In separating copper from silver or gold,
advantage is taken of this fact; for, although we cannot cupel an alloy
of copper and silver, it is easy to cupel an alloy of copper, silver and
lead. If, however, the lead is not present in sufficient quantity, the
whole of the copper will not be removed, and the button of silver, still
retaining copper, will be found embedded in a coating of black oxide of
copper. Copper oxidises less easily than lead does; and, consequently,
the alloy which is being cupelled becomes relatively richer in copper
as the operation proceeds. It is on this account that the ill-effects of
the copper make themselves felt at the close of the operation, and that
the oxide of copper is found accumulated around the button of silver.
Tin and antimony, on the other hand, are more easily oxidised; and the
tendency of their oxides to thicken the slag makes itself felt at the
commencement: if the button of alloy once frees itself from the ring or
crust of unfused oxide first formed, the cupellation proceeds quietly,
and leaves a clean button of silver in the centre. But in either case
the cupellation is imperfect, and should be repeated with a larger
proportion of lead. An unfused and, consequently, unabsorbed slag tends
to retain small buttons of alloy or metal, and thus cause serious loss.
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