When metallic sulphides are present in the ore, an excess of oxide of
lead helps to keep the sulphur out of the button of metal. In addition
to the oxide of lead required as a flux, some will be required to
provide the lead in which the silver is to be collected. Oxide of lead,
mixed with charcoal or flour, yields, when heated, a multitude of minute
buttons of metal uniformly distributed through the mass of the charge;
as the charge melts down these run together and fall to the bottom; this
shower of lead collects the silver more easily than a single button at
the bottom of the crucible could do. Only that portion of the oxide of
lead which remains in the slag can be considered as a flux; very often
the first indication of an excessive reduction of lead is the pastiness
of the slag rendered thick by the withdrawal of the oxide of lead which
would have kept it fluid. If, in an assay, it is found that 5 parts of
flux are not sufficient for 1 part of ore, the remedy lies in using a
different flux rather than in taking a larger quantity.
_On the Reducing Effect of Charcoal, Flour, and Tartar._--The weight to
be got from a given charge will depend (provided sufficient oxide of
lead is present) upon the proportion of the reducing agents in it. We
have thought it well to illustrate this part of the subject by a series
of experiments which the learner will do well to practise for himself
before proceeding to the assay of actual ores.
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