERIALS 63

10 gr. on a flat dish for two hours at 110° ; when free
sulphur is present, however, the temperature should not
exceed 60°, or the drying should be done in vacuo.
Moisture is harmful in that it is the cause of blowing
and porosity during vulcanization, and is particularly
dangerous when the goods are open-cured, i.e. not in
moulds. In general, 2 per cent, may be allowed as a
limit. Powders containing more than this amount of
moisture should be stove-dried before being put into the
mixings. All filling materials, whatever other tests be
applied, should be assayed for moisture as a matter of
course.

Acidity, by which is meant free sulphuric acid, is an
old-established bugbear of the rubber industry, but it is
lawful to believe that very often spoilt goods in which
acid is found owe their acidity not to faulty raw materials
but to oxidation at a later stage. Acidity need be looked
for only in materials not of a decidedly basic nature, and
even then will not cause trouble in mixings containing
basic ingredients ; in other mixings it is certainly capable
of bringing about bad cures and eventual rapid decay.
Free acid is easily detected and determined in aqueous
extracts of the material under test (cf. p. 92) ; it is
advisable not to allow more than Ol per cent., calculated
as H2SO4.

Copper, even in very small quantity, is supposed to
exert a peculiar catalytic action leading to the break-down
of rubber goods in the cure and after. To be on the safe
side, compounding materials should be condemned when
their acid solutions or extracts show a perceptible blue
colour or addition of ammonia and filtration.

64 INDIA-RUBBER LABORATORY PRACTICE CHAP.

Accelerators.

There are three substances in common use, viz.,
litharge, caustic lime, and magnesia, which have the
property of accelerating vulcanization, even when present
to the extent of only a few per cent. ; the effect is more
pronounced, the greater the proportion of accelerator.
This is stated to be due, at any rate as regards litharge,1
to a heating of the dough above the actual vulcanization-
temperature caused by reaction between litharge, sulphur,
and rubber-resin, but the matter can hardly as yet be
regarded as fully cleared up. The possibility of thus
shortening the time, or lowering the temperature, of
vulcanization is of incalculable technical importance : and
many of the inferior rubber sorts cannot without accelera-
tors be satisfactorily cured at all. On the other hand, the
danger of over-curing has to be guarded against with
especial care. Litharge is more active, but only slightly
more, than lime and magnesia as an accelerator ; the
two latter are about equal in potency.

Litharge, PbO (Ger. Bleiglatte, Fr. Litharge), sp. gu. 94,
is commonly supplied in a state of high chemical purity,
but varies somewhat in degree of fineness and therefore
in colour. It should be soluble in cold dilute nitric acid
without notable effervescence or residue, and especially
without leaving black flakes of the noxious lead peroxide.
A little metallic lead in fine division, which is easily
detected and determined with the aid of acetic or very
dilute nitric acid, does no harm. The solution should
show no reaction for copper.

Litharge is a decided pigment, very small admixtures
of it producing a black rubber owing to formation of lead

1 E. Seidl, Gummi-Ztg., 25 (*1911), pp. 710, 748.

iv SOLID COMPOUNDING MATERIALS 65

sulphide. Very many, if not most, black and grey rubber
goods are coloured by means of litharge.

White Lead, basic lead carbonate (Ger. Bleiweiss, Fr.
Ceruse), sp. gr. 6-1-6-2, possesses the accelerating and
pigmentary properties of litharge in a reduced degree.
Being in its original condition a strong white pigment, it
tends to impart a bluish-grey tint to vulcanized rubber.
Like all substances containing water of hydration it is
liable, in unfavourable circumstances, to cause blowing.
Sublimed White Lead, which is comin increasingly into
favour, is an anhydrous basic sulphate of lead containing
roughly 65-75 per cent, of PbS04 with 20-30 per cent, of
PbO and a few per cent, of zinc oxide. It is a little
denser than ordinary white lead, but serves much the
same purposes. Red Lead, Pb3O4 (Ger. Mennige, Fr.
Minium), sp. gr. 8-6, has a powerful accelerating action,
largely due, no doubt, to -the heat evolved by its reaction
with sulphur ; at the same time its oxidizing propensities
are apt, unless kept within bounds, to extend to the
rubber. It finds a limited use, chiefly in special quick-
curing mixings. The cheaper kinds of red lead are subject
to adulteration with white powders (calcium carbonate,
barytes, &c.) and with ferric oxide. Organic dyes, which
also occur as adulterants, may be detected by extraction
with alcohol, dilute acid, or dilute ammonia.

Lime, Ca(OH)2 (Ger. Aetzkalk, Fr. Chaux), sp. gr. 2-1,
is almost always nowadays used in the form of slaked
lime, which is easier to store and has a finer grain than
quicklime. It is added to mixings in small quantities
only, and has practically no pigmentary effect. Lime
should be nearly free from silica and carbonate, and
should show an ignition loss corresponding within close
limits to that of Ca(OH)2, viz. 24-3 per cent. Iron in

66 INDIA-RUBBER LABORATORY PRACTICE CHAP.

minute amount does no harm, but manganese, which
sometimes occurs in lime, shares the evil reputation of
copper and should be present in no more than traces.

Magnesia, Calcined Magnesia, MgO (Ger. Magnesia
Usta, Fr. Magnesie Calcinee), sp. gr. 3*2 — 3*6, behaves
much like lime in rubber mixings but, besides being
heavier, has a rather coarser grain. It is prepared by the
ignition of magnesite or artificial magnesium carbonate,
and seldom consists of magnesium oxide pure and simple.
Magnesias always show an ignition loss ranging from
about 2 to about 20 per cent, according to the degree of
calcination and of subsequent exposure to the atmosphere ;
hence also the bulkiness and the true density are apt to
vary. Calcium and silica should not be present beyond
one or two per cent., and it is well to test for manganese.
When a given grade of magnesia is used for given
mixings, successive deliveries should not be allowed to
vary seriously in ignition loss. Magnesium Carbonate,
which is mainly employed as an indifferent filler, has also
a slight but definite accelerating effect on vulcanizations.

Fillers.

Powders coming under this heading are white, but
have too little covering powder to be effective pigments.
They may thus occur, even in tolerably large proportions,
in red and black rubber goods. The chief members of this
group are barytes, calcium and magnesium carbonates,
and siliceous substances.

Barytes, BaSO4 (Ger. Schwerspat, Fr. Baryte), sp. gr.
4-3 — 4'6, ranks among the most largely used materials,
being a cheap, weight-giving, and perfectly indifferent
body. Almost all the barytes which comes into the
rubber industry consists of the mineral heavy-spar,

iv SOLID COMPOUNDING MATERIALS 67

ground fine. It invariably contains a little fluorspar,
CaF2, — up to 10 per cent. — which is quite harmless
except in that it reduces the specific gravity slightly.
Inferior grades may further contain as impurities silica
and iron ; the yellow tinge imparted by the latter is
sometimes cloaked by an addition of ultramarine. Barytes
should be examined for its content of barium sulphate by
boiling 2 gr. for half an hour with dilute hydrochloric acid,
cooling, filtering, and weighing the ignited residue in a
platinum crucible. A few drops of pure hydrofluoric
acid are then added and fumed off, whereupon the cru-
cible is ignited and re-weighed ; the presence of any
serious amount of silica is thus indicated by a loss in
weight. Lead is a very undesirable impurity ; in the
absence of iron it may be detected by the formation of
lead sulphide when a little of the powder is mixed with
ammonium sulphide on a watch-glass. Precipitated
Barium Sulphate, which is comparatively seldom em-
ployed, is finer in grain and more of a pigment. It should
be examined for free acid and for calcium sulphate.

Whiting, CaC03 (Ger. Kreide, Fr. Blanc d'Espagne, de
Meudon, &c.), sp. gr. 2'7 — 2-9, is another very widely used
material, its chief recommendation being cheapness. It
commonly consists of the mineral chalk, ground and levi-
gated, and is sometimes far from dry. The better grades
should be as free as possible from silica and iron.
Manganese is not unknown as an impurity.

Silica, in the form of naturally occurring minerals
(infusorial earth, kieselguhr, &c.)or of chemical precipitates,
is supplied to the rubber industry in very pure white
varieties, sp. gr. 1-8 — 2-0, largely under fancy names such
asAtmoid. Owing to its fineness and low specific gravity
it is specially useful in rubber mixings as a stiffening

68 INDIA-RUBBER LABORATORY PRACTICE CHAP.

agent. The analytical examination comprises moisture,
ignition loss (i.e. water of hydration), and bases, the
difference being silica. One gramme is weighed in a
platinum crucible and dried at 110° ; it is next ignited ;
finally a sufficiency of pure dilute hydrofluoric acid is
added and a drop or two of sulphuric acid, the liquid is
evaporated, and the residue is ignited. This residue,
though consisting partially of sulphates, approximately
represents the metallic oxides present ; should it amount
to more than 2 per cent., it is advisable to determine and
deduct the combined sulphuric acid. Infusorial earth is
apt to contain a good deal of moisture, anything up to
15 per cent. The ignition loss ranges from 3 to 4'5 per
cent. ; bases should not exceed 4 per cent. Adulterations
or natural admixtures of calcium carbonate are generally
apparent at first sight, and are easily dealt with analytic-
ally. Talite is a fine white silica, very similar to the
above but denser, sp. gr. 2-2. It contains bases up to
4 per cent, and very little combined water, the ignition
loss being well under 1 per cent.

Kaolin, China Clay, hydrated aluminium silicate (Ger.
and Fr. Kaolin), sp. gr. 2 '3 — 2-6, was formerly a very
popular indifferent filler, and is still used in fairly large
quantities. It should show 11 — 14 per cent, ignition loss,
46 — 48 per cent, of silica, and 38 — 40 per cent, of
alumina, and should undergo only slight decomposition
when attacked by dilute acid. Kaolin is apt to be very
damp, moisture-contents of 20 per cent, being not unusual.
Slate Powder, which is occasionally used as a filler, and
Ochre (cf. p. 80), are also alumino-siliceous minerals, but
with a fairly high content of iron.

Talc, French Chalk, hydrated magnesium silicate
(G?r. Talk, Fr. Talc), sp. gr. 2-7, is an indispensable

iv SOLID COMPOUNDING MATERIALS 69

material in rubber manufacture. Owing to its peculiar
unctuousness, it finds the widest possible application for
"pouncing," or dusting over, rubber surfaces intended to
be non-adhesive, also moulds, boards, &c., intended to
come into contact with rubber without risk of sticking.
A thin film of talc will render the tackiest rubber surfaces
mooth and repellent. In addition, talc is used to a
considerable extent as a compounding material, to impart
smoothness or stiffness, or in cable-coverings to enhance
electrical insulation. Talc is graded commercially by its
colour, the inferior qualities having a greyish or yellowish
tone ; but from the rubber point of view this matters less
than the degree of unctuousness, which also is subject
to slight variations. Samples for comparison may be
tested by rubbing between finger and thumb. Chemically,
talc shows about 6 per cent, ignition loss, 60 per cent, of
silica, 9 per cent, of alumina, and 25 per cent, of mag-
nesia ; it should be quite indifferent towards moderately
dilute acids. A rather common adulterant is calcium
carbonate, which is easily detected and determined by
the aid of dilute acid. Mica Powder is also sometimes
used in electrotechnical rubbers, on account of its high
insulating properties.

• Asbestos (Ger. Asbest, Fr. Amiante), sp. gr. 2-9 — 3*2, is
supplied both as fibre and ground into powder. In the
fibrous form it is one of the most important constituents
of steam- jointings and " mechanicals " generally. The
special virtue of asbestos lies in the tough, felted, un-
yielding structure which it imparts to rubber ; hence the
suitability of asbestos-filled rubbers for high temperature
work on the one hand, and for exerting friction, e.g. in
brake-blocks, on the other. Asbestos contains water of
hydration, .silica, iron, and magnesia in somewhat

70 INDIA-RUBBER LABORATORY PRACTICE CHAP.

fluctuating proportions, and is by no means resistant, at
any rate so far as concerns the magnesia, to acid attack.
It does not readily lend itself to adulteration, Pumice
Powder and Ground Glass are types of filling material
serving special ends. The use of the former is mainly,
and that of the latter wholly, confined to rubbers for
abrasive purposes, e.g., pencil erasers.

Magnesium Carbonate (Ger. Kohlensaure Magnesia, Fr.
Carbonate de Magnesie), sp. gr. 2-2, has of late years
come into favour as a light filler. It is a bulky, fluffy
powder prepared synthetically in the wet way, and is
usually of high chemical purity. In composition it
approximates to the formula, MgC03.H2O. The dried
powder generally shows total ignition loss 55 — 57 per
cent., combined water 16 — 18 per cent., and CO2 36 — 41
per cent. Combined water and C02 may be determined
in one operation by heating O2 gr. of substance in a short
tube-furnace, after the fashion of an organic combustion-
analysis. Light magnesium carbonate differs markedly
from ground magnesite, which is comparatively seldom
used as a rubber material. Magnesite is much denser
(sp. gr. 3'0), is free from combined water, and contains
siliceous and metallic impurities.

Pigments.

It is required of rubber pigments that they shall suffer
no change during vulcanization. For all the more serious
classes of rubber goods, the public are accustomed to four
colours only, viz., black, grey, white, and red, and expect
clean and vivid tints. The palette, so to express it, of the
rubber manufacturer is therefore not quite the same as
that of the painter, and is practically limited to the short
list of pigments reviewed below. Lead-colours, qua

iv SOLID COMPOUNDING MATERIALS 71

pigments, are excluded, at least as regards heat-cured
goods ; on the other hand, antimony red is a pigment
which is peculiar to the rubber industry.

The exact effect of a pigment in rubber can only be
gauged by making an experimental mixing, the more so
since rubbers themselves are for the most part by no
means colourless. Two or more pigments of the same
class may be compared by painter's test, thus : — the
powders are worked into a thick paste with boiled linseed
oil, deposited in lumps of equal area on glass plates, and
inspected from the back, i.e., through the glass. Speci-
mens so prepared may be allowed to harden and can be
preserved for reference. Pigments other than white should
be diluted with a standard zinc white, blacks with 50 — 60
parts, other colours with two parts. Conversely, the
covering power of whites may be compared by dilution
with a standard lampblack. Conclusions as to the effect
of pigments in vulcanized rubber from their effect in oil,
must, however, be drawn with some caution.

White Pigments.

Zinc White, ZnO (Ger. Zinkweiss, Fr. Blanc de Zinc),
sp. gr. 5-5 — 5-6, has long been and remains one of the
most popular of compounding materials. Regarded
merely as a pigment, it leaves something to be desired :
its covering power is but moderate, and in vulcanizations
conducted at medium to high vulcanization-temperatures
it has a tendency to assume a yellowish tinge. The best
zinc white is made from the previously isolated and
purified metal and is supplied as a fine, white powder
which is almost chemically pure ZnO. It should be
completely soluble, without effervescence, in dilute (10 per
cent.) acetic acid. This and other white pigments should

72 INDIA-RUBBER LABORATORY PRACTICE CHAP.

be free from iron, copper, and lead ; to test for the latter,
a moderately acid solution in hydrochloric acid is treated
with hydrogen sulphide, when the liquid should remain
colourless. Inferior zinc whites are sometimes adulterated
with barytes, whiting, kaolin, etc., and, in zinc whites
intended primarily for paint-making, white lead is a
possible adulterant. A defect of zinc white, not shared
by zinc sulphide, is that it is liable to be leached out of
rubber goods by dilute acids. For this reason zinc white
is in some countries prohibited (equally with lead com-
pounds) for articles which come into contact with foods
and beverages.

Lithopone, sp. gr. 3-8 — 4-2, is a synthetic product
obtained in the wet way by precipitating barium sulphide
with zinc sulphate ; theoretically it should be a mixture
of 70*5 per cent, of BaSO4 with 29*5 per cent, of ZnS,
but the conditions of manufacture admit of fluctuations.
Originally a paint-making material, lithopone has become
one of the most important of rubber pigments ; it posesses
a high covering power and does not discolour in the cure.
The valuable ingredient is the zinc sulphide, according to
the content of which lithopones are commercially graded
under the denomination of variously-coloured seals. The
standard quality for rubber mixing is " Bed Seal," which
is guaranteed to contain 30 per cent, of zinc sulphide.
The full analysis of lithopone takes into account soluble
salts of barium and zinc, barium carbonate, and other
impurities. For rubber purposes it suffices to determine
moisture, acid-insoluble matter, and sulphide. One
gramme of the dry material is boiled with concentrated
hydrochloric acid diluted with twice its volume of water
for half an hour, or until no more hydrogen sulphide is
evolved, The liquid is completely cooled and the solid

iv SOLID COMPOUNDING MATERIALS 73

residue is filtered off, ignited in a platinum crucible, and
weighed. A little pure hydrofluoric acid is then added
and fumed off, and the crucible is again ignited and
weighed ; adulteration with silicates, e.g. kaolin, is thus
detected. In order to ascertain whether the acid-soluble
portion contains non-zinciferous bodies or zinc compounds
other than sulphide, the volumetric assay of sulphide
described in the next paragraph is carried out on a
separate quantity of 0'5 gr. For controlling current
deliveries it suffices to decompose with acid and weigh
the residue, which should not exceed 70 per cent.

Zinc Sulphide, ZnS (Ger. Schwefelzink, Fr. Sulfure de
Zinc), sp. gr. 3*3, has latterly come into vogue as a light
pigment of high covering power. The zinc sulphide of
commerce is apt to be largely contaminated with rela-
tively less valuable zinc compounds, especially zinc oxide ;
some qualities may contain not much more than half
their weight of sulphide. It is well, therefore, to deter-
mine the content of ZnS in the commercial article, as
follows : — 0-15 gr. of dried material is weighed into a
stoppered bottle and shaken up with 50 c.c. of JV/10
iodine solution. Five c.c. of concentrated hydrochloric
acid are added and the bottle is allowed to stand, with
occasional vigorous shaking, for an hour or two. When
decomposition is complete, no white clots should be
visible, but only yellow films and skeletons of sulphur.
The unused iodine is then titrated back with thiosulphate
solution, 1 gr. of iodine being equivalent to 0*384 gr. of
ZnS. The better qualities of zinc sulphide should contain
not less than 90 per cent, of ZnS. Matter insoluble in
strong acid should be absent.

74 INDIA-RUBBER LABORATORY PRACTICE CHAP.

Red Pigments.

Antimony Bed, Golden Sulphide (Ger. Goldschwefel,
Fr. Soufre Dore d'Antimoine), sp. gr. 3-1 — 4-2, is one of
the oldest and most important accessories of rubber manu-
facture. Its colouring principle is antimony sulphide in
the finely-divided form in which that compound is pre-
cipitated from solutions. The smoothness, brilliancy, and
high covering power of antimony red make it perhaps the
most satisfactory of all rubber pigments.

Antimony red occurs in two main varieties, orange and
crimson. The latter, which is little used by itself, is
prepared by boiling antimony trichloride with thio-
sulphate solutions ; chemically it is a trisulphide of anti-
mony containing a few per cent, of oxygen, probably in
the form of oxysulphide. Tints ranging from crimson
through scarlet to orange are obtained by varying the
conditions of precipitation, or by mixing orange and
crimson sulphides. The orange variety is that which
yields the characteristic colour associated in the public
mind with red rubber goods. It is prepared by boiling
powdered stibnite with polysulphide solutions and pre-
cipitating the resulting sulphantimonate solutions with
mineral acids. The essential constituent is an antimony
sulphide or mixture of sulphides in which the combined
sulphur fluctuates between 3 and 3*6 atoms to 2 atoms
of antimony. The orange antimony pigment falls into
two subdivisions, according as it has been co-precipitated
with hydrated calcium sulphate ("plastered antimony")
or not. The plastered variety is cheaper to make and,
although comparatively poor in antimony, is not so
deficient in pigmentary power as might be expected.
This, consequently, is on the whole the most popular

iv SOLID COMPOUNDING MATERIALS 75

form of antimony red. It usually contains from 30 to
50 per cent, of CaSO4.2H20.

Not all makes of antimony red keep their colour
satisfactorily in vulcanization. This highly important
point can be tested only by actual vulcanization experi-
ments. The chemical composition of antimony red is
summed up by (1) free sulphur, (2) antimony, (3) calcium
sulphate, and (4) sulphide-sulphur.

1. A certain amount of free sulphur is invariably
co-precipitated with antimony sulphide. It is for this
reason that antimony red was long regarded, erroneously,
as in itself a vulcanizing agent, and even now many
manufacturers are accustomed to introducing their vul-
canizing sulphur into red mixings in this form. Con-
sequently the pigment is commonly supplied as containing
so and so much of free sulphur, even up to 40 per cent.,
the required excess of which is mixed in by the makers.
In modern scientific rubber manufacture an exact know-
ledge of the free sulphur content is, needless to say,
indispensable. To determine free sulphur, 2 gr. of
antimony red are weighed into a filter-thimble which is
plugged with fat-free cotton wool. The material is then
subjected to extraction with carbon disulphide in a Soxhlet
extractor during eight hours. After evaporation of the
solvent, the flask is dried for an hour at 60° before being
weighed. The thimble with its contents is also dried,
first in the open air, then at 60°. Carbon disulphide, if
pure, keeps very well in a red bottle ; but if there be any
doubt about the solvent, it should be redistilled before
being used for extractions.

2. For the determination of metals it is best to take
extracted material, because free sulphur, by enclosing
particles of unchanged substance, is apt to impede the

76 INDIA-RUBBER LABORATORY PRACTICE CHAP.

dissolving action of acids. Half a gramme of dried
extracted pigment is dissolved in 10 c.c. of concentrated
hydrochloric acid, and the hydrogen sulphide is boiled off.
Two grs. of tartaric acid and 200 c.c. of hot water are
added. If there remains a siliceous residue on boiling,
this is filtered off and weighed ; but there is rarely
enough to make it worth while. Antimony is precipitated
by means of hydrogen sulphide and may then be deter-
mined gravimetrically or volumetrically.

The gravimetric method is "on the whole the more trust-
worthy, especially when antimony determinations are
not a daily recurring task. The precipitated sulphide is
rinsed from the filter into a large tared porcelain crucible,
which is placed on a water-bath. One or two c.c. of con-
centrated nitric acid are added. During the effervescence
which sets in, the crucible is temporarily covered with a
watch-glass ; the contents are then evaporated to dryness.
A few drops of red fuming nitric acid are added to the
solid residue, and the crucible is gently warmed over a
naked flame, heated more strongly to drive off sulphur
and acid fumes, and lastly ignited. The antimony is
then present as SbO2.

To effect a volumetric determination, the sulphide is
rinsed into a beaker and decomposed with concentrated
hydrochloric acid. Hydrogen sulphide is driven off by
boiling nearly to dryness. The residue is dissolved in
water by the aid of tartaric acid and made up to 250 c.c.
in a measuring-flask. Of this solution 100 c.c. are
neutralized with sodium carbonate, cooled, rendered
alkaline with 10 c.c. of saturated sodium bicarbonate
solution, and titrated with JV/10 iodine solution. Antimony
can also thus be determined directly in the extracted or
(provided there be no difficulty in dissolving) in the original
pigment. One gr. of I corresponds to 0-473 gr. of Sb.

iv SOLID COMPOUNDING MATERIALS 77

3. The filtrate from the precipitated sulphide is
rendered alkaline with ammonia. A little iron may here
present itself as a green colloidal solution of ferrous
sulphide ; this is coagulated by boiling and filtered off.
The iron itself is of no great importance, but it must be
removed before precipitation of calcium, which latter, on
account of the high factor of conversion, needs to be deter-
mined with precision. Calcium is brought down as usual
by means of ammonium oxalate and is weighed as CaO.

4. It is not practicable to determine sulphide-sulphur
directly as such. Instead, total combined sulphur is
determined in the extracted pigment and the necessary
deductions for calcium sulphate are subsequently made.
Either of the following methods may serve : —

(a) Not more than J gr. of extracted substance is
dropped little by little into 5 c.c. of concentrated sulphur-
free nitric acid contained in a 50 c.c. Erlenmeyer flask.
The acid is meanwhile kept cool by shaking the flask
in a basin of water. Antimony is thus completely
oxidized, but a certain amount of sulphur remains in the
form of yellow clots. The flask is now gently heated on
a water-bath, care being taken to avoid fusion of the
sulphur, and a pinch of powdered potassium chlorate is
added from time to time until all the sulphur has dis-
appeared, whereupon the mixture is transferred to a small
basin and evaporated on the water-bath nearly to dryness.
The residue is lixiviated with hot water ; the filtrate,
which should be made up to about 400 c.c., is boiled up
with the addition of a little hydrochloric acid (and
tartaric acid, if necessary), and barium sulphate is brought
down in the usual way.

b. The potash-peroxide fusion method (cf. p. 131) is
very expeditious and not less accurate than any other.
The substance (0'3 to 0-5 gr.) is melted down in an iron

INDIA-RUBBER LABORATORY PRACTICE

CHAP.

bowl with 5 gr. of stick potash and about 1 c.c. of water.
When the melt no longer effervesces on addition of a little
sodium peroxide, enough peroxide to produce the dark
ferrate colour is added and thoroughly melted in. On
lixiviation, acidification with hydrochloric acid, and
boiling, it sometimes happens that everything goes into
solution. More usually, however, there is a flocculent
antimonial residue, which is filtered off. The clear liquid
is precipitated with barium chloride as usual.

Analyses of antimony red, conducted as above, are
rather apt to give figures adding up to more than 100.
This is often, if not mostly, due to the calcium sulphate
being present in an aggregate state of hydration falling
short of CaSO4, 2H2O.

Some typical analyses of antimony reds : —

Orange,
English I.

Orange,
English II.

Orange,
French.

Orange,
German.

Free Sulphur ....
Antimony
Sulphide-Sulphur . . .
Calcium Sulphate . . .

19-1
235
11-2
46-4

17-9
22-2
10-3
50-4

7-3
62-3
27-5
3-2

15-2
54-7
25-0
5-3

100-2

100-8

100-3

100-2

Scarlet, French.

Crimson, English.

Free Sulphur

10-6

15-6

Antimony . . .

41-4

62-1

Sulphide-Sulphur ....
Calcium Sulphate ....
Oxygen (by difference) . .

17-8
30-6

19-6
2-7

100-4

100-0

iv SOLID COMPOUNDING MATERIALS 79

Moisture in antimony red is determined upon 5 — 10 gr.,
not by oven-heating, but by drying in a vacuum
exsiccator. Normally it should not exceed 1 per cent.

Acidity may be determined as in sulphur (see p. 92),
except that the titration is performed on an aliquot portion
of clear liquid, e.g., one-half of the total volume, decanted
from the solid matter. Calculated as H2SO4, it should
not exceed 0'06 per cent.

Running deliveries of antimony red should be watched
for uniformity of colour. Free sulphur should invariably
be* determined and it is well, also, to check the contents
of antimony and plaster by the following handy method
due to F. Jacobsohn1 : —

1. One gr. of pigment is oxidized in a porcelain crucible
with a few c.c. of fuming nitric acid, evaporated, and
ignited. This gives SbO2 plus CaS04.

2. One gr. of original pigment (not, as proposed by
Jacobsohn, of the oxidized matter from the above opera-
tion) is mixed in a porcelain crucible with 2 gr. of
resublimed ammonium chloride, fumed off, and ignited.
Antimony being thus volatilized, there remains CaSO4
plus any siliceous matter present. A slight conversion of
calcium sulphate into chloride may take place, but not
enough to disqualify the method for technical use.

Rouge, Oxide of Iron, Fe2O3 (Ger. Eisenoxyd, Fr. Oxyde
de Fer), sp. gr. 5-0-5-2, is inferior in attractiveness to
antimony red, giving at best dull and brownish tones.
Its chief uses are for non-poisonous articles ; for ebonites,
in the vulcanization of which it behaves better than anti-
mony red ; and for heat-resisting goods generally. Eouge
is prepared by calcining iron sulphate, and the better
qualities are almost chemically pure ferric oxide. A
1 Chem..Ztg., 32 (1908), p. 984.

8o INDIA-RUBBER LABORATORY PRACTICE CHAP.

number of rouges supplied as rubber pigments, however,
contain much added matter. To produce a more brilliant
colour, the rouge is oftened calcined in company with
calcium or barium sulphate, and calcium carbonate or
other diluent is sometimes added, so that rouges contain-
ing as little as 30 per cent, of Fe2O3 are not unknown.

Rouge is examined as follows. Iron is determined by
boiling 0'5 gr. with 500 c.c. of hydrochloric acid (1 : 20),
adding excess of ammonia, and filtering off and igniting
the precipitate plus undissolved residue. Silica is then
detected by evaporating with hydrofluoric and sulphuric
acids, igniting, and re-weighing. The filtrate is divided
into equal halves, and in the one calcium is determined,
in the other sulphuric acid. Barium sulphate, if present,
can generally be observed as a heavy white admixture
when the pigment is boiled up with acid ; it is separated
from ferric oxide by fusing with potassium bisulphate and
leaching out the iron with hot dilute acid. Eouge is
subject to considerable variation in tint, according to the
mode of preparation ; the pigmentary effect of a given
specimen in the rubber can be judged with certainty only
by experimental vulcanizations. The colour of running
deliveries should be controlled by painter's test.

Eed Ochre, a pigment prepared by calcining yellow
ochre (see p. 83), is practically a rouge much diluted with
siliceous matter and is consequently of inferior covering
power.

Vermilion, HgS (Ger. Zinnober, Fr. Vermilion), sp. gr.
8-1, is by far the most brilliant and powerful of red pig-
ments, but also one of the most costly ; its use is limited
to very special articles, such as dental rubbers. It should
be tested for free sulphur and ash, both of which should
be practically absent. The presence of soluble mercury

iv SOLID COMPOUNDING MATERIALS 81

salts is easily detected, and should on no account be
tolerated. Vermilions are occasionally adulterated with
organic dyestuffs to heighten the colour.

Black Pigments.

Lampblack, amorphous carbon (Ger. Euss, Fr. Noir de
Fumee), sp. gr. 1-8, is the soot prepared by burning oily,
resinous, and tarry matter, natural gas, or acetylene.
Carbon in one form or another is the only black pigment
(excepting lead sulphide, cf. p. 64) suitable for rubber.
Different makes of lampblack vary enormously in bulki-
ness, but they are much alike in covering power, weight
for weight. Lampblack should be assayed for greasy
matter by extraction of 1 — 2 gr. with acetone, and also for
ash; gritty particles contained in the latter are to be
regarded as deleterious. With the bulkier sorts of lamp-
black, acetone extraction is best carried out thus : — 2 gr.
are shaken with acetone in a 100 c.c. flask and made up
to about 1 c.c. above the mark. Should a persistent
suspension be formed, a drop of dilute hydrochloric acid
may be added to effect coagulation. After settling (which
may be hastened by means of the centrifuge), 50 c.c. of
the clear liquid are pipetted off and evaporated in a tared
Soxhlet flask.

Good qualities of lampblack show up to 5 per cent, of
grease and mere traces of ash. Inferior grades may con-
tain several per cent, of mineral matter and may possess
a brownish discoloration, which is sometimes cloaked by
means of ultramarine or other blue pigment.

Bone-black, which is a product of charring, not of com-
bustion, consists of amorphous carbon together with
calcium phosphate and carbonate. The mineral matter
usually amounts to about 90 per cent. ; owing to its pre-

82 INDIA-RUBBER LABORATORY PRACTICE CHAP.

sence and to a certain coarseness of grain, the application
of bone-black is comparatively limited. The percentage
of carbon may be approximately arrived at by determin-
ing ash and acetone extract. It is more accurate, how-
ever, to determine carbon directly by dissolving out
mineral matter with hydrochloric acid and weighing the
residual matter on a tared filter ; the ash of this residue
is then determined and deducted.

Black Hypo is less in vogue now than formerly ; it is a
complex mixture of varying composition, prepared by
calcining litharge together with sulphur, with or without
the subsequent addition of bone-black or lampblack.
Beside litharge and lead sulphide, black hypo contains the
lead salts of several sulphur oxy-acids. Analysis yields
little information as to its value in rubber mixings;
but the content of free sulphur should invariably be
determined.

Graphite (Ger. Graphit, Fr. Plombagine), sp. gr. 2-2
upwards, is in request rather as a filler than as a pigment.
It has a considerable stiffening effect, and its remarkable
lubricating properties, by preventing the rubber from
sticking to metal, render it valuable in steam-jointings
and other high-temperature goods. Like talc it serves,
with certain classes of goods, for pouncing doughs and
moulds. Graphite is supplied in widely varying degrees
of purity, according to the amount of associated earthy
matter, which ranges from one or two per cent, to thirty
or forty per cent. The value of a graphite is in general
proportional to its content of graphitoidal carbon. The
degree of fineness, also, is not a matter of indifference.
Flake graphites, which can scarcely rank as black pig-
ments, are more highly prized than powder graphites, on
account of their mechanical properties. Chemical ex-

iv SOLID COMPOUNDING MATERIALS 83

amination need seldom be carried beyond an assay of the
carbon, which is effected with sufficient accuracy for most
purposes by incineration in a platinum crucible. Carbon
in the form of graphite burns away very slowly, and it is
advisable to turn over the powder occasionally during
ignition. High-class flake graphites containing very little
ash cannot be dealt with by open incineration, but must
be placed in a boat and burnt with the aid of oxygen in a
small tube-furnace.

Other Pigments.

Yellow, green, and blue pigments are of minor import-
ance. Their chief use is for toys, surgicals, and tiling ;
latterly also yellow pigmentation has come into some
prominence in connexion with balloon fabrics. Beside
mineral pigments, a number of organic dyestuffs are in
use for uncured and cold-cured goods, especially in the
form of superficial applications. The most brilliant effects
are obtained with dyes soluble in naphtha, which are
incorporated with the rubber in the form of solutions.
To some extent, also, organic dyes are employed in the
form of lakes deposited upon alumina, barium sulphate,
&c., which are added to the mixings as powders. Organic
dyes for use in rubber should be resistant to acids and
totally insoluble in water.

Yellow Ochre is a natural product, consisting of clayey
and other siliceous matter with 20 — 30 per cent, of
hydrated iron oxide. It is not a strong pigment and
lacks brilliancy. Chrome Yelloiv, PbCrO4, is very satis-
factory as a colour in the rare cases when its lead content
permits it to be used. Cadmium Yellow, CdS, is much
the best yellow pigment, as it gives a fine colour and is
not in the least affected by vulcanization ; but it is about

84 INDIA-RUBBER LABORATORY PRACTICE CHAP, iv

as dear as, or dearer than, rubber itself. Precipitated
Arsenic Sulphide, As2S3, is an excellent pigment and
resists heat-curing, but has the defect of being extremely
poisonous.

Chrome Green is the chief green pigment of general
applicability. It consists almost wholly of chromium
sesquioxide, and is quite indifferent to vulcanization.
Binmann's Green (zinc-cobalt oxide) is also in use for
cold-cured goods.

Prussian Blue and Ultramarine serve as blue pigments,
the latter being preferred owing to its greater permanency.
Where resistance to acids is essential, ultramarine is out
of court ; Prussian blue, on the other hand, is decomposed
by alkalis, and is also somewhat apt to suffer in
vulcanization.

CHAPTEE V

MISCELLANEOUS ACCESSOEIES

Naphtha.

VAST quantities of rubber solvent are used up in the
manufacture of rubber goods. By making rubber, pre-
viously mixed with compounding materials, into a
treacly " solution " with naphtha, textile threads and
fabrics can be so impregnated or coated that the rubber
adheres in a continuous form after evaporation of the
solvent and vulcanization. The " proofing," i.e. water-
proofing, of fabrics in this manner is one of the very
oldest branches of the rubber industry. In more recent
times impregnation by means of solutions has come to
play an important part in the manufacture of composite
objects such as tyres, hose, belting, overshoes, &c. The
only rubber solvents economically applicable on the large
scale are coal-tar (benzenoid) and petroleum (paraffin
and olefine) hydrocarbons. Petroleum, or Mineral, Naphtha
(Ger. Benzin, Fr. Essence Minerale), once largely used, is
a medium in which rubber does not swell so readily as in
benzene homologues, but has the advantage of being less
apt to impart a foreign odour to the goods ; it has now-

86 INDIA-RUBBER LABORATORY PRACTICE CHAP.

adays fallen very much into the background, mainly
owing to considerations of price. The popular rubber
solvent is Coal-Tar Spirit (Ger. Benzol, Fr. Essence de
Houille), especially the xylene fraction, known in the
trade as solvent naphtha, which is not excessively volatile
in the open air. Solvent naphtha generally boils to the
extent of 95 per cent, between 125° and 155°, and consists
principally of the three xylenes flanked by minor pro-
portions of toluene, pseudocumene, and mesitylene.
This is not, however, the only coal-tar fraction which
finds application in rubber manufacture. In a smaller
way, "nineties" benzene (B.P. 80°— 120°), of which 90
per cent, distils up to 100°, and " fifties " benzene (B.P.
80°— 130°), of which 50 per cent, distils up to 100°, are
also used.

Specific Gravity. — The determination to three places
of decimals is effected with great ease and rapidity by
means of hydrometers (see Chap. VI). Standard temper-
ature is largely a matter of convention : 15° is on the
whole the most usual for density statements, but 18° and
20°, as being more convenient practically, are also
favoured. If determined at some other known tempera-
ture, specific gravities can be corrected to standard by
the aid of coefficients of expansion (see p. 102). Specific
gravities serve chiefly as a rough preliminary test to
detect irregularities ; also, they afford valuable indications
as to the presence of coal-tar naphtha in petroleum
naphtha, or vice versa, since benzenoid hydrocarbons are
considerably heavier than paraffin hydrocarbons of about
the same boiling-point. The specific gravity of solvent
naphtha is 0-860 — 0*870 at 15° ; that of nineties benzene
(0-880—0-885) and that of fifties benzene (0-870—0-880)
are slightly higher.

v MISCELLANEOUS ACCESSORIES 87

Distillation Test. — All naphthas, whether single samples
or running deliveries, should be examined by distillation.
The quantitative results of fractional distillation vary
considerably according to the modus operandi. Placing
the thermometer in the boiling liquid, for instance, affects
the results in one sense, and the use of an efficient
dephlegmator in the opposite sense. Whenever it is
necessary to compare figures with those obtained by some
standardized method, the prescibed form of apparatus and
procedure must be adopted. For general purposes, how-
ever, one may as well employ the most accessible form of
apparatus and the simplest and least unscientific mode
of distillation. The following procedure may be recom-
mended : —

One hundred c.c. of naphtha are introduced into an
ordinary distillation-bulb of 150 c.c. capacity, which is
fitted with a thermometer (bulb about half-way down the
vapour- column) and connected to a rather short Liebig's
condenser having an inner tube of uniform diameter from
end to end. A few grains of pipeclay or pumice are
added and the naphtha is distilled, over a steady flame,
into a 100 c.c. measuring-jar at about the rate of two
drops per second. The stem of the distillation-bulb
should be 16 — 20 mm. wide and its height between bulb
and tubulure 70 — 80 mm. The exact initial temperature of
distillation is of no significance, because the first few c.c.
of distillate are always contaminated with water and have
therefore an abnormally low boiling-point. The ther-
mometer is imagined divided into spaces of 5° above and
below 100° ; as soon as the first of these points after
commencement of distillation has been reached, the
volume of distillate is read off and is regarded as belonging
to the preceding interval of 5°. For example, if nothing

INDIA-RUBBER LABORATORY PRACTICE CHAP.

has begun to come over at 115°, but 9 c.c. have been
collected when the thermometer reaches 120°, then 9 c.c.
are set down as coming over in the interval 115° — 120°.
As distillation proceeds, readings of the volume distilled
are taken every 5°, and the quantity coming over between
each point is calculated by subtraction. In this way a
lucid and comparable picture of the composition of the
naphtha is obtained. Distillation is continued up to 95 or
96 per cent, of the total volumes.

The following figures represent distillation-tests of
typical solvent naphthas : —

II.

III. IV.

110°— 115°

115°— 120°

—

120°— 125°

125°— 130°

130°— 135°

135°— 140°

140°— 145°

145°— 150°

150°— 155°

Above 155°

'. 3

2 (155°-

-160°)

100 100 100 5 (above 160°)
100

For purposes of rubber manufacture the initial boiling-
point matters little, except in so far as the naphtha will
be more volatile in the open air, the lower it is. Naphtha
III is rather exceptional, containing, as it does, a very
considerable proportion of toluene. More importance
attaches to the final boiling-point. The highest boiling
fractions are comparatively difficult to dry out of the
rubber, and are then apt to give trouble by retarding
vulcanization. Moreover, they often contain malodorous
bodies which impart an obstinate smell to the finished

v MISCELLANEOUS ACCESSORIES 89

goods. Hence it is a sound rule to reject naphthas of
which more than 5 per cent, boils above 155°. Many
naphthas, like IV, are currently used, of which 5 per
cent, or so boils above 160° ; but if a temperature-limit
is to be set at all, it had better be set at 155°.

The residue in the distillation-bulb is turned into a dish
and allowed to evaporate in the open with little or no
heating. Any excessive amount of greasy matter in
solution will thus reveal itself, and naphthalene, if present,
will be left in the form of crystals. Naphthalene is to be
regarded as an undesirable impurity, on account of its
clinging smell. Impurities due to inadequate refining,
such as phenols and pyridines, are detected by their odour
when the naphtha is allowed to evaporate on filter-paper
or on the palm of the hand. Carbon disulphide, which,
however, rarely occurs, may be tested for as below.

The more volatile coal-tar naphthas, nineties or fifties
benzene, commercial toluene, &c., are subjected to distil-
lation-test in the same way. Being fractions which come
over, in the coal-tar distilleries, before solvent naphtha is
collected, they usually boil to the extent of 95 per cent,
below 130°. They are very liable to contain carbon
disulphide, an impurity which is objectionable, not indeed
on technological, but on hygienic, grounds. To determine
carbon disulphide, about 1 c.c. of phenylhydrazine is added
to 100 c.c. of the naphtha, and the liquid is allowed to
stand for at least ten hours. A voluminous crystalline
deposit of the phenylhydrazine salt of phenylthiocarb-
azidic acid separates out ; this is filtered off, air-dried,
detached from the filter, and weighed. By applying the
factor 0*26 the weight of carbon disulphide is found. No
naphtha intended to be used in workshops where it is
evaporated into the atmosphere should contain more than

90 INDIA-RUBBER LABORATORY PRACTICE CHAP.

2 gr. of carbon disulphide per litre. This and other
malodorous impurities are also highly undesirable in
rubber solutions intended for sale.

Petroleum Naphtha occurs in a greater variety of ranges
of boiling-point than coal-tar naphtha. Much, if not most,
of that which comes into the rubber industry boils between
80° and 140°. Any residue above 140° will consist, in
part, of heavy matter having the character and smell of
lamp-oil, and should therefore be kept low. On. the other
hand, a large proportion distilling below 100° is not less
undesirable, on account of its volatility in ordinary
handling. Distillation tests are made exactly as with
coal-tar naphtha.

Shale Naphtha is a rubber solvent which, being produced
in limited quantity, seldom travels far from its place of
origin (Midlothian and Lanarkshire, in the British Isles).
It is composed of olefine and paraffin hydrocarbons in
approximately equal parts, and commonly boils between
75° and 155°.

Carbon Disulphide (Ger. Schwefelkohlenstoff, Fr.
Sulfure de Carbone) is used as a diluent and impregnating
agent in the cold-cure process ; it boils at 46° — 47° and has
a specific gravity of 1-27. Commercial carbon disulphides
are frequently not far from chemically pure, but the
inferior grades contain traces of organic sulphur-com-
pounds which give them a villainous odour, and sometimes
a more or less yellow tint. Unless freshly rectified, carbon
disulphide invariably holds a slight amount of sulphur in
solution, which is easily determined by distilling 100 c.c.
out of a tared flask. For rubber work, carbon disulphide
should not be grossly malodorous, and should show at
most 2 gr. of distillation-residue per litre.

Carbon Tetrachloride (Ger. Tetrachlorkohlenstoff, Fr.

v MISCELLANEOUS ACCESSORIES 91

Tetrachlorure de Carbone) serves, alone or mixed with
coal-tar naphtha, for making non-inflammable rubber
solutions, and also as a diluent in the cold-cure process.
The pure substance, sp. gr. 1-63, boils at 77°. Good
commercial carbon tetrachloride should distil between 75°
and 78°, and leave no residue. Adulterations with hydro-
carbons are best detected through the specific gravity.
From its mode of manufacture, carbon tetrachloride is
usually contaminated with carbon di sulphide ; this is
detected and determined as in coal-tar spirit.

Vulcanizing Agents.

Next to rubber itself the most important raw material
of the rubber industry, though not necessarily the one
used in largest quantity, is sulphur. When mixed with
sulphur and heated to temperatures above 120°, raw
rubber enters into chemical combination with a greater
or less amount of sulphur and so becomes vulcanized.
The product, vulcanized or "cured" rubber, differs from
the uncured material in that it is stronger mechanically,
is insoluble (though it swells up to some extent) in rubber
solvents, is practically indifferent to changes of tempera-
ture within fairly wide limits, and is a more stable
substance generally. The discovery of vulcanization in
the early forties of last century may be regarded as the
true starting-point of rubber manufacture on the large scale.

Sulphur for rubber mixings takes the form, for the
most part, of Flowers of Sulphur (Ger. Schwefelblumen,
Fr. Fleurs de Soufre), sp. gr. 2-0. This is a fine powder
obtained by sublimation and consists of a mixture of
ordinary X-sulphur and /x-sulphur, the latter of which is
insoluble in carbon disulphide and makes up about
three-quarters of the flowers. Coarse mechanical impuri-

92 INDIA-RUBBER LABORATORY PRACTICE CHAP.

ties being supposed absent, flowers of sulphur always
contain free sulphuric acid and a little moisture. The
latter is determined by drying 5 gr. on a flat dish in a
vacuum-exsiccator. To determine acidity, 10 gr. are
mixed with enough alcohol (previously neutralized) to
moisten the powder thoroughly; 100 c.c. of water and
some phenolphthalein are added and titration is carried
out with N/1Q caustic soda solution. There should not
be present at the very outside more than 0-2 per cent, of
acid, calculated as H2S04. Flowers of sulphur are
practically free from ash; any notable incineration-
residue will be due to adulterations, e.g. with infusorial
earth. A finer but more expensive form of sulphur, which
is used in comparatively small quantities, is Precipitated
Sulphur. This should contain not more than 0-05 per
cent, of free acid and 1 per cent, of ash. In some
varieties there is an admixture of calcium sulphate, due
to the process- of manufacture. Precipitated sulphur is
apt to contain several per cent, of moisture. Another
form of sulphur sometimes met with is a greenish
powder, which is obtained by grinding impure solidified
sulphur and shows up to 10 per cent, of ash consisting of
ferruginous clayey matter. In addition, a number of
mixtures under fancy names are, or used to be, on the
market ; these very commonly contain paraffin or other
waxy matter and various mineral loadings.

Sulphur is also employed in rubber works for sulphur-
baths, i.e. baths of molten sulphur in which articles to be
vulcanized are immersed either naked or in moulds. The
sulphur in the latter case acts merely as a carrier of heat,
in place of the more usual high-pressure steam or hot air.
Sulphur for use in baths is bought in stick or lump form
and need be examined only for ash.

v MISCELLANEOUS ACCESSORIES 93

Sulphur Chloride, S2C12 (Ger. Chlorschwefel, Fr.
Chlorure de Soufre), sp. gr. 1-68, is another body
possessing the property of combining with rubber to
produce a material which closely resembles sulphur-
vulcanized rubber in physical respects. Unlike sulphur
in elementary form, sulphur chloride acts instantaneously
in the cold ; consequently the vulcanizing effects obtain-
able by it are only skin-deep. Vulcanization by means of
sulphur chloride, or " cold-curing," was discovered a few
years later than heat-curing and is still much practised,
though necessarily on a somewhat restricted scale.
Sulphur chloride is usually applied to the articles to be
cured (i.e. thin sheet, tubing, or thread, proofed fabrics,
&c.) in dilute carbon disulphide or carbon tetrachloride
solution. It is also applied in the form of vapour, by
which method peculiarly glossy surfaces can be produced.
Moisture or moist air must be rigorously excluded in
any process in which sulphur chloride comes into
play.

Sulphur chloride should always be examined by
distillation. The boiling-point of the pure substance,
viz., 137°, is greatly affected by impurities. In a reason-
ably pure commercial article, a range of 130°-140° may
be allowed, with a residue of 5 per cent., which may
consist principally of sulphur. Dissolved sulphur is
determined by rinsing this residue with carbon disulphide
into a tared Soxhlet flask, drying at 110°, and weighing.
The commonest impurities are either chlorine (in the
form of SC12), which is highly noxious, or dissolved
sulphur, which in small amount is harmless. Too much
dissolved sulphur, e.g. more than 5 gr. per 100 c.c., may
cause the cured goods to sulphur up. On the assumption
that no elements other than sulphur and chlorine are

94 INDIA-RUBBER LABORATORY PRACTICE CHAP.

present, excess of either may be determined by C. O.
Weber's 1 method, as follows : —

A solution of the material is first prepared by accurately
weighing about 5 gr. into a 100 c.c. flask and making up
to the mark with dry benzene. Of this solution, 10 c.c.
are pipetted into 25 c.c. of N/l alcoholic potash solution
in a wide-mouthed flask. The mixture is digested on a
water-bath for an hour, whereupon the solvent is distilled
off and expelled by heating the flask in the oven at 110°.
The residue is taken up with 100 c.c. of hot water and
rendered slightly acid with nitric acid. Ten c.c. of a 10
per cent, copper sulphate solution are added to eliminate
sulphide- sulphur, and in the filtrate chlorine is determined
gravimetrically in the usual way. Chemically pure
sulphur chloride contains 52-5 per cent, of chlorine.

Should naphtha, or carbon disulphide or tetrachloride,
be present, the above method might lead to erroneous
conclusions. Such impurities, however, are likely to
occur only in recovered sulphur chloride. They may
easily be detected by decomposing the sulphur chloride
with a large excess of dilute aqueous caustic soda solution
and submitting the liquid to distillation. The impurities
in question will then come over with steam in the first
runnings, and may be measured and identified.

Oils and Waxes.

Various oils, both glyceride and hydrocarbon, play a
modest part in the compounding of manufactured rubber.
Oils serve on the one hand to facilitate the mixing and
sheeting of low-grade doughs, and on the other hand to
impart a certain softness and suppleness to the finished
goods. They are added in quite small proportions,
1 The Chemistry of India Rubber, London, 1902, p. 182.

v MISCELLANEOUS ACCESSORIES 95

seldom exceeding 5 per cent, of the rubber. The glyceride
oils most in use are linseed, cottonseed, rape, and castor
oils. As a rule no special chemical examination is
required, provided the oil be what it purports to
be.

Mineral Oils ranging from light spindle oil to semi-
solids of the vaseline type are used for much the same
purposes as the above, and in similar small doses. It is
well to test mineral oil for free acid, i.e. sulphuric acid
introduced by the refining process. In the case of pale
oils this is done by shaking up 10 gr. of material with a
previously neutralized mixture of alcohol and ether (4 : 1)
and titrating with N/10 caustic soda and phenol-
phthalein. With dark oils the phenolphthalein colour-
reaction by this method is almost or completely obliterated.
If too dark for direct titration, the oil is well shaken with
three 20 c.c. lots of hot 90 per cent, alcohol (previously
neutralized), and the united alcoholic extracts are titrated
with phenolphthalein as indicator. Or the oil may be
directly titrated in ether-alcohol solution with the aid of
Alkali Blue 6B (Hochst), an indicator which strikes blue
with acid and red with alkali.

Mineral Waxes, viz., paraffin (made from petroleum,
shale, or lignite) and ceresine (made from ozokerite), are
very extensively added to rubber mixings in small pro-
portions, the maximum being about 10 per cent, on the
rubber. In soft rubber goods generaUy, the effect aimed
at is reduced micro-porosity, whence increased air- and
water-tightness ; in cable-coverings mineral wax serves
the further purpose of enhancing insulation-resistance.
Although ceresine and paraffin are closely similar chemi-
cally, they differ widely in consistency, ceresine having a
tough, amorphous structure like beeswax, whilst paraffin

96 INDIA-RUBBER LABORATORY PRACTICE CHAP.

is "short" and subcrystalline. The specific gravity of
all these waxes varies between 0*89 and O92.

Mineral waxes, provided they melt to clear white or
yellowish liquids, are unlikely to contain deleterious
impurities. The temperature at which they liquefy is a
matter of no great moment from the rubber point of view,
but is important on the score of cost, since mineral waxes
are mostly sold on a melting-point basis, the highest-
melting ones being the most prized. Ceresines of the
best quality show melting points approaching 75°; the
medium qualities, M. P. 60° — 70°, are mostly mixtures of
true ceresine with paraffin ; cheap grades are apt to be
adulterated with rosin. Paraffin may be approximately
determined by E. Graefe's l method based on the partial
precipitation of ceresine (but not paraffin) from carbon
disulphide solutions by ether- alcohol. Eosin is isolated
quantitatively by boiling out with alcoholic potash.

The best hard paraffins melt at a few degrees above 60°,
and qualities melting down to 50° are current which are
still hard and homogeneous; the lower-grade varieties,
such as paraffin scale, contain a small amount of heavy
mineral oil which beyond certain limits does not form a
homogeneous solid solution, but is ready to exude on to any
absorbent surface. These oily paraffins, in fact, represent
a transition-stage between hard paraffin and vaseline.

The melting-points of mineral waxes are anything but
sharp, and it is impossible to obtain comparable results
unless a standard method be adopted. On the whole the
most serviceable method hitherto proposed is that of
L. Ubbelohde ; 2 it requires, indeed, a special form of
apparatus, but in return yields definite and reproducible

1 See J. Marcusson and H. Schliiter, Chem.-Ztg., 31(1907), p. 348.

2 Zt. Angew. Chem., 18 (1905), p. 1220.

MISCELLANE 0 US A CCESSORIES

figures. A thermometer A (Fig. 17) and a tiny glass
funnel B are so connected by a split brass tube C that the
thermometer-bulb always occupies a fixed position with
respect to the funnel when the latter is
pressed home. The funnel is charged with
melted wax and attached to the thermometer
before the wax has set. The whole apparatus
is then fixed in an air-bath consisting of a
test-tube of 4 cm. diameter and 23 cm.
length, which dips into a large beaker of
water. Heat is applied in such a way that
the thermometer rises about 1° per minute.
The temperatures at which the wax com-
mences to sag out of the bottom of the
funnel (softening-point) and at which the
first drop of molten material disengages
itself (dropping-point) are both noted ; they
may lie from 2° to 6° apart.

Fabrics,

The textile fabrics which come into use
in rubber manufacture are mostly of cotton,
and in a few cases of flax or hemp. They
range from fine dense tissues for surgical
sheeting and waterproof garments to the
coarsest ducks for belt and tyre insertions.
The suitability of a fabric for a given purpose is mainly
a question of weight per unit area, spacing and thickness
of threads, strength, and behaviour under heat. Chem-
ical examination is concerned chiefly with such sizing
preparing, and loading materials as may be present, any
of which is undesirable in excessive amount. Sizing is
determined by extracting 5 gr. of air-dry fabric with hot

98 INDIA-RUBBER LABORATORY PRACTICE CHAP, v

water and evaporating and drying the nitrate on a flat
porcelain dish ; 4 per cent, of aqueous extract may be
allowed. Greasy matter is extracted out of 10 gr. by means
of carbon disulphide ; it is usually under 1 per cent, and
should not exceed 2 per cent. Mineral loading, apart
from the small amount of mordant which may occur in
dyed fabrics, should be altogether absent ; its presence is
revealed by the results of incineration, allowance being
made for the fact that 1 — 2 per cent, of ash is a normal
constituent of undyed cotton. The ash should in any case
be free from copper.

CHAPTEE VI

SPECIFIC GEAVITIES

THE specific gravity, or density, of a substance is
strictly defined as the weight in vacua of a cubic centi-
metre of the substance. In practice it is usual to define
it as the weight in air of the substance divided by the
weight in air of an equal volume of water at 4°. The
error thus involved is insignificant for practical purposes.
Liquids have comparatively high coefficients of expansion,
and their densities should never be stated without speci-
fying the exact temperature ; with solids at ordinary
temperatures there is no need for any such refinement.

Liquids.

Organic solvents and oils are the liquids of which the
density is of most interest in rubber manufacture. There
are three instruments for determining the densities of
liquids : in ascending order of convenience and descend-
ing order of precision these are (1) the pycnometer, (2)
the hydrostatic balance, and (3) the hydrometer. For
technical purposes hydrometers are the most generally
useful, on account of the ease and despatch with which
determinations are made. Those on which specific

w H 2

loo INDIA-RUBBER LABORATORY PRACTICE CHAP.

gravities are directly read off are much to be preferred.
A hydrometer, if properly made and calibrated, should
give results correct to O001 of specific gravity. To attain
this degree of accuracy, it must have a scale sufficiently
open to be divided visibly into O001 units ; consequently
there must be available not one but several spindles, each
with a short range of 0*06 or O'l of specific gravity. For
solvent naphthas 0-800— O900 or 0-820 — 0-880 will be the
range ; for petroleum naphtha 0-650—0-800 in two or more
spindles. Very long spindles are unhandy ; 20 cm. is a
convenient length. The scale-marks of a bought hydro-
meter should never be taken on trust. Calibration is
effected as follows: —

Two liquids, pure or mixed, are chosen such that their
densities correspond to points near either end of the
scale. Their temperature having come into equilibrium
with that of the room, readings of the hydrometer are
taken when floating in each ; the exact density of each is
then determined by means of the pycnometer, care being
taken that there has been no change of temperature in
the meantime. The whole operation should be repeated
at least once, preferably at another temperature. If now
it is found that the difference, for the two liquids, between
the hydrometer results is not the same (to within O'OOl of
specific gravity) as the difference between the pycnometer
results, it will be necessary to draw up a full table con-
necting scale-readings with true specific gravity ; on the
whole, however, it is simpler to reject the hydrometer.
If these differences are identical, but there is a constant
discrepancy between scale-readings and true specific
gravity, then this discrepancy is noted as a correction to
be applied to the hydrometer readings ; it may conveni-
ently be recorded on the hydrometer-case. If, lastly,

vi SPECIFIC

this discrepancy does not amount to O'OOl, then the
hydrometer is as accurate as such an instrument can be.

Hydrometer readings should be observed from below
the level of the liquid, with the spindle floating freely out
of reach of the walls of the vessel. About 300 c.c. of
liquid should be taken, in a cylinder at least 5 cm. wide.
It is well to stir up with the thermometer before intro-
ducing the spindle.

The hydrostatic balance (Mohr's balance) has for its
principle the weighing of a glass plummet, commonly of
5 or 10 c.c. displacement, when suspended in the liquid
under test. This is done either by laying ordinary weights
on a pan or by means of riders corresponding to O'l, 0*01,
0-001, &c., of specific gravity, which are adjusted on a
beam bearing a scale of ten divisions. Whatever be the
form of apparatus used, it should be so designed that
specific gravities are read off directly. Calibration is
easily effected by means of distilled water, and there is
generally no difficulty in meeting errors of instrument by
making an alteration on the balance itself rather than
applying corrections. Mohr's balance is available for
liquids of any density, and has the advantage of working
with small quantities — 50 c.c. or even less — of liquid. It
may be relied upon for three places of decimals ; a yet
higher accuracy can be attained, but only by taking
special precautions with regard to the suspension-wire
of the plummet.

The pycnometric method consists in the weighing of a
vessel of known volume when filled with the liquid. By
the use of pycnometers of the Sprengel tube type, and
temperature-adjustment in a thermostat, the method may
be driven to an accuracy of O'OOOOl, but it is useful also
on a lower level of precision for check determinations, for

LABORATORY PRACTICE CHAP.

calibrating hydrometers, and for determining the specific
gravity of solids (see below). Pear-shaped pycnometers
with perforated ground-in stoppers — the specific gravity
bottles of commerce — may be conveniently employed,
and they should be calibrated in grammes of water
at 4°. Counterpoises are usually supplied with these
vessels and are a great boon, but it is imperative to
adjust them from time to time. The most generally
suitable sizes of pycnometer are from 10 to 50 c.c. For
taking the density of liquids a 10 c.c. capacity suffices,
and, if the volume happen to be exact to within a milli-
gramme, determinations involve no calculation whatever.
Very viscous liquids should invariably be dealt with by
pycnometry, the other methods described above being
unsuitable.

It often occurs that the density _DT at a standard
temperature T° is required, but that it is experimentally
more convenient to determine the density Dt at a not too
remote temperature t°. If the coefficient of expansion k
of the liquid (see Appendix, Table V) be known, the
requisite correction is given by the formula

DT = A {1 + k (t - T}}

Pulverulent Solids.

Matter in the solid state presents an important dis-
tinction between true and apparent specific gravity :
the former refers to the volume actually taken up by the
solid material and nothing else, the latter to the volume
outlined by a solid which may enclose air in vacuoles or
cushioned between discrete particles. Thus the apparent
specific gravity of a spongy mass or a fluffy powder may
be as little as a tenth, or less, of the true specific gravity.

For determining the true specific gravity of powders

vi SPECIFIC GRAVITIES 103

the most generally applicable method is the pycnometric
one. A pear-shaped 50 c.c. pycnometer is charged with
about one-quarter its capacity of substance and weighed.
Enough water to half-fill the vessel is added and thoroughly
shaken up with the powder. The vessel is warmed on a
water-bath to about 50°, placed under a bell- jar or in an
exsiccator, and subjected to evacuation until the water
boils briskly ; when this point is reached the vacuum
must be more cautiously regulated to avoid spirting.
After its contents have been in ebullition for some
minutes, the pycnometer is cooled, filled up with water,
stoppered, and weighed, the temperature of the water
being noted. An alternative procedure is to weigh the
powder into a beaker, boil over the flame with a small
quantity of water, cool, rinse into the pycnometer, and
continue as above. It is of course assumed that the
powder is neither insoluble in, nor otherwise affected by,
water ; otherwise recourse must be had to alcohol,
benzene, or other mobile liquid, the density of which
must be accurately determined. Calculations are made
by the formula : —

Sp.Gr.= w-^_

where Wl = weight of solid

W2 = weight of liquid, aqueous or otherwise, filling

the pycnometer, the solid being absent.
W8 = weight of solid plus liquid filling the pycno-
meter.

d = density of liquid at temperature of experiment,
against water at 4°.

The result, it will be observed, is referred not to c.c.
but to grammes of water at 4° ; the difference, however,

104 INDIA-RUBBER LABORATORY PRACTICE CHAP.

vanishes in the experimental uncertainties, nor need the
thermal expansion of the powder be taken into considera-
tion. Owing to the difficulty of disengaging the entangled
and adsorbed air from a powder it is seldom possible, unless
very refined methods be applied, to state more than two
places of decimals accurately in the specific gravity.
Generally speaking, organic liquids have the property of
wetting solids better than water, and therefore drive out
the air more effectively. Thus for sulphur, graphite, etc.,
or fine greasy powders, water cannot satisfactorily be used.

A second method, as accurate as the above but less
widely applicable, consists in floating the powder in a
liquid having the same density as itself. The powder is
shaken up in a cylinder, or boiled if necessary, with a
liquid heavier than itself, which is then diluted with a
light liquid until the powder rises. When the mixture
has been so adjusted that the bulk of the powder floats
between the bottom of the vessel and the surface of the
liquid, the specific gravity is taken in situ by means of
Mohr's balance. The method fails with powders which
are mixtures or which have a specific gravity exceeding
3*3. Heavy liquids available for this method are : —

Cadmium boro-tungstate, sp. gr. 3-28, to be diluted with
water. Attacks carbonates.

Methylene iodide, sp. gr. 3*33, to be diluted with
benzene, May be decolorized, at need, by shaking with
mercury.

Potassio-mercuric iodide, sp. gr. 3 -19 (54 pts. HgI2,
43 J pts. KI, 10 pts. water), to be diluted with water.

India-Rubber, etc.

The method in general use for ascertaining the specific
gravity of rubber, resins, pitches and waxes is that of

vi SPECIFIC GRAVITIES 105

hydrostatic weighing, which is fairly expeditious and
leaves nothing to be desired in point of accuracy. It is
the apparent, not the real, specific gravity which is thus
determined, unless indeed special measures be taken to
expel enclosed air before weighing in water. Determina-
tions are carried out as follows : —

A piece of the material, preferably of not less than
5 gr., is weighed by itself. It is then speared on a small
brass skewer attached to one end of a suitable length of
horsehair or very fine wire, the other end of which is tied
to a small hook. Both the hook and the skewer are
easily improvised from ordinary domestic pins. The
object is then taken in a pair of tongs, immersed in a
beaker of water, and cleared of the numerous air-bells
which are sure to cling to it by brushing all over with a
camel's hair or fine bristle pencil ; this operation should
be conducted in a good light. The beaker is placed on a
wooden support straddling over the pan of the balance ;
the object is suspended from the hook above, and its
weight under water is determined. The tare of the
horsehair and hook, with the skewer immersed to a
similar depth in water, must also be known. We have
then weight in air, plus tare, minus gross weight in water
as representing the weight of water displaced by the
object, i.e. its volume ; from this and the weight in air
of the object we deduce its specific gravity. Temperature
correction — which, however, affects only the third place
of decimals — is made by multiplying the specific gravity
so found by the density of water at the temperature of
experiment.

Materials lighter than water are dealt with in the same
way, except that the object must be weighted with a
sinker. This may consist of a smooth piece of metal to

106 INDIA-RUBBER LABORATORY PRACTICE CHAP.

which a looped pin-point is soldered, or which is slung
from the object by a thread, or, yet more simply, of a piece
of lead wire wrapped round the object. Needless to say,
the tare of the sinker under water must be known and
added to that of the suspension.

Eubber in small fragments or dust must be treated like
a powder, i.e., by pycnornetry. For vulcanized rubber
generally, hydrostatic weighing is to be recommended
wherejver practicable, as being less troublesome than the
pycnometric method and more trustworthy than any
flotation method, howrever ingenious or fascinating. In
solids cooled from fusion without crystallizing, and in
properly made rubber goods not purposely intended to
be porous, true and apparent specific gravities may be
accepted as identical. Unvulcanized rubber in crape
form is quite amenable to the hydrostatic weighing
method, provided it be boiled out in vacuo with especial
thoroughness ; if in lump form it should first be rolled
into thin sheet or cut into narrow strips.

The specific gravity of pure resin-free rubber ranges
from 0-91 to O96, largely according to botanical origin.
Variations within the third place of decimals may depend
upon crop, age of trees, mechanical treatment, &c.
Plasticizing on the hot rollers generally raises the specific
gravity about O005. Eesin in any considerable proportion
tends to raise the specific gravity, since the resins them-
selves are nearly as heavy as water, or slightly heavier.
The rubbers from Hevea, Castilloa, and Ficus, whether
wild or plantation, have a specific gravity of 0'91 to 0*92.
Manihot rubbers are invariably somewhat heavier and
show a greater range, viz., O93 to 0-96. The deresinified
rubber of African sorts is equally heavy, seldom showing
less than O93 but not often more than O95 ; in presence

SPECIFIC GRAVITIES 107

of their resin, however, the more highly resinous sorts
may reach as high a specific gravity as 0'98.

Vulcanization involves a condensation of matter, hence
the specific gravity of vulcanized rubbers is always
greater than that of the uncured doughs. Starting from
a mixture of rubber with nothing but sulphur, this
shrinkage is only O007— 0-013 at soft cures (2°— 4°), but
amounts to 0-05-0-07 for ebonites (20°— 50°).

Diluents and charge being absent, soft-vulcanized
rubbers are still, for the most part, lighter than water,
but ebonites are invariably heavier. The specific gravity
of vulcanized rubber goods containing auxiliary materials,
as determined by hydrostatic weighing, may be calculated
additively from the components, assuming always that
there are no vacuoles in the rubber. A quality made up
of a parts of vulcanized rubber, sp. gr. A, b parts of a
filler, sp. gr. B, c of another filler, sp. gr. C, and so on,
will have the specific gravity

a+b+c+

a b c
A+B+C+

Goods ordinarily met with in trade may thus have
specific gravities ranging from less than unity to upwards
of 2. In the old days before the rise of rubber chemistry,
specific gravity was almost the only criterion of the quality
of rubber goods which could be expressed in figures, and
now as then experienced practical men can draw astonish-
ingly far-reaching conclusions from inspection combined
with a knowledge of the specific gravity. Apart from
this, specific gravity is of no small direct importance in
the manufacture of rubber goods. Most such are made
up to definite limits of specific gravity, either to meet

io8 INDIA-RUBBER LABORATORY PRACTICE CHAP.

peculiarities of popular demand, or to fit in with require-
ments as to weight in composite structures containing
rubber, or because the goods are sold by weight, or for
other reasons. Especially important is the distinction
between floating (i.e. lighter than water) and non-floating

Filler.

Sp. Gr.

Sp. Gr.
of Rubber.

Litharge

9-4

1 '006

Zinc White . . .

5-5

10
30

1-043
1-194

1-173

4-5

100
30

1-590
1*161

Lithopone \
Antimony Red j-
(not plastered)]

Zinc Sulphide ^
Magnesia
Antimony Red I
(plastered) J

Talc \

4-1

3-3

2-7

100

10
30
100

5
10
30
100

1-541

1 -031
'1-154
1-519

0-997
1 -027
1-139
1 -456

1-120

Whiting/

Magnesium Carbonate)
Graphite J ' '

Lampblack . . , .

2-2

1-8

100

10
30

1-390

1-013
1-099

1-004

1-072

goods. Air-chambers of tyres, rubber strip and bands,
golf-balls, certain kinds of sheet and tubing, and a host
of minor articles, are commonly demanded in floating
quality. The admixtures introduced into such goods
must be regulated accordingly. Thus, starting from Para

vi SPECIFIC GRAVITIES 109

rubber vulcanized with 10 per cent, of sulphur (sp. gr.
0-968), the floating limit is reached with about 9 per cent,
of lampblack, 5 — 7 of antimony red, and only 4 of zinc
white or litharge. On the other hand, no limit is set
by waxes, oils, floating factice, or, within practicable
proportions, bitumen.

The table on p. 108 shows the specific gravities of
vulcanized rubbers made up of 100 parts of Para, 10 parts
of sulphur, and n parts of some of the commonest pulveru-
lent fillers.

In order to ascertain the true specific gravity of rubber
goods which are blown or spongy, the pycnometric method
must be resorted to, the sample having previously been
ground fine on the rollers. Eubbers which cannot be
reduced to crumb are cut by hand into the finest possible
shreds. Thorough boiling out in vacua is essential. In
works practice it is often expedient to control internal
blowing in vulcanized rubber goods, both qualitatively
and quantitatively, by comparing the true "with the
apparent specific gravity of the goods.

CHAPTER VII

ANALYSIS OF MANUFACTURED RUBBER : ORGANIC

MOST people have at one time or another seen a
pneumatic inner tube, a rubber heel, and an ebonite pipe-
stem ; and there is no need to insist on the vast diversity
in character of manufactured rubber articles. Neverthe-
less, essentially the same system of quantitative analysis
applies throughbut. Analysis may have for its aim
(1) the detection or quantitative control of specified
constituents, e.g., ash, total acetone extract, free sulphur,
total sulphur, alcoholic potash extract, etc., or (2) the
elucidation, item by item, of the mixing from which the
article was made. In the former case all is tolerably plain
sailing ; but the complete proximate analysis of rubber
goods, though there is nothing particularly arduous or
esoteric about the analytical procedure, involves difficulties
of interpretation which are hardly ever absent and are
sometimes insuperable.

Heat-cured rubber goods are made by compounding
rubber, sulphur, diluents, and charge, and vulcanizing the
mixture to a low (soft rubber goods) or a high degree
(ebonites). After vulcanization, they preserve their con-
no

CHAP, vii ANALYSIS OF MANUFACTURED RUBBER in

stituents practically unaltered, except that part of the
sulphur has gone into combination with the rubber
and possibly also, though to an inconsiderable extent,
with the charge. Cold-cured goods form a minor class
confined to thin sheet, tape, or thread, thin- walled tubing,
waterproofed fabrics, etc. ; they are compounded without
sulphur and are superficially vulcanized by means of
sulphur chloride. No chemical changes in the charge,
provided it be properly selected, are brought about by
cold-curing, but the rubber goes into combination with
both sulphur and chlorine.

Insertions and other inhomogeneities apart, the quali-
tative composition of manufactured rubber is summed up
by the following scheme : —

Article.

Necessary Constituents.

Possible Constituents.

Rubber proper.

Heat-cured goods

Rubber-Resin and -Ash.

and Ebonites.

Sulphur combined with

rubber.

Free Sulphur.

Rubber proper.

Diluents : —
Waxes.

Rubber-Resin and -Ash.

Oils.

Cold-cured

Sulphur combined with

Foreign

Goods.

rubber.

Resins.

Chlorine combined with

[ Factice.

rubber.

Bitumen, &c.

Rubber proper.
Rubber-Resin and -Ash.
If intended for vul-

Charge : —
Inorganic
Powders.
Carbon Black.

Doughs and
Solutions.

canization : —
Free Sulphur.

Fibres.
Organic Dust.

In the case of solu-

tions : —

Naphtha or other sol-

vent.

Ii2 INDIA-RUBBER LABORATORY PRACTICE CHAP.

The primary operations in rubber analysis are concerned
with separating and isolating the above-named main
classes of constituents. As regards diluents (and free
sulphur), this is effected by extraction with a sequence of
solvents. The charge, as a whole, is isolated by removing,
and incidentally decomposing, all the organic components,
and may then be subjected to further analysis. Eubber
proper, once combined with sulphur or chlorine, cannot
be isolated unchanged and is determined by difference or,
in the rare cases where that is possible, by the bromine
method. The sulphur and chlorine combined with rubber
can be determined by special assays.

Before being subjected to analysis, rubber goods must
be suitably comminuted. Certain classes of goods, viz.,
ebonites and highly charged rubbers, are readily reduced
to coarse powder by the aid of a rasp. Most soft rubbers,
however, cannot thus be dealt with. Whenever factice is
known to be absent or does not need to be determined,
it is quite satisfactory to cut up the rubber with a pair of
scissors into snippets approximating to 2 mm. cubes.
Where a set of mixing rollers is available, there is no
better way of comminuting rubber goods than by " crumb-
ing " them on the closely-set cold rollers. Goods rather
highly vulcanized or containing much charge can thus be
brought into the state of a woolly mass or even of a
powder. High-class elastic rubbers, on the other hand,
cannot be fully disintegrated, but go into the form of
crape. If this crape be thin enough, it need only be cut
up with scissors and is then suitable for any analytical
operation. A certain amount of heating inevitably takes
place when rubber is sent through the rollers, but great
care must be taken that it does not go too far. As a
substitute for orthodox rollers, the small metallurgical

vii ANALYSIS OF MANUFACTURED RUBBER 113

mill mentioned on p. 26 (Fig. 9) may be made to do good
service.

Manufactured articles made from a single mixing are
usually homogeneous enough to render averaging of
samples unnecessary. Parti-coloured goods, however,
will have to be averaged from a fairly large bulk if they
are to be analysed as a whole. In dealing with composite
rubber goods, e.g. tyres, cable-coverings, etc., it will
generally be desirable to analyse each section or layer
separately; since these cannot, given a well-made article,
be separated by stripping, there is nothing for it but to
cut out the various samples as cleanly as may be with the
aid of a sharp wet knife and a pair of scissors. Rubber
laid upon fabric can similarly, if thick enough, be shaved
off with a wet knife. When the fabric is rubbered on one
side only, it can often be stripped away after wetting with
steam or boiling water. Another artifice, which, however,
involves slight losses of extractive matter, consists in
soaking the rubbered fabric in naphtha and shaving off
the swelled rubber, which is then dried until free from
solvent. In the case of rubbers consisting of a single
mixing and spread upon fabric in a thin layer, trouble
may often be saved by analysing rubber and fabric
together, due allowance being eventually made for the
latter. In order to ascertain the relative proportions of
fabric and rubber, the fabric is isolated in a clean or
nearly clean condition by boiling in petroleum (see p. 141)
followed by copious washing with naphtha ; any rubber-
fillers remaining entangled with the fabric may be deter-
mined and isolated by incineration.

114 INDIA-RUBBER LABORATORY PRACTICE CHAP.

I. Moisture.

Manufactured rubber may in general be assumed to be
free from moisture. Eubber which has from accidental
or extraneous causes become wet may be dried in vacua
or in an air-oven at 60°.

II. Acetone Extraction.

The first operation in rubber analysis, which is to be
applied in all cases without exception, consists in extrac-
tion with boiling acetone. One or two gr. of material are
placed in a filter-thimble, covered (if in the state of
powder) with a plug of fat-free cotton wool, and extracted
in a Knofler tube for ten hours. In the case of ebonites it is
safer to extract for twenty hours. Acetone which has
stood for a long time may contain terpenoid bodies and
must be redistilled before use. During extraction, note
should be taken whether yellowish matter is being
dissolved out slowly and hesitatingly ; if in that event
the extracts are barely fluorescent, bitumen is indicated ;
if strongly fluorescent, coal-tar pitch. A nearly colourless
fluorescent extract points to mineral oil. Before being
distilled, the extract is allowed to cool completely, and
any deposition which may take place is observed.
Sometimes free sulphur crystallises out in long mono-
clinic prisms. A bulky translucent crystalline growth
will be due to paraffin or ceresine. Oily drops will in
most cases consist of factice-extract. All these substances
dissolve easily in hot, sparingly in cold, acetone ; hence
the necessity of extracting with boiling solvent. When
determinations other than that of free sulphur have
eventually to be made in the extract, two or more separate
extractions of the same rubber must be carried out.

vii ANALYSIS OF MANUFACTURED RUBBER 115

The extract, in its 100 c.c. or 150 c.c. Soxhlet flask,
is distilled from a water-bath, dried, and weighed. Free
sulphur, if present, is apt to give trouble by causing
violent spirting in the later stages of distillation ; when
spirting sets in, it is advisable to stop the distillation, to
add a little benzene or carbon disulphide, and to continue
evaporating with or without condensation. The flask is
dried, to avoid loss of sulphur, for not more than one
hour at 110° or for three hours at 60° ; but if there is a
large amount of extract, say over J gr., it is well to
prolong these periods.

Substances completely extracted by acetone : —

Free Sulphur

Eubber-resin and other resins
Oils, mineral and fatty
Paraffin and ceresine.

Acetone extracts may in addition contain the acetone-
soluble matter accompanying
Factice
Eeclaim
Bitumens and pitches.

Free Sulphur is, of all rubber components, the one most
liable to uneven distribution. After vulcanization, the
sulphur left uncombined is evenly distributed throughout
the mass in a colloidal or other metastable condition.
Sooner or later, however, and to a greater or less extent,
it tends to work to the surface and there ultimately to
assume the stable crystalline (rhombic) form ; this con-
stitutes the familiar phenomenon of " sulphuring up."
Some judgment, therefore, is necessary in sampling goods
which have visibly sulphured up. It should also be
borne in mind that goods which have undergone super-

ii6 INDIA-RUBBER LABORATORY, PRACTICE CHAP.

ficial wear will have lost that portion of their free sulphur
which has thus effloresced.

Free sulphur in* acetone extracts is determined by one
of the following methods : —

1. Differential Solubility Method, by which sulphur
is isolated and weighed as such. To the extract in its
flask are added 20 c.c. of petroleum spirit (B.P. 60° — 100°)
saturated in the cold with sulphur. After shaking and
standing, the resin, etc., is dissolved out and the sulphur
left behind. The clear liquid is separated and the residue
washed two or three times with fresh reagent, and finally
with pure solvent free from sulphur. If the residual
sulphur is well crystallized, these operations can be ef-
fected by simple decantation ; if not, it will be necessary
to pass the liquids through a filter and collect the retained
sulphur by the aid of carbon disulphide. Finally the
sulphur is dried in the original flask at 60° and weighed.
The success of this method depends on whether all the
non-sulphurs go readily into solution, and from this
point of view petroleum spirit is a much better solvent
than acetone itself, which in the cold does not easily take
up waxes, factice-extracts, and bitumen-extracts, whereas
petroleum spirit does. The solubility of sulphur in cold
petroleum spirit is trifling, viz. about 3 gr. per litre.
Paraffin in considerable quantity may give trouble in
refusing to dissolve, as also the extracts from coal-tar and
other pitches and those from the fancy resins sometimes
occurring in ebonite. Though inapplicable in these ex-
ceptional cases, the method is in general accurate enough
for most practical purposes.

2. Thiocyanate Method, based on that of C. Davis and
J. L. Foucar.1 The extract is boiled under reflux with

1 Journ. Soc. Chem. Ind. 31 (1912), p. 100.

vii ANALYSIS OF MANUFACTURED RUBBER 117

25 c.c. of ordinary (95 per cent.) alcohol and about 1 gr. of
potassium cyanide in lump form, during half an hour, or
longer if there be much sulphur. 'What remains of the
cyanide is fished out, and the alcohol is driven off by
distillation. The sulphur has" now been converted into
potassium thiocyanate. About 50 c.c. of water, a slight
excess of nitric acid, and some iron alum are added.
Titration is then carried out with N/W or N/5 silver
nitrate solution to disappearance of the red colour.
The last-named operations should be conducted in the
draught-cupboard. One gr. Ag = 0-297 gr. S.

3. Oxidation Method. — To the extract are added 2 — 5 c.c.
of sulphur-free nitric acid and a pinch of powdered
potassium chlorate. The flask is covered with a watch-
glass and warmed on a water-bath for about six hours. If
much sulphur be present, it may be necessary to add
more chlorate from time to time ; care should be taken
not to let the sulphur melt. When all the sulphur has
been oxidized, the liquid is diluted with much water
containing a few c.c. of hydrochloric acid, filtered cold,
and precipitated with barium chloride. The method is
more especially useful when quite small amounts of
sulphur have to be determined with accuracy.

Mineral Oil, Vaseline, Paraffin, etc. — These hydro-
carbons may be isolated by destroying the rest of the
extract by means of sulphuric acid, the method being
adapted from one which has long been in use for the
assay of ceresine in crude ozokerite. When isolated, the
hydrocarbons, whether oil, vaseline, or wax, can usually
be identified by their appearance and consistency. To
the extract are added 1| — 2 c.c. of concentrated sulph-
uric acid, and the flask, covered with a watch-glass,
is heated in an oven at 110° for 3-4 hours. The right

ii8 INDIA-RUBBER LABORATORY PRACTICE CHAP.

dose of acid varies with the amount of resin to be decom-
posed, and should be so regulated that the final magma
flows with difficulty. Thirty c.c. of petroleum ether
(B.P. 40° — 60C) are then poured in and boiled under
reflux for an hour. The solution is decanted into a
separating funnel and the flask is rinsed out with solvent,
transference of acid being carefully avoided. The
petroleum ether solution is now thoroughly washed with
caustic soda solution (10 — 20 per cent.) to which half its
volume of alcohol has been added. Finally the solution
is evaporated in a tared flask, which is then dried for
2 — 3 hours at 110° and weighed. The method fails when
very large excesses of resin are present, but is otherwise
quite satisfactory, with perhaps a slight tendency to high
results. Mineral oils and waxes in acetone extracts, it
may be observed, sometimes originate not as direct com-
pounding materials but as factice-ingredients (see p. 41).
A portion of the extract from bitumens, also, is of the
nature of hydrocarbons resistant to sulphuric acid.

Paraffin and Ceresine. — No analytical distinction
between these two waxes can advantageously be made
(but see p. 96). When the wax is isolated, there is often
no difficulty in recognizing the one or the other by its
consistency. An old-established method of determining
solid hydrocarbons, when these alone are present, rests
on the principle of differential solubility and may be
carried out as follows : —

The extract is boiled under reflux with 30 c.c. of
alcohol, which should be as nearly as possible of 97 per
cent, strength (e.g., about equal volumes of absolute and
ordinary alcohols). If globules of molten substance
remain undissolved, more alcohol must be added. The
solution is decanted from any solid residue and cooled

VII

ANALYSIS OF MANUFACTURED RUBBER 119

under running water. Ice-cooling is a refinement which
adds but little to the accuracy, such as it is, of the assay.
The waxy matter which crystallizes out is received on a
filter, washed with alcohol (90 per cent.), and rinsed with
the same into a tared Soxhlet flask. After removal of the
liquid by evaporation, the residue is dried and weighed
as usual. A slight plus correction may be made on
account of the paraffin remaining dissolved in the mother-
liquor. The conditions of solubility of hard paraffin in
alcohol are approximately these : —

Alcohol.

Solubility
at boiling-point.

Solubility at 18°.

100 per cent.
97 „
96 „
95

4-0 gr. per 100 c.c.
1-6
0-7
0-3

0'20 gr. per 100 c.c.
O'll
0-08
0-04

It follows from these data that to dissolve 0*2 gr. of
paraffin (more than this is not likely to occur in an
acetone extract), 30 c.c. of boiling 97 per cent, alcohol
should suffice, and that 0'033 gr. of paraffin will then
escape into the mother- liquor. Against the latter item is
to be set the sulphur which, provided that free sulphur
be present in appreciable quantity, will find its way into
the deposited paraffin and call for a slight minus correc-
tion. This sulphur, which may be anything up to 5 per
cent, of the wax, can be determined in the weighed
paraffin by the thiocyanate or nitric acid method.

Other Components, of the acetone extract present un-
solved analytical problems, so far as generally applicable
methods are concerned. Rubber-Resin itself is so variable

120 INDIA-RUBBER LABORAl^ORV PRACTICE CHAP.

a substance that there is no means of determining it
directly. Fatty Oils, again, cannot be estimated in
presence of rubber-resin by the simple process of taking
the saponification number, because some resins (from
Para and plantation sorts, especially) have a high
saponification number of their own, whilst others (from
African sorts, especially) have next to none. The same
argument applies to Factice- Extract, with the added
complication that this class of body itself shows a con-
siderable range of saponification numbers. No direct
method for the determination of Bitumen- Extract is as
yet known. When either of the two last-named is
present, an approximate estimation can be made on the
basis of the figures subsequently found for factice and
bitumen insoluble in acetone. In the case of factice, the
assumption may be made that roughly 10 per cent, of
white or 20 per cent, of brown factice are acetone-soluble
and go into the present extract. As for bitumen, the
estimation of extract on these lines is even more un-
certain, seeing that anything between 50 and 80 per cent.
of the total bitumen may be dissolved out during acetone
extraction ; but 70 per cent, may be regarded as a rough
average (see p. 54).

Rubber-Resin. — In the simplest cases, which embrace
most of the better-class rubbers, acetone extract consists
of rubber-resin plus free sulphur, plus, possibly, mineral
wax. Sulphur can be determined accurately, mineral
wax with tolerable accuracy ; hence rubber-resin can be
estimated by difference with no great error. The
importance of this lies in the fact that by calculating
it as a percentage of resin plus rubber proper (as subse-
quently determined), the resin-content of the raw rubber
or rubbers originally employed is arrived at. The

vii ANALYSIS OF MANUFACTURED RUBBER 121

original resin-content is not affected by vulcanization,
except in so far as a small part of it is rendered insoluble
in acetone ; the more, the higher the vulcanization. This
insolubilized portion may amount to several per cent, in
ebonites, where it eventuates in the alcoholic potash
extract ; in soft rubbers, however, it is so small as to be
practically negligible. Original resin-content, thus
ascertained, often gives useful information for identifying
the raw rubber sort, at any rate within limits. Occasion-
ally also rubber-resins possessing characteristic odours
(Para, Guayule, certain Africans, etc.), can be recognized
in the acetone extract. The polariscopic test (p. 16)
effects little more than a classification into rubbers poor
and rich respectively in resin. There is, in fact, no
certain way of identifying original rubber, either through
the rubber itself or through its resin. The best one can
hope for is an indication pointing to this or that class of
rubber, which, to an experienced rubber analyst, may
often be of considerable value.

When factice or bitumen is present in even small
amount, the estimation of rubber resin by difference is
attended with some uncertainty. To allow for factice-
extract, the best plan is to regard 10 per cent, of the
factice subsequently extracted by alcoholic potash, if
white, or 25 per cent, if brown, as belonging to the
acetone extract. With bitumen the uncertainties are
even greater. Eoughly speaking, the acetone extract is
about five times the carbon disulphide extract subsequently
determined (see p. 54). In inferior rubbers containing
large admixtures of these diluents, conventional ratios
between matter soluble and insoluble in acetone are no
longer to be trusted as aids to estimating rubber-resin by
difference. When reclaim is present, all ordinary calcula-

122 INDIA-RUBBER LABORATORY PRACTICE CHAP.

tions are upset, and each case must be judged, as
shrewdly as may be, on its merits.

III. Carbon Bisulphide Extraction.

This operation concerns bitumens and pitches only.
To ascertain whether there is any need for it, a sample of
the original or acetone-extracted material is covered in a
test-tube with carbon disulphide ; the formation of a
decidedly brown solution indicates presence of bitumen
or pitch. A qualitative distinction between these two
may be made by means of pyridine, which has no effect
on bitumen but readily dissolves coal-tar pitch, yielding
reddish solutions with a strong green fluorescence. In
general, the presence of bitumen or pitch will already
have been remarked in the course of acetone extraction
(see p. 114). Formerly it was a safe rule that these
diluents could occur only in black or grey rubbers ;
nowadays, however, bitumen of the " mineral rubber "
type is sometimes to be found, in very small proportion,
in reds and whites.

The residue from acetone extraction, in its filter- thimble,
is dried in the open air or by being put for about a quarter of
an hour in the oven. It is then plugged with cotton wool
(this is indispensable owing to the high specific gravity of
the solvent) and extracted in a Soxhlet or Knofler tube with
fresh carbon disulphide. Usually a few hours suffice,
but in presence of a large proportion of bitumen it may
be necessary to extract for ten hours or longer. The
operation is at an end when the liquid surrounding the
thimble remains quite colourless after standing for half
an hour. The extract is distilled, dried for 1 — 3 hours at
110°, and weighed. Carbon disulphide will dissolve out
no rubber, unless the material be under-cured. To meet

vii ANALYSIS OF MANUFACTURED RUBBER 123

this eventuality, the dried extract is treated repeatedly
with cold carbon disulphide in small doses until no more
coloured matter goes into solution. If a pellicle of rubber
is now left behind, it is dried and weighed, and its weight
is subtracted from that of the total extract.

It should be borne in mind that the bitumen thus
found represents quite a small fraction (10 to 30 per cent.,
according to circumstances) of the bitumen originally
added to the rubber. More than half of the total bitumen
will have been already removed by acetone. On the
other hand, a certain proportion, of the same order of
magnitude as the carbon disulphide extract itself, resists
solvents altogether and passes on into the final residue
of vulcanized rubber. Some data as to the distribution of
bitumen which, if used with caution, may serve as the
basis of calculations for the present case, are given on
pp. 54—55.

IV. Alcoholic Potash Extraction.

The next and last process of extraction consists in the
removal of factice, or rather of the essential, fully
vulcanized, constituent of factice. This is effected by
saponification, whereby the factice is converted into
glycerine and alkali-soaps soluble in alcohol. Too much
stress cannot be laid on the necessity of having a finely-
divided sample, procured either by rasping or crumbing,
for this determination. Alcoholic potash penetrates
rubber with such difficulty that it may very well extract
only a fifth or a tenth of the factice actually present, if
the rubber be merely cut up into snippets.

The rubber left over from the previous extractions is
spread on a dish, freed from solvent by air-drying, and
transferred to a 100 c.c. flask ground-in to a reflux con-
denser. The apparatus designed for the toluene extraction

I24 INDIA-RUBBER LABORATORY PRACTICE CHAP.

of gutta-percha (Fig. 22fr) serves equally well for the present
purpose. Fifty c.c. of N/5 alcoholic potash, or 10 c.c.
of Nj~L suitably diluted with alcohol, are added, and the
mixture is boiled on a sand- or water-bath for three hours.
The reagent employed should not be dark in colour, but
yellow at most ; it should be made up with clean spirits
of wine (not methylated spirit) and kept in a well-lighted
place. In taking up factice, it assumes a more or less
deep brown colour.

The liquid is poured away from the rubber and dis-
tilled nearly to dryness from a water-bath. Meanwhile
the rubber is boiled up with three relays of 20 c.c. of
water and wrung out, the combined washings being
added to the distillation-residue. The clear soap-solution
thus obtained is transferred, when cool, to a separating
funnel, acidified with hydrochloric acid, and extracted
two or three times with ether. The ethereal extract is
distilled in a tared Soxhlet flask, dried for an hour or
two at 110°, and weighed.

The brown evil-smelling mass thus obtained consists
not of factice but of factice-acids, and requires a small
plus correction. On the average, the yield of acid from
acetone-extracted brown factice is 94 per cent., from
white factice 88 per cent. In order, then, to arrive at
the weight of unsaponified factice, the weight of factice-
acids is multiplied by 1'136 if chlorine (indicating white
factice) is found in the alcoholic potash extract, or by
1-064 if chlorine is absent. The older method of factice-
determination, by which the rubber was weighed before
and after extraction, is open to more than one serious
objection and is not to be recommended.

It is often desirable to determine the sulphur of
vulcanization combined with the extracted factice. To

vii ANALYSIS OF MANUFACTURED RUBBER 125

this end a separate extraction must be made, because a
certain amount of sulphur (and most of the chlorine) is
split off in the operations described above. A separate
alcoholic potash extract, accordingly, is evaporated in an
iron bowl, and the sulphur is determined by the potash-
peroxide fusion method (see p. 43). So also both sulphur
and chlorine may be determined in the same extract, but
there is little to be gained from chlorine determinations
unless the simultaneous presence of white and brown
factice be suspected, since white factices do not vary
notably in sulphur- and chlorine-content (cf. p. 45).

The percentage of factice determined by means of
alcoholic potash falls short of the factice originally present
by so much as has already been dissolved out by acetone.
In calculating back to mixings, then, the factice now
found should be increased by one-fourth in order to arrive
at original brown factice, and the same quantity should
be subtracted from the acetone extract of the rubber. In
the case of white factice the allowance will be one-ninth
of the factice found.

V. Combined Sulphur.

After these various extractions, what we have left is
rubber hydrocarbon, plus the sulphur of vulcanization
combined with it, plus the charge. Before going on to
the analytical treatment of charge, it will here be well to
consider the determination of combined sulphur. In the
majority of heat-cured soft rubber goods the sulphur of
vulcanization ranges between 2 and 4 per cent, of the
rubber proper, and when it is desired merely to obtain
approximate information as to the original mixing, it
does well enough to assume that for 100 parts of rubber
hydrocarbon there are 3 parts of combined sulphur

126 INDIA-RUBBER LABORATORY PRACTICE CHAP.

present. This sulphur, added to the free sulphur previously
found, gives total sulphur as compounded in the mixing.

From another point of view, however, sulphur of
vulcanization calls not only for direct determination, but
for accurate determination. The coefficient of vulcaniza-
tion of any rubber, which may more briefly be denominated
degree of sulphur and be expressed in degrees, is denned
as the parts of rubber-combined sulphur present per 100
parts of pure sulphur-free rubber hydrocarbon. This
quantity is very characteristic of the rubber and the cure.
In any given mixing, it increases with the temperature of
the cure, the duration of cure, and the proportion of
sulphur incorporated; moreover, all these conditions
remaining constant, it varies with the species of rubber
employed. What is considered correct vulcanization for
any given mixing can be, as it were, registered by means
of the degree of sulphur. There is no need, therefore, to
insist on the importance of determining sulphur of
vulcanization in connexion with manufacturing practice.

The determination of degree of sulphur involves three
distinct assays, viz. : —

(a) That of total combined sulphur.

(b) That of inorganically combined sulphur, if present.

All these being expressed as percentages of original
material, we have

8 = _100(*-a) degrees.

Assay (b) is carried out on a portion of the isolated
charge (see p. 141). Eubber proper is usually estimated
by difference (see p. 13^) and in complicated mixtures is
liable to come out with an error of several per cent.

vii ANALYSIS OF MANUFACTURED RUBBER 127

In order not to aggravate this error, it is well to deter-
mine sulphur as accurately as possible. The percentage
of combined sulphur in high-class soft rubbers ought to
be stated correctly within ± 3 to 4 units in the second
place of decimals. In highly charged goods a somewhat
wider limit of error must be allowed. With ebonites,
which may be cured up to 40° or higher, the second
place of decimals may be disregarded.

Total combined sulphur in rubber goods is determined
in the residue remaining after all the extractive operations
have been brought to an end. If it be intended to weigh
out a portion only of the residue in hand, this residue
must previously be completely dried and then weighed.
Extracted rubber has a peculiar tendency to take up
oxygen, so that the drying must be done with care. The
best thing is to heat in the air-oven at 110° until the
water or organic solvent is nearly expelled, and to finish
in the vacuum-oven. Generally speaking, however, it
makes for accuracy to weigh out a separate sample of
original substance, to subject it to the various extractions,
and to use it bodily, after cursory drying, for the deter-
mination of combined sulphur.

Of the various methods available, two may be specially
recommended. By the classical method of Henriques,
now over thirty years old, the rubber is nitrated and the
organic matter then destroyed by fusion with alkaline
nitrate. Very accurate results are obtained, but the
method is tedious and much at the mercy of accidents,
and is useful rather as a check method than for routine
work. The second method, in which sodium peroxide is
the oxidizing medium, dates from the comparatively
recent time when that reagent became cheap and familiar.
It has for some years past been extensively practised in

128 INDIA-RUBBER LABORATORY PRACTICE CHAP.

numerous modifications, amongst which the one described
below is as rapid, accurate, and handy as any.

The amount of material to be weighed out should be
1 gr. of high-class (especially floating) qualities, O5 gr. of
heavily charged qualities, or 0'3 gr. of ebonite.

1. Nitrate Fusion Method. — The rubber is placed
together with 15 c.c. of sulphur-free concentrated nitric
acid in a 25 c.c. porcelain crucible of the wide-mouthed
type, which is covered with a watch-glass and warmed,
cautiously at first, on a water-bath. When the evolution
of nitrous gases has ceased, the watch-glass is removed
and the mixture is evaporated. Should fragments of
rubber remain undecomposed, evaporation with more
nitric acid will be necessary. The contents of the crucible
now consist of a yellow slime, to which carbon black
or bitumen, if present, may impart a black discolor-
ation. Whilst the sticky mass is still warm, 5 gr. of a
finely-powdered mixture of sodium carbonate and po-
tassium nitrate (5 : 3), both sulphur-free, are added in
small portions at a time and uniformly incorporated
with the aid of a thick iron wire hammered flat at the
end. The crucible and its contents are then dried in
the air-oven ; practised analysts, however, will combine
this with the next operation, taking care that condensed
moisture is not allowed to remain on the crucible-cover.
The mixture (which, it will be noted, is nothing more or
less than a kind of slow gunpowder) is now very cautiously
heated over a small flame until reaction sets in, when the
flame is at once withdrawn. The crucible meanwhile is
covered with its lid, or with a semi-flat porcelain dish,
bottom downward ; in case solid particles are thrown on
to the cover, they can subsequently be scraped off and
added to bulk. Keaction should take the form of a

vii ANALYSIS OF MANUFACTURED RUBBER 129

slow and mild deflagration with evolution of bluish
smoke. Provided the reagents be finely powdered and
intimately mixed, and not heated too quickly, there is no
risk of anything so violent as an explosion. Eeaction
over, the flame is urged and the contents of the crucible
are brought to fusion and well stirred with the iron wire
previously used. After remaining in fusion for a short
time, as much as possible of the melt is poured and
scraped on to an iron plate and then thrown, whilst still
hot, into a beaker of water. The cooled crucible also is
digested with hot water. When the melt has been com-
pletely lixiviated, the liquid is filtered. The residue,
which contains inter alia carbonates of all the lead, zinc,
barium, calcium, and magnesium present in the rubber,
is worth preserving in case check determinations of these
metals should be required. The filtrate is strongly
acidified with hydrochloric acid, evaporated to dryness,
taken up with water, and filtered ; whereupon precipi-
tation by means of barium chloride is effected in the
usual way.

2. Potash-Peroxide Fusion Method. — The most con-
venient vessels for this fusion are hemispherical bowls of
spun sheet-iron, about 10 cm. diameter, such as are com-
monly used for sand- or oil-baths. They are cheap, and
last out twenty or thirty fusions. Large nickel crucibles
serve equally well, but are both dearer and less durable.
The rubber is placed in a bowl with 16 gr. (usually corres-
ponding to 16 cm.) of stick potash or less, according to
the amount of rubber proper present, and 2 c.c. of water ;
more water than this is both superfluous and undesirable.
The potash is dissolved by gentle heat and boiled down
at a simmer. Now and afterwards the contents of the
bowl should be kept well mixed, either by means of a

130 INDIA-RUBBER LABORATORY PRACTICE CHAP.

long iron spatula or by agitating the bowl held in a pair
of crucible tongs. When the rubber begins to melt, the
flame is so regulated that decomposition, accompanied by
evolution of a bluish fume, takes place at a steady and not
too rapid pace. After 15: — 20 minutes, volatile matter will
cease to be given off and the melt will consist of a homo-
geneous dark mass. The spluttering which continues
up to this point does not seriously affect the accuracy
of the assay, but is very disagreeable to the operator, who
will do well to wear gloves. A lid of sheet iron may be
used, but the process is less well under control, especially
as regards keeping the melt properly mixed. Sodium
peroxide is now added in pinches, and thoroughly incor-
porated. The amount required varies between 2 and 5 gr.
and may be gauged by the circumstance that when
oxidation is complete the melt changes from a light green
to a dark purple colour due to ferrates and permanganates.
Should effervescence take place on adding the peroxide,
it is a sign that the fusion with potash alone has not been
urged far enough. Care must be taken that the oxidizing
magma comes into contact with the whole inner super-
ficies of the bowl.

After cooling, but whilst still warm, the bowl is three-
quarters filled with water. The dissolved contents are
rinsed into a beaker, made up to 400 c.c., treated with
concentrated hydrochloric acid until the ferric oxide is
just dissolved, further acidified with 5 c.c. of the same,
and boiled. In the absence of barium the liquid will be
perfectly clear, though strongly coloured by ferric chloride.
Whether barium be already present or not, the boiling
liquid is now precipitated with a decided excess of barium
chloride and allowed to cool completely ; this latter pre-
caution is necessary, because barium sulphate is by no

vii ANALYSTS OF MANUFACTURED RUBBER 131

means insoluble in the hot, acid, highly saline liquid.
The precipitate is filtered off and weighed as usual. From
beginning to end the whole assay can, if desired, be
carried out within 2J hours. Should the stick potash
contain sulphur, the requisite correction is determined
once for all by a blank assay ; but most of the better
brands are practically free from sulphur.

This method (but not the preceding one) lends itself
also to the determination of chlorine of vulcanization in
cold-cured goods, precisely as in the case of white factice
(see p. 44). If, however, the rubber has previously been
extracted with alcoholic potash, the determination will
have no quantitative value, since cold-cured rubber gives
up most of its chlorine to alkali. With a cold-cured
rubber containing white factice, the best course is to
determine total chlorine immediately after acetone extrac-
tion and to deduct 8 parts of chlorine per 100 of factice
separately determined by means of alcoholic potash.

Mineral Combined Sulphur. — By this is meant the
total combined sulphur contained in the pulverulent
constituents (charge) of the rubber, and it is determined
directly in a portion of the charge isolated by the method
described in the next chapter. As much as can be spared
of the charge (previously well mixed) of black and grey
rubber may be weighed out, but in the case of whites and
reds not more than J gr. should be taken. The powder is
boiled down with 5 gr. of stick potash and oxidized with
sodium peroxide as above ; there being little or no organic
matter to get rid of, the whole business is over in less than
ten minutes. Or, still more simply, the powder is directly
mixed with about 3 gr. of peroxide in a small iron or
nickel crucible and heated to fusion ; but if the powder
contains a high proportion of sulphides or of carbon black,

132 INDIA-RUBBER LABORATORY PRACTICE CHAP.

it must be diluted with a little magnesia to mitigate the
violence of the reaction. The melt is dealt with as before.
Mineral sulphur thus found is calculated as a per-
centage of the original rubber and subtracted from total
combined sulphur similarly calculated ; this gives sulphur
of vulcanization. Sulphur of vulcanization calculated as
a percentage of rubber proper (itself a percentage of
original material) constitutes the degree of sulphur.

When a rubber is to be examined for degree of sulphur
only, one gramme is weighed out, extracted, and treated by
the nitrate or potash-peroxide method. Another gramme
(except when mineral ingredients are altogether absent)
is dissolved as on p. 141, and the whole of the charge
isolated from it is similarly treated. The result of the
latter assay is simply subtracted from that of the former.
This double operation is, or should be, a very frequent
item of factory routine in cases where the percentage of
rubber proper in the mixing is already known. In these
cases, moreover, the separate determination of mineral
sulphur can often be dispensed with; but it should be
noted that it is a little unsafe to calculate mineral sulphur,
if there be much of it, from the compounding materials
as specified on paper, and that when lead compounds are
present the actual determination of mineral sulphur in
each sample is a matter of necessity.

VI. Determination of Rubber.

Although much work has been expended on the deter-
mination of rubber by direct methods, there can be no
doubt that, as matters now stand, the only trustworthy
general way of arriving at the percentage of rubber in
vulcanized goods consists in simple estimation by differ-
ence. In former times, when inorganic matter could be

vii ANALYSIS OF MANUFACTURED RUBBER 133

determined only by incineration, the difference method
was liable to rather large errors, but the application of
centrifugal machinery has altered all that. Eubber proper,
then, is determined by adding up the percentages of the
various extracts, total charge, and sulphur of vulcaniza-
tion, and subtracting the sum from 100. As a rule, it is
quite feasible thus to estimate the content of rubber
proper within one or two per cent, of itself. When
bitumen is present, a certain amount of vulcanized
bitumen will remain with the rubber and evade direct
determination ; the best that can be done in such cases
is to apply an approximate correction (see p. 54).

Vulcanized rubber combines with bromine as readily as
unvulcanized ; the presence of mineral fillers, however,
militates against the accurate determination of rubber as
tetrabromide, and the sulphur of vulcanization also intro-
duces some uncertainty. When there is no mineral
matter, or none except barytes, silicates, and carbon,
rubber can be determined, after extraction, in both soft
rubbers and ebonites by the tetrabromide method (see
p. 14). Under the action of bromine, part of the com-
bined sulphur is split off as sulphur bromide, whilst
the remainder persists in the rubber bromide.1 For soft-
cured goods of 2° — 4° of sulphur, the same factor of
conversion, viz., 0'42, holds approximately good as for
unvulcanized rubber. For ebonites the factor ranges
between 0'6 and 1-0, according to the degree of
vulcanization.

VII. Analysis of Unvulcanized Goods.

This is a minor branch of rubber manufacture, limited
to certain special articles. Unvulcanized insulation-strip
1 W. A. Caspar!, Le Caoutchouc et la Guttapercha, 8 (1911), p. 5289.

134 INDIA-RUBBER LABORATORY PRACTICE CHAP.

is usually free from mineral charge, but may contain
factice and waxy hydrocarbons. Among more or less
highly charged doughs sold as such may be mentioned
tyre-repairing compositions, in which much litharge or
other accelerator may be expected ; doughs for surgical
and dental ebonites ; bands or cords for sealing tins, &c.
Exactly the same mode of analysis is applicable to all
these as to vulcanized goods, from which they differ only
in containing no rubber-combined sulphur. Certain
modifications are, however, dictated by the fact that
uncured dough is apt to melt and clog when slightly
warmed, and so to resist extraction by boiling solvents.
When the material is not already in the form of sheet or
cord, it should be rolled into sheet of not more than
1 mm. thickness. This is cut into strip, weighed out,
and wrapped spirally in stout gauze, whereupon it is
ready for extraction with acetone and alcoholic potash.
When the dough is available only in lump form, it is cut
up as finely as possible, and the snippets, after being
wrapped layer-wise in filter-paper or gauze, will yield
their extractive matter readily enough to acetone — but
not always quantitatively to alcoholic potash. The latter
difficulty may be overcome by the following artifice : —
the acetone-extracted material is swelled and dissolved
in ten parts of benzene, and the magma is mixed with
alcoholic potash solution (which precipitates the rubber
in porous clots) and boiled as usual. The extract is
transferred to a separating funnel and shaken with a few
c.c. of water ; the bulk of the benzene then separates
out and is rejected. At least one more extraction of the
rubber clot is carried out, and finally the united extracts
are further treated as on p. 124.

Carbon disulphide extractions being impracticable in

vii ANALYSIS OF MANUFACTURED RUBBER 135

the case of uncured dough, bitumen, if present, must be
approximately estimated from the acetone extract.
Failing this possibility, the best plan is to vulcanize a
portion of the sample, e.g. by pressing into a test-tube and
heating in an oil bath, and then to proceed as usual.

VIII.— Analysis of Rubber Solutions.

The rubber solutions current in trade and within
factories range from dilute solutions containing 5 — 10 per
cent, of rubber and little or no charge to thick pastes or
cements in which there is much pulverulent filling and
comparatively little solvent. The solvents in use are
coal-tar hydrocarbons, petroleum hydrocarbons, carbon
disulphide, and carbon tetrachloride, or mixtures of two
or more of these. Rubber solutions may be analysed as
follows : —

1. Fifty gr., or as much as can be spared, of solution
are steam-distilled until the runnings are no longer turbid.
The non-aqueous portion of the distillate is separated off
and subjected to fractional distillation, specific gravity
determination, and chemical tests. The nature of the
solvent or solvents is thus ascertained.

2. Five to twenty gr., according to concentration, of
well mixed solution are spread on a wide flat porcelain
dish, which is covered with a clock-glass and weighed.
The dish is dried in the vacuum oven, so that the dissolved
matter forms a thin elastic film on the bottom of the
dish. This film is drawn off and turned upside down,
and drying is continued to constancy of weight. The loss
in weight gives the percentage of solvent, which cannot
be determined accurately by steam-distillation owing to
the difficulty of expelling the last traces. The residual

136 INDIA-RUBBER LABORATORY PRACTICE CHAP.

film of rubber thus obtained is examined by the usual
methods of rubber analysis.

In some solutions and pastes, especially those intended
for cementing solid tyres, rubber is replaced wholly or
partially by gutta-percha or balata. A quantitative
separation of these hydrocarbons from rubber is im-
practicable, but information as to the presence of gutta
can generally be obtained by isolating and examining the
pure hydrocarbons ; to this end, the solution is suitably
diluted andcentrifuged, and the clear liquid is precipitated
with alcohol. As distinguished from indiarubber, gutta-
percha and balata are more plastic and almost destitute
of tensile elasticity, and they are much less viscous in
solution.

IX. — Analysis of Ebonite.

Ebonite is the product obtained when rubber is mixed
with 30 — 50 parts of sulphur and subjected to prolonged
cure at high temperature. It not only has a high degree
of sulphur but also contains rather large percentages of
uncombined sulphur, which does not effloresce to the
surface. Superior ebonites, especially those which take a
high polish, are made with little or no filling matter ; in
the cheaper kinds, diluents and pulverulent ingredients
play an important part, and the proportion of rubber may
tail off to near vanishing-point.

In all essentials, the analysis of ebonite is conducted on
the same lines as that of soft rubber. The material is
easily comminuted by rasping. Acetone extraction
should be continued for 20 hours. Alcoholic potash,
even if no factice at all be present, will invariably extract
a few per cent, of saponifiable matter which has been
formed by the action of sulphur upon resin, etc. Mineral
matter must be determined by ash-analysis (see p. 158),

vii ANALYSIS OF MANUFACTURED RUBBER 137

antimony and mercury being separately determined, as
also carbon.

A frequent ingredient of ebonite is added resin. So far
as this consists of rosin or rubber-containing resins, it
finds its way completely into the acetone extract. The
hard varnish-maker's resins, however (copal,- shellac, etc.,
etc.), are insoluble, or only partially soluble, in acetone.
In order to obtain information as to the presence of these
latter, the ebonite is subjected, immediately after acetone,
to extraction with epichlorhydrin, B.P. 116°— 117°. This
is best done by boiling out the material in a flask under
reflux condensation with two or three portions of solvent.
From the united extracts the bulk of epichlorhydrin is
removed by distillation from an oil-bath, and the re-
mainder by adding one or two relays of clean alcohol to
the residue and distilling off. Casein, which sometimes
occurs as a filler, is easily detected by the soda-lime test
for nitrogen and determined by a Kjeldahl assay ; as factor
of conversion of nitrogen to «casein, 6*7 may be taken.
Cork Dust, a not uncommon filler, is more insidious
from the analyst's point of view. Taken alone, it yields
5 — 10 per cent, of extractive matter to boiling acetone
and subsequently 40 — 50 per cent, to alcoholic potash.
The fatty acids from the latter extraction form a brown,
amorphous mass of firmer consistency than those from
factice, and, unlike these, insoluble in cold petroleum
spirit; in boiling petroleum spirit they are slightly
soluble, a white subcrystalline deposit separating out on
cooling. After extraction with alcoholic potash, cork
dust still yields a little matter to dilute aqueous caustic
lye ; the red solutions so obtained give, on acidification, a
light yellow precipitate almost insoluble in ether. By
the aid of these data, cork dust in ebonite may be detected

138 INDIA-RUBBER LABORATORY PRACTICE CHAP.

and roughly determined, it being borne in mind that one
or two per cent, of organic acids which are derived
neither from factice nor from cork dust are always likely
to be found in the alcoholic potash extract.

X. Reclaim and Ground Waste.

No general method has as yet been devised for deter-
mining reclaim or ground waste in vulcanized rubber
goods, any more than for separating two or more raw
rubber sorts, once vulcanized together. Even qualita-
tively it is by no means always possible to make sure of
the presence of reclaim. In all cases, rubber analyses
should be gone through in the normal way, regardless of
its presence or absence ; conclusions as to one or the
other may afterwards be drawn from the analyses and
from other tests.

The main chemical characteristic of reclaim in a vul-
canized mixing is its comparatively high degree of
sulphur. Whereas raw rubber in soft-cured goods is
seldom vulcanized above 4°, reclaims themselves show
originally from 2° to 5°, and after re-vulcanization rise to
a correspondingly higher degree. Hence from an ab-
normal degree of sulphur in a not obviously over-cured
article the presence of reclaim may reasonably be
suspected. A physical peculiarity of vulcanized reclaim
is its relatively low capacity for absorbing solvents. In
general, and with any kind of vulcanized rubber, tendency
to swell in solvents decreases as the degree of sulphur
rises ; but as regards reclaim the purely physical con-
dition of the material is an additional factor. Eubber
goods made largely or wholly from reclaim, then, swell
up much less when immersed in naphtha than those
made from ra\v rubber. In goods containing both, a

vii ANALYSIS OF MANUFACTURED RUBBER 139

curious pitted formation may often be observed when a
perfectly plane surface is allowed to swell in naphtha.
Another side of the same phenomenon is the circumstance
that vulcanized reclaim is much less readily dissolved at
high temperatures, e.g. by boiling petroleum (cf. p. 141)
than ordinary vulcanized rubber. By its behaviour
under this treatment, reclaim can frequently be detected
with tolerable certainty, and it is conceivable that an
approximate separation of raw rubber from reclaim in
rubber goods, based on selective decomposition with
solvents at different temperatures, may some day be
worked out.

Little, if any, evidence of the presence of reclaim is to
be gained from the accompanying non -rubbers. When,
however, there is found an unexpectedly large variety of
mineral fillers, some of them in quite small quantity, the
presence of reclaim may well be suspected, since the
tendency in compounding raw rubber mixings is rather
to simplify than to complicate the charge.

The mechanical properties of rubber goods are of
course affected to a considerable extent by the presence of
reclaim, which can often be detected by merely fingering
and inspecting the sample. Tensile strength and, still
more, elongation at rupture, are much reduced. So,
above all, is resiliency : the material feels more or less
" dead " when pulled or bent, and recovery from deform-
ation takes place sluggishly. At the same time, these
effects may be just as well produced by inferior raw
rubbers, or excess of foreign materials, or faulty cure ; so
that conclusions must be drawn with caution.

CHAPTEE VIII

ANALYSIS OF MANUFACTURED RUBBER : INORGANIC

HAVING dealt with rubber diluents and sulphur, we
now come to what is in the main a branch of inorganic
analysis, viz., examination of the solid compounding
materials ; and the first task will be to determine their
aggregate percentage in the rubber. Until recent times
there was no way of doing this save by incineration,
which involves numerous inaccuracies, gives no informa-
tion as to organic pulverulent fillers, and, by disguising
the mineral combined sulphur, renders the determination
of degree of sulphur difficult, if not impossible. By now,
however, methods by which the rubber is dissolved away
and the residual charge collected have been gradually
brought to perfection. Of the numerous solvents by
which rubber can be destructively dissolved, the one
earliest proposed, viz., petroleum, is on the whole the
best. The chief obstacle to be surmounted has been
rather the difficulty of collecting the charge, which is
obtained in very fine suspension and, in most cases,
resists all attempts at filtration. This problem has been
satisfactorily solved by the application of centrifugal
machinery.

J40

CHAP, vni ANALYSIS OF MANUFACTURED RUBBER 141

I. Determination of Charge.

In presence of little or no free sulphur, it is immaterial
whether the original rubber or such as has undergone
extraction be taken ; when the total sample available for
analysis is scanty, the latter is obviously to be preferred.
Preliminary extraction is advisable in presence of much
free sulphur, in order that
possible after - vulcanization
during the hot petroleum
treatment may be avoided.
The rubber must in any case
be as finely divided as possible.
One or two grammes are
weighed into a 50 c.c. Erlen-
meyer flask, and covered with

10 c.c. of a petroleum fraction
(distilled from ordinary lamp-
oil) of B.P. 200° upwards.
Soaking overnight in the
cold greatly facilitates the
subsequent operation, but is
not absolutely necessary. The
rubber is brought into solu-
tion by heating in a bath of

011 or fusible metal to not
much over the initial B.P.
of the solvent, i.e., the liquid
should be just on the simmer.

position of apparatus is shown in fig. 18. The flask is
held by a short stalk of glass tubing acting as reflux
condenser, and is well shaken from time to time. Dis-
solution is completed in ^ hr. to 2 hrs., according to

FIG. 18.
A convenient dis-

142 INDIA-RUBBER LABORATORY PRACTICE CHAP.

the degree of vulcanization ; lightly vulcanized goods,
especially if previously soaked, break up very rapidly.
When the magma seems smooth and homogeneous it is
cooled and settled, and the fluid portion is decanted off.
Should undissolved lumps of rubber remain, more petrol-
eum is added and heating is continued. Ultimately the
whole magma is rinsed into a tared pear-shaped flask
(see Fig. 12) with ordinary ether or light petroleum spirit,
and is made up to about 50 c.c. Thick- walled cylinders of
the Nessler-glass type, if not too heavy, are also quite
suitable, but Erlenmeyer flasks should not be used in the
centrifuge, as they can seldom be trusted to stand the
strain. The flask or glass is counterpoised (to within
Ol gr.) against a similar vessel holding the same kind
of contents or merely water, and the two are whirled
for 20 — 30 minutes in a centrifuge (see Figs. 10 and 11)
running at 1500—2000 revolutions per minute. All solid
matter, excepting negligible quantities which may remain
in colloidal suspension, will now have gone to the bottom.
If of a heavy and not too finely-divided character, the
charge will often settle in so compact a mass that the
supernatant liquid can be poured away to the last drop.
As a rule, however, the liquid will have to be removed by
siphoning, for which the apparatus shown in Fig. 19 may
be used. The siphon, which should be of about 2 mm.
bore, is started by sucking at C. Whilst at work the
siphon is gradually lowered through the rubber joint D
until it approaches the level of the solid deposit in B to
within practicable limits. Eventually the volatile solvent
is recovered by distillation from the receiver A. The
charge is broken up with a wooden spatula, thoroughly
shaken with 50 c.c. of fresh ether or petroleum spirit, and
again whirled and separated. The same process is repeated

viii ANALYSIS OF MANUFACTURED RUBBER 143

once or twice again, and the flask is then dried at 110°
and weighed.

The feasibility of dissolving vulcanized rubber by means
of hot petroleum reaches its limit at a degree of vulcan-
isation somewhere between 5° and 10°. On ebonite, hot
petroleum has no dissolving action worth mentioning.

FIG. 19.

Since revulcanized waste and reclaim are apt to attain
somewhat high degrees of sulphur, it often happens that
rubbers containing these ingredients refuse to go com-
pletely into solution. The isolated charge will then be
contaminated with rubber, and will come out too high.
After the subsequent treatment of the charge with hot

144 INDIA-RUBBER LABORATORY PRACTICE CHAP.

acid, this impurity can sometimes be skimmed off, dried,
and weighed ; more usually it will have to be filtered off
together with insoluble mineral matter and determined
by ignition loss. Should a really important proportion
of the original rubber remain undissolved in the form of
flakes after petroleum treatment, the presence of ground
waste or reclaim may safely be concluded; it is often
worth while to isolate these flakes and examine them
separately.

II. Analysis of Charge.

In most cases the insoluble matter isolated as above
consists mainly of inorganic substances. There may also
be present lampblack, graphite, coal-tar dyes in the form
of lakes, vegetable fibres, cork dust, starch, &c., not to
mention residues of undissolved rubber. The charge is
scraped out with a wooden spatula and closely inspected
with the aid of a lens. Asbestos or cellulosic fibres,
flake graphite, metallic powders, &c., will be recognized at
once ; lampblack also can often be recognized, and various
clues as to the inorganic constituents may be obtained.
The charge, unless it is to be analysed as a whole, must
be ground and mixed thoroughly before being weighed
out in portions, since the centrifugal process brings about
a grading of the several powders in layers. Starch,
which will now be present in the form of dextrin, is
extracted by means of boiling water ; it is a very unusual
filler and occurs only in low-grade complex qualities.
Tests for organic dyes, which occur more often in
cold-cured than in heat-cured goods, are made by the
usual methods ; the dyes may, however, have escaped
into the hot petroleum, or may have been extracted in
the course of the earlier stages of analysis. The sys-

viii ANALYSIS OF MANUFACTURED RUBBER - 145

tematic examination of charge, in all ordinary cases, is
conducted as follows : —

1. The material is treated with dilute hydrochloric
acid, evolution of carbon dioxide or hydrogen sulphide
being noted, and evaporated to dryness. The residue is
again brought into solution by boiling with moderately
strong (1 : 1) hydrochloric acid, then adding 200 — 300 c.c.
of water, and boiling once moj*e. In the case of red
rubbers, a sufficiency of tartaric acid is added. If much
lead be present, it may be desirable to decant and boil
with fresh portions of water, without acid. The totally
insoluble residue is filtered off, tested for absence of lead
by a drop of ammonium sulphide, ignited in a platinum
crucible, and weighed. In presence of carbon black,
vegetable fibres, cork, ebonite, or residual rubber, the
filter is previously tared (e.g. in the crucible itself) and is
dried and weighed before ignition. To the ignited
substance are added a sufficiency of pure hydrofluoric
acid and a drop or two of sulphuric acid. After being
evaporated down, the contents of the crucible are
re-ignited and re-weighed.

In the first ignition, provided the residue be small or
consist wholly of barium sulphate, the ignition loss will
represent accurately enough the aggregate organic matter
present. When there is silica in the residue, a certain
amount of water, corresponding to the combined water of
the siliceous matter, is included in the ignition loss and
may be allowed for if the nature of the siliceous matter
be known. Amorphous or graphitoidal carbon may thus
be determined by ignition if uncontaminated with other
organic matter ; but if such be present, carbon must be
determined separately by the nitric acid method (see
p. 155) either in the rubber or in the isolated charge.

146 INDIA-RUBBER LABORATORY PRACTICE CHAP.

Loss on evaporation with hydrofluoric acid indicates
silica. Should it be small in amount, the loss may
be set down to siliceous impurities or rubber-ash, and the
residue (if white) regarded as pure barium sulphate. If,
however, siliceous matter be present in any considerable
quantity, the final residue in the platinum crucible should
be fused with sodium carbonate or potassium bisulphate
an'd assayed for barium and other bases in the usual way.
The result, together with that of the hydrofluoric acid
treatment ( = SiO2), provides the requisite quantitative
data as to barytes and siliceous fillers.

Ferric oxide (rouge) and chromic oxide (green pigment)
are in the main resistant to acid attack and behave in the
same way as barium sulphate.

2. The filtrate from acid-insoluble matter is treated
with hydrogen sulphide, when lead or antimony sulphide
may come down. A precipitate of the former need be
expected only in black or grey goods, of the latter only in
red goods. Small quantities of lead are best determined
gravimetrically : the precipitate together with the filter
is moistened with concentrated nitric and sulphuric acids
in a porcelain crucible, gently charred and burnt, ignited,
and weighed as PbS04. Larger amounts may be
determined volumetrically by the following method l : —

The precipitate is decomposed with strong hydrochloric
acid, diluted to 50 c.c., and treated with ammonia in very
slight excess. Five c.c. of acetic acid are added, whereby
the lead is again brought into solution. The liquid is
heated to boiling and titrated with ammonium molybdate
solution (8-6 gr. per litre), which is standardized against
lead nitrate solution (16 gr. of Pb (NO3)2 per litre, 1 c.c. =
0-01 gr. of Pb). A dilute solution of tannin (5 gr. per litre) »
1 J. F. Sacher, Chem.-Ztg. 33 (1909), p. 1257.

vni ANALYSIS OF MANUFACTURED RUBBER 147

sterilized with alcohol, serves as external indicator,
giving a yellow coloration when the end-point is
reached.

When the hydrogen sulphide precipitate consists of
antimony sulphide, the metal is best determined gravi-
metrically as on p. 76. Lead and antimony are seldom,
if ever, present together. In rubber compounded with
black reclaims, however, a little antimony may occur side
by side with lead and may be detected by extracting the
mixed sulphides with sodium sulphide solution.

Vermilion is recognizable in rubber goods by its
brilliant reds and pinks free from the slightest yellow or
brown tone. In its presence, nitric acid must be added
when the charge is decomposed by hydrochloric acid,
otherwise it will not dissolve completely. All the mercury
is then brought down by hydrogen sulphide. Mercury
and antimony sometimes occur together, in which case
the sulphides are separated by means of sodium sul-
phide.

3. The filtrate from the foregoing is boiled until free
from hydrogen sulphide, filtered if necessary, and rendered
alkaline with ammonia. An inconsiderable precipitate at
this stage is usually due to mere impurities. A decided
precipitate of alumina may represent a portion, extracted
by acid, of the siliceous fillers present, and should be
weighed as such. Iron is mostly a mere impurity, but in
the case of goods pigmented with rouge or ochre is to be
regarded as part of the pigment. Precipitates of calcium
fluoride and phosphate are also possibilities ; the former
will have been introduced by barytes, the latter by bone-
black.

4. In the filtrate, zinc is precipitated by boiling with
an excess of ammonium sulphide. There are few pre-

148 INDIA-RUBBER LABORATORY PRACTICE CHAP.

cipitates so slow and difficult to filter as zinc sulphide,
and therefore it saves time, at this stage, to halve the sub-
stance analysed, as follows : —

Precipitation having been effected, the whole liquid (of
which the volume is presumed less than 500 c.c.) is im-
mediately washed into a J-litre measuring flask with
hot water, made up to rather over the mark, and well
shaken. The flask is left to itself until the precipitate
has settled (1 — 2 hours) and is then further cooled with
water, if necessary. Of the clear supernatant liquid 250
c.c. are siphoned off. Should the original volume of liquid
fall short of 250 c.c., a J-litre flask may be made use of
in analogous fashion. In the clear half, calcium and
magnesium, which are now the only metals left, are
precipitated successively by means of ammonium oxalate
and sodium phosphate ; the results so obtained are
multiplied by two. For greater accuracy, in measuring
the hot liquid as above, a flask having 0'2 c.c. divisions
on the stem may be used, the temperature of the liquid
being noted ; allowance ought also to be made for the
volume of zinc sulphide, sp. gr. 3-3. For ordinary pur-
poses, however, it suffices to fill up to an improvised mark
corresponding to 15 c.c. in excess of the 500 c.c. bulk.

All the zinc is now left as sulphide at the bottom of the
flask and may, if desired, be determined by the ordinary
gravimetric methods. Filtration is best effected by
means of a thick filter paper, or, still better, a nest of
paper pulp. The sulphide may then be re-dissolved and
the zinc re-precipitated as carbonate and weighed as oxide.
A not much less accurate procedure consists in roasting
the precipitated sulphide directly to oxide, after detaching
the filter-paper as far as possible and burning it separately.
Several hours' ignition in an open crucible, until the

viii ANALYSIS OF MANUFACTURED RUBBER 149

smell of sulphur dioxide has passed away, will be re-
quired.

It is far more expeditious to determine the zinc volu-
metrically, for which the ferrocyanide method may be
recommended. The whole remaining contents of the
measuring flask are transferred to an ordinary flask,
boiled up with a slight excess of hydrochloric acid to
dissolve the sulphide, and titrated whilst hot with a
solution of potassium ferrocyanide (35 gr. per litre,
1 c.c. = 0'1 gr. ZnO approximately). The end-point is
indicated by means of a 5 per cent, solution of uranyl
nitrate, used as an external indicator : when all the zinc
is precipitated, the red colour of uranyl ferrocyanide
appears. The titrating solution is standardized against a
solution of 10 gr. of pure zinc oxide in a litre of acidulated
water ; it keeps fairly well if preserved in the dark.

III. Special Determinations in the Charge.

When the charge has been submitted to the systematic
analysis described in the foregoing section, the results
will add up to less than full weight if the charge contains
mineral matter other than barytes and simple oxides.
Certain supplementary determinations may therefore
have to be carried out, notably that of sulphide-sulphur
and that of carbonic acid. Without a knowledge of these
constituents it is often impossible to render a correct
account of the qualitative nature of the compounding
materials. Supplementary determinations postulate a
good supply of isolated charge. When they are per-
formed upon aliquot parts of a single lot of charge, it is
most important that the charge be thoroughly ground
and mixed before being broken into.

Combined Water (from silicates, calcium sulphate,

150 INDIA-RUBBER LABORATORY PRACTICE CHAP.

magnesium carbonate, etc.), can usually be estimated in-
directly by calculation, and need be determined only in
exceptional cases. The determination is carried out,
preferably on not less than J gr., by ignition in a current
of dried air in a short tube-furnace (see p. 35), the water
being caught in a calcium chloride tube. Organic matter
in the charge will vitiate the results. In presence of
volatile inorganic matter, e.g., antimony, a plug of asbestos
should be placed in the cool fore-part of the ignition
tube.

Total Sulphur is determined as on p. 131, and will then,
include the sulphur belonging to both sulphides and
sulphates. To determine total sulphur other than that
existing as BaS04 or PbSO4, 0-2 — O5 gr. of charge is
assayed for acid-soluble total sulphur by the nitric acid
method given for antimony pigment on p. 77. Acid-
soluble sulphur minus sulphide-sulphur gives soluble
sulphate- sidphur, which is usually present, if at all, as
calcium sulphate.

Sulphide- SulphiiT. — This may be present in large
amount as zinc sulphide and in small amount as lead
sulphide, either of which sulphides gives up practically
the whole of its sulphur as hydrogen sulphide when
decomposed by acid. In the case of antimony sulphide,
the disengagement of hydrogen sulphide is not quite
quantitative. To determine sulphide-sulphur, 0*2 — O5 gr.
(or more, according to circumstances) of charge is weighed
into an evolution apparatus fitted with a reflux condenser.
The well-known Corleis flask (Fig. 20) does very well for
this. Cheaper and equally serviceable is the apparatus
shown in Fig. 21, which is made up from a dropping
funnel, a ground-in wash-bottle, and a glass worm. The
exit is connected with two successive absorption flasks,

vni ANALYSIS OF MANUFACTURED RUBBER

each containing 50 c.c. of 10 per cent, zinc acetate
solution. Air having first been expelled by a current of
carbon dioxide, hydrochloric acid (1 : 1 for zinc sulphide,
pure for lead sulphide) is introduced little by little and
slowly brought to the boil. When all the hydrogen
sulphide has been boiled off and driven over by means of

FIG. 20 (Scale 1 : 6). FIG. 21 (Scale 1 : 5).

carbon dioxide, the united contents of the absorption-
flasks are thoroughly shaken with 50 c.c. of A^/10 iodine
solution, the excess of which is titrated back with thio-
sulphate. One gr. of iodine = 0-1263 gr. of sulphur.

Carbonic Acid maybe present as calcium or magnesium
carbonate, exceptionally as lead carbonate ; its determina-
tion is at least as important as that of sulphide-sulphur

152 INDIA-RUBBER LABORATORY PRACTICE CHAP.

and is carried out by means of the same apparatus. The
substance is first covered with cupric sulphate solution —
to retain hydrogen sulphide — and then decomposed with
dilute (1 : 10) hydrochloric acid, very slowly added. The
liquid is eventually boiled and the carbon dioxide swept
over by means of purified air. Absorption is effected in
Geissler bulbs or soda-lime tubes, which must be preceded
by at least one calcium chloride tube to retain moisture. .
Phosphoric Acid, which is but rarely present, may be
determined in the usual way by precipitating a nitric acid
extract of the charge with ammonium molybdate re-
agent.

IV. Notes on the Interpretation of Charge- Analyses.

Of the various organic substances which may be em-
braced in the ignition loss of the acid-insoluble portion,
finely-divided undissolved rubber points to the presence of
reclaim or ground waste, unless the sample as a whole is
obviously over-cured. Its amount, when ascertained,
should be deducted from charge proper. Carbon may be
separated from rubber, and other organic matter, by the
nitric acid method (p. 155), and should be determined
separately whenever it does not happen to be the sole
combustible constituent. Carbon may have been imported
by coal-tar pitch (see p. 56) or black hypo (see p. 82).
Amorphous carbon may be distinguished from graphite by
the relatively difficult combustibility of the latter. Among
less common organic substances may be mentioned saiv-
dust, cork dusty ebonite dust, cellulosic fibres, etc., which
sometimes occur in brake-blocks and the like. These
remain with the charge when the rubber is broken down
by means of petroleum and can then be approximately
determined by ignition loss. The original mineral con^

viii ANALYSIS OF MANUFACTURED RUBBER 153

stituents of ebonite dust, like those of reclaim or ground
waste, will in most cases, however, prove inextricable
from the mineral charge proper.

In the acid-insoluble portion of the charge we have all
the barium (which hardly ever occurs in rubber goods
except as sulphate), all the silica, and most of the bases
combined with silica as silicates. If silica be present
uncombined, atmoid or talite is indicated. When alumina
is the principal silicate-base in a white residue, kaolin is
indicated ; in coloured residues, ochre or slate-powder.
Magnesia as a base points to talc ; asbestos, which can
usually be recognised as such, sends most of its magnesia
into acid solution. Fancy silicates such as pumice, mica,
glass, etc., are peculiar to special classes of goods and
can often, moreover, be detected by inspection under the
lens.

The form in which lead occurs in a rubber article is in
nine cases out of ten litharge. This may be detected as
a heavy yellow powder, and thus distinguished from white
lead, when the isolated charge is shaken up with a liquid,
as in the course of the isolation process itself. White lead
is less easy to make sure of ; but in many cases the
carbonic acid and sulphate- sulphur afford a clue. All
heat-cured goods made up with lead compounds contain
a small quantity of lead sulphide. Antimony and
mercury are never present except as sulphides ; the
former will frequently be found to be accompanied by
calcium sulphate.

Zinc may be present in the form of oxide or sulphide
(as such or in lithopone) or both. When there is little or
no sulphide-sulphur in the charge, -zinc white may be
confidently diagnosed. So much sulphide-sulphur (apart
from lead sulphide, which is mostly negligible) as is not

154 INDIA-RUBBER LABORATORY PRACTICE CHAP.

accounted for by antimony or mercury may be presumed
to be in combination with zinc. If, then, this sulphur
and the zinc present are in approximately equimolecular
proportion, zinc sulphide alone is indicated ; if there is a
deficiency of sulphur, zinc oxide and sulphide side
by side. Barium sulphate and zinc sulphide in equimole-
cular proportion point, almost certainly, to lithopone.

The variety of forms in which calcium may occur is
apt to give trouble in interpretation. Sulphate-sulphur
soluble in acid may be assumed to be combined with
lime ; calcium sulphate very commonly occurs in red
rubbers, but seldom elsewhere. Calcium in large amount
points to whiting, and this can be checked (in absence of
magnesium) by the carbonic acid present. Small amounts
(2 — 5 per cent.) are probably present as caustic lime, so
far as they are not of the nature of impurities. It is not
easy to make sure of this body in company with car-
bonates ; useful qualitative indications can often be
obtained by testing an aqueous extract of the charge for
alkalinity.

Of magnesia it may similarly be said, in general, that
considerable quantities indicate magnesium carbonate,
small quantities calcined magnesia. Some control can
be effected by careful determination of the carbonic acid,
it being remembered that commercial light magnesium
carbonate contains only about 40 per cent, of CO2. When
asbestos is present as a filler, magnesia derived therefrom
(up to about 30 per cent, of MgO upon the silicate) may
be encountered in the final stage of mineral analysis.

V. Special Determinations in Original Material.

1. Sulphur. — The total sulphur in a manufactured
rubber article is determined by treating the comminuted,

vni ANALYSIS OF MANUFACTURED RUBBER 155

but otherwise untouched, material by the nitrate or
peroxide method, as described on pp. 128 — 131.

The necessity for determining total sulphur, however,
seldom arises. On no account should sulphur present in
a given form (e.g. sulphur of vulcanization) be estimated
by the difference between total sulphur and sulphur in
other forms : the chances of large error greatly outweigh
those of accurate determination. It may here be useful
to recapitulate the various forms in which sulphur may
occur in rubber goods :—

1. Free (pp. 115-117).

2. Combined with factice (p. 125).

3. Combined with rubber and bitumen (pp. 128-131).

4. In mineral combination (p. 131) : —

a. Barium sulphate (p. 150).

b. Sulphides (p. 150).

c. Soluble sulphates (pp. 150-151).

The sum of Nos. 1 and 3 constitutes sulphur incor-
porated as such in the original mixing.

2. Carbon. — When quantitative data as to the amor-
phous carbon or graphite in a rubber are required, it is
often advisable to make a special determination starting
from original material. To this end, advantage may be
taken of the fact that the organic portion, and much of
the mineral matter, is converted into soluble bodies by
the action of nitric acid, whilst carbon is not attacked to
any serious extent. The comminuted rubber (1 — 2 gr.)
is evaporated with 20 c.c. of nitric acid in a small
porcelain basin on a water-bath. Evaporation is repeated,
if necessary, with fresh acid until no more lumps of
undecomposed rubber can be detected. The contents of
the dish are rinsed into a beaker and boiled with 400 c.c.

1 56 INDIA-RUBBER LABORATORY PRACTICE CHAP.

of water, when rubber-nitration products and more or
less inorganic matter go into solution. The settled liquid
is sent through a tared filter ; the residue is twice washed
by decantation with hot water, brought on to the filter,
and thoroughly washed out with hot water. The contents
of the filter are now rinsed back into a small beaker and
warmed with 50 c.c. of dilute ammonia, which dissolves
the remaining nitro-compounds. A pinch of ammonium
chloride should be added to precipitate colloidal carbon.
Any floating grease (derived from bitumen or paraffin) still
present is removed by extraction with ether. After settling,
the clear liquid is poured through the original filter, and
the process of boiling up the residue with ammonia and
ammonium chloride, settling, and decanting, is repeated
so long as yellow solutions are formed. Finally the resi-
due is boiled up with 100 c.c. of dilute hydrochloric acid,
returned to the original filter, well washed, dried at 110°,
and weighed. We now have carbon plus mineral matter,
the latter consisting essentially of barium sulphate and
silicates. The weighed filter is therefore incinerated and
the weight of mineral matter duly subtracted.

Sufficiently accurate results can thus be obtained by
weighing the actual carbon in all cases when there is a
reasonably large amount of it, and the residual minerals
consist mainly of barium sulphate. Siliceous matter
introduces an error owing to its water of hydration.
When there are several centigrammes of siliceous matter,
and in general when the minerals are much in excess
of the carbon, the latter should be determined by com-
bustion. For this purpose the insoluble residue is
collected, not on a paper filter, but on ignited asbestos in
an untared Gooch crucible or similar contrivance. Asbestos
and substance are then dried, transferred unweighed to a

vni ANALYSIS OF MANUFACTURED RUBBER 157

boat, and burnt in a small tube-furnace, the carbon being
weighed as CO2.

3. Antimony and Mercury .— The necessity frequently
occurs of determining these metals by a separate assay,
especially in ebonites and in other materials where the
bulk of the mineral charge is dealt with by ash-analysis.
The first thing to do, in such cases, is to destroy the
rubber and other organic matter in the wet way. Several
methods, each of which has its devotees, have been
proposed, e.g. by J. Eoth l (sulphuric and nitric acids),
F. Frank and C. Birkner2 (ammonium persulphate and
nitric acid), and W. Schmitz3 (sulphuric acid and mercury).
Sulphuric acid acts satisfactorily enough, and the addition
of mercury may well be dispensed with. The following
procedure may be recommended : —

Two gr. of rubber are heated with 25 c.c. of concen-
trated sulphuric acid in a long-necked Kjeldahl flask,
loosely stoppered with a small funnel. After the first
effervescence has subsided, the liquid is left to boil briskly,
over the free flame, for two or three hours. It is then
cooled, and 1 gr., or more if necessary, of potassium
permanganate crystals is cautiously added. On boiling
again for a short time, the liquid assumes a light reddish-
yellow tint, and there is now no need to continue the
destructive process. The cooled magma is taken up with
300 c.c. of water, boiled up, and filtered. In the clear
filtrate, antimony or mercury is precipitated by means of
hydrogen sulphide. If antimony alone be present, it is
determined in the usual way. In presence of both
metals, the sulphide precipitate is collected on a tared

1 Chem.-Ztg. 33 (1909), p. 679.

2 Chem.-Ztg. 34 (1910), pp. 34, 49.

3 Gummi-Ztg. 25 (1911), p. 1928.

158 INDIA-RUBBER LABORATORY PRACTICE CHAP.

filter, washed, rinsed into a small beaker, and warmed
with sodium sulphide solution. The undissolved mercury
sulphide is returned to the filter and washed, and in the
filtrate antimony is reprecipitated by acidification.
Meanwhile the bulk of the mercury wall have remained,
together with other fillers, in the undissolved sulphate
residue ; this is now therefore gently boiled for some time
with moderately strong hydrochloric acid. After filtering,
and testing the residue with ammonium sulphide, the
clear solution is treated with hydrogen sulphide ; the
precipitate of mercuric sulphide is added to that which is
already on the tared filter, washed with water slightly
acidified with hydrochloric acid, then with alcohol, then
with carbon disulphide, dried, and weighed.

VI. Ash-Analysis.

Any unimpeachably correct method for ascertaining
the amount and composition of the charge in a vulcanized
rubber would have to be based on the destructive dissolu-
tion of the rubber-substance. But such a process as that
described on pp. 141 — 143 is comparatively troublesome
and necessitates the use of costly apparatus. On the
other hand, incineration is a rapid, easy, and, within its
limits, exact operation, but it has the defect of obscuring
the true nature of the charge to some extent. The effect
of incineration on the more usual filling materials may be
expressed by the following classification : —

No change in the main : — Barytes, Zinc White, Iron
Oxide, Calcium Carbonate, Magnesia.

Loss of combined water : — Calcium Sulphate, Lime,
Infusorial Earth, Kaolin, Asbestos, Talc.

Loss of carbonic acid and water : — Magnesium Car-
bonate, White Lead.

vin ANALYSIS OF MANUFACTURED RUBBER 159

Volatilization : — Sulphur, Vermilion, Lampblack,
Graphite, Organic Fibres and Powders.

Partial Reactions :— Litharge combines with sulphur
and oxygen to form lead sulphate ; so much as is present
in excess of the available sulphur remains unchanged,
being sometimes partially converted into minium. Zinc
Sulphide is more or less roasted to oxide, according to
the duration and intensity of ignition ; a little sulphate
may also be formed. Antimony Sulphide is converted
into tetroxide ; occasionally the metal is recovered
quantitatively from the ash, more usually it undergoes
partial volatilization. Whenever accurate information as
to its amount is desired, antimony, like mercury, should
be determined separately in the rubber itself. Caustic
Lime and Magnesia combine with as much sulphur as
they can seize and yield sulphates to a corresponding
extent ; the same tendency, in a comparatively modest
degree, is observed with Zinc White and Magnesium
Carbonate. A certain amount of reaction in this sense,
negligible for ordinary purposes, will have already taken
place during vulcanization (cf. p. 111). Lime, so far as
it is not sulphated, is converted into carbonate.

It will be gathered from the above that the percentage
ash of a compounded rubber will generally come out
slightly lower, less usually slightly higher, than the true
total charge. Unless, however, there be much magnesium
carbonate, carbonaceous matter, or vermilion present, or,
on the other hand, much sulphur in conjunction with
basic materials, the difference will not be intolerably
great, as rubber standards go. This discrepancy affects
the determination not only of charge but also of rubber ;
but it can to a large extent be neutralized by a knowledge
of the nature of rubber goods generally and by certain

160 INDIA-RUBBER LABORATORY PRACTICE CHAP.

supplementary analytical operations. In commercial
specifications it is still customary to impose limits on the
percentage, not of true charge, but of ash ; which, in view
of the ease and cheapness of ash-determinations, seems
quite proper. As regards the detailed composition of the
original charge, the direct results of ash-analysis
undoubtedly yield but a distorted image. The same,
however, applies to some extent to analysis of the charge
isolated as such. An analyst who is capable of interpret-
ing the one will probably be equal to the other, so far as
exact interpretation is possible at all. On the whole,
then, the cases in which ash-analysis, plus supplementary
determinations, fails to yield all such information as is
obtainable are less common than might be supposed.
Even such lacunae as sulphide-sulphur and carbonic acid
can generally be filled up deductively. Almost the only
determination for which isolation of the charge is
absolutely indispensable is that of sulphur of vulcaniza-
tion in goods containing inorganically combined sulphur.
In the incineration of vulcanized rubber it is desirable
to avoid high temperatures and lengthy ignition as far as
possible, so as to reduce decomposition of calcium
carbonate and partial volatilization of metals to a
minimum. The dissipation of volatile organic matter
should be carried out slowly and without allowing the
vapours to burst into flame, since excessive local tempera-
tures might then be produced at the surface. Of heavy
rubber 1 gr., of medium 2 gr., and of floating 5 gr., cut
into smallish pieces or crumbed, are weighed into a tared
porcelain dish of about 8 cm. diameter. The best form
of dish is neither flat nor hemispherical, but the inter-
mediate shape. As the dish is cautiously heated over a
Bunsen flame, the rubber, unless very highly charged,

vin ANALYSIS OF MANUFACTURED RUBBER 161

melfe and presently fumes off with ebullition. There
need be little or no charring, since rubber, resins, waxes,
and bitumens are volatile with very little organic residue.
When no more fumes are disengaged, the dish is trans-
ferred to a muffle and ignited at a low red heat until the
ash is free from carbonaceous matter. As a rule, neither
the first nor the second operation need last more than ten
minutes. The clearing of the ash by ignition proceeds
rapidly, owing to the porosity of the mineral matter and
its catalytic effect on combustion. Litharge should not
be allowed to fuse nor calcium carbonate to dissociate ;
that is, the temperature should not rise much above 700°.
Magnesium carbonate is wholly reduced to oxide. Lamp-
black burns off very readily, but in presence of graphite,
which is less easily combustible, there is nothing for it
but to take the risks attendant on prolonged ignition.
Zinc reveals itself by a brilliant yellow coloration of the
hot ash, which disappears on cooling. In a busy labora-
tory it is well to number the dishes, — best with an
argentiferous marking ink, — and keep a list of their
approximate tares near the balance.

The contents of the dish are treated with cold dilute,
hydrochloric acid. Effervescence of carbon dioxide
indicates calcium carbonate. If the ash goes completely
into solution, lead, antimony, and silicates are absent ;
if there is a residue, strong hydrochloric acid is added.
Evolution of hydrogen sulphide at this stage generally
indicates zinc sulphide ; but the presence of this body in the
original rubber is to be inferred only if the disengagement
of hydrogen sulphide is somewhat considerable, since
sulphides in small amount may be produced during
incineration. Evolution of chlorine points to minium,
probably of secondary formation. The contents of the

1 62 INDIA-RUBBER LABORATORY PRACTICE CHAP.

dish are transferred to a beaker, diluted, and boiled for
some time to bring lead and antimony into solution.

A trouble often experienced at this stage is caused by
the difficult solubility of antimony tetroxide. When there
is a large ballast of other inorganic matter, the antimony
is usually so finely divided that it can be dissolved by pro-
longed boiling with hydrochloric and a little tartaric acid.
But with goods containing little else than antimony-red as
charge (e.g. red inner tubes, high-class sheet, &c.), the ash
is apt to sinter into a hard enamel which is practically in-
soluble, though relief may sometimes be obtained by fusion
with potassium bisulphate. The best plan in such cases
is to cover the ash with powdered ammonium chloride and
re-ignite ; most of the antimony is thereby volatilized,
whilst the little that remains no longer offers an obstacle
to the determination of non-antimonial matter.

The insoluble residue, consisting of barium sulphate
and silicates, is filtered off, washed, and tested for lead,
by means of ammonium sulphide. As a rule, the appear-
ance of the residue will afford a clue as to whether much
siliceous matter is present. Should this not seem to be
the case, the residue is ignited and weighed in a platinum
crucible, then evaporated with hydrofluoric and sulphuric
acids, re-ignited, and re-weighed. The difference in
weight, if considerable (e.g. more than a centigram),
will represent something more than ordinary siliceous
impurities, and in this case it is safer, in order to deter-
mine barium sulphate accurately, to fuse the residue in
the crucible and determine barium in the lixiviated
melt. Total acid-insoluble matter other than barium
sulphate and coloured sesquioxides may be set down to
silicates ; but it is to be noted that additional instalments
of the silicates originally present are almost sure to be

vin ANALYSIS OF MANUFACTURED RUBBER 163

encountered in the subsequent group-separation, hydrous
silicates being much more susceptible to acid attack after
ignition than before. The nature of the silicates may be
gathered from their qualitative composition, as ultimately
ascertained.

Should the acid-insoluble portion of the ash be sus-
pected from the beginning of being largely siliceous, it
is advisable, instead of treating with hydrofluoric acid, to
fuse with sodium carbonate directly after igniting and
weighing, and to analyse as usual.

The acid-soluble portion of the ash is dealt with by the
ordinary group-separation methods sketched out on
pp. 145—149.

We now have to consider the relations existing between
the results of ash-analysis and the original mineral com-
ponents of the rubber. The task is a double one. In
the first place, corrections have to be applied to each
analytical item so • as to translate ignited into terms of
original charge. In the second place, the amount and
nature of the several compounding materials have to be
deduced from the immediate analytical data. As aids to
recalculation, the following points may be noted : —

Barium sulphate may be set down to barytes or
lithopone.

Siliceous matter covers the ash of raw rubber, im-
purities generally, and the usual siliceous fillers. Most
silicates lose some combined water (4 — 12 per cent.) on
ignition ; when, therefore, the identity of the silicate has
been decided upon, the proper plus correction should be
made.

Lead, in the great majority of cases, exists in the ash as
sulphate but was originally present as litharge ; a corre-
sponding minus correction is therefore to be made. If

M 2

164 INDIA-RUBBER LABORATORY PRACTICE CHAP.

the ash visibly contains much litharge as such, the
best course, on the whole, is to make no correction for
sulphatization ; in case there is no other sulphate or
sulphide present beside barium sulphate, the actual
amount of sulphated lead can, however, be ascertained
by a determination of total sulphur in the ash. Bed lead,
white lead, black hypo, and lead sulphide are comparatively
uncommon fillers. When they do occur they cannot well
be traced in the ash, but indications may often be obtained
by testing the shredded rubber with concentrated hydro-
chloric acid.

Antimony generally comes out too low in the ash, and
should be separately determined. It may be calculated to
Sb2S3 or Sb2S4. Whenever antimony occurs, it is well to
look out for calcium sulphate.

The Group III (ammonia) precipitate yielded by a
white ash is in most cases insignificant and due mainly
to impurities. A respectable precipitate of alumina,
carrying with it some silica, points to silicates ; kaolin,
for instance, sends almost the whole of its alumina into
acid solution after ignition. Phosphate and fluoride
precipitates (from bone-black and fluorspar respectively)
are easily recognized and may be dealt with by the
usual methods. Rouge and ochre, which give rise to
precipitates in this group, will already have revealed
themselves in the ash if not in the rubber itself.

With zinc it is sometimes difficult to decide whether
the oxide or the sulphide was originally present, more
especially when the quantity is small. The complete
conversion of sulphide into oxide requires very prolonged
ignition ; hence, given a reasonable quantity of zinc, a
decided evolution of hydrogen sulphide with acid indicates
zinc sulphide, whilst little or no evolution indicates zinc

viii ANALYSIS OF MANUFACTURED RUBBER 165

white. Barium and zinc in approximately equimolecular
proportions point to lithopone. This latter pigment often
occurs together with zinc white, fairly often with barytes,
but hardly ever with added zinc sulphide.

Calcium may have been present originally as carbonate,
hydroxide, or sulphate. The sulphate is very rarely met
with except as a constituent of antimony red. The ash
of red rubbers will therefore frequently contain calcium.
When Ca is found in any amount up to one-half of the
Sb, it may fairly safely be regarded as belonging to the
pigment, and may be calculated as CaS04,2H2O. Calcium
in excess of this limit will probably be due to whiting.
In rubber-ash generally, soluble sulphates, so far as they
cannot be accounted for as reaction-products of litharge,
lime, and magnesia, may be ascribed to calcium sulphate.
Carbonic acid determined in the ash may be taken to
represent whiting, with the proviso that any hydroxide
originally present will in most cases emerge from the
incineration process in the form of carbonate. Hence, if
caustic lime and whiting occur together, they cannot be
disentangled in the ash. Small quantities of calcium,
say up to 5 per cent, of CaO on the rubber, indicate
caustic lime, and may be calculated to Ca(OH)2. Larger
quantities are best calculated to CaC03, and assumed to
be due to whiting alone. Special quick-curing rubbers,
howrever, may contain rather considerable percentages of
caustic lime.

With magnesium, again, there is often a doubt as to
whether oxide or carbonate was originally present, since
both are found as oxide in the ash. Here, as in the case
of calcium, one must be guided by the fact that the oxide
is essentially an accelerator of vulcanization and the
carbonate a filler. Calcined magnesia is seldom put into

1 66 INDIA-RUBBER LABORATORY PRACTICE CHAP, vm

a mixing other than that of the quick-curing type to more
than 10 per cent., whilst carbonate may occur in any
quantity. Much magnesium carbonate (30 per cent, or
over), unlike whiting, makes a very stiff rubber, and may
therefore be suspected from the beginning. On the other
hand, it is unsafe to assume that small quantities of
magnesium necessarily indicate oxide rather than car-
bonate. In the absence of calcium, magnesium car-
bonate may be detected in the finely-divided original
rubber by means of dilute hydrochloric acid.

Both calcium (in very small amount) and magnesium
originating from silicates may find their way into the acid
extract of an ash. Similarly, a portion of the calcium of
bone-black may persist into the oxalate precipitate.

CHAPTEE IX

GUTTA-PEECHA AND BALATA

THE industries of which these gums are the raw
material are commonly carried on side by side with the
manufacture of rubber goods, though the technique is in
almost all respects quite different. Gutta-percha and
balata are obtained from the latex of trees which are
botanically unrelated to the rubber- bearing species, and by
a distinct method of collection. The former gum is also
to some extent extracted, by mechanical means and
by the aid of solvents, from leaves and other parts
of the tree. Gutta-percha comes from the Malay
Peninsula, Sumatra, Borneo, and neighbouring
countries, whilst balata has its place of origin in
Venezuela and the Guianas. Gums passing under the
name of balata, but differing somewhat from Venezuela
balata, especially in the associated resins, are moreover
imported from the Amazon Valley, West Africa, and
elsewhere. Gutta-percha and balata consist essentially
of an amorphous colloidal hydrocarbon chemically indis-
tinguishable from that of india-rubber, together with a
large proportion of resin. The specific gravity of the
isolated hydrocarbon is 0'96, that of the resinous gum

1 68 INDIA-RUBBER LABORATORY PRACTICE CHAP.

being usually just short of unity. In spite of the
chemical similarity, these hydrocarbons differ widely
from rubber hydrocarbon in physical respects. When
cold, they are of a firm tough consistency suggestive of the
denser sorts of leather ; under shearing stress they are
eminently pliable, and recover from deformation incom-
pletely and somewhat sluggishly ; tensile and compressile
elasticity are all but absent. This peculiar mechanical
" deadness" is much enhanced by the presence of resin.
At temperatures well below 100°, gutta-percha and balata
soften without developing the marked tackiness of un-
cured rubber, and in that condition they are highly plastic
and ductile and lend themselves admirably to all kinds of
moulding operations. This property, together with their
toughness and high electrical insulation when cold, con-
stitutes their chief claim to utilization in the arts. Balata
hydrocarbon shows considerably more springiness under
shearing stress than gutta ; but for many practical pur-
poses no very sharp distinction is made between the two
gums. By far the greater part of the gutta-percha
imported into Europe goes into cable-coverings, especially
for submarine work ; the remainder is made into various
small articles of surgical, chemical, and mechanical utility.
Balata, as such, is notably applied to the manufacture of
lined canvas belting. The technology of gutta-percha
and balata resolves itself into processes of washing and
drying, softening by heat, shaping, and cooling. Vulcani-
zation is not practised. Compounding materials are
only exceptionally added, and usually in small proportion.
On a moderate scale the de-resinification or " hardening "
of gutta-percha and balata by extraction with solvents is
carried out industrially, raw gutta-percha of low resin-
content being much scarcer now than formerly.

ix GUTTA-PERCHA AND BALATA 169

Crude gutta-percha is composed of twists, sausage-
shaped rolls, and large lumps of rounded or rectangular
contour, always containing moisture and a good deal of
bark and other dirt. It is even more inhomogeneous than
crude rubber, because the separate pieces in a package
commonly differ not only in washing loss but also in
quality and resin-content. Attempts at sampling crude
gutta-percha are therefore apt to be illusory, and it
is advisable to postpone analysis until the whole package
has been washed and sheeted. Washing losses usually
run from 20 to 50 per cent. In dealing with a hand-
sample, a proportional fragment may be cut from each
piece, so as to make up a sample for analysis ; but even
in this case" it is sounder practice to determine moisture
on the whole lot, then mix on the rollers, and then con-
tinue the analysis. Extracted gutta-percha usually takes
the form of neat blocks homogeneous in composition and
nearly free from moisture and dirt. Crude Ealata of the
Venezuelan type comes over in large thick slabs and blocks,
which are as a rule much cleaner and more homogeneous than
crude gutta-percha. A considerable quantity of recovered
gutta-percha, mainly cable-strippings, comes into the
market and is used up again without any special treatment
save removal of mechanical impurities. Old gutta-percha
which has served on land will generally have become
partially resinified, whilst with submarine goods this
effect is only slight ; but in either case the gutta hydro-
carbon will have deteriorated by partial loss of plasticity
and cohesion.

The commercial value of gutta-perchas and balatas
obviously depends in a high degree on washing loss and
resin-content ; but apart from this there are notable
variations in the mechanical quality of the gutta hydro-

i;o INDIA-RUBBER LABORATORY PRACTICE CHAP.

carbon, as also in the insulation-resistance of the washed
and dried gums.

Analysis of Gutta-Percha and Balata.

The analytical procedure is the same for raw and
manufactured material, and consists in the determination
of moisture, resin, dirt, and hydrocarbon (termed " gutta "
for short). No analytical distinction is made between
the hydrocarbons of gutta-percha and balata, but there
are various indications on which an identification can
often be successfully based, provided that the one or the
other be present alone or in decided predominance.

Moisture. — One to five gr. (or more in the case of crude
gum), cut into snippets, are spread on a flat dish and
dried to constant weight in the vacuum-oven at 95°.
Three hours should be given as a minimum, but with
comparatively moist samples a much longer period may
be requisite. An alternative method consists in heating
for three hours and upwards at 110° in an atmosphere
destitute of oxygen, the apparatus being of the type
referred to on p. 24. Dried carbon dioxide is the
indifferent gas ordinarily used, but dried coal gas serves
practically as well, since gutta-percha is much less apt to
absorb matter from it than india-rubber.

Resin. — The ratio of gutta to resin in crude gutta-percha
varies between wide limits, from about 3 : 1 in the finest
reds to about 1:3 in low whites. In balata the ratio is
about 1:1. Manufactured gutta-percha of ordinary
quality may be expected to show ratios ranging between
two of gutta to one of resin and equal parts. The deter-
mination of resin is carried out by either of the two
methods described below. Original material may be
taken for assay if the moisture-content amounts to only

ix GUTTA-PERCHA AND BALATA 171

a few per cent., as is the case with the majority of manu-
factured goods ; otherwise the material should first be
dried.

1. Extraction Method. — The material is cut with the
aid of a knife and scissors into the finest possible snippets.
Of these, one or two gr. are extracted with acetone
exactly as in the case of rubber, except that a Soxhlet
extractor is employed. At the comparatively low tem-
perature of the Soxhlet, gutta-perchas do not, unless
abnormally soft, cake by partial fusion. Ten hours
suffice for complete extraction, whereupon the extract is
evaporated and dried for three hours at 110°. This
method of resin-assay is not only the least laborious but
also the most accurate ; its only drawback is that a good
deal of time elapses between start and finish.

2. Precipitation Method. — One gr. of material, which
need not be finely comminuted, is placed in a 100 c.c.
Erlenmeyer flask with 20 c.c. of redistilled toluene or
solvent naphtha, heated to 100° on a water-bath, and
vigorously shaken until solution is complete. The liquid
is poured, with agitation, into 50 c.c. of ordinary alcohol
in a rather larger Erlenmeyer, whereby gutta is pre-
cipitated ; the contents of the first flask are also shaken
up with alcohol and added to bulk. The last traces of
gutta come out of solution somewhat reluctantly ; it may
therefore be necessary to warm and shake very thoroughly
until the solution is clarified. The clear liquid is poured
off and set aside, and the residual clot of gutta is washed
by kneading with alcohol, redissolved, and again pre-
cipitated. With highly resinous gutta-perchas the
process should be repeated once more. The united clear
alcohol-toluene liquors are distilled from a capacious tared
Soxhlet flask, and the residual resin is dried to constancy

172 INDIA-RUBBER LABORATORY PRACTICE CHAP.

and weighed. The precipitated gutta, which carries with
it all the dirt originally present, may be dealt with
quantitatively as described below.

The resin obtained from ordinary gutta-perchas is of a
pale amber colour and semi-hard consistency ; that from
inferior white sorts is sometimes completely hard and
brittle. Gutta-perchas extracted from leaves and twigs
yield soft resins of a characteristic
gree i colour. Balata resin, when
fresh, is a thick treacly fluid. Eesins
extracted from manufactured goods
may be affected as to colour and
smell by the presence of a little
tarry or bituminous matter.

Dirt and Fillers. — Gutta-percha
owes its brown colour to the invari-
able presence of a few per cent, of
finely-divided humus, which cannot
be completely removed by washing.
Balata contains less of this impurity,
or none at all. The presence of
pulverulent compounding materials
can generally be recognized at first
sight ; the most usual ones are the
pigments zinc white, lithopone, anti-
mony red, and carbon black, also
talc, kaolin, and other silicates acting merely as fillers.
With the exception of the three pigments last-named,
mineral admixtures are most conveniently determined by
incineration. By far the greater number of manufactured
goods contain no intentionally added solid matter. Dirt
in gutta-percha may be determined as follows :—

I, Pontio's Method. — Thi§ is based on the fact that

FIG. 22a.

GUTTA-PERCHA AND B ALA 7 A

173

gutta-percha is readily liquefied by toluene in the form of

vapour, and is a remarkably convenient and accurate,

though somewhat slow, method. The material, which

must contain only a few per cent, of moisture, is roughly

cut up, and 1 gr. is weighed into a tared 7 cm. ashless

filter of medium density (e.g. Schleicher and Schiill's

" White Band >;) folded into the usual conical form. The

filter is fitted into a cage of copper

or nickel wire, which is suspended in

the vapour of toluene boiling under

reflux condensation. For one filter

only, the simple outfit shown in fig.

22a serves very well. A is a 250 c.c.

Soxhlet flask, aperture about 4 cm.,

in which 50 c.c. of toluene are kept

boiling on a sand-bath; B is a

bayonet-shaped adapter, the sole aim

of which is to prevent condensed

drops from falling into the filter ;

C is an air-cooled condenser which

may consist of a simple upright tube

or a spiral worm. Embedded in the

lower cork is a wire stirrup which

carries the cage. If the bottom of

the adapter be kept flush with this

cork, or slightly higher, the condensed

toluene will return down the sides

of the apparatus and there will be no danger of splashing.

Fig. 226 shows a ground-in modification of the same

apparatus, with which the inevitable slow loss of solvent

attendant upon the use of cork is avoided ; this apparatus

also does very well for alcoholic potash extractions

(of, p. 35). For conducting three, four, or six dirt-

FIG. 22Z>.

174 INDIA-RUBBER LABORATORY PRACTICE CHAP.

determinations at once, large boiling-vessels of the type
of Fig. 23& or Fig. 236 are used, the filter-cages being
suspended from six-rayed stars of stout wire. Apparatus a
is really a laboratory vacuum-still ; the flange of the glass
upper half is clamped to that of the metal or porcelain
lower half by means of three small wooden vices ; a
leather washer is interposed, or, if the flange-surfaces be
very true, the connexion can be rendered tight with a
mixture of glucose syrup and glycerine. In apparatus b

FIG. 23a (Scale 1 : 6). FIG. 23k (Scale 1 : 7).

the thick glass globe, aperture about 11 cm., is fitted with
a flat glass ground-in stopper having a central tubulure.
The six-filter apparatus devised by M. Pontio, which is
intended for other determinations beside that of dirt,
consists of apparatus b with an extensive superstruc-
ture. Mercury lutes are undesirable in vapour-extraction
apparatus, since drops of volatilized and condensed metal
are apt tq find their way into the filters.

Under the action of toluene vapour, the gutta-percha

ix GUTTA-PERCHA AND BALATA 175

is liquefied and gradually passes through the filter, leaving
solid impurities behind. The time required is rarely less
than ten hours and sometimes considerably longer. It
may happen that the solids are so fine as to choke the
filter and prevent the solution from traversing it in reason-
able time ; in this case relief may often be obtained by
substituting xylene or solvent naphtha for toluene, so
that the filter is exposed to higher temperatures. Again,
some materials form solutions too viscous to be filtrable ;
this is especially likely to occur with balatas and
" hardened " gums. Dirt will then have to be determined
by the alternative method given below. These exceptions
apart, the method works smoothly and is widely applicable.

2. Sedimentation Method. One or two grammes of
material are dissolved in 100 c.c. of hot benzene or similar
solvent and rinsed into a wide test-tube or Nessler glass,
which is then corked and left to itself on a water-bath.
The dirt sinks to the bottom within two or three hours.
As much as possible of the transparent solution is
siphoned off without disturbing the sediment, which latter
is brought on to a tared filter or Gooch crucible and
thoroughly washed with solvent, or— still better — exposed
for half an hour to toluene vapour. The filter is dried at
110° and weighed. This method, though comparatively
laborious, yields results within a few hours, and may be
yet further expedited by the use of a centrifuge, in which
case the volume of solvent should be cut down to
50 c.c.

Whilst washed balata is remarkably poor in dirt, washed
gutta-percha always contains from 2 to 6 per cent, of
impalpable humus. These natural impurities usually
contain about 20 per cent, of mineral matter. The ash
of a gutta-percha incinerated as such generally comes

176 INDIA-RUBBER LABORATORY PRACTICE CHAP.

out a little higher than that of the isolated dirt, because
traces of metallic humates and resinates are dissolved out
by toluene. Gutta-percha solutions are always slightly
coloured, whereas balata solutions are almost water-white.
In any case, the total ash of washed gutta-percha should
not exceed 1'5 per cent.

Pulverulent fillers may be determined together with
the dirt, but it is to be noted that the more fine-grained
sorts of pigment are apt to pass through the filter in
colloidal suspension. Accurate determinations are best
effected by the aid of the centrifuge (cf. p. 142).

Factice is determined after acetone extraction exactly
as in the case of rubber (see p. 123). A few tenths per
cent, of saponifiable matter are almost always extracted
by alcoholic potash from factice-free material. As an
admixture, factice is rather uncommon, and is never
present in any considerable proportion.

Bitumen and Pitch, unless present in mere traces, are
detected by the brown colour which they impart to
solutions of the material. Pitch, moreover, is indicated
by its fluorescence in solution and by the presence of
finely-divided carbon. Approximate quantitative deter-
minations are effected by the method described under
" Chatterton's Compound," due regard being had to
the fact that more than one-half goes into the acetone
extract.

Gutta. In most cases it suffices to estimate gutta by
difference. When it is desired to determine gutta directly,
the method to be adopted consists in precipitating the
hydrocarbon from benzene or toluene solution by means
of alcohol. Solid matter other than mere dirt should
first be eliminated. This is best done by rinsing the
solution into a measuring cylinder or flask, making up to

IX GUTTA-PERCHA AND BALATA 177

the mark, and allowing the insoluble to settle out ;
an aliquot part of clear liquid is pipetted off, and in this
the gutta is precipitated. Precipitation is effected as on
p. 171, and should be repeated at least once. Ultimately
the clot of gutta is boiled out with clean alcohol, trans-
ferred to a flat tared dish, dried in vacuo or in indifferent
gas, and weighed. The gutta will contain all the solid
impurities which were present when it was precipitated,
so that these may have to be determined in the weighed
gutta and duly taken into account. The guttas isolated
respectively from gutta-percha and balata may be distin-
guished, apart from the peculiarities of colour mentioned
above, by the fact that in equivalent solutions balata
hydrocarbon is about twice as viscous as gutta.

Chatterton's Compound. — This is a watertight cement
prepared by boiling gutta-percha with Stockholm tar,
often with the addition of rosin or bituminous substances.
Its value depends mainly on the amount of gutta present.
Volatile matter, which will include moisture, is deter-
mined by the method described under " Moisture." By
acetone extraction gutta-percha resin and added rosin are
isolated, together with so much of the tar as does not
consist of volatile matter or carbon. The extracted
residue is dissolved in ten parts of benzene ; if the settled
solution is light in colour, it will contain nothing but
gutta, apart from the sediment. If the colour is brown,
the presence of bitumen is indicated. In that case the
solution is reprecipitated three times with .just twice its
volume of alcohol ; the clear united benzene-alcohol
solutions will then contain bitumen insoluble in acetone,
whilst gutta and dirt compose the residue. Dirt, which
is determined as usual, may include a little carbon in-
troduced by the tar.

APPENDIX

TABLE I.
ANALYTICAL CHARACTERISTICS OF CRUDE RUBBER SORTS.

THE number of wild rubbers figuring in trade as
distinct sorts and sub-sorts runs to two hundred or
over. Many of them, especially the African ones, are so
ill-defined and variable that anything like a sound
comprehensive classification based on quantitative data
is impossible. The subjoined table gives no more than a
generalized conspectus of some of the principal varieties.
For fuller information the reader is referred to Weber's
The Chemistry of India Rubber (London, 1902), now
slightly out of date in some respects, and particularly to
Gimimi-Kalender (Berlin), 1913. For details of the several
Congo sorts, see J. Liebschiitz, Gummi-Zeitung, 21
(1907), p. 336.

W.L. = Percentage Washing Loss.

R. = Percentage of Resin in the washed and dried rubber.
A. = Ash

Designation.

W.L.

Para Hard Fine, Acre Para,
Bolivian Fine (Hevea) . . .
Manaos Scrappy (ffevea) . . .
Para Negrohead, Sernamby
(Hevea)
Canieta (ffevea)

16—21
20—25

30-40
45_50

2-5-35
1-6—2-0

3-6
1-4 i-g

0-2—0-4
0-6—0-7

0-5—1-5

0'5 0'8

Mattogrosso Virgin (Hevea)

18—22

2-5—3-0

0-5-07

181

1 82 APPENDIX

ANALYTICAL CHARACTERISTICS OF CRUDE RUBBER SORTS.— (contd.).

Designation .

W.L.

Mattogrosso Negrohead (ffevea)
Mollendo Fine (Hevea) ....

20—25
15—20

1-5—2-0
1-8—2-0

1-5—2-0
0-2—0-3

Mollendo Coarse (Hevea) . . .

12—18

2-0—2-5

0-3—0-4

Caucho Ball, Peruvian Ball

(Castilloa) .

20—30

3—5

0'5 — 1-5

Mani£oba (Manihot) . .

25—35

2-8—3-0

3-0—4-5

Ceara Negrohead (Manihot) . .

20—30

4-5

1-0—1-5

Jequie (Manihot)

25—30

4-5—5-5

1-5—2-0

Mangabeira (Hancornia) . . .
Ciudad Bolivar, Orinoco (Hevea)

30—40
15—20

20—25

7—8

1-0—1-5
0-4—0-7

Central American (Castilloa) .

20—40

4 — 7

1-0—2-5

Guayule (Parthenium) ....

22-26

20—35

1-0—1-5

Sierra Leone, Conakry, Massai,

Soudan (mostly Landolphia)

15—30

5—7

0-4—1-0

Bassam, Cape Coast, Accra,

Lahou, Ivory Coast, Gold

Coast (mostly L. ) . .

25—40

7-11

07—1-0

Second Accra Lump, Saltpond,

&c. (mostly L.)

30—40

29—38

1-4—2-0

Gambia, Bissao (mostly L ) . .

30—50

5-6

1-0—2-0

Gaboon, Loango, Ogowe, Congo

Ball (mostly L.)

25—35

8-18

0-6—1-0

Lagos, Niger, Benin (Landol-

phia and Funtumia) . . .

30—40

10—25

0-3-0-7

Batanga, Cameroon (L. and F.)

25—35

10-15

0-5-1-0

Lower Congo, Wamba (mostly

L )

10 — 20

5—6

0.5 ro

Angola, Loanda. Benguela

(mostly L )

25—40

5 7

1-0 2-0

Upper Congo sorts (mostly L) .

5—15

4—10

0-5—1-5

Mozambique, Beira (mostly L.)

7-15

5—8

0-1—0-8

Madagascar, Tamatave, Ma-

junga (mostly L.)

20—30

7—10

0-2—0-5

Assam (Ficus) . ....

15—35

5—11

0-5 — I'O

Penang ( Ficus, Willuf/hbeia, &c. )

15—30

5—7

0-3—0-7

Borneo (mostly Urceola) . . .

35—45

10—11

0-4—0-6

Java Plantation (Ficus) . . .

1—2

5—6

0-3—04

Ceylon and Malayan Planta-

tion (Hevea)

1—2

2-5—3-5

0-2—0-6

Plantation Rambong (Ficus) .

1—2

7—8

0-2—0-3

APPENDIX

183

TABLE II.
COMPARISON OF PRICES IN DIFFERENT MEASURES AND CURRENCIES.

The German rate of exchange is taken at £1 = 20 m. 40 pf.,
the French rate at £1 =25 fr. 25 c.

Per Ib.

Per ton avoird.

Per kg.

S. ((.

& s. d.

M. pf.

Fr. c.

10 0

1,120 0 0

22 50

27 82

9 0

1,008 0 0

20 25

25 03

8 0

896 0 0

18 00

22 25

7 0

784 0 0

15 75

19 47

6 0

672 0 0

13 50

16 69

5 0

560 0 0

11 25

13 90

4 51

498 1 3

10 00

12 37

4 0

448 0 0

9 00

11 12

3 7

402 7 10

8 08

10 00

3 0

336 0 0

6 75

8 35

2 0

224 0 0

4 50

5 56

1 0

112 0 0

2 25

2 78

102 13 4

2 06

2 55

93 6 8

1 87

2 32

84 0 0

2 08

74 13 4

1 85

65 6 8

1 62

56 0 0

1 39

49 16 1

1 24

46 13 4

1 16

40 4 9

1 00

37 6 8

28 0 0

18 13 4

968

4 19 7

4 13 4

406

i '

268

1 84

APPENDIX

TABLE III. — CONVERSION OF CRUDE
If a crude rubber priced at A shillings per Ib. has a Washing Loss of

cashing, &c. , being ignored) is _ — — - shillings per Ib. This value is

100 — .D

10%

12%

14%

16%

s. d.

s. (Z.

s. d.

s d.

g d

s d

* d

6 0

6 14

6 3

6 44

6 64

6 8

6 10

6 llf

7 1|

5 10

5 114

6 1

6 24

6 4

6 5f

6 74

6 94

6 114

5 8

5 94

5 lOf

6 04

6 2

6 34

6 54

6 7

6 9

5 6

5 74

5 8f

5 104

5 11|

6 14

6 3

6 4J

6 6|

5 4

5 5|

5 6|

5 8

5 94

5 11

6 Of

6 24

6 44

5 2

5 3£

5 44

5 6

5 74

5 9

5 104

6 0

6 1|

5 0

5 14

5 24

5 3|

5 54

5 6|

5 84

5 9f

5 114

4 10

4 114

5 04

5 1|

5 3

5 44

K O1

5 6

K OJ^

5 74

^ Q

5/i 1

4 6

4 7

4 8*

4 94

4 10|

0 Z%

5 0

ft 3%
5 14

5 2|

o§

5 44

4 4

4 5

4 64

4 74

4 84

4 9|

4 11|

5 04

5 2

4 2

4 3

4 4

4 5j

4 64

4 74

4 8|

4 10

4 114

4 0

4 1

4 2

4 3

4 44

4 5i

4 64

4 7|

4 9

3 10

3 11

4 0

4 1

4 2

4 34

4 44

4 54

4 6|

3 8

3 9

3 10

3 10|

3 11|

4 1

4 2

4 34

4 44

3 6

3 7

3 7|

3 8|

3 94

3 lOf

3 11|

4 Of

4 2

3 4

3 4|

3 5|

3 64

3 74

3 84

3 94

3 104

3 114

3 2

3 2|

3 34

3 44

3 54

3 64

3 74

3 8

3 94

3 0

3 Of

3 14

3 24

3 3

3 4

3 5

3 6

3 7

2 10

2 lOf

2 114

3 0

3 1

3 If

3 24

3 34

3 44

2 8

2 8|

2 94

2 10

2 lOf

2 114

3 04

3 H

3 2

2 6

2 64

2 74

2 8

2 84

2 94

2 10

2 11

2 11|

2 4

2 44

2 5

2 5|

2 64

2 7

2 7|

2 84

Q Ql

2 2

2 24

2 3

2 34

2 4i

2 5

2 54

2 64

2 7

2 0

1 1^

2 1

2 14

2 2

2 2|

2 34

2 4

2 44

1 10

1 11

2 0

2 04

2 1

2 14

2 24

1 8

i ^4

1 8|

1 9|

1 104

1 10|

1 11|

1 6

1 64

1 6{

1 74

1 8

1 84

1 4

1 4|

1 54

1 5|

1 64

1 2

i 24

1 24

1 34

1 4

1 0

1 10*

i 04

1 1

1 14

104

10|

lli

—

APPENDIX

185

INTO WASHED RUBBER PRICES.

B per cent. , then the actual cost of the clean dry rubber (cost of

worked out in the subjoined table for a series of values of A and B.

18%

20%

22%

24%

26%

28%

30%

32%

s d

s. d.

s. d

8. rf.

S. (Z

s. d

7 4

7 6

7 8J

7 lOf

8 1*

8 4

8 7

8 10

6 0

7 14

7 34

7 5f

7 8

7 10*

8 H

8 4

8 7

5 10

6 11

7 1

7 3i

7 o|

7 8"

7 104

8 1

8 4

5 8

6 84

6 104

7 04

7 2f

7 74

7 10i

8 1

5 6

6 6

6 8

6 10

7 Oi

7 5

7 74

7 10

5 4

6 ::.•

6 5*

6 74

6 94

7 2

7 44

7 7i

5 2

6 li

6 3"

6 5

6 7

6 9

6 114

7 If

7 4£

5 0

5 lOf

6 04

6 24

6 4J

6 64

6 84

6 11

7 14

4 10

5 8i

5 10

5 11|

6 If

6 34

6 6

6 8

6 104

4 8,

5 6

5 74

5 9i

5 11

6 1

6 3

6 5

6 74

4 6

5 34

5 5

5 6f

fi 84

5 10i

6 Oi

6 2J

6 44

4 4

5 1

5 24

5 4

5 5f

5 74

5 94

5 114

6 14

4 2

4 10i

5 0

5 14

5 3

5 5

5 6|

5 84

5 104

4 0

4 8

4 94

4 11

5 04

5 2i

5 3f

5 5f

5 74

3 10

4 6|

4 7

4 84

4 10

4 114

5 1

5 3

5 4f

3 8

4 3*

4 44

4 6|

4 7i

4 8f

4 10i

5 0

5 If

3 6

4 Of

4 2

4 3J

4 44

4 6

4 74

4 9

4 lOf

3 4

3 10J

3 114

4 Of

4 2

4 34

4 4f

4 6i

4 8

3 2

3 8

3 9

3 10i

3 114

4 04

4 2

4 34

4 5

3 0

3 54

3 6i

3 74

3 8f

3 10

3 Hi

4 04

4 2

2 10

3 3~

3 4-

3 5

3 6

3 7i

3 84

3 9f

3 11

2 8

3 0*

3 14

3 24

3 34

3 44

3 5f

3 6f

3 8

2 6

2 10"

2 11

3 0

3 Of

3 If

3 3

3 4

3 5

2 4

2 7|

2 84

2 9J

2 10J

2 11

3 0

3 1

3 2J

2 2

2 5i

2 6

2 6|

2 74

2 84

2 9J

2 10i

2 Hi

2 0

2 2f

2 3|

2 4|

2 5

2 5f

2 64

2 74

2 84

1 10

2 04

2 1

2 If

2 2i

2 3

2 3f

2 44

2 54

1 8

1 10

i 104

1 11

1 llf

2 OJ

2 1

2 If

2 24

1 6

1 74

1 8

1 84

1 9

1 91

1 10i

1 11

1 H4

1 4

1 5

1 54

1 6

1 64

1 7

1 74

1 8

1 84

1 2

1 3

1 04

1 34
1 Of

1 3f
1 1

1 4i
1 14

1 4f
1 2

1 5
1 2i

1 54
1 2|

1 0
10

101

104

10|

114

1 86

APPENDIX

TABLE III.— CONVERSION OF CRUDE
If a crude rubber priced at A shillings per Ib. has a Washing Loss of

washing, &c., being ignored) is shillings perlb. This value is

100 - B

34%

36%

38%

40%

42%

44%

46%

s, d.

s. d.

*. d.

s. d.

*. d.

s. d.

6 0
5 10

9 1

8 10

9 44
9 li

9 8
9 5

10 0

9 8|

10 4
10 01

10 84
10 5

11 11

10 9|

5 8

8 7

8 10t

9 li

9 5t

9 91

10 H

10 6

5 6

8 4

8 7

8 104

9 2

9 of

9 10"

10 2

5 4

8 1

8 4

8 7

8 104

9 21

9 6i

9 101

5 2

7 10

8 1

8 4

8 71

8 11

9 21

9 6f

5077

7 9f

8 Of

8 4

8 71

8 11

9 3

4 10

7 4

7 64

7 9£

8 Of

8 4

8 71

8 111

4871

7 3t

7 64

7 94

8 01

8 4

8 71

4 6

6 9|

7 0|

7 3

7 6

7 9

8 0^

8 4

4 4

6 6|

6 9j

6 llf

7 2*

7 5f

7 9"

8 01

4 2

6 4f

6 6

6 8f

6 Hi

7 11

7 51

7 81

4 0

6 Of

6 3

6 54

6 8

6 lOf

7 If

7 5

3 10

5 9f

5 llf

6 21

6 4f

6 71

6 10

7 1

3 8

5 6f

5 8f

5 11

6 H

6 4

6 64

6 91

3 6
3 4

5 34
5 0*

5 2!

5 7f
5 4i

5 10
5 6f

6 01
5 9"

6 3
5 111

6 5f
6 2

3 2

4 9i

4 ll|

5 li

5 3}

5'51

5 8

5 101

3 0

4 64

4 81

4 10

5 0

5 2

5 41

5 6f

2 10

4 34

4 5

4 7

4 8f

4 101

5 Of 5 3

2 8

4 04

4 2

4 34

4 6J

4 7i

494 111

2 6

3 9*

3 11

4 04

4 2

4 3f

4 51

4 71

2 4

3 64

3 7f

3 9

3 101

4 01

4 2

4 4

2 2

3 3}

3 44

3 6

3 74

3 8f

3 101 40

2 0

3 04

3 14

3 2f

3 4

3 51

3 7

3 81

1 10

2 94

2 104

2 11|

3 04

3 2

3 31

3 4f

1 8

2 61

2 71

2 81

2 9|

2 101

2 llf

3 1

1 6

2 31

2 4

2 5

2 6

2 7

2 81

2 91

1 4

2 01

2 1

2 If

2 2f

2 31

2 41

2 54

1 2

1 9*

1 10

1 10*

1 114

2 01

2 1

2 2

1 0

1 6

1 6f

1 7j

1 8

1 8|

1 91

1 101

1 3

1 3*

1 4

1 41

1 51

1 5f

1 64

1 0

1 Of

1 1

1 14

1 If

1 21

1 2f

101

lOf

APPENDIX

187

INTO WASHED RUBBER PRICES — (continued).

B per cent., then the actual cost of the clean dry rubber (cost of

worked out in the subjoined table for a series of values of A and B.

48%

50%

52%

54%

56%

58%

60%

s. d.

11 6£

12 0

12 6

13 Oi

13 7|

14 31

15 0

6 0

11 2f

11 8

12 If

12 8

13 3

13 101

14 7

5 10

10 10$

11 4

11 9£

12 4

12 101

13 6

14 2

5 8

10 7

11 0

11 51

11 Hi

12 6

13 1

13 9

5 6

10 3

10 8

11 1|

11 7

12 11

12 8i

13 4

5 4

9 114

10 4

10 9

11 2f

11 9

12 31

12 11

5 2

9 71

10 0

10 5

10 104

11 41

11 11

12 6

5 0

9 31

9 8

10 1

10 6

11 0

11 6

12 1

4 10

8 Hi

9 4

9 81

10 If

10 74

11 14

11 8

4 8

8 8

9 0

9 41

9 Hi

10 2|

10 81

11 3

4 6

8 4

8 8

9 04

9 5

9 10

10 3|

10 10

4 4

8 04

8 4

8 8

9 01

9 51

9 11

10 5

4 2

7 8i

8 0

8 4

8 8*

9 1

9 64

10 0

4 0

7 44

7 8

8 0

8 4"

8 8*

9 11

9 7

3 10

7 01

7 4

7 7i

7 llf

8 4

8 8f

9 2

3 8

6 9

7 0

7 34

7 74

7 101

8 4

8 9

3 6

6 4|

6 8

6 iii

7 3 7 7"

7 114

8 4

3 4

6 1

6 4

6 64

6 101 7 2i

7 61

7 11

3 2

5 94

6 0

6 3

6 64 6 9^

7 2|

7 6

3 0

5 5|

5 8

5 lOf

6 2

6 54

6 9

7 1

2 10

5 H

5 4

5 6f

5 9i

6 Of

6 44

6 8

2 8

4 9f

5 0

5 2i

5 54

5 84

5 Hi

6 3

2 6

4 6

4 8

4 10£

5 1

5 41

5 6f

5 10

2 4

4 2

4 4

4 64

4 8|

4 11

5 2

5 5

2 2

3 104

4 0

4 2

4 44

4 61

4 9

5 0

2 0

3 64

3 8

3 9f

3 11£

4 2

4 41

4 7

1 10

3 21

3 4

3 5f

3 71

3 9i

3 Hi

4 2

1 8

2 101

3 0

3 H

3 3

3 5

3 7

3 9

1 6

2 6|

2 8

2 9f

2 lOf

3 04

3 2

3 4

1 4

2 3

2 4

2 54

2 6i

2 7f

2 94

2 11

1 2

1 11

2 0

2 1

•2 2"

2 34

2 41

2 6

1 0

l 74

1 8

1 8f

1 9f 1 lOf

1 llf

2 1

1 3i

1 4

1 4|

1 51 1 64

1 7

1 8

1 0

1 0^

11 1 H

1 24

1 3

TABLE IV.

CORRESPONDING TEMPERATURES AND SATURATED STEAM PRESSURES.
1 Atmosphere = 1*033 kg. per sq. cm. = 14-72 Ib. per sq. in.

De-
grees
C.

Degrees
F.

Atmo-
spheres.

Pounds
per
sq. inch

De-
grees
C.

Degrees
F.

Atmo-
spheres.

Pounds
sq. inch.

100
101

212-0
213-8

1-00
1-03

14-7
15-2

141
142

285-8
287-6

3-68

3-78

54-2
55-7

102

215-6

1.07

15-7

143

289-4

3-89

57-3

103

217'4

I'll

16-3

144

291-2

4-00

58-9

104

219-2

1-15

16-9

145

293-0

4-11

60-5

105

221-0

1-19

17-5

146

294-8

4-22

62-2

106

222-8

1-23

18-1

147

296-6

4-34

63-9

107

224-6

1-27

18-7

148

298-4

4-46

65-7

108

226-4

1 31

19-3

149

300-2

4-58

67-5

109

228-2

1-36

20-0

150

302-0

4-71

69-4

110

230-0

1-41

20-7

151

303-8

4'84

71-3

111

231-8

1-46

21-4

152

305-6

4-97

732

112

233-6

1-51

22-2

153

307-4

5-10

75-1

113

235-4

1-56

23-0

154

309-2

5-24

77-1

114

237-2

1-62

23-8

155

311-0

5-38

79-2

115

239-0

1-67

24-6

156

312-8

5-52

81-3

116

240-8

1-72

25-4

157

314-6

5-67

83-4

117

242-6

1-78

26-2

158

316-4

5-82

85-6

118

244-4

1-84

27 vl

159

318-2

5-97

87-8

119

246-2

1-90

28-0

160

320-0

6-12

90-1

120

248-0

1-96

28-9

161

321-8

6-28

92-4

121

249-8

2-02

29-8

162

323-6

6-44

94-8

122

251-6

2-08

30-7

163

325-4

6-60

97-2

123

253-4

2-15

31-7

164

327-2

6-77

99-7

124

255-2

2-22

32-7

165

329-0

6-94

102-2

125

257-0

2-29

33-7

166

330-8

7-11

104-8

126

258-8

2-36

34-8

167

3326

7-29

107-4

127

260-6

2-43

35-9

168

334-4

7-47

110-0

128

262-4

2-51

37-0

169

336-2

7-65

112-6

129

264-2

2-59

38-1

170

338-0

7-84

115-3

130

266-0

2-67

39-3

171

339-8

8-03

118-1

131

267-8

2-75

40-5

172

341-6

8-23

121-0

132

269-6

2-83

41-7

173

343-4

8-43

123-9

133

271-4

2-92

43-0

174

345-2

8-63

126-9

134

273-2

3-01

44-3

175

347-0

8-84

130-0

135

275-0

3-10

45-6

176

348-8

9-05

133-1

136

276-8

3-19

47-0

177

350-6

9-26

136-3

137

278-6

3-28

48-4

178

352-4

9-48

139-5

138

280-4

3-38

49-8

179

354-2

9-70

142-8

139

282-2

3-48

51-2

180

356-0

9-93

146-1

140

284-0

3-58

52-7

188

APPENDIX

TABLE V.
PHYSICAL CONSTANTS OF VARIOUS LIQUIDS.

Substance.

Boiling point,
Degrees C.

Sp. Gr. at 15°.

Coeff . of
Expansion
at ord. temp.

Bromine . .

3-15

0-00116

Sulphur Chloride ....

137

1-68

o-ooioo

Carbon Disulphide . .

1-27

0-00120

Pentane ... ...

0-630

0-00160

Hexane

0-664

0-00144

Heptane . •

0-688

0-00121

Octane

125

0-703

0-00112

Nonane

149

0-721

Decane

173

0-734

0-00101

Mineral Naphtha . .

70—150

0-74—0-76

0-0012

Petroleum (Lamp Oil) . .

150-300

0-79—0-82

o-ooio

Benzene

0-880

0-00124

Toluene . .

110

0-871

0-00110

o-Xylene

142

0-878

0-00097

m-Xylene . .

140

0-869

o-ooioo

p-Xylene

138

0-864

0-00101 .

Pseudocumene

170

0-872

0-00095

Mesitylene

164

0-865

—

Solvent Naphtha ....

125—160

0-86—0-87

o-ooio

Nitrobenzene

205

1-19

Pinene (Turpentine) . . .

160

0-860

0-00097

Pyridine

117

0-986

—

Carbon Tetrachloride . .

1-60

0-00124

Chloroform .......

1-49

0-00127

Dichlorethylene ....

1-25

Trichlorethylene ....

1-47

—

Pentachlorethane ....

159

1-70

0-00091

Epichlorhydriii

116

1-19

—

Ethyl Alcohol ....

0-794

0-00110

Do. 95 per cent.

—

0-816

0-00108

Do. 90

—

0-833

0-00105

Methyl Alcohol

0-797

0-00126

Diethyl Ether

0-720

0-00161

Acetone

0-795

0-00149

Ethyl Acetate . .

0-895

0-00138

Amyl Acetate

148

0-865

0-00116

Acetic Acid (glacial) . . .

119

1-056

0-00107

Glycerine .

290

1-267

0-00051

Fatty Oils

0-915—0-930

0-0007

1 9o

APPENDIX

TABLE VI.

SPECIFIC VOLUME (INVERSE DENSITY) OF WATER FROM 0° TO 31°
REFERRED TO WATER AT 4°.

In making use of this table for applying corrections, it is
convenient to add or subtract parts per 10,000 to or from the
quantity under correction, as one would add or subtract a
percentage. A comma is accordingly inserted between the fourth
and fifth place of decimals.

Temperature.

Specific Volume.

Temperature.

Specific Volume.

Degrees.

1-0001,3

Degrees.
16

1-0010,3

1-0000,7

1-0012,0

1-0000,3

1-0013,8

1-0000,1

1-0015,7

1 0000,0

1-0017,7

1-0000,1

1-0018,5

1-0000,3

1-0022,1

1-0000,7

1-0024,4

1-0001,3

1-0026,8

1-0001,9

1,0029,4

1-0002,7

1-0032,0

1-0003,7

1-0034,7

1-0004,8

1-0037,5

1-0005,9

1-0040,5

1-0007,3

1-0043,5

1 -0008,7

1-0046,5

APPENDIX

191

TABLE VII.
ANALYTICAL FACTORS OF CONVERSION.

Element.

Found.

i
Required.

Factor.

Aluminium

A1203

0-5303

Antimony . ...

Sb02

0-7897

Sb2S3

0-7142

Barium

BaS04

BaO

0-6571

BaSO4

BaCOs

0-8455

Bromine

AgBr

0-4255

Calcium

CaO

CaC03

1 -7843

CaO

Ca(OH)2

1-3212

CaO

CaS04

2-4271

CaO

CaS042Aq i

2-7481

C02

CaC03 i

2-2750

Carbon

C02

0-2727

Chlorine

AgCl

0-2472

Iron

Fe,0o

0-6996

Magnesium

•"• ^2 o

Mg2P207

MgO

0-3624

MgO

MgCOa

0-2090

C02

MgC03

0-1917

MgO

0-6036

Lead

PbS04

PbO

0-7357

PbS

PbO

0-9328

PbO

0-9282

PbS

7-4536

Mercury

HgS

1-1603

HgS

7-2383

Sulphur

BaS04

0-1373

BaS04

S03

0-3429

BaS04
BaS04

S()4
H2S04

0-4114
0-4201

Tin

Sn02

0-7880

Zinc

ZnO

ZnS

1-1973

ZnO

0-8034

ZnS

3-0372

INDEX

Accelerators, 64-66, 134, 165
Acetone Extract, see also Resin

— of factice, 41

— of reclaim, 49

— of rubber goods, 55, 115
Acidity in factice, 44

- in powders, 63

— in sulphur, 92
Acroides, 57
Air-pump, 21, 23
Alumina, 147, 153, 164
Antimony, determ. of, 76, 147, 157
Antimony Red, 62, 74-79

— in gutta-percha, 172

— in rubber, 157, 159, 164
Arsenic Sulphide, 84
Asbestos, 69, 144, 154
Ash, determ. of, 8, 160

- analysis of, 161-163

— of crude rubber, 8, 15

— of factice, 44

— of gutta-percha, 176

— of reclaim, 50

— of rubber goods, 158-159
Asphaltene, 52

Atmoid, 67, 153

Balance, 36

— hydrostatic, 101
Balata, see Gutta-Percha
Barium Sulphate, 35, 72, 80, 83,

145, 153

Barium Sulphate, determ. of, 67,
80, 162

— precipitated, 67
Barytes, 65, 66

— in rubber goods, 146, 158, 163
Benzene, 86, 89

Besk, 57
Bitumen, 51—56

— determ. of, 115, 120-123, 135,
176

Black Hypo, 62, 82, 152, 164
Bone-Black, 81, 147, 164

Cadmium Yellow, 83
Calcium, determ. of, 77, 148

— carbonate, see Whiting

— sulphate, 74, 78, 80, 150, 164,
165

Carbohydrates, 8
Carbon Black, see Lampblack,
Borie-Black, Graphite

— determ. of, 56,82, 145, 155-157
Carbon Bisulphide, 89, 90, 122-

123, 135

Carbon Tetrachloride, 90, 135
Carbonic Acid, determ. of, 151
Casein, 137

Centrifuge, 10, 28-31, 142, 175
Ceresine, see Paraffin Wax
Charge, 38, 111, 126

— anal, of, 144-154

— determ. of, 141-144

194

INDEX

Chatterton's Compound, 177
China Clay, see Kaolin
Chlorine, determ. of, 43, 131
Chrome Green, 84, 146
Chrome Yellow, 83
Coal-tar Naphtha, 86, 135
Coefficient of expansion, 102

— of vulcanization, 54, 126
Cold-Cure, 93, 111
Combined Sulphur, 43, 77

— in rubber, 125-132

— mineral, 131

Combined Water, see Water of

hydration
Compounding, 37
Copal, 57, 137
Copper, 63, 64, 98
Cork Dust, 137, 144, 152
Crape, 2, 3, 21, 106, 112
Crimson Sulphide, 74
Curing, see Vulcanization

Dammar, 57

Dead Borneo, 57

Degree of Sulphur, 54, 126

Density, see Specific Gravity

Deresinification, 168

Diluents, 38

Dirt, determ. of, 59, 172-176

Distillation Test, 87

Dough, 37

- anal, of, 12, 134-135
Drying Ovens, 21-24
Dyestuffs, 65, 81, 83, 144

Ebonite, 37, 107, 112, 121, 143

— anal, of, 127, 136-138

- Dust, 145, 152
Epichlorhydrin, 137
Extraction with acetone, 7, 41, 49,

59, 81, 114, 171

— with alcoholic potash, 49,
123-124, 134, 136, 176

— with carbon disulphide, 75,
98, 122

— with epichlorhydrin, 137

— with toluene vapour, 10, 59,
172-175

Extractor, Soxhlet's, 33

— Knofler's, 31-33

Fabrics (textile), 97

— stripping of , 113
Factice, 39-47

— in doughs, 134

— in gutta-percha, 176

— in reclaim, 49

— in rubber goods, 115, 121,
123-125

Ferric Oxide, 79, 146

Fibres, 144, 152, 159

Fillers, 66-70, 165, 172

Filter-pump, 23

Fineness, degree of, 62

Floating (rubber) goods, 39, 50,

108

Flotation, 104, 106
Fluorspar, 67, 164
Free Sulphur. 41, 49, 75, 116
French Chalk, see Talc

Glass, powdered, 70, 153
Golden Sulphide, 74
Graphite, 82, 144
Gutta, determ. of, 176
Gutta-Percha, 167-170
- anal, of, 170-177

Hardening, 168
Heat-cure, 110
Hydrometer, 86, 99-101

Infusorial Earth, 67, 92
Insoluble Matter, 9-11

Jelutong, 57

Kaolin, 68, 72, 153, 164, 172

Lakes, 83, 144

Lampblack, 71, 81, 144, 159

Latex, 1

Lead, 64, 70, 72, 146

- sulphide, 64, 153, 164
Lime, 65

— in factice, 44

— in rubber goods, 154, 165
Litharge, 64, 134, 153, 161, 163
Lithopone, 62, 72, 154, 163, 165,

172

Magnesia, 44, 66, 154, 165

INDEX

195

Magnesium Carbonate, 66, 70, 154,

166

Manganese, 66, 67
Manufactured Rubber, 37, 60

— constituents of, 111
Mercury, 9, 147, 157, 175
Metals, powdered, 144
Mica, 69, 153

Mineral Naphtha, 85, 90, 100
Mineral Rubber, 51, 54
Mineral Wax, see Paraffin Wax
Moisture, 6, 44, 53, 58, 62, 92,

114, 170, 177
Muffle, 35

Naphtha, 85-90

— specific gravity of, 86, 100
Naphthalene, 89
Nitrate-fusion, 128
Nitrogen in rubber, 9, 16

Ochre, 68, 153

— red, 80

— yellow, 83
Oil, fatty, 94, 120

— mineral, 42, 95, 114, 117

— vulcanized, 39, 43, 45

Painter's Test, 71
Palembang, 57
Para Frangais, 39, 46
Paraffin Wax, 41, 95-97, 156

— determ. of, 43, 117-119
Pectous rubber, 10, 17
Peroxide Fusion, 43, 77, 129-131
Petrolene, 52

Petroleum, 11, 141, 143
- Ether, 43, 118

— Naphtha, see Mineral Naph-
tha

- Spirit, 42, 52, 116
Phosphates, 81, 147, 152, 184
Pigments, 70-84, 172
Pitch, 56,114, 176
Pontianak, 57

Pontio's Method, 59, 172-176
Protein, 2, 15, 16
Prussian Blue, 84
Pumice, 70, 153
Pycnometer, 101-104

Reclaim, 48-51

— in rubber goods, 138-139, 143,
144

Red Lead, 65, 164

Resin, 15, 56, 120, 137, 170, 172

— determ. of, 7, 59, 114, 120, 137,
171

— rubber-containing, 57-59
Rinmann's Green, 84
Roller Mills, 24-27

Rosin, 56, 96, 137, 177

Rouge, 79, 146, 164

Rubber, determ. of, 11-15, 50, 58,

132-133

- tetrabromide, 13, 133

Sampling, 3-6

Sandarac, 57

Sawdust, 152,

Shale Naphtha, 90

Shellac, 57, 137

Silica, 67

Silicates, 68-70, 146, 162, 163,

172

Slate Powder, 68, 153
Softening-point, 53, 56, 97
Solutions, 12, 51, 85, 135-136
Solvent Naphtha, 86, 89, 100
Specific Gravity, true and appar-
ent, 102, 109

— of liquids, 99-102

— of naphtha, 86, 100

— of powders, 102-104

— of reclaim, 49

— of rubber, &c., 104-109, 167

— bottle, see Pycnometer
Starch, 144
Substitute, 39

Sulphide-sulphur, 75, 78, 150
Sulphur, 91-92

— in rubber goods, 155
Sulphuring-up, 115
Sulphur Chloride, 93-94

Talc, 34, 68-69, 153, 172

Talite, 68, 153

Tetrabromide Method, 14, 133J

Thiocyanate Method, 42, 116, 119

Toluene, 89

Tube-furnace, 35, 70, 83, 150

196 INDEX

Ultramarine, 67, 81, 84 Water of hydration, 65, 68, 69, 70,

145, 156

TT v ^ n« n- White Lead, 65, 153, 164

Vaseline, 42 5 96 ,117 _ subiimed, 65

Vermilion, 80, 147 Whiting, 67, 154, 165

Washing Loss, 2, 3, 18-21, 169 Zinc, determ. of, 148-149

Waste, ground, 47, 138, 144 — Sulphide, 73, 154, 161, 164

Water, see Moisture — White, 71, 153, 161, 172

RICHARD CLAY AND SONS, LTD. BRUNSWICK ST., S.E., AND BUNGAY, SUFFOLK.

CHEMICAL TECHNOLOGY AND
ANALYSIS OF OILS, FATS & WAXES.

BY DR. J. LEWKOWITSCH, M.A., F.I.C.

Fifth Edition entirely rewritten and enlarged. Illustrated.

Medium 8vo.

VOL. I. 25S. NET. VOLS. II. AND III. (In Preparation.)

. — " The arrangement of the subject-matter remains much
as before, but its bulk has increased considerably, in spite of every
endeavour to compress it and eliminate what has become antiquated.
Due note has been made of recent progress in the chemistry and
technology of fats and oils, so far as the scope of the present volume
allows ... In short, there is evidence that the volume has undergone
a thorough revision in bringing it up to date. It is to be hoped that
the lamented decease of the author will not necessitate any considerable
delay in the completion of the new issue. The work was his magnum
opus, and will remain a worthy memorial of his industry and knowledge."

CHEMICAL ENGINEERING.— "The volume bears on every
page the impress of the true technologist. . . It is quite indispensable
to the worker in oils, fats, and waxes/'

LABORATORY COMPANION TO FATS
AND OILS INDUSTRIES.

BY DR. J. LEWKOWITSCH, M.A., F.I.C.

8vo. 6S. NET

OIL AND COLOURMAJSPS JOURNAL.—" The practical man,
whether student or operative, will appreciate the carefully compiled
tables."

BRITISH TRADE REVIEW. — "Consists almost exclusively of
tables, which can hardly fail to be of great value to works chemists
who require reliable data in a clear and handy form."

LONDON: MACMILLAN AND CO., LIMITED.

A Selection of Works on Applied Chemistry

THE PIGMENTS AND MEDIUMS OF
THE OLD MASTEBS. With a Special

Chapter on the Microphotographic Study of Brushwork. By Prof.
A. P. LAURIE, M.A., D.Sc. Illustrated. 8vo. [Spring, 1914.

FACTS ABOUT PROCESSES, PIG-
MENTS, AND VEHICLES. A Manual for

Art Students. By Prof. A. P. LAURIE, M.A., D.Sc. Crown
8vo. 35. net.

STUDIES IN WATER SUPPLY. By

A. C. HOUSTON, D.Sc., M.B., C.M. With Diagrams and
Charts. 8vo. 5*. net.

FRACTIONAL DISTILLATION. By

SYDNEY YOUNG, D.Sc., F.R.S. Crown 8vo. &r. 6d.

GAS ANALYSIS. By Prof. L. M. DENNIS.

(Being The Second Edition of Hempel's "Methods of Gas
Analysis.") Crown 8vo. gs. net.

THE CHEMISTRY OF PHOTO-
GRAPHY. By Prof. RAPHAEL MELDOLA, F.R.S.

Illustrated. Crown 8vo. 6s.

JENA GLASS & ITS SCIENTIFIC AND
INDUSTRIAL APPLICATIONS.

By Dr. H. HOVESTADT. Translated by J. D. and ALICE
EVERETT. 8vo. 15-5-. net.

PRINCIPLES OF DYEING. By G. S.

FRAPS, PH.D. Crown 8vo. js. net.

THE DIETETIC VALUE OF BREAD.

By JOHN GOODFELLOW, F.R.M.S. Illustrated. Crown 8vo. 6s.

THE POLARISCOPE IN THE CHEMI-
CAL LABORATORY. An Introduction to

Polarimetry and Related Methods. By GEORGE WILLIAM
ROLFE, A.M. Crown 8vo. 8s. net.

LONDON: MACMILLAN AND CO., LIMITED.

Standard Works on Chemistry

A COMPLETE TREATISE ON INOR-
GANIC & ORGANIC CHEMISTRY.

By The Right Hon. Sir H. E. ROSCOE, F.R.S., and C. SCHOR-
LEMMER, F.R.S. 8vo.

VOL. I. THE NON-METALLIC ELEMENTS. Fourth
Edition. Revised by The Right Hon. Sir H. E. ROSCOE assisted
by Dr. J. C. CAIN. 2is. net.

VOL. IT. THE METALS. Fifth Edition. Revised by Sir H. E;
ROSCOE and others. 30^. net.

VOL. III. ORGANIC CHEMISTRY : THE CHEMISTRY OF
THE HYDRO-CARBONS AND THEIR DERIVATIVES. Parts II.,
IV., and VI. 2is. each. Part III. iSs.

CHEMISTRY OP THE PROTEIDS. By

Dr. GUSTAV MANN. Based on Professor COHNHEIM'S " Chemie
der Eiweiss Korper." 8vo. 15^. net.

ANALYTICAL CHEMISTRY. By Professor

A. MENSCHUTKIN. Translated by J. LOCKE. 8vo.iyjr.net.

HISTORY OF CHEMISTRY FROM
THE EARLIEST TIMES TO THE
PRESENT DAY. By ERNST VON MEYER,

Ph.D. Third Edition. Translated by Dr. G. McGowAN. 8vo.
17^. net.

ESSAZS IN HISTORICAL CHEM-
ISTRY. By Sir EDWARD THORPE, C.B., LL.D.,

F.R.S. Third Edition. 8vo. I2j.net.

THEORETICAL CHEMISTRY. By Pro-
fessor WALTER NERNST, Ph.D. Revised in accordance with the
sixth German Edition by H. T. TIZARD. 8vo. 15^. net.

CRYSTALLINE STRUCTURE AND
CHEMICAL CONSTITUTION. By Dr.

A. E. H. TUTTON, F.R.S. 8vo. 55-. net.
LONDON: MACMILLAN AND CO., LIMITED.

Standard Works on Chemistry

INTRODUCTION TO PHYSICAL
CHEMISTRY. By Prof. JAMES WALKER,

F.R.S. Sixth Edition. 8vo. los. net.

SCIENTIFIC FOUNDATIONS OF
ANALYTICAL CHE MIST BY. By Prof.

WILHELM OSTWALD. Translated by G. McGowAN, Ph.D.
Third edition. Crown 8vo. 6s. net.

PRINCIPLES OF INORGANIC CHEM-
ISTRY. By Prof. WILHELM OSTWALD. Translated

by A. FINDLAY, D.Sc. Third edition. 8vo. iSs. net.

OUTLINES OF GENERAL CHEM-
ISTRY. By Prof. WILHELM OSTWALD. Translated

by W. W. TAYLOR, D.Sc. Third edition. 8vo. i-js. net.

GASES OF THE ATMOSPHERE: THE

HISTORY OF THEIR DISCOVERY. By Sir WILLIAM RAMSAY,
K.C.B. Third edition. Extra crown 8vo. 6s. net.

THE EXPERIMENTAL STUDY OF

GASES. By Dr. MORRIS W. TRAVERS, F.R.S.
With Introduction by Sir W. RAMSAY, K.C.B. 8vo. 10^. net,

THE THEORY OF ELECTROLYTIC
DISSOCIATION AND SOME OF ITS

APPLICATIONS. By Prof. HARRY C. JONES.
Extra crown 8vo. 7.r. net

INTRODUCTION TO PHYSICAL
CHEMISTRY. By Prof. HARRY C. JONES.

Crown 8vo. 7.?. net.

PRINCIPLES OF INORGANIC CHEM-
ISTRY. By Prof. HARRY C. JONES. 8vo. 12$. 6d.

net.

A TEXT-BOOK OF ELECTRO-CHEM-
ISTRY. By Prof. MAX LE BLANC. Translated
from the 4th enlarged German edition by WILLIS R. WHITNEY,
Ph.D., and JOHN W. BROWN, Ph.D. 8vo. 115-. net.

LONDON : MACMILLAN AND CO., LIMITED.

UNIVERSITY OF CALIFORNIA LIBRARY

15234

1303070

O? CAU*ORNIA

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "India-rubber laboratory practice"

See other formats