Take out
the rod, washing it in the slag, and, in a minute or two, pour. Clean
and cupel the button of lead.
~General Remarks on the Fusion.~--Other things being equal, the smaller
the quantity of the slag the better, provided there is sufficient to
cover the metal. The presence of peroxides of the heavy metals is
prejudicial, since they tend to increase the quantity of silver retained
in the slag. It may be given as a general rule that when iron, copper,
manganese, &c., are present, there is a more than ordinary need for
cleaning the slags, and care must be taken to keep these metals in the
state of lower oxide.
In selecting the fluxes, it should be remembered that soda is the best
for quartz, and borax for lime and metallic oxides. And that with ores
almost free from gangue some quartz or glass should be added to protect
the crucible. Two parts of soda are enough to flux 1 part of quartz;
whilst of borax, or oxide of lead, 4 parts are barely sufficient. Oxide
of lead has the advantage of being heavy and so does not occupy much
space in the crucible; on the other hand, if the melting down be
performed too quickly, or if oxide of lead only is used, this high
specific gravity is a disadvantage, for the lighter earthy matter floats
as a pasty mass on the more fluid oxide of lead, and thus escapes its
action.
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