.

(_a_) _Pasting or Saponification._--The melted fats and oils are
introduced into the soap-pan and boiled by means of open steam with a
caustic soda lye 14 deg. to 23.5 deg. Tw. (10 deg. to 15 deg. B.). Whether the fatty
matters and alkali are run into the pan simultaneously or separately is
immaterial, provided the alkali is not added in sufficient excess to
retard the union.

The commencement of the saponification is denoted by the formation of an
emulsion. Sometimes it is difficult to start the saponification; the
presence of soap will often assist this by emulsifying the fat and thus
bringing it into intimate contact with the caustic soda solution.

When the action has started, caustic soda lye of a greater density, 29 deg.
to 33 deg. Tw. (18 deg. to 20 deg. B.), is frequently added, with continued boiling,
in small quantities as long as it is being absorbed, which is
ascertained by taking out samples from time to time and examining them.

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 deg. Tw. (24 deg. 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 t