The
process leaves too much to the judgment of the vanner. It is more than
probable that in practice the returns from the dressing-floors check the
assayer, instead of, as should properly be the case, the assayer
checking the returns. It is only when this last is done that any control
is had over the system of dressing. A correct assay of this ore is a
matter of some importance, because of the high price of the metal.
The method of assaying the black tin is a dry one, and consists of
mixing it with "culm," and submitting it in a black-lead crucible to the
highest temperature of a wind furnace. The sample is taken wet as it
arrives at the smelting house, and is assayed direct. The product of the
assay is examined, and a deduction of a considerable percentage is very
properly made for impurities, since the assay really determines the
percentage, not merely of tin, but of the bodies present which are
reducible at a white heat. The judgment as to how much is to be deducted
is assisted partly by an examination of the metal got from the assay,
and partly by the experience acquired in smelting similar ores. The
produce, which is that of the impure tin, is stated in parts in twenty;
thus a produce of 14 is equivalent to 70 per cent., or to 14 cwt. per
ton.
[Illustration: FIG. 57.]
MECHANICAL SEPARATION.
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