The liquid lead is run into large cone-shaped moulds on either side of
the pot; and a wrought iron ring being cast into the blocks thus formed,
they are readily lifted, when set, by the crane. To give some idea of
the rapidity of the process, it may be mentioned that from the time the
lead is melted and fit to work in the big pot, to the time that it is
crystallized and ready for tapping, is, in the case of a 36 ton pot,
from thirty-five to forty-five minutes; and the time required for
tapping the liquid lead into the large moulds is about eight minutes.
Before the lead begins to crystallize, the upper pot is charged with
lead of half the richness of that in the lower pot. Thus, when the
liquid lead has been tapped out of the lower pot, it is replaced by a
similar amount of lead of the same richness as the remaining crystals,
by simply tapping the upper or melting pot, and allowing the contents to
run among the crystals.
The same operation is repeated from time to time, until the crystals are
so poor in silver that they are fit to be melted, and run into pigs for
market.
The large blocks of partially worked lead are placed by the crane in
a semicircle round it, and pass successively through the subsequent
operations. The advantages of the steam process, as compared to the old
six-ton Pattinson pots formerly used by the writer's firm, are: (1) a
saving of two-thirds amount of fuel used; (2) the saving of cost of
calcination of the lead to the extent of at least four-fifths of
all that is used; (3) above all, a saving in labor to the extent of
two-thirds.
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