THE COLLOID-CHEMISTRY OF SOAP MANUFACTURE 171 



obtain* (! are not always so satisfactory as when the process is 

 carried out at a higher temperature. It is also a matter of empiric 

 knowledge that certain fats readily yield satisfactory soaps when 

 used in the cold while others, it might almost be said, never do 

 so. The extremes are represented, on the one hand, by the use 

 of such a fat as castor oil, cottonseed oil or even palm kernel oil; 

 on the other hand, by the use of hydrogenated cottonseed oil, 

 stearin or Japan wax. 



What happens becomes intelligible when the qualities and 

 quantities of the fatty acids found in these fats and oils are com- 

 pared with the physico-chemical and colloid properties of soaps 

 which are produced from the different fatty acids as illustrated 

 IM Figs. 1 to 13. * Soaps are readily made by the cold process only 

 from such fats as are approximately liquid or yield fatty acids 

 which are liquid at the temperature at which the soap is manufactured. 

 I n t lie older schemes of soap manufacture this process was regularly 

 employed with such oils as olive, cocoanut and cottonseed. Partly 

 because larger and larger quantities of these materials are now 

 used for food, and partly because the soaps from these materials 

 are relatively soft and " wash away " easily (and for toilet 

 purposes, for example, a somewhat firmer product is desired) 

 the original oils have, with time, had admixed with them larger 

 and larger fractions of fats with higher melting points. As this 

 has happened the difficulty of making the soap by the " cold " 

 process has increased, for the fats rich in palmitic acid, stearic 

 acid, etc., cannot be thus saponified. At higher temperatures 

 it is, of course, an easy matter. Since saponi ft cation represents 

 an exothermic reaction, considerable heat is produced which 

 warms the soap mix lure. For this reason fatty acids melting 

 at temperatures considerably above that of the surrounding 

 medium can still be saponified in the " cold." The higher solid) 

 fatty acids also saponify more slowly than do the lower ones, 

 whence the common practice of allowing the necessary fat-alkali 

 reaction mixtures, when soap is produced l.y the tl cold " process, 

 to Stand several days, while, to conserve the liberated heat, the 

 vats or frames are protected with mattresses. 



The now almost universally employed " hot" process of soap 

 m.-inufact ure may l>e dismissed with the remark that at the higher 

 temperature employed all the oils and fats used (or, in the Twi r u 

 1 See pages 10 to 29. 



