The fusion should be
made at a high temperature, so as not to occupy more than from 20 to 25
minutes. Thirty grams of ore is taken for a charge, mixed with 20 grams
of cream of tartar, and 10 grams each of dried borax and soda. If the
gangue of the ore is basic, carrying much oxide of iron or lime, silica
is added, in quantity not exceeding 10 grams. If, on the other hand, the
gangue is mainly quartz, oxide of iron up to 7 grams must be added.
_Example._--Twenty grams of copper pyrites, known to contain 27.6 per
cent. of copper, gave by the method first described 5.22 grams of
copper, equalling 26-1/8 per cent. Another sample of 20 grams of the
same ore, calcined, fused with 40 grams of nitre, and washed to ensure
the removal of arsenic and sulphur, and treated according to the second
method, gave a button weighing 5.27 grams, equalling 26-3/8 per cent.
The ore contained a considerable quantity of lead. Lead renders the
assay more difficult, since after calcination it remains as lead
sulphate, and in the fusion for coarse copper reappears as a regulus on
the button.
~The Estimation of Moisture.~--The Cornish dry assayer very seldom makes
a moisture determination. He dries the samples by placing the papers
containing them on the iron plate of the furnace.
It is well known that by buying the copper contents of pyrites by
Cornish assay, burning off the sulphur, and converting the copper into
precipitate, a large excess is obtained.
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