LEATHER MANUFACTURE. This re markable substance, which is universally em ployed throughout the civilised world, is prepared from the skins of animals, or, it would perhaps be more correct to say, consists of that substance after it has been chemically changed by the process of tanning. This change is effected by means of a sub stance residing in several vegetable mat ters, to which the name of tannin has been given. When this tannin, which is soluble in water, is applied to the hides of animals from which the hair, epidermis, and any fleshy or fatty parts adhering to them are removed, and which hides then consist wholly of gelatin, also soluble in water, these two soluble sub stances so unite chemically as to form the wholly insoluble substance called leather. Of the ox-hides which are converted into leather, those supplied by bulls are thicker, stronger, and coarser in the grain than those of cows ; while the hides of bullocks are intermediate between those of the bull and the cow. Such leather is employed for the soles of boots and shoes ; for most parts of harness and sadlery ; for making leather trunks, buckets, hose for fire-engines, and pump-valves ; for the thick belts used in military accoutrements ; and for the gloves of cavalry. The thick buff-leather, formerly used as armour, and which was pistol proof, and would resist the edge of a sword, was made from the hide of the urus or wild bull. The skins of calves, though thinner than those of cows, are thicker than most other kinds of skin which are converted into leather.
The process necessary to convert hides into the thick hard leather used for the soles of boots and shoes is as follows :—Tho horns are removed from the hides ; and the latter are scraped, steeped, and sweated, and the hair removed. The hides are then immersed for a few days in a liquid which opens the pores and fits them for the action of the tan ning ingredients. In the old method of tanning, which is not yet entirely abandoned, the hides and powdered bark were laid in alternate layers in the tan-pit, which was then filled with water to the brim. After some
months the pit was emptied and re-filled with fresh bark and water, and this process was repeated whenever the strength of the bark was exhausted. In this way the time required for impregnating the hides varied, according to their thickness and other circumstances, from one to four years. The process has been expedited by the use of a concentrated solo. tion of bark instead of mere layers of bark in water. The variations of practice among dif ferent tanners extend to the substance used as an astringent, as well as to the manner of applying it. Ground oak-bark, which was for merly the only material in common use, and is still the most general, produces good leather of a light fawn-colour. Valonia, of which considerable quantities are imported for the use of tanners, produces leather of great solidity and weight, the colour of which is inclined to gray, and which is more impervious to water than that made with oak-bark. Catechu, or Terra Japonica, the inspissated extract of the Acacia Catechu, produces lea ther of a dark reddish fawn colour, which is light, spongy, and very pervious to water. Another substance which has been used of late years is a kind of bean-pod called divi divi. These substances may be used either individually or in various combinations; and they are prepared with plain water or with ooze, with hot water or with cold, according to the judgment of the tanner. In whichever way the tanning is effected, the hide is sub jected to the action of solutions increasing progressively in strength, until it is so perfectly penetrated, that when cut through it presents a uniform brown colour; any appearance of a light streak in the middle of its thickness being an indication of imperfect tanning. When the process is complete, the hides are hung up in a shed, and allowed to dry slowly, and, while they are drying, they are compres sed by heating or rubbing, or by passing them between rollers, to give them firmness and density.