first essential of a milling base is that the saponification should
be thorough and complete; if this is not ensured, rancidity is liable to
occur and a satisfactory toilet soap cannot be produced. The soap must
not be short in texture or brittle and liable to split, but of a firm
and somewhat plastic consistency.

(i.) _Drying._--The milling-base, after solidification in the frames,
contains almost invariably from 28 to 30 per cent. of water, and this
quantity must be reduced to rather less than half before the soap can be
satisfactorily milled. Cutting the soap into bars or strips and open
piling greatly facilitates the drying, which is usually effected by
chipping the soap and exposing it on trays to a current of hot air at
95-105 deg. F. (35-40 deg. C.).

There are several forms of drying chambers in which the trays of chips
are placed upon a series of racks one above another, and warm air
circulated through, and Fig. 21 shows a soap drying apparatus with fan
made by W. J. Fraser & Co., Ltd., London.

The older method of heating the air by allowing it to pass over a pipe
or flue through which the products of combustion from a coke or coal
fire are proceeding under the floor of the drying chamber to a small
shaft, has been superseded by steam heat. The air is either drawn or
forced by means of quickly revolving fans through a cylinder placed in a
horizontal position and containing steam coils, or passed over
steam-pipes laid under the iron grating forming the floor of the
chamber.

[Illustration: FIG. 21.--Soap-drying apparatus.]

It will be readily understood that in the case of a bad conductor of
heat, like soap-chippings, it is difficult to evaporate moisture
without constantly moving them and exposing fresh surfaces to the
action of heat.

In the Cressonnieres' system, where the shavings of chilled soap are
dried by being carried through a heated chamber upon a series of endless
bands (the first discharging the contents on to a lower belt which
projects at the end, and is moving in the opposite direction, and so
on), this is performed by intercepting milling rollers in the system of
belts (Eng. Pat. 4,916, 1898) whereby the surfaces exposed to the drying
are altered, and it is claimed that the formation of hardened crust is
prevented.

In the ordinary methods of drying, the chips are frequently moved by
hand to assist uniform evaporation.

The degree of saturation of the air with moisture must be taken