LEATHER AND LEATHER DYEING
So far as can be learned, in every part of the world, the first
materials used by man for clothing and coverings were the skins of
animals. In its natural condition, however, the hide stripped from a
dead animal has certain properties which greatly interfere with such
use. When dry it is stiff and hard; when moist it rapidly decomposes,
and when exposed to hot water it swells and in time dissolves. These
difficulties had to be overcome before skins and furs could be
properly utilized. And, accordingly, in the history of every nation
and race, one of the very earliest of all developing industries was
the art of leather making; that is, of converting the hard and easily
decomposed rawhide into a soft, pliable, and comparatively permanent
substance, well suited for the use of man.
In most uncivilized nations this conversion was accomplished by
rubbing and working some oily or greasy substance into the hide,
until it was thoroughly soft and flexible. Thus, in our Indian
tribes, the old squaws would turn the deer skins and the pelts of
various fur-bearing animals into beautifully soft and strong leather,
by rubbing and working into them the brains of the animals. The
Esquimaux and other Northern tribes from time immemorial, too, have
worked out this method with great perfection. Indeed without it they
would have been unable to survive at all.
In other parts of the world it was discovered that rawhide could be
made more durable by treatment with metallic salts, especially with
alum, and then, by softening this product by rubbing in some oily
material, a very fair leather could be produced. On the other hand,
in warmer climates, as for instance among the Egyptians, the very
earliest records show the use of vegetable extracts, containing the
substances now known as tannins, for softening and preserving skins;
and these races understood the art of dyeing, painting, gilding,
and embossing the leather thus made, and used it for shoes, straps,
aprons, and harness.
The Romans and Babylonians were famous for their leather industry,
and the ancient Romans not only imported but manufactured it
themselves, and used it freely. In the Middle Ages the greatest
developments in the art were made by the Moors in Spain, whose
leather, commonly called Cordovan leather, from the city which was
the centre of the industry, has probably never been equalled for
beauty and importance. This Cordovan leather, of which fine specimens
are still to be found in museums and private collections, was made of
sheepskin, tanned with bark. It was ornamented with silver foil, laid
on a backing of size, and covered with a yellow varnish or lacquer,
sometimes tinted with bitumen. This protected both the leather and
design very perfectly from injury by air or moderate moisture, and,
being done on a large scale with splendid designs, was used largely
for handsome wall coverings, competing favorably with tapestries
manufactured in France and elsewhere for the same purposes.
PREPARATION OF LEATHER.
In general, we may say that at the present day there are the same
three classes of leather as in the days of the ancients, according to
whether the hide is treated with oil or fatty materials, with alum or
other metallic salts, or with the bark of trees or other vegetable
substances containing the compound known as tannin.
=1. Oil Tanning.=—This, while of less importance than the other two
methods, is still used in considerable quantities for lighter and
cheaper qualities of leather. The process most commonly used is often
called chamoising, or “shamoying,” because it is used principally for
the production of “chamois leather” or wash leather. The hides used
for this form are usually thin and light, the flesh sides of split
sheepskins being the commonest, and the resultant leather is not only
soft and flexible and strong, but is also unaffected by water. For
this reason it is more difficult to dye than other varieties.
=2. Mineral Tanning or Tawing.=
_Alum._—For thousands of years it has been known that if a solution
of alum is rubbed or soaked into a raw hide the fibres of the leather
become changed to an insoluble and permanent condition, and by
afterwards rubbing and rolling, and working in some greasy material,
like the yolk of eggs, a useful variety of leather can be produced.
The alum in this case does not form a permanent compound with the
animal fibres, but can be washed out by working in warm water.
Chemists have agreed, therefore, to call this temporary reaction by
the name “tawing” as opposed to “tanning” where the chemical action
is a permanent one. The “kid” leathers used for gloves are commonly
made by this process.
_Chrome._—During the last few years a new process has been
introduced, based upon the use of chromium salts, which are absorbed
by the hide in the form of the yellow or orange-colored salts,
chromate and bichromate of sodium, and then are reduced in the fibres
to a green compound by the use of hydrosulphite of sodium, or some
other strong but harmless reducing agent.
This chrome leather is extremely valuable, and is freely used,
especially for the “uppers” of good quality in the boot and shoe
trade. This leather is very strong, and is water-proof, but possesses
a serious disadvantage for the dyer, in that when it is once dry it
can never be again wetted, and therefore it must be dyed fresh from
the tannery wash tanks, or not at all.
=3. Vegetable, or Bark Tanning.=—At some very early period in the
world’s history it was discovered that certain vegetable extracts,
possessing in general a peculiar “puckery” taste, also possessed
valuable properties in the treatment of raw hide. This process was
certainly well known to the Romans, for Pliny mentions, as tanning
materials, the three great sources of tannin to-day, namely, gall
nuts, the bark of trees, and sumach. These and many other vegetable
materials, used for tanning, all contain a peculiar substance, known
as “tannin” or tannic acid, which gives them their useful properties.
The tannins from different plants are not identical, although closely
related to each other. They all have a strong astringent taste, and
dissolve readily in water, forming weak acid solutions. They make
dark-colored compounds with iron salts, and convert the hide tissue
of animals into a tough, insoluble, and comparatively indestructible
material which, when loosened and softened by some mechanical action,
is known as leather.
_Tannin._—Pure tannin can best be obtained from gall nuts—small
excrescences on the leaves and twigs of certain plants caused by the
puncture of some insect preparing to deposit its eggs there. The best
varieties, called Aleppo galls, come from Turkey and Austria, where
they are found on oak trees, and contain from 60 to 70 per cent. of
tannic acid. From these it can be extracted in a very pure form,
and it comes to market as an extremely light, fine, grey or light
tan-colored powder, which dissolves in very little water to an almost
colorless solution. Tannin in this form is largely used for dyeing,
especially in the dyeing of cotton or linen goods with the Basic
colors.
For tanning purposes it is customary to use the bark of various
trees, oak bark being the most esteemed in Europe and, in this
country, hemlock bark being the most used. These contain from 12 to
15 per cent. of tannin, as a rule, with a moderate amount of brown
coloring matter. Pine bark is also frequently used, and the bark of
fir, spruce, and larch, while, in Russia especially, much willow bark
and birch bark is used for light grades, the so-called Russia leather.
The next most valuable source of tannin is known as sumach,
consisting of the finely-ground twigs and leaves of several species
of that plant. The American sumach contains more tannin—18 to 25 per
cent—than other varieties, but it is less valuable than the Sicilian
sumach, which contains less coloring matter, and therefore can be
used for tanning light shades of leather. All the materials can be
used in the tannery either directly, or in the form of previously
prepared extracts. From the Far East come some very important sources
of tannin, used for dyeing as well as for leathermaking, in the
form of dried extracts of various plants. One of these is Catechu
or Cutch, now of value only for its tannin contents, but in former
years used as a brown dyestuff as well. A similar product, known as
Gambier, is still imported on a large scale from Singapore and other
Eastern ports. It contains less tannin than Cutch, but less coloring
matter as well. It is used not only for leather but for black silk
dyeing with logwood.
=The Tanning Process.=—Without going too much into detail, the
conversion of raw hide into leather by means of tannin is a very
lengthy and mechanical process. The hides are first softened by
soaking in water, and then are dehaired, usually by steeping in a
bath of slaked lime until the hair is loosened and can be scraped off
with a blunt knife.
This lime must then be extracted by steeping in an acid bath,
preferably containing some organic acid like lactic or acetic acid;
some manufacturers, for the sake of cheapness, use dilute sulphuric
acid for this purpose, with the invariable result of making the
leather brittle and rotten when it is fully dried.
After the acid has been rinsed off, the hides are placed in the tan
liquor, made either by dissolving one of the extracts in water, or
by mixing the finely-ground bark or sumach with water and placing
the hides in the mixture. The tanning process is a very slow one,
especially for heavy hides, and it may take several months before
the tannin penetrates to the center of the goods. When that time has
come, the hides are taken out, brushed off, rinsed with cold water,
drained off on horses, and then hung up in a drying shed to slowly
dry.
When in the proper condition they are thoroughly rolled by hand or
machinery, to break up any adhesions, and to make the leather soft
and flexible. Then they are ready to be finished, are dyed to the
required shade, rubbed down and polished with wax or varnish, grained
by being run through rollers with engraved patterns, and otherwise
prepared for the trade.
DYEING AND STAINING OF LEATHER.
_General._—It has been mentioned, in previous chapters, that animal
fibres of all sorts, such as wool, silk, feathers, etc., seem
to possess at the same time both acid and basic properties, and
therefore they combine readily with dyestuffs belonging to the Basic
and also to the Acid class. This at once distinguishes animal fibres
from vegetable fibres such as cotton, linen, and paper, which, being
practically neutral in composition, will not combine with either
Basic or Acid dyestuffs without the assistance of mordants.
This same rule applies to leather, and we are therefore able to dye
leather successfully with either Acid or Basic dyestuffs, using a
dye-liquor made acid with, preferably, a volatile organic acid such
as acetic or formic acid.
_Acids._—For Basic colors acetic acid is generally used, as being
cheaper than the other, and quite as good for dissolving the dyes
for the dye-bath. For Acid colors it is generally best to use formic
acid, for acetic acid in many cases fails to liberate the color-acids
from the dyes, and then the colors fail to “bite.”
Professional leather dyers, for the sake of economy, often use a
little sulphuric acid in the dye-bath, a practice which is one of the
chief causes of the short life of modern leathers.
With other animal fibres, such as wool and silk, the Acid colors
take quite as readily as the Basic; but with leather, there is some
little difference, according to the way in which the leather has been
prepared.
For our purposes it is hardly worth while to discuss the dyeing of
chrome leather or of chamois leather. The leather almost universally
employed for hand work has been bark tanned, excepting where very
white goods are used, of rather light quality. These are generally
tawed with alum, and for this reason have a greater affinity for the
Acid colors than when the fibres have already been fully charged with
tannic acid, which at the same time, it will be remembered, acts as
an excellent mordant for the Basic colors.
_Dyestuffs._—Accordingly, while Acid colors may be used, they do not
act nearly so readily as the Basic colors. For this reason, except
for special shades such as a clear sky-blue or a pure scarlet,
which can hardly be obtained excepting by the use of Acid dyes,
or where special fastness to sunlight is required, the best Basic
colors, such as Methylene Blue, Methylene Heliotrope, Thioflavine
T (for yellow) and Safranine (for red) are usually employed. For
black, it is well to use one of the many Leather blacks, made by
mixing together powerful Basic dyes. For brown, the standard leather
color, used in enormous quantities for gloves and the like, is the
well-known Bismarck Brown, or for more orange shades, the closely
related dyestuff, Chrysoidine. And, although neither of these colors
is as fast to light as the Basic dyes mentioned above, they give
very satisfactory results. These colors should be dissolved in water
acidified with a little acetic acid.
The greatest pains must be taken in each case to see that the color
is all in solution, and that no specks of undissolved color are
allowed to come in contact with the leather. The leather must be very
carefully and thoroughly moistened by soaking, if necessary over
night, in lukewarm water softened, if the surface of the leather
seems to demand it, with a few drops of ammonia water.
=Dyeing Leather and Staining Leather.=—As regards the application of
the color; dyers generally make a distinction between leather that is
_dyed_ and leather that is _stained_.
In _dyeing_ leather the moistened goods are placed in a tray or pan
(agateware is most convenient for small pieces) and floated backwards
and forwards in the dye-liquor, which should be deep enough to fully
cover them. The liquor is usually about lukewarm on starting, and
may be heated very gradually and gently to about 120° or 130°, if
desired. For light shades, however, this is not at all necessary,
and indeed the color, as a rule, penetrates deeper and is laid on
more evenly when the bath is kept cold. The leather is kept in the
dye-bath until the desired shade is reached, which should be at the
end of half an hour or so.
When dyed in this manner, the dyestuff has a chance to soak into the
leather, and so, when finished, the color is not so liable to be
affected by rubbing or by wear. The leather should come out evenly
coated on both sides, shaded effects if desired being produced later,
by the staining process.
_Stained Leather._—In staining leather, on the other hand, the color
solution is applied directly to the surface of the damp—not wet—goods
by means of a brush or soft sponge, or a little pad of cloth.
Accordingly, no matter how carefully the leather has been softened
and moistened beforehand, the color does not penetrate far, and is
found only on the particular surface where it has been applied.
For flat, even shades, the dyeing process is usually preferable,
but by staining, it is possible for the craftsman to work on the
surface of the leather, as an artist does on paper with water colors,
and beautiful effects can be produced. Oil paint is often used for
decorating leather, and when applied skilfully in thin layers, the
effects are good. But staining with dyestuffs is usually preferable,
as showing more of the grain of the leather, and being more
transparent.
The staining of leather may either be done free-hand, or else by the
filling in of set designs, marked out previously by tooling or some
other method; or, as will be discussed later, by the use of stencils.
In any case success chiefly depends upon the condition of the surface
that is to receive the dye. The surface of the leather should be
dampened, thoroughly and evenly, so that the dye will adhere, and
even penetrate a little; but it must not be so wet that the colors
will run.
To get this exactly right requires considerable practice. As a rule,
the leather is, first, carefully and evenly soaked in water or, if it
is at all greasy, in water with a little ammonia in it. When this has
been thoroughly done, the leather is taken out and dried off, first
on one side and then on the other, with pieces of cloth and then
later with blotting paper. After this it is exposed to the air for a
little time until the exact point of dryness has been reached.
The color solution should be applied with a camel’s hair brush or
a small, soft pad of cotton, and any excess of liquid wiped off,
or soaked up with blotting paper, and the color rubbed in with the
fingers or pad, as soon as possible.
=Acid Dyes for Leather.=—As above mentioned, certain shades are
hard to obtain without the use of Acid colors. This is particularly
true in the case of blue. For the lighter and brighter shades it
is necessary to use one of the Acid blues such as Cyanole FF.
(_Cassella_), or Patent Blue (_Metz_). These are applied in exactly
the same way as the Basic colors. Some of the Acid reds, too, will be
found valuable for certain shades of scarlet, etc., that can hardly
be reached with Safranine. Among the best of the fast Acid colors for
leather may be mentioned:
_Red._—Fast Scarlet, BXG, _Badische_; Biebrich Acid Red, 2B,
_Kalle_, and Fast Acid Red, M, _Metz_.
_Yellow._—Tartrazine, _Badische_; Wool Yellow, 1A, _Kalle_, and
Fast Acid Yellow, 3G, _Metz_.
_Blue._—Wool Fast Blue, BL, _Badische_; Nerocyannic Blue, B,
_Kalle_, and Fast Acid Blue, BB, _Metz_.
When using these Acid dyes side by side with the Basic colors, it
will be noticed that the latter, as a rule, are far more powerful,
and color the leather much more rapidly than the Acid dyes.
Accordingly for _staining_ leather the Basic dyes are the most
satisfactory. On the other hand in _dyeing_ leather, where the
dye-liquor is allowed to act longer on the goods, the Acid colors are
more valuable, not only because they are fast to light, but also
because they will penetrate more deeply and more evenly.
=Finishing Leather.=—After coloring the leather it is necessary to
finish it carefully, to get a smooth surface and to protect it from
injury by rubbing or moisture. Some workers simply let the leather
dry and then rub down the surface (without using any wax or oil)
with the finger or the palm of the hands. Usually the grain or hair
side of the leather is rubbed down with a little wax, the white or
yellow wax, used largely as a finishing polish for tan shoes, being
frequently employed for this purpose. It can be readily obtained from
almost any good shoe store or, if desired, can be made by mixing
together equal quantities of beeswax and carnauba wax in a molten
condition, and thinning the mixture with a little turpentine.
A recipe used with success by many leather workers calls for a
mixture of beeswax, turpentine, and neatsfoot oil. The wax is
carefully melted, mixed with a small amount of turpentine, and then
enough oil is stirred in to make it soft. When used upon embossed or
figured leather this wax is never applied directly, but is placed
inside a little bag of soft muslin, and rubbed on and into the
leather with a circular motion—the palm of the hand being often used
to finish the waxed surface.
=Bronze Effects.=—An interesting point in connection with the use of
the Basic dyes, and some of the Acid dyes, too, for staining leather
is that, when applied in a strong solution, as is very likely to be
the case when one is trying to get dark shades with an application
of the brush or pad, they quite frequently, on drying, show a very
marked metallic lustre. This is due to the formation of minute,
bright-colored crystals, which reflect the light, thus imparting to
the fabric colors which have nothing to do with the shade produced by
the dyestuff itself. Thus, Cyanole FF, _Cassella_, when dissolved, or
when dyed on leather or any other material, gives a rather greenish
shade of blue. But it gives a very brilliant old gold effect, almost
as bright as gold leaf, when applied in a strong solution and allowed
to dry quickly.
When this effect is not desired it can be avoided by building up
the dark shades by successive applications of weak solution, and
by rubbing down the little crystals with, if necessary, a little
moisture, whenever they appear to be forming.
In some cases, however, this bronzing property is of some value, and
enables the skilful craftsman to obtain interesting and effective
results with a minimum of trouble and expense. By painting on a
strong solution of dyestuff, and letting it dry quickly, the bronze
effect will be produced, and then by rubbing in portions, the true
coloring of the dyestuff will be brought out in strong contrast to
the crystal-covered surface. Unfortunately, these bronze effects
are not fast to either rubbing or moisture, and even dry rubbing
will break down the crystals, while rubbing with a damp cloth or a
moist finger will dissolve the color off in blotches. To render this
bronze effect more durable, it is possible to make a regular bronze
lacquer, by adding varnish or gum like orange shellac or gum benzoin
to a strong alcoholic solution of a Basic dye. The bronze varnish
thus produced will, when dry, stand light finishing with wax in the
usual way. The addition of a little benzoic acid to the solution
increases the lustre of the crystals.