5 grams of flour, and place in an E crucible (Battersea
round). Put these in the furnace at a red heat, cover the crucible, and
gradually raise the temperature until the whole charge has melted down
and is in a state of tranquil fusion. Pour into a mould, and replace the
crucible in the furnace. As soon as the lead is solid, detach the slag
and put it back into the crucible; and when it is again fluid, charge on
to it with a copper scoop a mixture of 20 grams of oxide of lead, and 1
gram of charcoal: when fusion has again become tranquil, pour and detach
the button of lead. The lead buttons should be hammered into discs with
rounded edges, and be freed from slag; if too big for a cupel they may
be scorified together in a small scorifier, but it is better to cupel
them separately.
~Ores containing Metallic Oxides.~--Peroxides of iron, manganese, and
copper interfere by counteracting the effect of the charcoal or flour,
and thus reducing the size of the lead button. Peroxide of iron will
reduce the weight of lead by a little more than its own weight; and
peroxide of manganese has about twice this effect. When these oxides are
present an additional quantity of flour must be used, and precautions
must be taken to prevent reoxidation of the slag by the furnace gases.
This may best be prevented by using a layer of common salt as a cover to
the charge.
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