134 ANIMAL PROTEINS 



" irreversible changes " subsequent to the mutual pre- 

 cipitation of oppositely charged gel and sol, are equally 

 applicable to the chrome tannages. Chrome tannage was 

 once thought to embrace such irreversible changes, but 

 the process can now be " reversed " with ease. The 

 reversibility of the chrome tannage is an easier proposition 

 than that of vegetable tannage, partly because the leather 

 is comparatively much less tanned, and partly because 

 the acidity or alkalinity of the stripping agent may be 

 adjusted, as desired, without the oxidation trouble. In 

 approaching this question from the theoretical side one 

 must consider mainly whether to solate the tanning agent 

 to a positive or to a negative sol. Our imperfect know- 

 ledge of the electrical forces in operation in the chrome 

 tannage is thus a serious drawback, but the evidence on 

 the whole points to the precipitation being effected by a 

 negative sol near its isoelectric point but in faintly acid 

 solution. Hence, we should theoretically expect that 

 reversion should take place into a negative sol in nearly 

 neutral or even faintly alkaline solution. Thus, suitable 

 stripping agents for chrome leather would be the alkali 

 salts of organic acids (especially if multivalent). Now, 

 Procter and Wilson have recently accomplished this strip- 

 ping of chrome leather by the use of such salts. They 

 approached the question from an empirical and practical 

 point of view and found that Rochelle salt, sodium citrate, 

 and sodium lactate would strip the chrome tannage with 

 ease. This important and very creditable achievement 

 will have great practical and commercial importance. 

 Procter and Wilson have deliberately and carefully re- 

 frained from offering an exact explanation of this reversible 

 action, but point out that all their stripping agents are 

 salts of hydroxy-acids, and strongly insist that these form 

 soluble complexes with the chrome. Whilst not denying 

 this in the least, the present author would point out that 

 according to the views advanced in this book, the salts 

 of organic acids which do not contain hydfoxyl groups 

 should, when combined with a monacid base, also strip 



