As a general rule, it may be stated that when a scorification is
unsatisfactory, what is wanted is more heat, more lead, or more borax.
It is a safe plan when work has to be done on a strange ore, to make
three or four assays with varying quantities of lead. The proportion of
lead is right when a further addition does not yield a higher result.
The proper proportion having been found, a note of it should be made for
future use.
POT ASSAYS.
The object of the fusion in a crucible, like that of scorification, is
to concentrate the silver in a button of lead which is to be
subsequently cupelled; and to retain the earthy and waste matters in the
slag. It is necessary to consider the quality of the slag and the weight
and quality of the lead. The slag when fused should be liquid and
homogeneous, and not too corrosive on the crucible. The button of lead
should be soft, malleable, and free from a coating of regulus.[10] In
weight it should not differ much from the ore taken. With 20 grams of
ore, for example, a button of lead weighing from 18 to 25 grams will be
satisfactory: less than this would leave an undue proportion of silver
in the slag; and more would be unnecessarily large for cupelling, and
would increase the loss in that operation.
With average ores, take 20 grams of the powdered ore and mix with 30
grams of "soda," 40 grams of red-lead or litharge, 5 grams of borax, and
from 2 to 2.
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