The methods of working vary according to the nature of the ores treated
and local circumstances. The dry method, or ordinary smelting, cannot be
profitably practised with ores containing less than 4% of copper, for
which and for still poorer ores the wet process is preferred.

_Copper Smelting._--We shall first give the general principles which
underlie the methods for the dry extraction of copper, and then proceed
to a more detailed discussion of the plant used. Since all sulphuretted
copper ores (and these are of the most economic importance) are
invariably contaminated with arsenic and antimony, it is necessary to
eliminate these impurities, as far as possible, at a very early stage.
This is effected by calcination or roasting. The roasted ore is then
smelted to a mixture of copper and iron sulphides, known as copper
"matte" or "coarse-metal," which contains little or no arsenic, antimony
or silica. The coarse-metal is now smelted, with coke and siliceous
fluxes (in order to slag off the iron), and the product, consisting of
an impure copper sulphide, is variously known as "blue-metal," when more
or less iron is still present, "pimple-metal," when free copper and more
or less copper oxide is present, or "fine" or "white-metal," which is a
fairly pure copper sulphide, containing about 75% of the metal. This
product is re-smelted to form "coarse-copper," containing about 95% of
the metal, which is then refined. Roasted ores may be smelted in
reverberatory furnaces (English process), or in blast-furnaces (German
or Swedish process). The matte is treated either in reverberatory
furnaces (English process), in blast furnaces (German process), or in
converters (Bessemer process). The "American process" or "Pyritic
smelting" consists in the direct smelting of raw ores to matte in blast
furnaces. The plant in which the operations are conducted varies in
different countries. But though this or that process takes its name from
the country in which it has been mainly developed, this does not mean
that only that process is there followed.

The "English process" is made up of the following operations: (1)
calcination; (2) smelting in reverberatory furnaces to form the matte;
(3) roasting the matte; and (4) subsequent smelting in reverberatory
furnaces to fine- or white-metal; (5) treating the fine-metal in
reverberatory furnaces to coarse- or blister-copper, either with or
without previous calcination; (6) refining of the coar