~Dry Assay.~--This assay is made up of two parts: (1) the concentration
of the silver in a button of lead; and (2) the cupellation of the
resulting alloy. The concentration of the button of lead may be
effected either by scorification or by fusion in a crucible.
The scorification assay is performed in a scorifier, which is a shallow
open-mouthed dish about 2-1/2 inches across, with a very thick bottom to
enable it to withstand the corrosive action of the slag. A charge of
more than 3 or 5 grams of the ore cannot be worked in one, and with such
small charges the unavoidable variations have a serious effect on the
figures reported. A difference of one milligram on the weight of the
button of silver got represents a difference of 6 or 10 ounces per ton.
With rich ores such variation is unavoidable under any conditions, and
the only safe plan is to take the mean of several assays. But with
poorer ores the accuracy of the assay, as well as convenience in
working, is much increased by working in a crucible with larger charges.
In scorification the proportion of lead required for scorifying 1 gram
of ore is in average cases from 10 to 15 grams, sinking in the case of
galena to 2 grams, and rising with earthy and refractory substances to
from 30 to 40 grams. But by fusing in a crucible with well-selected
fluxes, a proportion of 4 of flux to 1 of ore is generally sufficient;
and not only is the proportion of added matter less, but it is also
easier to manipulate large quantities in crucibles, so that, although in
some cases the crucible assay is more troublesome and less satisfactory,
yet with poor and earthy ores it is the best method of dealing with
them; while when properly worked it yields results as accurate as
scorification does.
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