TANNING MATERIALS. Figs. Nearly all plants contain an astringent princi ple known as tannin, which is distinguished by its property of forming with proteid matter, such as animal skins, an insoluble compound called leather, which is strong, flexible, and resistant to wear. Because of the many uses of leather, there is great demand for large quantities of tannin with which to make it. While many plants contain tannin in considerable quantities, practically all the tannin is secured from a few, and these few are as a rule those from which the tannin can be secured in commercial quantities most economically. In this country, bark of hemlock, chestnut or rock oak has been and still is the chief source of tannin, but as the supplies of these barks are exhausted and are farther removed from the tannery, other materials are used in constantly greater quanti ties, particularly in eastern tanneries where sup plies of bark are most difficult to secure. This con dition has encouraged the importation of foreign materials and the use of extracts which can be made where the tanning materials grow, and trans ported to the tannery much cheaper than the raw materials. In addition to hemlock and oak bark, the use of chestnut wood, quebracho, palmetto, mangrove and sumac extracts, as well as of other materials, is rapidly increasing, so that products at present but little known or used are men tioned in the following list, as the time is fast approaching when many of them will be used in considerable quantities. Very few plants are cultivated for their tannin and, with the possible exception of canaigre, none are cultivated in the United States.
Tanning extracts.
Until within recent years all tanneries prepared their own tanning liquors directly from the raw materials, each having its own leach house and maintaining immense ricks of bark. With rapidly decreasing supplies of bark and other native tan ning materials, this is no longer possible in some of the older settled parts of the country, and many tanneries rely in part or entirely on extracts which are simply tanning liquors made where the raw material is still accessible and cheap, and con centrated to a small bulk for the sake of economy in handling and transportation. The most com
monly used extracts produced in this country are made of chestnut oak and hemlock barks, chest nut and quebracho woods, sumac, palmetto and mangrove.
In preparing liquors or extracts, the material must first be ground ; how fine is largely deter mined by experience, the aim being to secure the maximum quantity of tannin at the lowest cost. If the material is too finely ground it will pack in the leaches and extraction will be too slow to be economical. The ground material is carried by conveyors to the leaches, which are large round wooden vats about fourteen feet high and fourteen feet in diameter, each holding about ten tons of bark. These leaches are provided with a false per forated bottom through which the tanning liquor can pass, an opening in the bottom through whick the exhausted material passes in emptying the leach, and which, when the leach is working, is closed with a long plug reaching to the top of the leach. Each leach also has a vertical spout rising from under the false bottom to near the top and connected with the next adjoining leach, so that the liquor from the bottom of one leach may pass to the top of the next succeeding one. The leaches are arranged in the form of a battery, and six to four teen leaches are used. In extracting the tanning material, very hot water is run on the top of the material in a leach which has previously been nearly exhausted of its tannin. This is known as the "tail leach." From the bottom of the tail leach the liquor passes to the top of the next leach, the material in which has not been so com pletely exhausted as that in the tail leach. The liquor passes successively from the bottom of one to the top of the next leach, each containing material less exhausted than that in the previous leach, until it passes on to the "head leach," con taining material from which no tannin has been removed.