The skins of animals, combined in a variety of ways with astringent and other matters, to adapt them to numerous purposes of utility. The art of preparing leather is very ancient, and is practised in almost every country of the world by nearly similar processes. The objects obtained by this art, are, the prevention of their destruction by putrefaction; the rendering them strong, tough, durable, and impervious to moisture; and in giving them a bright and beautiful appearance by dying and polishing; according as these qualities may be required. The preliminary operation in making all kinds of leather, is the separation of the fleshy and other foreign matters adhering to the skin, the animal juices retained in its pores, and also the cuticle with its hairy covering, excepting in those instances wherein the wool is required to be left on, as in the case of sheep-skin rugs. The skins, after being duly purified, and their texture opened so as to adapt them to imbibe other matters in solution, are made into leather by two different processes, one called tanning, and the other tawing; and both these processes are sometimes combined in sheep, goat, and deer skins, by tawing first and tanning afterwards, in a slight manner; and a large proportion of the tanned hides of the horse, ox, and other large animals, undergo an operation called currying, to render it flexible, and resist water.
There are many trifling variations in the processes adopted by different tanners and leather-dressers with respect to the same kind of skins, and each kind is treated differently in some respect, either in consequence of its natural peculiarities, or the application to which it is designed when finished. Our descriptions will, therefore, apply to the general mode of proceeding in the principal sorts of leather.
The thin skins of cows, calves, and others of a similar texture, are soaked for two or three days in a pit of water to free them from dirt, blood, and other matters that may slightly adhere to them. They are then taken out, and laid upon a horse or beam, (which is usually a semi-cylindrical piece of timber, or the rib of a whale,) whereon they are scraped and pared, to free them from any adhering flesh, fat, etc. The hides are next immersed in a pit containing milk of lime, wherein they are frequently stirred, and are allowed to remain until the cuticle of the skin is so far destroyed as to be easily rubbed or pared off along with the hair to which it is connected. When this is found to be the case, they are taken out, stretched upon the beam, and with a large two-handled blunt-edged knife the workman scrapes off the hair. In lieu of this liming process, in some places, the hides were formerly piled wet one upon another, and covered over with spent bark, (or otherwise kept warm in what was called a smoke-house,) until the cuticle and the hair would readily come off.
The absorption of lime in the before-mentioned process makes the skins hard and thick; to render them supple, and prepare them for receiving the tan liquor, they are thrown into a pit called the poke, or mastering-pit, which contains a quantity of putrescent dung diffused in water: the dung of dogs, pigeons, or sea-fowl, is preferred for this purpose, that from cows and horses not being sufficiently powerful. During the process they are frequently well stirred, and sometimes taken out of the pit, piled up, and put in again. When the skins have become perfectly soft, they are taken out of the putrescent pit, and cleansed on the beam, when they are ready for tanning. I he large thick hides of the ox or boar, intended for the toughest sole-leather, being not so liable to sudden injury as the thinner skins, are frequently cleared of their hair and other matter without resorting to the liming process. They are allowed to ferment, piled up in a warm place, and the putrefactive process is carried farther, that the cuticle and hair may be easily removed.
When this has been done, they are immersed for several days in sour liquor, made from fermented barley, or rye meal; the acid is generated in the process, and seems to be the active agent in softening and opening the texture of the skin, assisted by the continuance of the fermentation, of which the skin partakes. This process, which always precedes that of tanning, is called raising, as it has the effect of considerably swelling the skin. Instead of the foregoing acid, some tanners use very dilute sulphuric acid, in the proportion of about four pounds to a hundred gallons of water.
The process of tanning is essentially the same in all skins. It consists merely in immersing the skin for a sufficient length of time in an infusion of oak bark, or other vegetable astringent, until it is completely saturated with it. Hence the art of preserving the hides of animals by this method is one of the most ancient and universal of all manufactures, no apparatus whatever being required to perform it, except a pit or hole for water, in which the tanning vegetable may be put, and the skin thrown in along with it. Almost equal simplicity is observed in the most improved methods of tanning, the art mainly consisting in judiciously regulating the strength of the tanning infusion, and in the manipulation of stirring the hides in such a manner, that all that are in a pit may be equally impregnated.
The substance used in this country is chiefly oak bark, which is ground into a coarse powder, and is thrown into pits with water, by which an infusion of the tan, and other soluble parts, is made, which is technically called ooze. The hides, (previously prepared in one or other of the ways before mentioned,) are first put into small pits, with a very weak ooze, where they are allowed to macerate for some weeks, with frequently stirring, or handling, as it is termed. As the process of tanning proceeds, the strength of the different oozes is gradually increased, after which, the half-tanned hides, (if of the thick kind, intended for sole leather, and which require very complete tanning,) are put into larger pits, with alternate layers of ground bark, in substance, till the pit is filled, over which a heading of bark is also laid, and the interstices filled up with a weak ooze to the brim. The hides are by this arrangement supplied with a quantity of fresh tan in proportion as they absorb the tan, previously dissolved in the water.