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

XXVI.
DIAL-PLATE ENAMELLING.
The manufacture of dial-plates for clocks is a special
business, and generally consists in covering sheet copper with
white enamel. The thin sheet copper, cut to the desired size,
is made bright and coated on both sides with perfectly white
enamel, in fine powder mixed with water and laid on with a
brush. As the enamel used is always very rich in tin oxide
it covers well enough to be used in exceedingly thin layers.
Any white enamel of good covering power will do for the
purpose, the following being typical mixtures :
Sand .... 100 100 100
Lead oxide . . 50 108 167
Tin oxide ... 50 38 33
Potash ... 200 20 80
When the mass has been applied, the discs, laid side by
side, are thoroughly dried, and a large number are placed on
thin slabs of firebrick and inserted in a glowing muffle where
they are left until the enamel is fused. Hereupon they are
transferred, slabs and all, to a cooling oven. In this manner
a very large number of dials can be finished in the course of
a day.
The plates are then subjected to a searching examination,
and only such as are perfect and exhibit a perfectly uniform
lustrous surface are looked upon as finished. Frequently
some of them are slightly blistered or stained in places, and
these must be rectified before they are sent out. The simplest
way to remove the small blisters or bubbles is by scouring
the plate with fine sand, and reheating in the muffle until the
enamel has so far fused again that it forms a coherent lustrous