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

APPLIANCES FOE SMELTING THE ENAMEL MASS. 107
Consequently the porcelain crucibles for smelting enamels
must be prepared from a mass endowed with very highly
refractory properties and perfectly fireproof an end easy to
attain by reducing the content of felspar to a minimum, and
correspondingly increasing the proportion of kaolin.
Such a crucible, if properly treated at the first time of
using, becomes excellently adapted for the fusion of enamel
masses. When the first charge is smelted, the interior of the
crucible becomes coated with a thin layer of glaze more
refractory than the rest of the contents and remaining
unaffected by subsequent charges. Porcelain body being
always made from pure (i.e., iron-free) materials there is no
danger of the enamel mass being contaminated with iron,
even the first time of using the crucible.
Experience teaches that the crucibles last longest when
kept constantly hot and prevented from becoming cooled
through the admission of cold air or charging with unheated
materials. For this reason the furnaces should be in work
continuously, the crucibles not being removed or allowed to
cool until they are worn out or the furnace is to be shut
down for some time. In the latter event the dampers are
closed, so that the fire burns out slowly and the furnace afterwards
gradually cools down, the crucibles not being removed
until cool enough to handle ; they are then carefully exam-
ined, especially near the bottom, before being used again.
THE SMELTING FURNACE.
The furnaces formerly in general use for smelting enamel
masses were of the vertical type shown in section in Fig. 10.
In the centre of the hearth is a circular opening containing
a cylinder of fireproof material extending downwards through
the ashpit into a chamber underneath, whilst the upper end
projects a short distance into the furnace, and, being cut off
flat at the top, serves as a support for the crucible. The