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.