154 ANIMAL PROTEINS 



as to bring the tanning sol into condition for more per- 

 manent tannage. Neutralization gets rid of soluble chrome 

 salts as well as free mineral acid, and is the final stage in 

 rendering the tanning sol less positive, and perhaps even 

 negative. It is brought about by the use of weak alkalies, 

 of which borax is the easiest and safest, but not the 

 cheapest. Sodium silicate, phosphate, carbonate, and 

 bicarbonate have been also used, and a mixture of soda 

 and an ammonium salt has been suggested by Stiasny. 

 Whitening has also been tried, but is very slow-acting. 

 Considerable economy in alkali may be effected by a 

 thorough washing of the leather before using the alkali. 

 If the water be hard, so much the better, and if warm 

 water be available the process is hastened. For most 

 leathers it is necessary to remove excess of alkali just as 

 much as excess of acid, so that a thorough washing in 

 water generally follows the treatment with alkali. Any- 

 thing from J to 3 per cent, borax (or its equivalent) on 

 the pelt weight may be used, and, generally speaking, it 

 is better to use solutions as dilute as practicable in order 

 to avoid local over-neutralization and tender leather. 



Fat liquoring is a process which is very largely typical 



of chrome leather manufacture ; it consists in drumming 



the goods with an oil emulsion, the grease of which is 



entirely taken up by the leather. It thus strongly resembles 



drum stuffing (Part I., Section IV., p. 53) in method, but 



the " fat liquor " is such that it mixes easily with water, 



and usually contains soap in order to assist in this sense, 



and may sometimes indeed consist of soap only. Mineral 



oil is also used frequently in fat liquors. The object of 



fat liquoring is to give softness, pliability, or waterproof ness, 



and to feed the " empty " chrome tannage. It is also 



used as a preparation for more complete impregnation of 



grease, e.g. as in " stuffing " chrome harness, and in 



" dipping " chrome sole leather. Fat liquors are usually 



made by dissolving the soap in boiling water and gradually 



adding the oil with constant agitation. Perfect emulsifi- 



cation is essential, and this is assisted by the use of casein, 



