roceeds,
the temperature may be carefully raised, by means of the steam coil, to
120 deg. F. (49 deg. C.).

The first treatment having proceeded fifteen minutes, the contents of
the tank are allowed to rest; the settling should be prolonged as much
as possible, say overnight, to allow the impurities to precipitate well,
and carry down the least amount of entangled oil. Having withdrawn these
coloured "foots," the second portion of the weak caustic soda solution
is agitated with the partially refined oil, and, when the latter is
sufficiently treated, it is allowed to rest and the settled coloured
liquor drawn off as before. The oil is now ready for the final
treatment, which is performed in the same manner as the two previous
ones. On settling, a clear yellow oil separates.

If desired, the oil may be brightened and filtered, after refining to
produce a marketable article, but if it is being refined for own use in
the soap-house, this may be omitted.

The residue or "foots" produced during the refining of crude
cotton-seed oil, known in the trade as "mucilage," may be converted into
"soapstock" as mentioned in the preceding chapter, or decomposed by a
mineral acid and made into "black grease" ready for distillation by
superheated steam.

_Vegetable Oils._--The other vegetable oils come to the soap-maker's
hand in a refined condition; occasionally, however, it is desirable to
remove a portion of the free fatty acids, which treatment also causes
the colouring matter to be preciptated. This is effected by bringing the
oil and a weak solution of caustic lye into intimate contact. Cocoa-nut
oil is often treated in this manner. Sometimes it is only necessary to
well agitate the oil with 1-1/2 per cent. of its weight of a 12 deg. Tw. (8 deg.
B.) solution of caustic soda and allow to settle. The foots are utilised
in the soap-pan.

_Animal Fats._--Tallows are often greatly improved by the above alkaline
treatment at 165 deg. F. (73 deg. C.). It is one of the best methods and
possesses advantages over acid processes--the caustic soda removes the
free acid and bodies of aldehyde nature, which are most probably the
result of oxidation or polymerisation, whereas the neutral fat is not
attacked, and further, the alkaline foots can be used in the production
of soap.

_Bone fat_ often contains calcium (lime) salts, which are very
objectionable substances in a soap-pan. These impurities must be removed
by a treatment with hydroc