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2l6 MECHANICAL DENTISTRY AND METALLURGY.
If, for example, zinc or iron (jr l)()th of these metals are
present in small (juantities, any compound which yields
oxygen will, l)y virtue of the affinity of the latter for these
metals, effect their separation hy converting them into
oxids ; hence, when these metals are to be got rid of,
potassium nitrate is employed. But oxygen has a feeble
affinity for tin, and wdien this metal is present, its separation
is better eft'ected by some compound wdiich parts with
chlorin in the act of decomposition ; mercuric chlorid is
therefore used for the purpose. When the alloy of gold
contains a number of these metals at the same time, and is
very coarse, antimony sulphid, which is a very pow^erful
and efficient reagent, should be resorted to, unless the
operator should prefer, and which is the better way, to
reduce the alloy to pure gold by the " humid method."
The Dry Method.—After all traces of iron or steel have
been removed from the gold fragments and filings by
passing a magnet repeatedly through them the latter should
be placed in a clean crucible, lined on the inside with borax,
and covered either with a piece of fire-clay slab or broken
crucible. Sheet-iron has been recommended for the latter
purpose, but should iici'cr be used, as, when highly heated,
seales form on the surface, and are liable to drop in upon
the fused metals. If the operation is likely to be protracted,
an inverted crucible, with a hole in the bottom, may be
securely luted to the top of the one containing the metals,
the refining agents and fiuxes being introduced through
the opening in the upper crucible. These are then placed in
the furnace, on a bed of charcoal, or what is better, a mix-
ture of charcoal and coke, tlie latter l)eing built up around
the crucible, and over it when covered with a second cruc-
ible, care being taken that no fragments of fuel are per-
mitted to fall in upon the fused metals. The process is
as follows :
2l6 MECHANICAL DENTISTRY AND METALLURGY.
If, for example, zinc or iron (jr l)()th of these metals are
present in small (juantities, any compound which yields
oxygen will, l)y virtue of the affinity of the latter for these
metals, effect their separation hy converting them into
oxids ; hence, when these metals are to be got rid of,
potassium nitrate is employed. But oxygen has a feeble
affinity for tin, and wdien this metal is present, its separation
is better eft'ected by some compound wdiich parts with
chlorin in the act of decomposition ; mercuric chlorid is
therefore used for the purpose. When the alloy of gold
contains a number of these metals at the same time, and is
very coarse, antimony sulphid, which is a very pow^erful
and efficient reagent, should be resorted to, unless the
operator should prefer, and which is the better way, to
reduce the alloy to pure gold by the " humid method."
The Dry Method.—After all traces of iron or steel have
been removed from the gold fragments and filings by
passing a magnet repeatedly through them the latter should
be placed in a clean crucible, lined on the inside with borax,
and covered either with a piece of fire-clay slab or broken
crucible. Sheet-iron has been recommended for the latter
purpose, but should iici'cr be used, as, when highly heated,
seales form on the surface, and are liable to drop in upon
the fused metals. If the operation is likely to be protracted,
an inverted crucible, with a hole in the bottom, may be
securely luted to the top of the one containing the metals,
the refining agents and fiuxes being introduced through
the opening in the upper crucible. These are then placed in
the furnace, on a bed of charcoal, or what is better, a mix-
ture of charcoal and coke, tlie latter l)eing built up around
the crucible, and over it when covered with a second cruc-
ible, care being taken that no fragments of fuel are per-
mitted to fall in upon the fused metals. The process is
as follows :
