BLEACHING 243 



gelatine cover a very limited field. This limitation arises not 

 so much from the ineffectiveness of the bleach, as from the 

 other effects of these substances upon the purity of the pro- 

 duct and upon the elasticity of the gel which it can yield. 

 Especially important is the lyotrope influence of the bleaching 

 agent. Many reactive substances are ruled out simply be- 

 cause they either insolubilize the gelatine or weaken the gel 

 it makes. Others are inadmissible on account of their 

 poisonous nature. It must never be forgotten that whatever 

 is used in bleaching is, like the gelatine itself, much con- 

 centrated during evaporation and drying. Its possible 

 percentage in the finished product should be considered, 

 and also the possibility that in these finishing operations 

 what is present may not remain in solution, owing to super- 

 saturation. 



Bleaching by Reduction. Of all the reducing agents 

 suggested, sulphurous acid has proved to be much the 

 most suitable and successful. It has been used with equal 

 success both for bone and for skin gelatine, but on the whole 

 has proved more suitable for the former. 



Sulphurous acid can fulfil in this instance a double 

 function, viz. that of acid solvent for the bone phosphate, 

 and that of bleaching agent also. As it penetrates the bone 

 material, dissolving the phosphate, it also exercises its 

 bleaching influence on the gelatinous part of the material. 

 Changes of liquor tend to complete both actions, so that a 

 counter-current system is found most convenient. The 

 " acid process " for the manufacture of bone gelatine has 

 been previously described (Section II., pp. 224-227), and 

 the use of sulphurous acid in this connection is typified in 

 the Bergmann process (p. 227). In this process bleaching 

 is in effect merely a continued treatment. 



In the case of skin gelatine, also, sulphurous acid may 

 fulfil a double function, viz. that of deliming agent as well 

 as of bleaching agent. In such instance it is necessary to 

 use excess of bleaching acid, some acting as deliming 

 material and the remainder as bleaching agent. As it is 

 desirable to get rid of the lime and soda salts, several 



