combining with a strong caustic soda solution at a
low temperature, and evolving sufficient heat to complete the
saponification.

Sometimes tallow, lard, cotton-seed oil, palm oil and even castor oil
are used in admixture with cocoa-nut oil. The process for such soap is
the same as when cocoa-nut oil is employed alone, with the slight
alteration in temperature necessary to render the fats liquid, and the
amount of caustic lye required will be less. Soaps made of these blends
closely resemble, in appearance, milled toilet soaps. In such mixtures
the glycerides of the lower fatty acids commence the saponification, and
by means of the heat generated induce the other materials, which alone
would saponify with difficulty or only with the application of heat, to
follow suit.

It is necessary to use high grade materials; the oils and fats should be
free from excess of acidity, to which many of the defects of
cold-process soaps may be traced. Owing to the rapidity with which free
acidity is neutralised by caustic soda, granules of soap are formed,
which in the presence of strong caustic lye are "grained out" and
difficult to remove without increasing the heat; the soap will thus tend
to become thick and gritty and sometimes discoloured.

The caustic lye should be made from the purest caustic soda, containing
as little carbonate as possible; the water used for dissolving or
diluting the caustic soda should be soft (_i.e._, free from calcium and
magnesium salts), and all the materials carefully freed from particles
of dirt and fibre by straining.

The temperature, which, of course, must vary with the season, should be
as low as is consistent with fluidity, and for cocoa-nut oil alone may
be 75 deg. F. (24 deg. C.), but in mixtures containing tallow 100 deg. to 120 deg. F.
(38 deg. to 49 deg. C.).

The process is generally carried out as follows:--

The fluid cocoa-nut oil is stirred in a suitable vessel with half its
weight of 71.4 deg. Tw. (38 deg. B.) caustic soda lye at the same temperature,
and, when thoroughly mixed, the pan is covered and allowed to rest. It
is imperative that the oils and fats and caustic lye should be
intimately incorporated or emulsified. The agitating may be done
mechanically, there being several machines specially constructed for the
purpose. In one of the latest designs the caustic lye is delivered
through a pipe which rotates with the stirring gear, and the whole is
driven by means of a motor.