Lead Required.
5 6 grams
10 8 "
20 10 "
30 12 "
40 14 "
50-100 16-18 "
The alloy, in not too large pieces, is wrapped in the required weight of
lead foil and charged into the cupel at once; or the lead may be put in
first, and, when the cupellation has fairly started, the alloy may be
added wrapped in tissue paper; or a portion of the lead may be first
started and the alloy wrapped in the remaining lead and subsequently
added. The cupellation of large quantities of alloy or of alloys which
contain tin, antimony, iron, or any substance which produces a scoria,
or corrodes the cupel, must be preceded by a scorification. The
advantages of this are that the slag is poorer in precious metal than
that found on a cupel and is more easily collected and cleaned; that
larger quantities of metal can be treated, and that, even if the
substance is in part infusible, or produces at the start a clinkery mass
or scoria, the oxide of lead gradually accumulates, fluxes the solid
matters, and produces a good final result; but if the oxide of lead by
itself is not sufficient for the purpose, borax or some other flux can
be easily added.
If the button of silver got is very small its weight may be estimated
from its size; but it must be remembered that the weight varies as the
cube of the diameter.
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