THE COLLOID-CHEMISTRY OF SOAP MANUFACTURE 183 



There needs al><> to be explained the differences incident to the 

 salting-out of soap at a high temperature and at a lower one. 

 Soap being more soluble in water at a higher temperature, a higher 

 absolute concentration of salt is required to salt it out at such 

 than at a lower one. The slow reversal in the system from one of 

 soap-in-water to one of water-in-soap with lowering of tempera- 

 ture needs also to be kept in mind. This explains the slow gela- 

 tion commonly observed in the " lye " after the whole soap-water- 

 salt mixture has been allowed to stand for a number of hours. 

 Tin soaps of the lower fatty acids which are still soluble in water 

 at the higher temperatures, even when sodium chlorid or other 

 salt has been added to the point where most of the soaps are salted 

 out . slowly come out of solution, become hydrated, and if enough 

 of such are present, the lye itself gels. 1 



9. The Finishing of Soap 



Soap may be " finished " for market purposes in the soap kettle 

 itself. More commonly, however, the " curd " obtained after 

 complete sail ing-out of a soap is reheated, redissolved in more or 

 less water and salted out more or less completely a second time 

 by again being treated with sodium chlorid of different concen- 

 trations. 



Depending, on the one hand, upon the kind of fatty acids and 

 their quantitative relation to each other in a given mixture (fac- 

 tors commonly ignored), upon the ot her. on the amount of sodium 

 chlorid added, there may be obtained (1) a grained or curd soap, 

 (2) a settled soap, (3) a half-settled soap or (4) a soft soap. 



1 This is the system obtained in making a "settled" soap 'the K< TM-. -itV 

 auf I/'imnioderechlag" of the < ,Tin;m- :tml that upon which MIKKMN 

 made most of his observation- || ( . considers the gelatinous lye "a solution 

 of soaps in an alkaline lye containing salts;" the soap curd also "a solution 

 of soaps in an alkaline lye containing salts" and holds the two to represent 

 "phases" in equilibrium with each other i Fu \v;ois MBRKLKN : Die K.m 

 -eif.-ii 1:5. llalloa/S (1907)> \-ol.\ mi is from t heal >\ e. the "curd " phase is a 

 highly concentrated one of hydrated 'higher fatty acid -,,..,,, coiitainin: 



'T emulsified in it; the "lye" phase (when gelatinous > aKo one of 

 hydraled lo\\.-r t'attv acid- -<>ap of lower concent rat ion and containing :i 



high ficrccntagc of salt-water emulsified within it. See the succeeding sect H m 



