THE SCIENTIFIC ASPECTS OF TANNING. pr 



We have traced the hide or skin to that stage when, all the hair 

 being removed and superfluous flesh, it may be converted into 

 parchment or leather. 



We now proceed to consider the tanning process proper. The 

 hide is ready to imbibe and chemically unite with tannic acid to 

 form leather. When oak bark was the tanning material it was 

 comparatively a simple matter j but the modern man has tanning 

 agents by the score. Some of these will make his leather hard, 

 others equally soft: one will impart a red color, another a tawny 

 yellow. It is a source of no little anxiety to the tanner to know 

 which compound he shall use, at what stage of the process, and 

 the proportions of each. The majority use Bark and Valonia ; 

 others unite with these Myrabolams ; some prefer Mimosa ; 

 another tanner will employ in conjunction with one or two of 

 these a little Gambicej whilst a few are venturesome enough to 

 indulge in Divi Divi, or Hemlock extract. Whichever has been 

 the adirixture employed, it must be finished as nearly as possible 

 like bark tanned leather. 



Before passing into the tanyard the hide, if intended for sole 

 purposes, is transformed into a butt by having its thinner portions 

 removed to be tanned separately or converted into glue pieces, 

 the usual procedure being to place the butt into tan liquors, which 

 is partially exhausted of its tannic acid, so that its action on the 

 green pelt should be milder than otherwise it would be. Besides 

 which, these bark liquors contain a large percentage of Gallic and 

 Acetic acids, which although not tanning, help to maintain the 

 substance of the butt, as also eflSciently to discharge the lime 

 which may still remain in the pores of the skin. A strong tanning 

 solution presented at this stage would have the effect of case- 

 hardening both surfaces. This applies especially to the grain of 

 the butt, which if tanned too rapidly becomes hard and brittle, 

 never returning to its original condition. Some preparation to 

 shield the grain at this stage would enable the tanner to push on 

 the process more rapidly. This was the object of a patent taken 

 out in 1873, which has answered well when properly applied. 



