LEATHER. An insoluble compound of tannin and gelatine. Tannin is a very delicate test of gelatine, and if added to water containing the one five-thousandth part of gelatine it produces a cloudiness in it. It combines so energetically with gelatine, that if a piece of skin or bladder of sufficient size be suspended in a solu tion of tannin the whole of it is in time abstracted from the water, and the skin becomes converted into leather.
In the manufacture of leather the skins of animals are first steeped in lime water, in order that the hair and cuticle may be more easily separated, and then softened by allowing them to enter into a: certain state of putrefaction, or by the action of acids. They are then immersed in an infusion of oak bark, which contains tannin, and in time converts them into le,ather. The process is continued sometimes for several months, the infusion being renewed from time to time, and is not arrested until the entire substance of the skin has become brown throughout. Sometimes infusions of other astringent veget,
ables containing tannin, such as valonia, sumach, catechu, and the barks of the willow, birch, elm, and Spanish chesnut, are substituted for oak bark.
This is ordinary tanning : but there are other kinds of leather, as follow : Toyed leather. An insoluble compound of gelatine and chloride of aluminium. The skin is first steeped in an alkaline liquor, then in a solution of alum and common salt, which react on each other and produce chloride of aluminium and sulphate of soda. White glove leather is prepared in this way.
Wash leather. The skin is prepared and softened in the usual way, then steeped in oil, and afterwards in an alkaline solution.
Curried leather. Common tanned leather while still moist is besmeared with oil, which as the water evaporates, takes its place in the pores of the leather and renders it waterproof. This is also called " upper-leather."