o 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 deg. Tw., 20 deg. 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to rest; the separated half-spent lye is withdrawn and may be used for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.

Caustic soda lye (32-1/2 deg. Tw., 20 deg. B.) is gradually added until the soap
is again opened or grained, and boiling continued by the use of the dry
steam coil. As soon as the caustic soda lye is absorbed, another portion
is slowly added, and this is continued until the caustic soda or
"strength" remains permanent and the soapy mass, refusing to absorb
more, is thrown out of solution and grained. The granular mass will boil
steadily, and the boiling should be prolonged, as the last traces of
neutral oil are difficult to completely saturate with alkali. Thorough
saponification takes place gradually, and the operation cannot be
hurried; special care has to be bestowed upon this operation to effect
the complete combination of fat and alkali.

After resting for several hours, half-spent lye settles to the bottom of
the pan. In the case of yellow soaps or milling bases the settled lye is
removed to a suitable recept