If the button left from cupellation is yellow it is at least half gold,
and a rough guess as to the proportion of gold may be made from its
yellowness; the rest of the metal is generally silver. The presence of
platinum or one of the platinum group of metals makes the surface of the
button dull and crystalline. The native alloy of osmium and iridium does
not alloy with gold, however, but falls to the bottom of the molten
metal. It shows itself in the subsequent parting as a black spot or
streak on the under surface.
The buttons are removed from the cupel with a pair of pliers and then
brushed to remove adherent litharge and bone-ash. Some assayers advise
cleaning by dipping in warm dilute hydrochloric acid followed by washing
in water and drying. The button is next weighed. When the quantity of
silver obtained is not required to be known the weighing may sometimes
be omitted. The next operation in either case is parting either with or
without a previous inquartation.
_The loss of gold in cupellation_ is by no means always inconsiderable.
In three cupellations of 1 gram of gold with 20 grams of lead made
purposely at a very high temperature the cupel absorbed 6.04, 6.20, and
6.45 milligrams of gold. Hence at a high temperature there may easily be
a loss of more than half a per cent. of the gold.
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