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

XVIII.
PEEPAEING THE AETICLES FOE ENAMELLING.
Enamelled articles for technical purposes consist either of
iron ware (cast or wrought iron) or, less frequently, of copper
or brass utensils ;
and in order to enable the ground enamel to
adhere to the metal and prevent it chipping off, the latter
has to be prepared beforehand by a special treatment.
This is a very important matter, since if the operation is
not carefully performed, even the best-made enamel may
chip off in a very short time. Copper articles are much easier
to prepare than those made of iron, and it is never necessary
for the first-named metal to be made perfectly bright by
dipping in a pickling bath (dilute acid), since the enamel can
be applied direct, provided the articles are not greasy on the
surface.
The peculiar brownish-red colour generally exhibited by
copper ware is due to a thin layer of cuprous oxide, and
when enamel is fused on such a surface it adheres strongly
there on cooling. This is owing to the behaviour of the acid
substances (boric acid and silica) in the enamel mass, these
being in sufficient quantity to combine with a certain amount
of oxides on fusing ; so that when enamel is melted on an
uncleaned surface of copper the cuprous oxide is dissolved by
the mass, which then adheres firmly to the metal.
If a piece of copper, covered, for instance, with white
enamel, be examined by scraping off the enamel layer, it will
be found that the stratum nearest the metal is of a greenish