VEGETABLE TANNAGE 35 



and mangrove bark have been used, but are now made into 

 extracts (pp. 38 and 41). 



Leaching. Whatever class of leather is being made, 

 and whatever blend of tanning materials is being employed, 

 the tannins must be efficiently extracted by water in order 

 to make the tanning liquors. This process is called " leach- 

 ing." The tanning materials, after being ground, crushed 

 or shredded, are placed in large pits arranged in "rounds," 

 " sets," or " batteries " of 6, 8 or 10 units, through which 

 water is percolated systematically, so as to secure a con- 

 tinuous extraction. Water itself is added to only one of 

 the pits of material. The liquor produced is passed on to 

 the next pit, and then to the next, and is continually gather- 

 ing strength. After passing thus through the series, the 

 liquor becomes the source of the strong extracted tan 

 liquors which are used in the tannery proper. With this 

 system the stronger leach liquors are being acted upon by 

 fresh material, and the nearly " spent " material is being 

 acted on by the weakest liquors, and finally by water, thus 

 ensuring a complete extraction. In the press leach system, 

 which is now practically universal, the bottom of one pit 

 communicates with the top of the next, and the liquor 

 presses round by gravity flow caused by a few inches " fall." 

 Iiquor is thus constantly percolating downward through 

 the material in each pit. The " head leach " and " tail 

 leach " are always adjacent in a double row of pits, and 

 when the material in the latter is quite spent, it is " cast," 

 and the pit is filled with fresh material. The liquor is then 

 pressed round into this pit by adding water to the tail 

 leach. Hot water is used to secure better diffusion. At 

 least two such sets of leaches (" taps " and " spenders ") 

 are necessary to spend the material of the average tannery 

 and to obtain liquors of the necessary strength. 



The Manufacture of Extracts. In addition to the 

 use of the natural tanning materials described above, 

 modern leather manufacturers employ also a variety of 

 " tanning extracts," i.e. vegetable tanning materials in 

 which the tannin has been already extracted, and which 



