Tanning.—The larger and heavier skins operated upon by the tanner, as those of bulls, buffaloes, oxen, and cows, are technically distin guished as hides ; while the name skins is applied to those of smaller animals, as calves, sheep, and goats. The process necessary to convert hides into the thick hard leather used for the soles of boots and shoes, and for similar purposes, will be first noticed. The hides are brought to the tanner either in a fresh state, when from animals recently slaughtered, or, when imported from other countries, dried or salted, and sometimes both, for the sake of preserving them from decomposi tion. In the former case the horns are removed, and the hide is scraped to cleanse'it from any small portions of flesh or fatty matter which may adhere to the cutis ; but in the latter it is necessary to soften the hides, and bring them as nearly as possible to the fresh state, by steeping them in water, and repeated rubbing or beating. After this the hair is removed ; sometimes by steeping the hides for several days in a solution of lime and water, which has the effect of loosening the hair and epidermis, or outer skin ; and sometimes by suspending them in a close chamber called a amoke-house, heated a little above the ordinary temperature bf the atmosphere by means of a smouldering fire ; in which ease the epidermis becomes loosened by incipient putrefaction. In either case, when the hair and epidermis, or cuticle, are sufficiently loosened, they are removed by scraping with a curved knife, the hide being laid upon a' convex bench, or ' beam: The hides are prepared for the actual tanning, or immersion in a solution of bark, by steeping them for a few days in a pit containing a sour solution of rye or barley flour, or in a very weak menstruum consisting of one part of sulphuric acid mixed with from five hundred to a thousand parts of water. By this process, which is called ' raising,' the pores of the hides are distended and rendered more susceptible of the action of the tan, and the substance of the skin is apparently increased ; but, as the process does not add to the gelatin of the akin, a hide which is much thickened by the raising process loses its substance when condensed by the shoemaker's hammer.
Different tanners vary very much in the above processes. Some believe that much of the original gelatin of the hides is never com bined with the tannin, but is wasted, owing to defective modes of liming, unhairing, and cleansing. The German tanners often adopt a process called wearing, by laying the hides in a pack or pile, and covering them with spent tan to produce a fermentative heat. On these points much remains yet to be discovered. There are also many variations in practice, in relation to the actual tanning, or conversion of the prepared pelt into leather. Oak bark, crushed or ground to a coarse powder in a bark-mill, is the substance most commonly used to supply the astringent principle. 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 greatly expedited by the improvements introduced in consequence of the experiments of M. Seguin, a French chemist;
nevertheless it may be well to observe that skins which are quickly tanned, by the use of strong solutions, produce leather of less durable quality than that which is slowly formed : 100 lbs. of skin, quickly tanned in a strong solution of bark, will produce 137 lbs of leather, while the same weight of skin, slowly tanned in a weak solution, produces only 1174 lbs. ; the additional 194 lbs. in the former case tending to swell the tanner's bill, although it deteriorates the leather, and causes it to contain less of the animal solid.
• The variations of practice among different tanners extend to the substance used as an astringent, as well as to the manner of applying it. Ground oak-bark, which was formerly the only material in common use, and is still the most general, produces good leather of a light fawn colour. Valonia 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, pro duces leather of a dark reddish fawn colour, which is light, spongy, and very pervions 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. In the methods of preparing tanning solutions there is also considerable variety. Some tanners use cold water for the purpose, and others hot water or steam ; others again, instead of pure water, employ ooze, or tanning liquid which has been exhausted by use. A further point of difference is found in the strength of the solutions used, which vary exceedingly. When the im pregnation of the hides with tannin is effected by laying them flat in the tan-pits, they are frequently taken out to renew the solution • and the skins which have lain near the top of one.pit are laid near the bottom of the next, so as to equalise the amount of hydrostatic pressure. Some times the tanning is facilitated by suspending the skins vertically in the liquid, by which means they are penetrated quickly ; but the plan requires considerable room, and, unless the skins are frequently'moved, it occasions injurious folds in the leather. Another plan, which answers well for small light thins that require but a short time for tanning, is to sew up each skin into the form of a bag, to fill it with tan-liquor, and then immerse it in the pit. The great space required is the principal objection to this plan. In whichever of the above ways the tanning is effected, the hide is subjected 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 compressed by beating or rubbing, or by passing them between rollers, to give them firmness and density. A yellow deposit is now found upon the surface of the leather, to which the name of bloom or pitching is technically given; and, although this deposit is subsequently removed by the shoemaker in the operation of buffing, and forms a useless addition to the weight and cost of the leather, the prejudice of purchasers requires that it be left on the surface by the tanner.