244 ANIMAL PROTEINS 



changes of liquor are given to the goods, possibly with 

 intermediate washing. Here again approximation to a 

 counter-current system is of advantage, as the employment of 

 used bleach liquors for deliming purposes effects considerable 

 economy of sulphurous acid. Indeed, there need be no 

 waste acid at all. 



Whether the material be for bone or skin gelatine, 

 however, it will be seen that the extraction is conducted 

 in an acid condition and the resulting sol is also acid. Most 

 usually the decolorization and nitration processes are also 

 conducted with such an acid sol. From what has been 

 said (Section IV., p. 235) of the value of dibasic inorganic 

 acids as clarifying agents, it will be understood that the 

 presence of sulphurous acid at this stage is of great advantage 

 in the production of a clear and bright gelatine. Indeed, 

 it is well known in trade circles that sulphurous acid gelatines 

 are usually of exceptional clarity and brightness. 



The disadvantage of sulphurous acid processes is also 

 found in the same fact that both sol, gel and cake are in an 

 acid condition. To complete the bleach it is sometimes 

 necessary to add sulphurous acid to the sol after extraction, 

 or even after evaporation, but this is to be avoided if possible. 

 Usually the ideal attempted is that the bleaching action 

 should be as much as possible before extraction ; the excess 

 of sulphurous acid is then washed off just before the extrac- 

 tion, as far as practicable, and the rest is boiled off during 

 extraction. The ideal is practically never attained, for 

 the acid is strongly adsorbed, and the result is that the 

 finished article is always an acid gelatine, and sometimes 

 indeed very decidedly such. The acid condition is objection- 

 able in the case of some forms of filter press on account of 

 the solvent action on the metals, and is objectionable in 

 evaporation for similar reasons. Acid gelatines are also 

 objectionable for many purposes for which gelatine is usually 

 sold, and this limits the commercial possibilities of the 

 product thus obtained. 



Sulphurous acid is itself, of course, a gas, and whilst 

 the gas itself has been used for treating the material (e.g. 



