THE PITS.
Leather is a substance universally used amongst civilized and very generally amongst barbarous nations; it is made from the skins of animals, which are tanned, or prepared with some substance, having the power of converting the perishable skin, that decays readily when wet or moist, into a lasting and comparatively imperishable leather.
The preparation of skins by tanning or other similar processes has been practised from the earliest times; and although it has engaged the attention of several scientific men, and has been the subject of many curious experiments, it has received less alteration from recent improvements in chemical science than many other manufacturing processes. Several plans, which have been suggested with a view to expediting the process, which on the old system is a very tedious one, have been found to injure the quality of the leather, and have therefore been wholly or partially abandoned; and others, which appear to be more successful, are as yet adopted by a few manufacturers only.
The larger and heavier skins operated upon by the Tanner, as those of bulls, buffaloes, oxen, and cows, are technically distinguished 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 first be 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 decomposition. 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 that may adhere to the inner skin; 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 smoke-house, heated a little above the ordinary temperature of the atmosphere by means of a smouldering fire, in which case the epidermis is loosened by a very slight 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.
Unhairing the Hides. Striking the Hides.
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.
Oak-bark is the substance most commonly used to supply the astringent principle, and it is crushed or ground to powder in a bark-mill. 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 refilled 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 improvement, introduced in consequence of the experiments of M. Seguin, a French chemist, of tanning with concentrated solutions of bark, formed by passing water through a mass of powdered bark, until, by successive filtrations, it is completely deprived of its soluble tanning principle.
