There should be no greasiness in the sample, but when pressed between finger and thumb it must be firm and dry.
Boiling is continued until the faint caustic taste on applying the cooled sample to the tongue is permanent, when it is ready for "graining out". The pasty mass now consists of the soda salts of the fat (as imperfect soap, probably containing emulsified diglycerides and monoglycerides), together with water, in which is dissolved the glycerine formed by the union of the liberated glyceryl radicle from the fat with the hydroxyl radicle of the caustic soda, and any slight excess of caustic soda and carbonates. The object of the next operation is to separate this water (spent lye) from the soap.
(b) Graining Out or Separation.—This is brought about by the use of common salt, in a dry form or in solution as brine, or by caustic soda lye. Whilst the soap is boiling, the salt is spread uniformly over its surface, or brine 40° Tw. (24° B.) is run in, and the whole well boiled together. The soap must be thoroughly boiled after each addition of salt, and care taken that too large a quantity is not added at once.
As the soap is gradually thrown out of solution, it loses its smooth transparent appearance, and becomes opaque and granular.
When a sample, taken out on a wooden trowel, consists of distinct grains of soap and a liquid portion, which will easily separate, sufficient salt or brine has been added; the boiling is stopped and the spent lye allowed to settle out, whilst the soap remains on the surface as a more or less thick mass.
The separated spent lye consists of a solution of common salt, glycerine, and alkaline salts; the preparation of crude glycerine therefrom is considered in chapter ix.
The degree of separation of water (spent lye) depends upon the amount of precipitant used. The aim is to obtain a maximum amount of spent lye separated by the use of a minimum quantity of salt.
The amount of salt required for "graining out" varies with the raw material used. A tallow soap is the most easily grained, more salt is required for cotton-seed oil soap, whereas soaps made from cocoa-nut and palm-kernel oils require very large amounts of salt to grain out thoroughly. Owing to the solubility in weak brine of these latter soaps, it is preferable to grain them with caustic soda lye. This is effected by adding, during boiling, sufficient caustic lye (32-1/2° Tw., 20° B.) to produce the separation of the granules of soap. The whole is allowed to rest; the separated half-spent lye is withdrawn and may be used for the pasting of fresh materials.
After the removal of the settled lye, the grained mass is boiled with sufficient water to dissolve the grain and make it smooth ("close" it), and is now ready for the next operation of "boiling on strength".
(c) Boiling on Strength or Clear Boiling.—This is the most important operation and is often termed "making the soap". The object is to harden the soap and to ensure complete saponification.