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

104 ENAMELS AND ENAMELLING.
the materials of the Hessian crucible on the enamel mass is a
very important consideration. Masses rich in basic substances,
e.g., soda, attack these crucibles to a considerable extent and
form, at the surface of contact, highly refractory compounds.
Furthermore, the iron compounds present in the crucible
material make their way, to some extent, into the enamel and
produce coloration these results ensuing more particularly
while the crucibles are still new, since the highly refractory
layer, formed by combination with silica on the walls of the
crucible during the smelting of the first charge, constitutes a
kind of glaze which, in subsequent use, both preserves the
crucible from further corrosion and prevents the contents
from becoming contaminated.
The chief cause operating to spoil the crucibles is,
usually, the practice of refilling them with a fresh charge
whilst still in the furnace, immediately after the fused charge
has been run off through the aperture provided for the purpose
at the bottom of the crucible. At the moment of emptying,
the crucible is at the maximum temperature attainable in the
furnace, and if now the fresh charge of comparatively cold
materials be introduced at once, the result is that cracks are
liable to be produced, which constantly increase and often
terminate in the breakage of the crucible just at the moment
when it is full of fused material, thus causing a great waste of
time and stuff.
It not infrequently happens that a crucible is already
spoiled before it has been used at all, since when left exposed
to the air for some time the crucibles have a tendency to
absorb moisture, in a quantity sufficient though
in itself
small to cause them to crack if exposed to the heat of the
furnace without due precaution.
Thus, when put suddenly
into the hot furnace, the water contained in the pores is con-
verted immediately into steam, which by its rapid efforts to
escape gives rise to cracks in the mass of the crucible.