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

plug is made rather too large,
SMELTING THE CHARGE. 113
coming into contact with the cold damp
that the crucible cracks on
mass. To avoid this
evil the author generally uses well-fitting plugs of wrought
iron attached to wrought-iron rods, the lid of the crucible
being then provided with a slit extending nearly to the centre
or, better still, divided in two pieces through the middle
to admit of the insertion of the rod. Fig.
13 shows the
arrangement of a crucible fitted with such an iron plug.
To prevent any injurious results accruing from the presence
FIG. 13.
of iron in contact with the enamel mass, this rod and plug are
previously coated with an enamel of such refractory character
that, though fusible at a very high temperature,
it remains
solid at the temperatures attained in smelting ordinary
charges.
If, when the plug is raised to allow the molten enamel to
run out, it is lowered again before the crucible is quite empty,
the aperture will be effectually sealed by the solidification of
the residue on the introduction of the comparatively cool fresh
charge of materials. Subsequently, when the contents again