BIRCH BARK, from the white birch, _betula alba_, is another aromatic tanning material. It contains only about 5 per cent. of tannin, and is, therefore, generally used with other tanning materials. It contains a tar which imparts an agreeable scent to the leather that protects it from the ravages of insects. In conjunction with willow bark (_salix arenaria_) it is used in the tanning process for the real Russia leather. An oil containing the scent can be extracted from the birch bark by dry distillation, and this extract is sometimes used during the dyeing process, in the manufacture of imitation Russia leather, which, however, only retains the scent for a few months, whereas the real Russia leather has a permanent odour. CANAIGRE (_rumex hymenosepalum_) is the tuberous root of a dock plant indigenous to Mexico and the Southern States of America. It is fairly rich in tannin (25-30 per cent.) and yields a moderately firm leather. It contains too large a proportion of starch, however, and cannot be described as a really satisfactory tannin. Moreover, it is not harvested economically and the only way to make a satisfactory tannin of it is to convert it into extract and remove the starchy matters near the source of supply, if anyone dare take the risk of establishing a factory in Mexico. BABLAH or BABOOL (_acacia arabica_ or _acacia vera_) grows in India, Egypt, and the Sudan. The bark of the babool tree is one of the principal tanning materials used in India for hides, calf, and sheep skins, which are sent in large quantities to Great Britain in a rough-tanned state and dressed there. It contains 15-20 per cent. of tannin, which readily oxidises in the leather in contact with light, turning into a bright pink colour. It also seems to weaken the fibres of animal tissues, and, for that reason, babool-tanned leather was condemned by the Society of Arts Commission on Bookbinding. The pods contain from 20-30 per cent. of tannin and give a mellow and plump leather similar to that produced by gambier. The bleaching of the material is troublesome, and it is probable that its use would be increased if the tannin were prepared in the form of extract. CUTCH is a crude extract made from the Indian tree _acacia catechu_. This is the real cutch, as distinguished from the mangrove "cutch." It is very rich in tannin (50-60 per cent.), but contains a large proportion of insoluble matter and is, therefore, very little used for tanning. It is well adapted for the dyeing of dark colours or black with mineral strikers, such as chrome and iron salts; but its chief use is for tanning fishermen's nets, which it renders waterproof.