PART IV. MISCELLANEOUS TANNAGES 



SECTION I. ALUM TANNAGES 



THE use of alum for making pelt into leather is several 

 centuries old. It was the first case of what are called 

 " mineral tannages." The tannage is closely analogous 

 in theory to the chrome tannages discussed in Part III., 

 and if soda be added to ordinary potash alum in 

 solution, a basic alum liquor is obtained which is 

 quite capable of yielding a satisfactory leather, and 

 which is thus a strict analogy of the basic chrome alum 

 liquor described in Part III., Section II. The range of 

 basicity which is practicable is very limited, however, and 

 it is much more usual to use common salt with the 

 alum instead of soda. The alum is, of course, hydrolyzed 

 and free sulphuric acid is quickly adsorbed, whilst the 

 colloidal solution of alumina is adsorbed also but more 

 slowly. The adsorbed acid tends to swell the pelt and 

 to cause it to take up the alumina less readily. The function 

 of the salt is to repress the swelling by a pickling action. 

 The actual result is thus partly due to the alum tannage 

 and partly due to the temporary tannage given by the 

 pickle. Hence such tannages are not firmly " fixed/' nor 

 is the result water-resisting, for much of the tanning 

 material may be washed out. If, however, such leathers 

 be stored for a time in a dry condition, the alumina becomes 

 much more firmly fixed, owing probably to a further 

 dehydration of the alumina gel deposited upon the fibres. 

 The tannage is thus relatively more " irreversible," and 

 such storage is practised in commerce for this purpose, 

 being known as the " ageing " of the leather. It will be 

 understood that it is possible to use too great a proportion 



