GELATINE AND GLYCERINE 227 



gelatine is more thoroughly bleached. It is subsequently 

 well washed before extraction. 



Recovery and Purification of Grease. The de- 

 greasing operation, which is applied usually to bones (p. 

 224) and to skin glue scutch, was once brought about by 

 steaming only, but is now accomplished with the assistance 

 of fat solvents, though in the latter case steaming together 

 with mechanical centrifugal force has proved sufficiently 

 successful. On the Continent carbon disulphide was once 

 largely used as solvent, and in this country benzene has 

 been employed, but their low volatility and high inflamma- 

 bility, as well as their expense, make both these substances 

 somewhat unsuitable, and it is now usual to make use of 

 petroleum oils, whether Scotch, American or Russian. A 

 fraction which boils about the same temperature as water is 

 usually employed, and all of it must be volatile under 

 280 F. Before the actual grease extraction the bones 

 should be sorted over and unsuitable substances (horns, 

 gravel, iron, etc.) removed. They are also usually put 

 through a mill and roughly crushed or broken. The actual 

 grease extraction plant consists of large copper vessels which 

 will each take 5 tons of bones. These extractors are 

 arranged in sets so that the degreasing is proceeding in some 

 whilst the others are being emptied and recharged. The 

 doors for charging and emptying must be securely fastened. 

 When the extractor is charged the solvent is run in and heated 

 by a steam coil which eventually causes it to distil. After 

 some hours the remainder, which has dissolved much grease, 

 is run off, and a fresh lot of solvent is added and heated up. 

 After four such extractions only about J per cent, of grease 

 remains in the bones. To remove the remainder of the 

 solvent high-pressure steam (80 Ibs.) is blown through the 

 bones. The extractor is then opened and the degreased 

 and somewhat dried bones are mechanically conveyed to 

 the cleansing mill. The grease solutions obtained are 

 subjected again to steam with a view to removing the 

 solvent and obtaining it for repeated use in this sense. 

 The efficient distillation and recovery of the solvent 



