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

68 ENAMELS AND ENAMELLING.
Manganese dioxide imparts a violet shade to cobalt blue,
whereas the protoxide has no injurious influence on the colour.
If manganous oxide and ferrous oxide be both present they
neutralise each other, so far as their effect on the colour is
concerned.
Smalt.
Of all the cobalt compounds employed in colouring glass
or enamel masses, none is so widespread as that manu-
factured in Saxony and sold under the name of smalt.
Chemically considered, smalt is a glass coloured an intense
blue by means of cobalt, and most nearly resembling cobaltous
silicate in composition and pigmentary qualities.
The manufacture of smalt need not be gone into in the
present work, as enamel-makers will purchase their supplies
and not attempt to prepare the article themselves. We will
therefore merely give a general outline of the process.
The cobalt ore is roasted in such a manner that scarcely
anything but cobaltous oxide is produced. The other metals
are not oxidised but separate, mainly as arsenides,
in the
subsequent process of smelting, and constitute an important
source of nickel.
The roasted ore is then fused with potash and silica,
a cobaltous-oxide-potash glass being formed. This glass,
on being poured into cold water, solidifies as a brittle mass,
which is then stamped to fine powder and subjected to a very
tedious process of washing by sedimentation, whereby the
powder is separated into various degrees of fineness, a con-
dition which exercises some influence on the intensity of
colour. It is an essential condition, especially when fine
enamels are in question, that none but the finest grades of
smalt should be used ;
and by adhering to this rule an unim-
peachable product can be obtained by the employment of
this material in all cases where the attainment of the very
highest degree of beauty of colour is not in question.