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

56 ENAMELS AND ENAMELLING.
Instead of dissolving iron in sulphuric acid, as above, a
solution of green vitriol (ferrous sulphate), previously boiled
with a little nitric acid, may be used. This is treated with
ammonia so long as a precipitate continues to come down.
When washed and dried this precipitated ferric hydrate may
if a
be used just as well as the pure ferric oxide. Of course,
pure hydrate is to be obtained the ferrous sulphate taken
must also be pure, whereas the commercial salt very often
contains copper sulphate, the presence of which will spoil the
colour of the enamel. By reason of its very bulky nature
ferric hydrate is exceedingly troublesome to wash, for which
reason it is better to prepare the ferric oxide from the oxalate,
in the manner described above.
Uranium Oxide.
Uranium is a metal allied to iron, and is found as pitch
blende and uranite, as well as in other rare minerals, from the
first named of which it is generally prepared. The production
of uranium oxide on a small scale is attended with so much
complication that no enamel-maker should attempt to prepare
it ; besides it can now be obtained in commerce, in a pure
state and at comparatively low price, its productivity being so
great that 1 part is sufficient to colour 200 parts of glass a
deep yellow.
The colour imparted by uranium oxide to glass is highly
characteristic, being dichroic : a beautiful wine yellow by trans-
mitted light, and a handsome canary green by reflected light.
The only method of producing this colour on enamel is to
provide a fundamental layer of very refractory white enamel,
and cover this with a fused layer of potash crystal glass
coloured with uranium oxide. Even in this case, however,
but little of the dichroic appearance is generally perceptible
the glass exhibiting only a more or less intense yellow
coloration.