CHAPTER X

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.