From the head leach the strong tanning liquors run to the settling cooler where much suspended matter as well as that which is insoluble in cold water settles out, or the liquors are carried imme diately to the evaporating pans to be concentrated. In a long battery of leaches it is customary to pump the liquors from one leach to another and often to reheat them at least once. To avoid repeatedly reheating the liquors between the leaches, a copper coil is often placed in the bottom of each, and the contents heated by steam.
It is customary partly to decolorize extracts. For this purpose dried blood is chiefly used, though blood albumen, casein, and other albuminous ma terials, as well as lead acetate and salts of alumina, are used to a certain extent. In decolorizing, the dilute liquor from which the suspended matter has settled out is run into a vat provided with a stir ring gear and steam coil, and to the liquor the decolorizing material dissolved in a little water is added and the whole well stirred. The temperature is raised to 70° C., when the albumen coagulates and carries down part of the coloring matter with it. The solution is allowed to settle in another tank, the clear liquor drawn off and sent to the vacuum pans, and the sediment filter pressed to recover the remainder of the liquor as well as the tannin-blood compound which it contains and which is used as a fertilizer. Tanning liquors may also be decolorized, or rather bleached, by passing sulfur dioxid through them before concentrating. The color thus temporarily removed is likely to return.
The material that goes out of solution when the dilute liquor is cooled in the settling tanks con sists largely of tannin which is difficultly soluble, but is capable of tanning leather. After being decolorized, or directly from the leaches, the liquor passes to the vacuum pans where it is concentrated to about 45° Twaddle, for liquid, or until the extract will solidify on cooling, for solid extracts. To avoid excess of color and the destruction of tannin the concentration is done at low tempera ture and without access of air. Liquid extract is sold in barrels or in tank-cars, the solid extract in bags or bales.
Future tanning materials.
The native tan-barks of the eastern and northern part of the United •States are rapidly decreasing under a heavy demand, which amounted to 1,425, 000 cords in 1905, and it is only a question of com paratively few years when a large part of the supply must come from other sources. There are three ways in which the material may be supplied, and doubtless all of them will contribute a part. They are : (1) larger use of foreign and little-used materials ; (2) more careful handling of tan-bark trees ; and (3) cultivation of tannin-containing plants as regular farm crops.
The growing of plants primarily for the tannin they contain will probably develop slowly, because other crops pay better. For this reason canaigre has failed in the South and West. So, too, the growing of woods or of barks rich in tannin, except on land that cannot be otherwise regularly cropped, does not promise at present to be a profit able undertaking. At present the most promising plant for cultivation is sumac, which may be planted, cultivated and harvested by machinery and handled in much the same way as other farm crops. Its cultivation is conducted successfully in Italy, where labor is much cheaper than it is here, but it remains to be demonstrated that sumac can compete with other farm crops under conditions in this country. On lands not suitable for general agriculture chestnut wood and chestnut oak bark may be grown or allowed to reproduce profitably within a period of twenty to thirty years. It is probable, however, that the price of raw tanning materials must rise considerably before their culti vation will develop to any extent.
Wild-grown materials will undoubtedly continue to be the almost exclusive source of tannin, but to meet the demand many materials but little used will be developed, and more care be exercised in gathering and marketing all kinds of tanning materials now used.
Literature.
Davis, Manufacture of Leather, Philadelphia ; Proctor, The Principles of Leather Manufacture, London, 1903 ; Fleming, Practical Tanning, Phila delphia, 1903 ; Modern American Tanning, Chicago, 1905 ; Watts, Leather Manufacture, London, 1906.