_--The ore to be smelted or assayed
should be concentrated to say not less than 50 per cent. of metallic
tin; though to obtain satisfactory results it should be brought nearer
70 per cent., as with ore containing less than 40 to 50 per cent. of
metal there will be a considerable loss both in the assaying and in the
smelting. If the ore to be operated on does not contain this quantity of
metal, then the sample (if coarse) must be reduced to a fine state, the
gangue being removed by vanning, and the ore saved for the fire assay.
The method adopted for the determination of tin in the ore is as
follows:--About 2-1/2 ounces troy (1200 grains, or about 80 grams) of
the ore to be assayed is weighed out and mixed on a flat copper pan
(shaped with a long lip) with one-fifth of its weight (240 grains, or
15.5 grams) of powdered culm (anthracite). The mixture of ore and culm
is either transferred to a black-lead crucible before the latter is put
into the furnace, or, as some prefer, it is carefully swept into a
crucible which has been imbedded in the fire. Some assayers cover their
pots with a flat cover placed loosely on, while others leave the mixture
in the open pot. The furnace, which has been previously fired to a
strong heat, is then covered, and the sample is subjected to a sharp
fire for a period of from twelve to twenty minutes.
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