It consists in using two pots only, of which the
lower is placed at such a height that the bottom of it is about 12
in. to 15 in. above the floor level, while the upper is placed at a
sufficiently high level to enable the lead to be run out of it into
the lower pot. The capacity of the lower pot, in those most recently
erected, is thirty-six tons--double that of the upper one. Round each
pot is placed a platform, on which the workmen--of which there are two
only to each apparatus--stand when skimming, slicing, and charging the
pots. The upper pot is open at the top, but the lower one has a cover,
with hinged doors; and from the top of the cover a funnel is carried
to a set of condensers. At a convenient distance from the two pots is
placed a steam or hydraulic crane, so arranged that it can plumb each
pot, and also the large moulds which are placed at either side of the
lower pot. The mode of working is as follows:
The silver lead is charged into the upper pot by means of the crane.
When melted, the dross is removed, and the lead run into the lower, or
working pot, among the crystals remaining from a previous operation.
When the whole charge is thoroughly melted, it is again drossed; and in
order to keep the lead in a thoroughly uniform condition, and prevent it
setting solid on the top and the outside, a jet of steam is introduced.
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