The setting point of zinc being above that
of lead, a zinc crust is gradually formed, and this is broken up and
carefully lifted off into a small pot conveniently placed, care being
taken to let as much lead drain off as possible. The fire is again
applied strongly to the pot, and when the lead is sufficiently heated, a
further quantity of zinc, about one-third of the whole quantity used, is
added, when the same process of cooling and removing the zinc crust is
repeated. This operation is gone through a third time with the remaining
portion--1/4 per cent.--of zinc; and if each of these operations has
been carefully carried out, the lead will be found to be completely
desilverized, and will only show a very small trace of zinc. In some
works this trace of zinc is allowed to remain in the market lead, but
at Messrs. Locke, Blackett & Co.'s works it is invariably removed by
subjecting the lead to a high heat in a calcining furnace. The zinc
crusts, rich in silver, are freed as far as possible from the lead by
allowing this to sweat out in the small pot, after which the crusts are
placed in a covered crucible, where the zinc is distilled off, and a
portion of it recovered. The lead remaining, which is extremely rich in
silver, is then taken to the refinery, and treated in the usual manner.
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