0 deg. Tw. (33.3
B.) at 60 deg. F. (15.5 deg. C.). When this point is reached, the crude
glycerine is ready to be withdrawn into a tank, and, after allowing the
excess of salt to deposit, may be transferred to the storage tank.

The colour of crude glycerine varies from light brown to dark brown,
almost black, and depends largely on the materials used for soap-making.
The organic matter present in good crude glycerine is small in amount,
often less than 1 per cent.; arsenic, sulphides and sulphites should be
absent. Crude glycerine is refined in some cases by the producers
themselves; others sell it to firms engaged more particularly in the
refined glycerine trade.

_Distillation._--Crude glycerine is distilled under vacuum with the aid
of superheated steam. The still is heated directly with a coal or coke
fire, and in this fire space is the superheater, which consists of a
coil of pipes through which high pressure steam from the boiler is
superheated.

The distillation is conducted at a temperature of 356 deg.F. (180 deg. C.). To
prevent the deposition and burning of salt on the still-bottom during
the distillation, a false bottom is supported about 1 foot from the base
of the still. With the same object in view, it has been suggested to
rotate the contents with an agitator fixed in the still.

Every care is taken that the still does not become overheated; this
precaution not only prevents loss of glycerine through carbonisation,
but also obviates the production of tarry and other bodies which might
affect the colour, taste, and odour of the distilled glycerine. The
vacuum to be used will, of course, depend upon the heat of the fire and
still, but as a general rule good results are obtained with an 18 inch
vacuum.

There are quite a large number of designs for still heads, and
"catch-alls," having for their object the prevention of loss of
glycerine.

The distillate passes into a row of condensers, to each of which is
attached a receptacle or receiver. It is needless to state that the
condensing capacity should be in excess of theoretical requirements. The
fractions are of varying strengths and quality; that portion, with a
density less than 14 deg. Tw. (19.4 deg. B.), is returned to the treated-lyes
tank. The other portion of the distillate is concentrated by means of a
dry steam coil in a suitable vessel under a 28 inch vacuum.

When sufficiently concentrated the glycerine may be decolorised, if
necessary, b