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

PIGMENTS. 63
-cipitated from combination it is converted, on exposure to air,
into ferric oxide, which gives a brownish yellow. Consequently
the only way to employ ferrous oxide is in the condition of a
ferrous salt, taking care at the same time that the enamel
mass is free from any easily reduced metallic oxide (e.g., lead
oxide). The mass can only be used as a transparent coating
or " cover
The best form of ferrous salt is pure ferrous sulphate,
which can be most suitably prepared in the following manner :
Iron is dissolved in dilute sulphuric acid, the metal being-
kept in excess, and the reactions assisted finally by gentle
heat. The hot solution is filtered into a vessel containing
an equal bulk of very strong alcohol, which is kept stirred
during the operation in order to prevent
the formation of
large crystals. After leaving at rest, covered up, for twelve
hours, the fine crystalline deposit of pure ferrous sulphate is
dried quickly by pressing between layers of filter paper, and
is then stored in properly closed vessels. It is added to the
enamel, in a dehydrated condition.
This pure sulphate will yield an agreeable green, and one
that is particularly suitable for use in imitations of old works
in enamel.
Cupric Oxide.
Cupric oxide may be obtained by the direct method of heat-
ing fine filings of the metal to redness in a flat basin until they
have become black, the metal having absorbed oxygen from
the air and changed into cupric oxide. The complete con-
version of the metal in this manner is nevertheless a lengthy
operation ; the product thus obtained, however, may be used
to advantage as a green colouring matter for ordinary enamels.
Special beauty of tone is furnished by a product obtained
from chemically pure copper by the very simple method of
preparation described below, which may be universally recommended.