The crucible is removed and its
contents poured into an iron mould. When the slag is solid, it is taken
up with tweezers and quenched in water. The regulus is easily detached
from the slag. It should be convex above and easily broken, have a
reddish brown colour, and contain from 40 to 60 per cent. of copper. A
regulus with more than this is "too fine," and with less "too coarse." A
regulus which is too fine is round, compact, hard, and of a dark bluish
grey on the freshly broken surface. A coarse regulus is flat and coarse
grained, and more nearly resembles sulphide of iron in fracture and
colour.
If an assay yields a regulus "too coarse," a fresh determination is made
with more nitre added, or the roasting is carried further. With low
class ores a somewhat coarse regulus is an advantage. If, on the other
hand, the regulus is too fine, less nitre or less roasting is the
remedy. With grey copper ores and the oxidised ores, iron pyrites is
added.
~Calcining the Regulus.~--It is powdered in an iron mortar and
transferred to a small Cornish crucible, or (if the roasting is to be
done in the muffle) to a roasting dish or scorifier. The calcining is
carried out at a dull red heat, which is gradually increased. The charge
requires constant stirring at first to prevent clotting, but towards the
end it becomes sandy and requires less attention.
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