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

116 ENAMELS AND ENAMELLING.
This latter oxide being, moreover, disseminated throughout
the entire mass has only a very slight effect on the colour, and
therefore does not spoil the enamel for use as a ground layer.
It must nevertheless be borne in mind that the addition of
saltpetre will result in a modification of the fusibility of the
enamel, to correct which a corresponding amount of silica
should be added before the mass is re-melted. By this artifice
the original properties of the mass can be restored.
In most works it is customary to fuse the mass for ground
enamel only once, and let it run off into the water tubs under-
neath, some makers waiting until the molten mass has so far
loosened the silica plug at the bottom that the fluid begins to
trickle through. It is needless to say that this practice cannot
be commended, as in many cases it leads to a great waste of
time and heat merely for the purpose of loosening the plug,
and therefore crucibles should be preferably fitted with the
iron plugs already described.
When fine cover enamels are in preparation, it is scarcely
possible to finish them properly by a single smelting, since
they generally appear full of small bubbles when only once
fused, and consequently require melting a second (sometimes
even a third) time. However, by increasing the heat as much
as possible after the mass has become liquid in the second
fusing, it will generally
be found that this treatment suffices
to furnish a product free from bubbles, the mass being thereby
rendered thin enough to allow the bubbles to ascend and
dissipate.
the fused mass in cold water makes it brittle
Quenching
and easily reduced to powder by stamping and grinding.
The
usual course is to allow the quenched material to accumulate
for a certain time, then dry carefully, weigh, and send to the
stamping mill, after which it is ground and weighed again, the
frequent weighings being for the purpose of estimating the
percentage of unpreventible waste, which occurs during the