A quantity of
bone ash is carefully mixed with water so that no lumps are formed, and
the mixture is then worked up by rubbing between the hands. The bone ash
is sufficiently wet when its cohesion is such that it can be pressed
into a lump, and yet be easily crumbled into powder. Cupel moulds should
be purchased. They are generally made of turned iron or brass. They
consist of three parts (1) a hollow cylinder; (2) a disc of metal; and
(3) a piston for compressing the bone ash and shaping the top of the
cupel. The disc forms a false bottom for the cylinder. This is put in
its place, and the cylinder filled (or nearly so) with the moistened
bone ash. The bone ash is then pressed into shape with the piston, and
the cupel finished with the help of three or four smart blows from a
mallet. Before removing the piston, turn it half-way round upon its axis
so as to loosen and smooth the face of the cupel. The cupel is got out
by pressing up the disc of metal forming the false bottom; the removal
is more easily effected if the mould is somewhat conical, instead of
cylindrical, in form. The cupels are put in a warm place to dry for two
or three days. A conveniently sized cupel is 1-1/4 inches in diameter
and about 3/4 inch high. The cavity of the cupel is about 1/4 inch deep,
and something of the shape shown in fig.
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