620 



of England tannage played an important role in the early tanning 

 stages; its special characteristic was its swelling powers, thus 

 keeping the texture of the hides open and nullifying the contracting 

 action of the stronger tans. 



Acacia bark (Mimosa) from Australia and South Africa, dividivi 

 from India, algorabilla from South America, all of them contain- 

 ing over three times as much tannin as does oak bark, had a con- 

 siderable influence upon the changes that have taken place. 



Reference has been made above to the heavy layer of bark bet- 

 ween each hide ; this was afterwards burnt or thrown away half spent ; 

 but with stronger and better tanning materials in use, it became 

 general to put this dusting material into large pits, called leaches, 

 leeks and spenders, in which the tannin of the bark was extracted 

 by means of water. These leaching pits were arranged in series 

 and the liquor pumped from one pit to the other, until by pas- 

 sing through successive pits of partly spent material it gained suf- 

 ficient strength for use in saturating the bark between the hides, 

 producing liquors of much greater tanning strength. 



The introduction of Tanning Extract in the early seventies, 

 however, had more effect on the method of tanning than anything 

 else. It was first introduced from France made from chestnut wood 

 and later from Hungary, produced from oak wood. In the manu- 

 facture of this material, the wood is reduced to fine shreds by 

 powerful machinery and the rasped wood is afterwards extracted 

 in large wooden vats by the aid of hot water. The resulting liquor 

 is decolorised by means of blood which is added to the cooled 

 liquor ; the temperature is then raised and the coagulated albumen 

 separating out carries with it the darker colouring matter of the 

 bark together with some of the tan. The clarified liquor is then 

 concentrated in large copper vacuum pans at a comparatively low 

 temperature, and shipped to England in concentrated form in casks ; 

 the Extract contains upwards of 30 per cent, of soluble tannin ready 

 for immediate use, for blending in with the tanner's leach liquors. 



The effect of these stronger liquors was that of hide which had 

 hitherto been tanned in one piece the thin part became too much 

 tanned before the thinck part had had sufficient tan, with the re- 

 sult that " rounding " became more general. This brought about a 

 great change ; and square pits sunk in the ground became general. 



Among the points that especially require attention and chemical 

 control is the ratio of tan to non-tans in the early liquors, i. e., 

 the ratio in which the tannic acid exists in proportion to the na- 



