The resulting slag is strongly basic and very corrosive; no
crucible will long withstand the attack of a fused mixture of oxides of
lead and copper. With silicates, also, it forms very fusible double
silicates; but in the absence of silicates and borates it has no action
upon lime or magnesia. Whether the lead be added as litharge or as red
lead, it will exist in the slag as monoxide (litharge); the excess of
oxygen of the red lead is thus available for oxidising purposes. If this
oxidising power is prejudicial, it may be neutralised by mixing the red
lead with 1 per cent. of charcoal.
~Glass~: broken beakers and flasks, cleaned, dried, and powdered will
do. It should be free from lead.
~Fluor~: fluor-spar as free as possible from other minerals, powdered.
It helps to flux phosphate of lime, &c., and infusible silicates.
~Lime~: should be fresh and powdered. It must not be slaked. Powdered
white marble (carbonate of lime) will do; but nearly double the quantity
must be taken. One part of lime produces the same effect as 1.8 parts of
the carbonate of lime.
~Tartar~ and "black flux," are reducing agents as well as fluxes. The
"black flux," which may be obtained by heating tartar, is a mixture of
carbonate of potash and charcoal.
REDUCING AGENTS.--The distinction between reducing agents and fluxes
(too often ignored) is an important one.
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