SWEET-POTATO 



TANNING MATERIALS 



623 



control this pest. The larva is of a green color and 

 bears a dark stripe along the middle of the back. 

 Several kinds of tortoise beetles feed on the leaves 

 soon after the plants are set. As a protection the 

 plants may be dipped before setting in a solution of 

 arsenate of lead, one pound to twenty-five gallons 

 of water. Paris green of a strength of one-fourth 

 pound to forty gallons of water, to which is added 

 one-fourth pound of lime, is also effective. Flea- 

 beetles may be controlled by the arsenical treat- 

 ment, and sawflies by either the Paris green or 

 the arsenical treatment. 



Literature. 



Wilcox & Smith, Parmer's Cyclopedia of Agri- 

 culture ; Bailey, Cyclopedia of American Horticul- 

 ture ; Fitz, Sweet Potato Culture ; Price, Sweet 

 Potato Culture for Profit ; Farmers' Bulletins, 

 United States Department of Agriculture, Nos. 26, 

 129 ; Arkansas Experiment Station, Bulletin No. 

 72 ; South Carolina Experiment Station, Bulletin 

 No. 63. Numerous other publications can be traced 

 through the Experiment Station Record. 



TANNING MATERIALS. Pigs. 848-851. 



By F. P- Veitch. 



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 diflicult 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 oh 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 nex:t 

 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. 



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, 



