SECTION V. BLEACHING 



THE adsorption law indicates that however much colouring 

 matter is removed from the volume concentration (continuous 

 phase) there must always be some left. After all that the 

 decolorization processes can do, there still remains much 

 colour that can only be removed by a chemical action of the 

 ordinary sense. The amount of colouring matter of this 

 kind is not large, but it is a deep red-brown, and when the 

 gelatine sol has been evaporated and dried out the final 

 product, if untreated, possesses this typical colour, and is 

 known as glue. If, before gelation, a chemical bleaching 

 action is applied to destroy this pigment, the product may 

 be then dried out in a nearly colourless condition and is 

 known as gelatine. Gelatine, therefore, is simply bleached 

 glue. Many other definitions have been given, and many 

 elaborate distinctions drawn, but the fact of bleaching is 

 the essential difference. In these days when gelatine is so 

 valuable, the higher-grade products are nearly always 

 bleached, and the term "glue " is consequently more often 

 applied to a lower-grade product, and is sometimes used in 

 a sense implying this fact. 



If it be desired to manufacture gelatine, it is fairly 

 obvious that the task is lightened by observing the axiom 

 that prevention is better than cure. If steps, are taken to 

 prevent the presence or development of such colouring 

 matter, a great advantage is attained, for not only is the 

 problem of bleaching easier, but also quicker and less expen- 

 sive in chemicals. The nature of the colouring matters 

 is but imperfectly investigated, but in the case of skin 

 gelatine the pigment of the hair roots and epidermis is 



E. 16 



