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

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COMPOSITION OF COVER ENAMELS.
Chemically speaking, cover enamels are glasses made translucent
or even quite opaque by the addition of tin oxide or
other white opaque bodies. Since a principal requirement
exacted of these enamels is that they must be readily fusible
and distribute uniformly over the ground enamel when melted,
the composition must be regulated so as to enable these con-
ditions to be fulfilled.
For special reasons it is, however, important to keep their
fluidity within certain bounds, since if a cover enamel be
fused in a crucible, and, after continued heating, left to re-cool,
& peculiar result will be noticeable, viz. : that while the mass
at the bottom of the crucible is still pure white the intensity
of the colour diminishes towards the top, the upper layers
having the appearance of faintly turbid glass and being un-
suitable for use as enamel.
The cause of this is not far to seek, being due to the high
specific gravity of the tin oxide, which therefore settles down
to the bottom of the liquid mass and leaves the upper portion
almost free. Consequently, if the enamel is too fluid, it will
not yield a homogeneous product, whether the opaque material
employed be tin oxide or the cheaper lead oxide often used
in place of the former.
In this connection bone ash acts in a very favourable
manner, as, by reason of its low specific gravity,
it does not
.settle down even in extremely fluid enamels, and is also cheap.