These are all attacked by
nitric acid. They nearly all contain a small quantity of organic matter,
and frequently considerable quantities of lead, zinc, silver, gold,
arsenic, bismuth, &c.
The copper ores are often concentrated on the mine before being sent
into the market, either by smelting, when the product is a regulus or
matte, or by a wet method of extraction, yielding cement copper or
precipitate. A regulus is a sulphide of copper and iron, carrying from
30 to 40 per cent. of copper. A precipitate, which is generally in the
form of powder, consists mainly of metallic copper. Either regulus or
precipitate may be readily dissolved in nitric acid.
Copper forms two classes of salts, cuprous and cupric. The former are
pale coloured and of little importance to the assayer. They are easily
and completely converted into cupric by oxidising agents. Cupric
compounds are generally green or blue, and are soluble in ammonia,
forming deep blue solutions.
DRY ASSAY.
That, for copper, next after those for gold and silver, holds a more
important position than any other dry assay. The sale of copper ores has
been regulated almost solely in the past by assays made on the Cornish
method. It is not pretended that this method gives the actual content of
copper, but it gives the purchaser an idea of the quantity and quality
of the metal that can be got by smelting.
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