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.
Commercial tannic acid, used for medicinal purposes, is prepared from galls or excrescences on oak trees growing in Asia Minor. These galls are caused by an insect (cynips) puncturing the small branches and producing abnormal growth in the perforated parts. The acid is gallotannic, which, if used for making leather, would produce a soft, spongy, and nearly white leather. This tanning material is used a little by Near Eastern tanners, but the result is unsatisfactory.
Classification of Tanning Materials
Tanning materials are divided into two main classes: (1) Pyrogallol, (2) Catechol. The pyrogallol tannins give a bluish-black colour, and the catechol tannins a greenish-black, with iron salts. Bromine water does not precipitate pyrogallols, but causes a precipitate with catechols. Pyrogallols yield ellagic acid (called "bloom" in the trade), which improves the waterproof qualities of leather. On the other hand, catechols contain a large proportion of insoluble reds, or phlobaphenes, which are deposited between the fibres and thus give solidity to the leather. Pyrogallol tannins give a light coloured, soft leather, and can be used alone satisfactorily; but heavy leathers, such as sole and belting, need a blend of both kinds of tannin. The pyrogallol tannins include sumach, chestnut, myrobalans, divi-divi, oakwood, algarobilla, chestnut oak, willow, and galls. The catechols include quebracho, gambier, hemlock, mimosa or wattle, mangrove, larch, birch, canaigre, and cutch. Oak bark and valonia contain some of the properties of both classes of tannins.
There are sub-divisions of these two classes, arranged according to chemical tests, by which one tanning material can be distinguished from another.
Tanning Extracts
The manufacture of extracts from vegetable tanning materials has increased so rapidly of late years that the process of tanning has undergone radical changes; and, whereas the tanner was limited to three or four materials thirty years ago, he now has the choice of about twenty good materials. These tannins can be suitably blended to produce first-class leather. The processes in extract manufacture are few. The wood, bark, or fruit is broken up into small pieces and macerated in hot or cold water. The concentration of the liquid is done in vacuum, or in an evaporator. The latest method is to treat the tan liquor in a "triple-effect" evaporator (Fig. [4]), the object of using three compartments being to economise in steam. This apparatus makes liquid extracts; where a powdered or crystal extract is required, the liquid is afterwards treated in a vacuum drying apparatus. Some extracts contain a lot of colouring matter and insoluble substances. To overcome this defect, they are clarified with blood albumen, sulphites, casein, or acetate of lead.
