Page 230 - My FlipBook

21 8 MFXTIANTCAL DENTISTRY AND METALLURGY.
When silver is present, add to the molten alloy from two
to four times its weight of antimony sulphid, SboS., ; this
must be added carefully and a little at a time. The heat
decomposes the sulphids. The sulphur unites with the
silver and other base metals, forming sulphids, while the
antimony unites with the gold, forming a leaden-colored
alloy. When effervescence has ceased, remove the crucible
from the fire and allow it to cool. The antimony and gold
alloy will be found in the bottom of the crucible, and the
sulphids on the surface.
Miller's process of separating silver from gold, consists
in melting the alloy in a crucible, glazed internally with
borax, then by means of a fire-clay tube passing through
the lid of the crucible, a current of chlorin gas is forced
into the molten mass. The silver and other impurities are
converted into chlorids which rise to the surface. After
the gold cools the still molten chlorid is poured off. The
gold is cleansed from any adhering chlorid, then remelted
and cast.
To separate the antimony from the gold, remelt the
alloy and throw upon the molten mass a current of air from
a blowpipe. Antimony oxid, SboOo, is formed and volatil-
ized ; continue the process until fumes cease to be given off.
When Iridium is Present.—Professor Essig, in writing
upon the subject, says : " The little, hard grains occasionally
met with in gold, upon which the file makes no impression,
consist of iridium, or a native alloy of osmium and iridium
and are not combined with the gold, but merely dissemi-
nated through it. The only dry method of separating it
from gold consists in alloying the latter with three times
its weight in silver, by which means the specific gravity of
the metal is so much lowered that iridium, which is very
infusible and of a specific gravity of 21.1, will subside to
the bottom of the crucible, when the gold and silver alloy