If the temperature
during calcination has been too low sulphates are formed, which are
again reduced to sulphides in the subsequent fusion. To prevent this the
roasted regulus is recalcined at a higher temperature, after being
rubbed up with a little anthracite. The roasted substance must not smell
of burning sulphur when hot. It is practically a mixture of the oxides
of copper and iron.
~Fusion for Coarse Copper.~--The calcined regulus is mixed with a flux
consisting of borax and carbonate of soda, with more or less tartar
according to its weight. Some "assayers" use both tartar and nitre, the
former of course being in excess. The charge is returned to the crucible
in which it was calcined, and is melted down at a high temperature, and,
as soon as tranquil, poured. When solid it is quenched and the button of
metal separated.
The slag is black and glassy. The small quantity of copper which it
retains is recovered by a subsequent "cleaning," together with the slags
from the next operation.
The button of "coarse copper" obtained must be free from a coating of
regulus. It will vary somewhat in appearance according to the nature and
quantity of the impurities.
~Refining the Coarse Copper.~--The same crucible is put back in the
furnace, deep down and under the crevice between the two bricks.
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