SECTION III. VEGETABLE TANNAGE 



Aw, tannages have for their object the conversion of the 

 readily putrescible hide tissue of the corium (the pelt) into 

 an imputrescible, insoluble and permanent material called 

 " leather/' which, possessing considerable strength and 

 pliability, is capable of application to a variety of useful 

 purposes. The conditions necessary for this transformation 

 have been clearly stated by Procter. 1 For the production 

 of leather from pelt "it is not only necessary to dry the 

 fibres in a separate and non-adherent condition, but so to 

 coat them or alter their chemical character that they are 

 no longer capable of being swelled or rendered sticky by 

 water." Whatever substance will secure this permanent 

 dehydration of the hide fibres in a separate condition is 

 called a "tanning material." The change from pelt to 

 leather is known as " tannage," the process is termed 

 " tanning," and those who undertake it are " tanners." 



In " vegetable tannage " the tanning materials are of 

 vegetable origin, and contain a group of organic compounds 

 called " tannins " which are extracted by the infusion of 

 these materials with water. Pelt, when immersed in these 

 infusions, is converted into leather, rather slowly ; but a 

 gelatin solution gives an immediate precipitate of " amor- 

 phous leather," even if the tannin infusion be exceedingly 

 dilute. The tannins are aromatic compounds of phenolic 

 character, and contain carbon, hydrogen and oxygen only, 

 but our knowledge of their chemical constitution is exceed- 

 ingly small owing to their instability and colloid nature, 

 which make impossible their preparation in a pure state. 

 They are all, however, derived from either catechol or 

 pyrogallol, and yield these substances if carefully heated to 



1 " Principles of Leather Manufacture," p. 184. 



