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

XXI.
APPLYING AND FIRING THE COVER ENAMEL OR
GLAZE.
When the utensils are taken out of the muffle in which the
ground enamel has been fired on, the uncovered parts of the
metal will be found covered with black, or magnetic oxide of
iron resulting from the action of oxygen on the glowing
metal. As this layer of oxide easily falls off and will colour
any cover enamel it touches, it must be carefully removed
before the covering mass is applied. This operation is best
effected by scrubbing with steel-wire brushes, or sand paper.
The application of the diluted cover enamel mass is performed
in the same way as in the case of ground-enamel, the
extent of dilution having been determined by a preliminary
experiment, as the thinner the layer the handsomer and more
durable the enamel. The covering layer, must, however, be
thick enough to cover properly ;
and therefore, when the mass
is rich in tin oxide, it can be diluted to a greater extent than
otherwise feasible. Bone ash has less covering power than
tin oxide, and consequently a mass containing the former
must not be thinned down so much as if the latter body were
present. When the use of very dilute covering masses is
necessary, the greedy absorption of the contained water by
the porous ground layer entails great care in drying the
ware before firing ; otherwise the escape of steam from the
ground enamel will produce defects in the upper layer.