LEATHER, the skins of animals chemically modified by tanning and otherwise, so as to arrest that prone ness to decomposition which characterizes unprepared skins, and to give to the substance greatly increased strength, toughness, and pliancy, with insolubility and unalterability in water. Remains exist of tanned leather made in Egypt not less than 900 years B. c. There are three methods by which leather is pre pared: first, and by far the most im portant, with tan barks and other vege table substances containing tannin; sec ond, by tawing with alum, bichromate of potash, and other mineral salts; and third, by shamoying or impregnating the raw skin with oil. The skins of all ani mals used for leather making consist chiefly of a fibrous gelatigenous sub stance called collagen, which on being boiled forms the ordinary gelatin of commerce, with an interfibrous coin pound called coriin, insoluble in water, but which in common with collagen unites with tannin to form the insoluble and unalterable compound tanno-gela tin, the chemical basis of tanned leather.
The skins of all animals may be made into leather ; but in practice the raw ma terials of the manufacturer consist of the skins of certain animals which are reared and slaughtered primarily for other purposes, and of which the sup ply is sufficiently large to form the basis of a great industry. Large skins, it may be remarked, such as those of oxen and horses, are in trade termed hides; those of calves, sheep, goats, and other smaller creatures are called skins. Of all leather-making hides the most important are those of oxen, which are primarily distinguished as ox, cow, and bull hides, and calf skins. Leading sources of sup ply are Australia, the Cape of Good Hope, the River Plata and South America generally, and China and Japan. From the East Indies there come vast quantities of small hides termed kips, both salted and tanned. Buffalo hides are imported in large quantities from Singapore, Batavia, Borobay, Kurrachee, and Calcutta. Horse hides are brought in considerable quantities from South America. Sheep skins, from the vast quantities yearly available in nearly all parts of the world, are a most important source of leather. Goat skins and kid skins come from the Cape, the East In dies, Asia Minor, and Switzerland; but many of the East Indian and Asiatic skins are when imported already tanned, and require only dressing. Other skins which have only a local or a limited market are the walrus, rhinoceros, and elephant, from which leather of great thickness, suitable for polishing wheels and other mechanical purposes is ob tained; and hog or pig skin is an im portant source of leather for saddle mak ing and other purposes. The skins of various species of deer and antelope, por poise and kangaroo, are also sources of leather; and from the Cape there are occasionally sent to the London market skins of the gnu and quagga. As sources of leather for fancy articles there may be mentioned the alligator (a leather now extensively imitated), and certain snakes' and sharks' skins.
Tanning.—The operations of tanning and the duration of the process vary very widely. Oak tannage is a very tedious process, and the common practice is now to hasten the completion of the operation by mixed tannage, in which more rapidly acting agents play a part. In the United States hemlock bark from Abies canadensis is the most important tanning material; and the mimosa or wattle barks of Australia are very largely used in the Brif sh colonies as well as in Great Britain. Standard ex tracts containing a fixed percentage of tannin have also come into favor for rapid tannage. But, with all the devices
which have been suggested, tanning is essentially a slow operation. The great amount of poor leather in the markets is due to hurried preparation and use of chemicals that injure the texture.
In the treatment of ox hides for the production of, say, sole leather, the first object of the tanner is to clean and soften the hide. This is done by washing with water, and if necessary working the hide under stocks till the whole is uniformly soft and pliant. The unhairing and re moval of the scarf skin is the next opera tion, for which in English tanneries the hides are steeped in pits containing lime water, while in the United States the plan adopted consists of sweating the hides, or artificially heating them till in cipient putrefactive fermentation is set up. The hides are afterward stretched over a tanners' beam, and the hair. and scarf skin are removed by shaving with a fleshing knife. At the same time the flesh side is gone over, and any .frag ments of fiber or fat adhering to it are pared away. All traces of lime in the hides must be got rid of, and that some times is effected in the first tan pit, con taining acid liquors weak in tannin, and sometimes by "bating" in "pure"—which is a warm decoction of pigeons' or other fowls' dung. The methot: of actual tan ning varies endlessly, but in general it may be said to ccnsist in suspending or dPpositing in iayers the hides in a suc cessive series of pits containing tan liquor or ooze which is weak at first, but which as the tanning proceeds is made increasingly rich in tannin. To finish the hides they are damped and softened in water, scoured to remove the bloom from their surface, then liberally oiled and the whole surface worked over by pressure with a three-sided steel implement called a striking pin. This operation removes all creases and smooths out and solidifies the leather—an operation carried further and finished after renewed oiling, by rolling the hide on a smooth floor under a heavy hand roller. Morocco leather is a term which now applies rather to the finish of a certain class of goods than to the source of the skin of which it is formed. It is a richly grained and dyed leather, originally and properly made from goat skins tanned in sumach. Sheepskins roughly tanned and un dressed are termed basils; dressed and dyed as for morocco, but finished smooth, they form roans; and split sheep skins (the flesh sides of which go to be sham oyed to form wash leather) tanned and dressed are known as skivers. Russia leather is now any smooth finished thin leather, impregnated with the empyreu matic oil of birch bark, which gives the substance its peculiar odor and insect resisting qualities. Originally it was made in Russia of dressed calf skins.
Tawing.—Tawing consists in dressing skins with certain mineral salts, and is useful principally for glove leathers and the so-called kid leather employed for the uppers of ladies' boots. It is also by tawing. that furriers' skins are prepared, and hides and skins in the hair generally preserved.
Shamoying.—This consists simply in impregnating and saturating skins with oil. The name is derived from the fact that the process was originally applied for the preparation of the skins of the Alpine chamois. Shamoy leather now consists principally of the flesh splits of sheep skins. The oil is worked by means of stock slowly into the interstices of the skin and there becomes oxidized, form ing a kind of combination with the gela tinous constituents, and yielding a pecu liarly soft and spongy texture.