Enamels and enamelling; an introduction to the preparation and ...

PIGMENTS. 51
condense on the cooler upper sides of the crucible which
therefore should not be embedded in carbon and form
delicate lustrous needles, which are scraped off and removed
from time to time.
This oxide (Flores antimonii argentei, or Nix stibii) is also
impure and usually contains variable proportions of antimonic
acid and antimony antimonate. To restrict the formation of
the latter it is advisable to heat the crucible no higher than
is necessary to cause the antimony to ignite and continue
burning.
A better method than the foregoing is to raise a crucible to
.strong red by embedding it in glowing charcoal, and throw
in small quantities of the following finely powdered and
intimately mixed ingredients:
Antimony
37 parts by weight.
Saltpetre . . 20 ,,,',
Potassium bi-sulphate . . . . . 15 ,, ,, ,,
When the whole has been thrown in, the crucible is kept red
hot until the contents flow calmly, whereupon they are poured
out on to a cold stone slab. On breaking up the solidified mass
and extracting it with boiling water, nearly pure antimony
oxide is left.
The purest oxide, and that most suitable for fine enamels,
is obtained as follows:
Commercial antimony sulphide is reduced to fine powder
and boiled in a glass or porcelain vessel with hydrochloric
acid, in which it dissolves with evolution of sulphuretted
hydrogen. When perfectly cold the liquid is decanted from
the insoluble residue and transferred to a large vessel contain-
ing at least a twenty-fold quantity of water, into which the
solution is run as a thin stream with continual stirring.
The
entire liquid immediately turns milky white, and a heavy
white powder (powder of algaroth) is precipitated. This is
collected in a conical filter of closely woven linen and washed