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

22 ENAMELS AND ENAMELLING.
is a brick-lined pit filled with water. Immediately underneath
the top cover, the shaft communicates with a chimney capable
of producing a good draught.
The shaft is first charged, about one-third full with glow-
ing coal, which is then covered with a layer of the quartz to
be calcined. Another stratum of fuel is spread on the top of
this, and the shaft is filled up to the top with alternating
layers of quartz and fuel. The amount of the latter required
to properly calcine a given weight of quartz has to be deter-
mined by experiment.
As soon as the lower charge of fuel is consumed and the
glowing quartz has sunk to the bottom of the shaft,
it is
raked quickly out of the furnace and falls into the quenching
pit ; and in proportion as the lower charges of quartz are
removed, the shaft is replenished by adding more quartz and
fuel at the top. By this means the furnace is kept continuously
in action, so that very large quantities of quartz can be
quenched in a short time and with a minimum consumption
of fuel. The quenched quartz is next broken down into coarse
powder in a stamping mill, and is then screened, the coarser
particles being returned to the stamps, while the remainder,
passing through the fine-mesh screen, is ground to fine meal.
Although the mechanical reduction of such a hard mineral
as quartz is rather expensive, the operation is one requiring
extreme care since it facilitates the production of perfect
homogeneity in the subsequent fusion of the enamel mass.
THE ALKALIES.
The second class of indispensable constituents for glass
and enamel is that of the alkalies. As already mentioned,
silica, when fused with a single alkali, forms a mass which
bears the name of glass (water glass) but is distinguishable
from all other kinds of glass by its solubility in water ; and
it is only when an alkaline earth (lime) or an oxide of some