Silver or gold exposed to similar treatment does not oxidise,
but retains its metallic condition; so that an alloy of lead and silver
similarly treated would yield its lead as oxide, which would sink into
the support, while the silver would remain as a button of metal.
The porous support, which is called _a cupel_(fig. 5), should absorb the
slag (oxide of lead, etc.) just as a sponge absorbs water, but must be
sufficiently fine-grained to be impervious to the molten metal. At first
sight it appears difficult to filter, as it were, a fluid slag from a
fluid metal; but an ordinary filter-paper damped with oil will allow
oils to run through and yet retain the water; but damped with water it
will allow water to run through and retain oils. Similarly, fused slags
damp and filter through a cupel, but the molten metal not damping it
withdraws itself into a button, which is retained. Although, of course,
if the cupel is very coarse-grained the metal may sink into the hollows.
Copper, antimony, tin, and most other metals, form powdery oxides, which
are not of themselves easily fusible, and it is necessary when these are
present to add some solvent or flux to render the oxide sufficiently
fluid. Fortunately, oxide of lead is sufficient for the purpose; hence,
mixed oxides of copper and lead, provided the lead is present in proper
proportion, form a fluid slag.
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