The process has its disadvantages, and these are a larger
original outlay for plant, and a constant expense in renewals and
repairs. This is principally caused by the breakage of pots; but with
increased experience this item has been very much reduced during the
last two or three years.
The "zinc process" of desilverizing, which is largely used by Messrs.
Locke, Blackett & Co., and was patented in the form adopted by them
about fourteen years since. The action of this process is dependent on
the affinity of zinc for silver. The following is a brief description of
it:
A charge of silver lead, usually about fifteen tons, is heated to a
point considerably above that which is used in either the Pattinson or
the steam process. The quantity of zinc added is regulated by the amount
of silver contained in the lead; but for lead containing 50 oz. to the
ton, the quantity of zinc used is in most cases about 11/2 per cent, of
the charge of lead. The lead being melted as described, a portion of
this zinc, usually about half of the total quantity required for the
charge, is added to the melted lead, and thoroughly mixed with it by
continued stirring. The lead is now allowed to cool, when the zinc is
seen gradually to rise to the top, having incorporated with it a large
proportion of the silver.
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