TAN′NIN. See Tannic acid.
TAN′NING. When the skin of an animal, carefully deprived of hair, fat, and other impurities, is immersed in a dilute solution of tannic acid, the gelatin gradually combines with that substance as it penetrates inwards, forming a perfectly insoluble compound, which resists putrefaction completely; this is tanned leather. In practice, lime water is used for cleansing and preparing the skin, water acidulated with oil of vitriol for ‘raising’ or opening the pores, and an infusion of oak bark, or sometimes of catechu, or other astringent matter, as the source of tannic acid. The process itself is necessarily a slow one, as dilute solutions only can be safely used. Skins intended for the curriers, to be dressed for ‘uppers,’ commonly require about 3 weeks; and ‘thick hides,’ from 12 to 18 months.
Of late years various ingenious contrivances have been adopted, with more or less success, to hasten the process of tanning skins and hides. Among these may be mentioned the employment of stronger tan solutions, the application of a gentle heat, puncturing the skins to afford more ready access for the liquid to their interior parts, and maceration in the tan liquor under pressure, either at once or after the vessel containing them has been exhausted of air by means of an air-pump. On the merit of these several methods it has been remarked “that the saturated infusions of astringent barks contain much less extractive matter, in proportion to their tannin, than the weak infusions; and when the skins are quickly tanned in the former, common experience shows that it produces leather which is less durable than leather slowly formed.” (Sir H. Davy.) “100 lbs. of skin, quickly tanned in a strong infusion of bark, produce 137 lbs. of leather; while 100 lbs., slowly tanned in a weak infusion, produce only 1171⁄2 lbs.” “Leather thus highly (and hastily) charged with tannin is, moreover, so spongy as to allow moisture to pass readily through its pores, to the great discomfort and danger of persons wearing shoes made of it.” (Ure.)
According to Mr G. Lee, much of the original gelatin of the skin is wasted in the preliminary processes to which they are subjected, more especially the ‘liming’ and ‘bating,’
He says, that 100 lbs. of perfectly dry hide, cleaned from extraneous matter, should, on chemical principles, afford at least 180 lbs. of leather.
Morocco leather is prepared from goat or sheep skins, which, after the action of lime water and a dung bath, are slightly tanned in a bath of sumach. They are subsequently grained, polished, &c.
Russia leather is generally tanned with a decoction of willow bark, after which it is dyed, and curried with the empyreumatic oil of the birch tree. It is the last substance which imparts to this leather its peculiar odour and power of resisting mould and damp. See Leather, Tannic acid, Tawing, &c.
TANTALIC ACID. Syn. Tantalic anhydride; Columbic acid. Rose believed this substance to be a dioxide, to which he gave the formula TaO2; but the subsequent researches of Marignac, and the crystalline form of potassic tantalic fluoride 2KF, TaF5, seem to show that it is to be regarded rather as Ta2O5.
TAN′TALUM. Ta. Syn. Columbium. A rare metal discovered by Mr Hatchett, in 1801, in a mineral from Massachusetts; and by M. Ekeberg, in 1803, in tantalite, a mineral found in Sweden. It exists in most of its ores in combination with oxygen.
TAPEWORM. See Worms.