Hog or pig skins are imported from Russia and other continental countries, and many are supplied by Scotland; their use is chiefly in the manufacture of saddles for horses, etc.
Walrus and hippopotamus hides are tanned in considerable numbers for the use of cutlers and other workers in steel goods, "buffing-wheels" being made of them, often an inch thick, which are of great importance in giving the polish to metal and horn goods. Lately, belts for driving machinery have successfully been made from them.
Kangaroo-skins of various species are tanned or tawed in Australia, and form a kind of leather in great favor for gentlemen's dress-boots.
The first process in making tanned sole leather is to soak the skins or hides in water for a greater or less time, to wash and soften them; they are then laid in heaps for a short time, and afterwards hung in a heated room, by which means a slight putrefactive decomposition is started, and the hair becomes so loose as to be easily detached. This process of " unhairing" is mostly followed in America; but in Great Britain. milk of lime is used for soaking the hide till the hair loosens. Hides or skins intended for dress ing purposes, such as shoe, coach, harness, or book-binding, after the hair is taken off by the lime, have to be submitted to a process called "bating," for the purpose of reducing the thickening or swelling occasioned by the introduction of the lime, and for cleansing the skin from and other impurities. This is effected by working the skins in a decoction of pigeons' or dogs' dung and warm water, and no dressing-leather is ever submitted to bark or sumac without undergoing this process.
If the old method of tanning is followed, the hides, after unhairing, are placed in the tan-pits, with layers of oak-bark or other tanning materials between them; and when as many layers of hides and bark are arranged as the pit will hold, water is let in, and the hides remain to be acted upon by the tannin.. material for months, and even in some cases for years, being only occasionally turned. But this primitive process is now rarely carried out; so much improvement has been effected in the tanner's art since its chemi cal principles were discovered that much less time suffices; and materials are now used which act so much more quickly than oak-bark alone that, even if the old process is used, it is greatly accele'rated. The most useful of these materials are catechu and
cutch (of which 9,000 tons are annually imported into Great Britain from India and Singa pore), gambler (about 1200 tons from Singapore), divi-divi (3,000 tens from Maracaibo, etc.), valonia (the acorns of the Quercus lEgylops, 25,000 tons of which are yearly imported from Turkey), and sumac leaves (16,000 tons, chiefly from Turkey).
The first attempts at improvement in tanning were the method invented by Mr. Spilsbury in 1823, and the improvement on this method by Mr. Drake, of Bedminster, in 1831. . The principle consisted in causing the ooze or tan-liquor to filter through the hides under pressure. For this purpose, in Drake's process, the edges of the hides were sewed up so as to form a bag. The bags being suspended, were tilled with cold tan liquor, which gradually filtered through the pores of the hides, and impregnated them with the tannin. The processes by infiltration, however, have been entirely abandoned for heavy leather, as they have the, effect of rendering the leather porous and deficient in firmness.
Various patents for improvements in tanning have been in operation of late years. Herepath Cox, of Bristol, tied hides to each other to form a long belt, and pressed them between rollers to squeeze out the partially exhausted tan-liquor from the pores, so that a stronger liquor might be absorbed. Messrs. J. & G. Cox, of Gorgie near Edinburgh, made an improvement on this mode, by attaching the bides to a revolv ing drum, so that the hides press on each other on the top of the drum, but hang sus pended in the tan liquor from the lower part; and thus, by the hides being alternately in and out of the liquor, the tanning is quickly effected.
After the hides have become thoroughly tanned in the pit by the action of the tannic acid upon their gelatinous substance, and when partly dried (if for "struck" sole leather), they are operated upon by a two-handled tool with three blunt edges, called a pin, which, by being rubbed with great pressure backwards and forwards on the grain side of the leather, makes it more and more compact; and this is still further accom plished by submitting the leather to the action of a heavily loaded brass roller.