624 



TANNING MATERIALS 



TANNING MATERIALS 



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 



Fig. 848. Vacuum pans used In making tannin extracts. 



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. 



Sources op Tanning Materials 

 Conifers. 



Hemlock (Tsuga Canadensis). Hemlock bark is still the 

 chief American tanning material. It contains 8 to 14 per 

 cent of catechol tannin. The tree is native from Nova 

 Scotia to Minnesota and Wisconsin, and southward in the 

 Alleghany mountains to Northern Alabama and Georgia. 

 Michigan and Pennsylvania furnish about 60 per cent of 

 all the hemlock bark now secured. The bark is used 

 extensively alone or in combination with oak bark in the 

 production of sole leather. Hemlock leather is harder and 

 less pliable but more permeable to water than oak leather. 

 The total quantity of hemlock bark used in 1905 was 

 1,000,000 cords, worth $8,470,000. An extract is also 

 made of which about 52,000 barrels were used in 1905. 



Western hemlock (Tsuga heterophylla) is found from 

 Alaska to Idaho and Montana, and southward in the 

 Cascade and Coast ranges of Washington, Oregon and 

 California, where it may constitute 13 per cent of the 

 forest growth. The bark contains 8 to 20 per cent of 

 tannin and is somewhat thinner than that of eastern hem- 

 lock. The wood contains less than one per cent of tannin. 



California swamp pine (Pinus muricata) is native along 

 the coast of upper and lower California. The bark con- 

 tains about 13 per cent of tannin. 



Monterey pine (Pinus radiata) is native on the coast 

 of California. The bark contains about 14 per cent of 

 tannin. 



Pine bark is used largely in Austria, Bavaria and 

 southern Germany. Aleppo pine (Pinus Halepensis) con- 

 tains about 15 per cent Of tannin very similar to hemlock. 

 The inner part of the bark is called Snoubar and contains 

 as much as 25 per cent of tannin of lighter color than the 



