140 ANIMAL PROTEINS 



the volume and elasticity of the pelt. Whether also the 

 loss of hide be due to hydrolysis or solation, it is 

 increased by time, hence short processes are (other 

 things being equal) much to be preferred. Fermentive 

 hydrolysis is minimized by cleanliness, alkaline hydrolysis 

 by avoiding strongly alkaline liquors, and solation of 

 collagen is reduced by both, and also by avoiding, as 

 far as possible, the presence of calcium and ammonium 

 salts. 



Soaking should be quick and clean. The use of the 

 paddle or drum gives the greatest efficiency and also assists 

 in procuring the softness so essential to the bulk of chrome 

 leathers. 



lyiming chrome leather satisfactorily is almost an im- 

 possible ideal. Every conceivable arrangement has some 

 objection to it. The time of the process may be shortened 

 either by the use of sulphide or by the use of mellow or 

 old limes. To shorten time by the use of sodium sulphide 

 unfortunately involves the employment of more alkali 

 than is desirable, with a consequent plumping effect and 

 tendency to harshness. If sufficient sulphide be used to 

 make the liming very short, then the grease is not " killed " 

 (saponified or emulsified). If the harshness and alkalinity 

 be removed by using also an excess of calcium chloride, 

 then the lyotrope influence of this substance enhances the 

 solation of the hide gel. On the other hand the use of 

 old lime liquors avoids the plumping effect, but increases 

 considerably the bacterial activity, and the bacterial 

 enzymes produce both hydrolysis and solation of the pelt. 

 In practice what is generally done is to shorten time by 

 both methods and so to admit both disadvantages to a 

 limited extent. This is theoretically more sound than 

 would appear, for in mellow limes sulphide has less plump- 

 ing power but is just as strong a depilatant ; whilst, on 

 the other hand, a mellow liming shortened by sulphide is 

 less objectionable as there is some evidence that bacterial 

 activity is relatively less in the first few days. Hence a 

 mellow sulphide liming of 7-10 days is very common in 



