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

66 ENAMELS AND ENAMELLING.
and roasting them with unrestricted access of air, which
volatilises as white arsenic (arsenious acid) the greater
part of the arsenic usually present.
The residue, stirred up to a thick pap with concentrated
redness in a furnace
.sulphuric acid, is then heated to bright
.and extracted with water. The cobaltous sulphate resulting
from the sulphuric acid treatment has the power of with-
standing the action of very high temperatures, which decom-
pose the salts proceeding from the other oxides in the air,
and convert them for the most part into oxides or insoluble
basic salts.
During the extraction of the calcined mass, only a small
quantity of ferrous sulphate passes into solution along with
the cobaltous sulphate. The former may be removed by the
addition of small doses of sodium carbonate solution, which
throws down ferric oxide as a brown precipitate, the end point
,of this reaction being easily detected after a little practice,
since the cobaltous carbonate beginning to come down can be
immediately recognised by its pale blue colour.
The cobalt solution thus freed from iron is filtered from
the ferric hydrate precipitate, and is then treated with
.sodium silicate (water glass) to throw down cobaltous silicate.
This pale blue precipitate is washed, dried and fused, in
"which state it forms a dark pure blue mass particularly well
.adapted for colouring enamels. A very small quantity of
this silicate, when reduced to powder by quenching and grinding,
is sufficient to impart a beautiful blue to the enamel
mass.
Apparently the quality
of the enamel mass exerts con-
siderable influence on the intensity of the colour produced by
this preparation ; and
it is therefore necessary to ascertain
from time to time, by fusion tests, the exact amount of cobal-
tous silicate required for the development of any given shade.