The usual
explanation is that with sulphury ores compounds of metallic oxides and
sulphides (oxysulphides) are formed, which remain in the slag, retaining
considerable quantities of the precious metal. It is said that under
certain conditions such a slag may contain as much as 10 per cent. of
silver. An excess of lead and a high temperature prevents the formation
of these oxysulphides. But if much silver is present in the ore, the
slag cannot be safely thrown away, even if sulphur is absent, and the
process has been satisfactorily performed.
If the crust which appears on the surface of the lead does not clear,
add a small lump of borax and 20 grams more lead; then close the muffle,
and keep the temperature as high as possible. If the slag forms
properly, but shows unfused or only half-fused lumps, even when the
scorification has proceeded for some time, add more borax, and stir with
an iron rod. The slag adhering to the rod must be detached by hammering,
and replaced in the scorifier.
If the ore consists largely of quartz, soda should be added instead of
borax; or, if it contains much copper, powdered quartz may be used. If
the scorifier at the end of an operation is more than usually corroded,
the borax should be replaced in subsequent assays on similar ores by
powdered glass or quartz.
If a fairly fluid slag is formed which does not clear from the metal and
show the eye, more lead and a higher temperature is wanted.
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