How much this must be (if not already
known) may be approximately determined by fusing 3 grams of the ore and
3 grams of "soda" in a small crucible (C) with 50 grams of litharge (not
red lead) under a cover of salt, and weighing the resulting button of
lead. Subtract 3 from the weight of lead obtained, and the difference
multiplied by 1.3 will give the quantity in grams of nitre required. If
the button of lead weighs less than 3 grams flour must be added. If this
is not satisfactory repeat the assay, adding an extra gram of nitre for
each 4 grams of lead in excess of that required, or 1 gram of flour for
a 12-gram deficiency.
In the method in which iron is used as a de-sulphurising agent, only as
much oxide of lead should be added as will give a button of lead of the
required size. Rather a large button of lead should be got, and the slag
should be strongly alkaline; if the ore does not already carry a large
amount of sulphur some should be added. The fusion should be performed
at a low temperature (similar to that for a galena assay), and should be
continued for some time after it has become tranquil. Take 20 grams of
the ore, 40 grams of "soda," 40 grams of oxide of lead, and 5 or 10
grams of borax; place this mixture in a crucible (with a rod of iron, as
in the galena assay), cover, and fuse for about half an hour.
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