TANNING, broadly speaking, the art of converting the skins of animals into leather. The skin of most of the higher animals con sists from the tanner's point of view of two layers, the outer containing coloring matter, the roots of the hair or fur, and being cellular in structure; the inner being thicker and of fibrous structure. The outer layer is decom posed much more easily than the inner by the action of alkalis; the latter is only soluble in water after protracted boiling, yielding a solu tion which gelatinizes upbn cooling. Moist skin undergoes putrefaction when exposed to the air for some time. Dried skin is hard and brittle. In preparing leather the object of the tanner is, in the first place, to remove the outer layer of the skin together with all adhering hair, and, in the second place, to bring about such a change in the under layer as shall prevent it from putrefying in moist air, and at the same time render it indifferent to moisture, without, however, altering its tenacity and sup pleness. The process of tanning, therefore, divides itself into two parts: (1) Cleansing the skin and removing the outer layer; (2) con verting the inner layer into leather. Techni tally only the second part of this process merits the title of tanning. As supplementary to these there is involved the process of dressing and currying the leather. The skins used by the tanner are principally those of cattle; but the skins of horses, asses, pigs, goats, dogs, alligators and many other animals are also converted into leather. The quality of the hide varies in different species of animals, and also in the same species, depending upon the quality and amount of the food consumed, and to a still greater degree upon the vicissitudes of climate in which they are reared. Wild cattle are said to furnish hides superior to those of domestic cattle.
In the first stage of the process the skins, having been thoroughly washed and trimmed of tails, shanks and pates are soaked in water until they are sufficiently soft to -allow of the adhering flesh and muscle being scraped off by means of a blunt knife; this softening process is generally aided by beating the hides with hammers or sticks worked by machinery. It is
of great importance that the water be soft, or if necessary softened with borax. If the hides are green, 24 hours are allowed for the first soaking, and an equal period for a second soaking in a fresh bath. They are usually halved lengthwise between the two soakings. Dry hides are soaked 24 hours in water con taining sodium sulphide, then halved and run through a dry mill for nearly an hour and then stacked up in piles for another 24 hours. They are then put back into the same bath for 24 hours more, again milled, then fleshed and put into clean, cold water overnight. If the soaking is too prolonged the skin cannot be made into good leather. The hides are now generally placed in pits with milk of lime. whereby the hair and upper layer of skin is gradually loosened. This operation requires about six days, the hides being changed daily to a fresh lime bath. They are then again subjected to the action of the dressing knife. The final process preparatory to tanning con sists in bating the hides in a very dilute acid liquid in which a mild putrefactive fermenta tation is going on. This bath is commonly made with lactic acid and glucose, the latter furnishing the fermentative ingredient. In this bath, which continues for six days, the lime is entirely removed and the hides are con siderably softened and swollen. The pre pared hides may now be tanned — that is, en abled to withstand putrefaction without loss of suppleness—by the action of different materials. These materials may he broadly grouped as (1) tannin, (2) metallic salts, (3) oily matters. When tannin is used the process is always called tanning. When metallic salts are used the process is in some localities called tawing; and when oily matters are used, shamoying or oil-tawing.