1. The art of tanning consists in converting hides and skins into leather, by impregnating them with astringent matter.
2. It is impossible to determine the period at which the art of tanning was discovered. It was doubtless known to the ancients, and probably to the antediluvians, in some degree of perfection; since skins were applied as means of clothing the human body, before the arts of spinning and weaving were practised. It is likely, however, that they were applied to this purpose, for a considerable time, in their natural state; and that accident, at length, suggested the means of rendering them more applicable, by saturating them with certain mineral or vegetable substances.
3. Although the art of converting skins into leather was practised in remote ages, yet it was not until near the end of the eighteenth century, that the true principle of the process was understood. Before this time, it was supposed, that the astringent principle of the agents employed, was a resinous substance, which adhered mechanically to the fibres, and thus rendered them firm and insoluble. The correct explanation was first given by Deyeux, and afterwards more fully developed by M. Seguin. These chemists clearly proved, that the formation of leather was the result of a chemical union between a substance called tannin, and the gelatinous part of the skin.
4. The subject, however, was not thoroughly understood, and reduced to scientific principles, until the year 1803, when Sir Humphrey Davy gave it a careful investigation, in a series of chemical experiments. These inquiries resulted in the conviction, that the method of tanning which had been in general use, may, with a few alterations, be considered preferable to that by which the process is carried on with more rapidity.
5. The skin which envelopes the bodies of animals, consists of three layers. That on the outside is a thin, white, elastic membrane, called the cuticle, or scarf skin; that on the inside is a strong membrane, denominated the cutis, or true skin; between these two is a very thin membrane, to which anatomists have given the name rete mucosum, and in which is situated the substance which gives color to the animal. The cutis is composed of fibres, which run in every direction, and, being by far the thickest layer, is the one that is converted into leather.
6. The skins of large animals, such as those of the ox and horse, are denominated hides; and those of smaller animals, as of the calf, goat, and sheep, are called skins. Of the former description, is made thick, of the latter, thin leather. The process of tanning different skins varies in many particulars, according to the nature of the leather, and the uses to which it is to be applied.
7. The general process of changing thick hides into sole-leather, is as follows: They are first soaked in water, to free them from dirt and blood; and then, if rigid, they are beaten and rubbed, or rolled under a large stone, to render them pliable. They are next soaked in lime-water, or hung up in a warm room, and smoked, until a slight putrescency takes place. The hair, cuticle, rete mucosum, on one side, and the fleshy parts on the other, are then scraped off, on a beam, with a circular knife.
8. Nothing now remains but the cutis, or true skin. Several hides, in this state of preparation, are put together into a vat, for the purpose of impregnating them with tannin. This substance is found in astringent vegetables, and is obtained, in a proper state for application, by infusion in water. In that condition, it is called ooze, which is first applied in a weak state.
9. After the ooze, of different degrees of strength, has been renewed several times, they are put between layers of bark, and suffered to remain several months, fresh bark, from time to time, being supplied. The whole process generally occupies from twelve to sixteen months. When strong solutions of tannin are used, the leather is formed in a much shorter time; but, in that case, it is much more rigid, and more liable to crack. It is rendered smooth and compact, by beating it with a wooden beetle, or by passing it between rollers.
10. Oak bark, on account of its cheapness, and the quantity of tannin which it contains, is more extensively employed by tanners than any other vegetable substance. In sections of country, where this kind cannot be conveniently obtained, the bark of the hemlock, spruce, and chestnut, the leaves of the sumach, and various other astringents, are substituted.