TANNING, the art of manufacturing leather from raw hides and skins. Before we detail the process, it may be proper to observe, that raw hides and skins being composed of minute fibres intersecting each other in every direction, the general operation of tanning consists chiefly in ex panding the pores, and dissolving a sort of greasy substance contained in them ; and then, by means of the astringency and gummy resinous properties of oak bark, to fill and re-unite them, so as to give firmness and durability to the whole tex ture.. But this theory has been contro verted by some chemists, who suppose that the animal jelly contained in the skin. is not dissolved, but unites during the process with the astringent principle of the hark, and forms a combination insolu ble in water.
The process of tanning varies consider ably, not only in different countries, but even in different parts of the same coun try. Theionowing, is the method most approved and practised in London and its vicinity. The leather consists chiefly of three sorts, known by the name of Inittg, or backs, hides, and skins. Butts are ge nerally made from the stoutest and hea viest ox hides, and are managed as fol. lows : After the horns are taken oll; the hides are laid smooth in heaps for one or two days in the summer, and for five or six in the winter : they are then hung on poles, in a close room called a smoke house, in which is kept a smouldering fire of wet tan; this occasions a small degree of putrefaction, by which means the hair is easily gut off; by spreading the hide on a sort of wooden horse or beam, arid scrap ing it with a crooked knife. The hair be ing taken off; the hide is thrown into a pit or pool of water to cleanse it from the dirt,. &c. which being done, the hide is again spread on the wooden beam, and the grease, loose flesh, extraneous filth, &c.carefully scrubbed out or taken off; the hides are then put into a pit of strong liquor called ooze or wooze, prepared in pits called fetches or taps kept for the pur pose, by infusing ground bark in water ; this is termed colouring : after which they are removed into another pit called a scowering, which consists of water strong ly impregnated with vitriolic acid, or with a vegetable acid prepared from rye or bar ley. This operation (which is called rais ing,) by distending the pores of the hides, occasions them more readily to imbibe the ooze, the effect of which is to astringe and condense the fibres, and'give firmness to the leather. The hides are then taken out of the scowering, and spread smooth in a pit commonly filled with water, called a binder, with a quan tity of ground bark strewed between each.
After lying a month or six weeks, they are taken up ; and the decayed bark and liquor being drawn out of the pit, it is fill ed again with strong ooze, when they are put in as before, bark between each hide. They now lie two or three months, at the expiration of which the same ope ration is repeated ; they then remain fonr or five months, when they again undergo the same process ; and after being three months in the last pit, are completely tan ned, unless the hides are so remarkably stout as to want an additional pit or layer.
The whole process requires from eleven to eighteen months, and sometimes two years, according to the substance of the bide, and discretion of the tanner. When taken out of the pit to be dried, they are hung on poles ; and after being compress ed by a steel pin, and beat out smooth by wooden hammers called beetles, the ope ration is complete ; and when thoroughly dry, they are fit for sale. Butts are chief ly used liar the soles of stout shoes. The leather which goes under the denomina tion of hides is generally made from cow hides, or the lighter ox hides, which are thus managed. After the horns are taken off, and the hides washed, they are put into a pit of water saturated with lime, where they remain a few days, when they are taken out, and the hair scraped off on a wooden beam, as before described; they are then washed in a pit or pool of water, and the loose flesh, &c. being taken off, they are removed into a pit of weak ooze, where they are taken up and put down (which is technically termed handling) two or three times a day for the first week : every second or third day they are shifted into a pit of fresh ooze, somewhat stronger than the former ; till, at the end of a month or six weeks, they are put into a strong ooze, in which they are handled once or twice a week with fresh bark for two or three months. They are then removed into another pit, called a layer, in which they are laid smooth, with bark ground ve ry fine, strewed between each hide. After remaining here two or three months, they are generally taken up, when the ooze is drawn out, and the hides put in again with fresh ooze and fresh bark ; where, after lying two or three months more, they are completely tanned, except a few very stout hides, which may require an extra layer : they are then taken out, hung on poles, and being hammered and smoothed by a steel pin, are, when dry, fit for sale. These hides are called crop hides ; they are from ten to eighteen months in tanning, and are used for the soles of shoes.