It will sometimes, but not
often, happen that the two buttons together will slightly exceed in
weight the silver which was actually present. This is because of the
retention in the buttons of a small quantity of lead. It has been stated
that the proportion of lead thus retained may be as much as 1% of the
silver present; this, however, can only be under exceptional conditions.
A determination of the actual silver in the buttons got in the series of
cupellations quoted on pages 102, 103, gave an average percentage of
99.85, so that even with the larger buttons the effect of the retained
lead would be only to increase the weight by about 1 milligram. In the
method of working with checks, the retained lead has no disturbing
influence.
~The proportion of lead required~ for the cupellation of any particular
alloy requires consideration. With too much lead the time occupied in
the process is increased, and so is the loss of silver; on the other
hand, too little lead is of greater disadvantage than too much. From 8
to 16 parts of lead are required for each part of silver alloy, or, if
gold is present, about twice as much as this must be used. For the
cupellation of 1 gram of a silver copper alloy containing different
percentages of copper, the following quantities of lead should be
used:--
Percentage of Copper
in Alloy.
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