CLARIFICATION AND DECOLORIZATION 239 



usefulness. The ordinary processes of dyeing fabrics or leather 

 are adsorption processes, and the decolonization of gelatine 

 sols consists essentially of the same process, except that the 

 concentration of the dyestuff is much less, and the liquor 

 remaining, instead of the adsorbent, is the primary con- 

 sideration. 



Decolorization of gelatine sols may be effected by any 

 substance with a large specific surface (see p. 201). Indeed, 

 a great variety of adsorbents are actually used in practice, 

 and each factory has its favourite material or mixture, and 

 its favourite mode, place, and time of application, determined 

 partly by the nature of the adsorbent and partly by the 

 precise form of apparatus used. Amongst the adsorbents 

 which have received special favour are sand, kieselguhr, 

 asbestos, animal charcoal, wood pulp fibre, albumin and 

 alumina. Sand is very effective, but a comparatively large 

 weight is needed, and its cleansing for repeated use is trouble- 

 some. On the other hand, it may be completely renovated 

 by ignition. Kieselguhr is a very powerful adsorbent, and 

 only a little will do much good ; it is, however, hardly 

 sufficient alone. Animal charcoal has great specific surface, 

 but its pores are very small for viscous liquors; and its use is 

 less suitable in the case of gelatine than in the decolorization 

 of liquors which may be boiled. Wood pulp fibre is a very 

 popular decolorizing material, not only in gelatine but also 

 in other trades. Its short, woolly fibres give a clarifying 

 as well as a decolorizing effect. It may thus act as a 

 mechanical filter for suspended matter and grease, as well as 

 an adsorbent for colouring matters present as sols. Its two 

 functions, however, are often confused. It may be re- 

 generated for repeated use by careful washing, and special 

 pulp- washing machines are manufactured and sold for the 

 purpose. Detergents are usually employed in the wash 

 waters. Asbestos is also a good adsorbent, and its long 

 fibres make it much less liable to non-operating " channels " 

 and " bursts." It also has the advantage that, if desired, 

 it may be regenerated by ignition. It forms a very useful 

 mixture with pulp fibre. 



