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

THE ENAMEL MASS. 101
suffered from the defect that the glaze was soon rendered
unsightly by the aforesaid cracks, and quickly began to chip
off a behaviour which greatly retarded the general extension
of enamelled goods for domestic purposes, by reason of the
lack of durability exhibited by this somewhat expensive
ware.
For a long time endeavours have been made to produce
enamel masses possessed of sufficient properties of expansion
and not liable to crack and chip off; but this object was not
attained until the practice was adopted of replacing a single
layer of enamel by two, of different molecular and chemical
properties, the one mass being termed the ground, and the
other the covering layer.
THE GROUND LAYER.
As its name implies, the ground layer of enamel is the one
applied direct to the surface of the metal, and separating the
latter from the covering layer. With regard to the properties
of the ground layer it should be noted that the mass should
be of a very refractory character, and only sintered, without
being actually fused, upon the surface of the metal, so that it
covers the latter, not as a perfectly coherent coating but as a
porous mass. It is as a result of this porosity
that the metal
is able to expand and contract rapidly without causing a
separation of the constituent particles of the ground layer of
enamel.
THE COVERING LAYER.
This layer, which acts as a glaze towards the ground layer, is
invariably compounded in such a manner as to be more readily
fusible than the latter, and to unite therewith at the surface
of contact, so that the enamel exhibits, in the finished state, a
gradual transition from the covering layer, through an intermediate
mixed stratum, to the ground layer.
The method of treating the masses intended to form the