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

PREPARING THE ARTICLES FOR ENAMELLING. 137
or reddish colour, due to the dissolving of the layer of oxide
at the surface of the metal. If this oxide layer contained
much cupric oxide, the adjacent enamel will appear bluish-
green, whilst if chiefly cuprous
oxide the colour will be of a
reddish cast. In order to obtain a pure white coating of
enamel under these circumstances, the cover enamel must
contain sufficient tin oxide to completely mask the ground
layer ; otherwise the colour of the latter will show through
and affect the shade of the superior layer. Thus, copper can
be enamelled without having been pickled, provided the
expense of preparing
too high.
The conditions are, however, totally
enamelling of iron is in question,
such a cover enamel is not considered
different when the
since that metal is never
combined with other substances.
chemically pure but is always
For instance, cast iron may be regarded as iron containing
variable quantities of iron carbide, iron silicate, etc., in solu-
tion, and also embedded crystals of carbon (graphite).
In cast
iron also the outer layer or " casting skin " must be dis-
tinguished from the inner substance of the metal, exhibiting
the above-named character. This outer skin consists of ferric
oxide, and of particles of carbon derived from the mould in
which the metal was cast these moulds being dusted over
inside with charcoal powder, before use. Small crystals of
graphite also separate out in the external layer
during cooling.
of the iron
Experience shows that only certain kinds of iron can be
enamelled with ease, this property depending on the method
pursued in making the cast iron. For example, iron that is
drawn off from the blast furnace whilst still in an extremely
hot (molten) condition, cools rapidly in the moulds and then
forms the so-called " white "
iron, which is distinguished by
its steel-grey colour, great hardness, and by containing, in &
very finely-divided state, all the carbon originally dissolved in