These sulphates may be decomposed by a higher temperature
towards the end of the operation; their removal is rendered more certain
by rubbing up the calx with some culm and re-roasting, or by strongly
heating the calx after the addition of solid ammonic carbonate. In
roasting operations, as large a surface of the substance as possible
should be exposed to the air. If done in a crucible, the crucible should
be of the Cornish type, short and open, not long and narrow. For
calcinations, _roasting dishes_ are useful: these are broad and shallow,
not unlike saucers, but unglazed. In those cases in which the products
of the roasting are liquid at the temperature used, a _scorifier_ (fig.
38) is suitable if it is desired to keep the liquid; but if the liquid
is best drained off as quickly as it is formed, a _cupel_ (fig. 5)
should be used.
[Illustration: FIG. 5.]
A scorifier is essentially a roasting dish sufficiently thick to resist,
for a time, the corrosive action of the fused metallic oxides it is to
contain. The essential property of a cupel is, that it is sufficiently
porous to allow the fused oxide to drain into it as fast as it is
formed. It should be large enough to absorb the whole of the liquid; and
of course must be made of a material upon which the liquid has no
corrosive action. Cupels do not bear transport well; hence the assayer
generally has to make them, or to supervise their making.
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