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

EAW MATERIALS FOR THE MANUFACTURE OF ENAMELS. 19
preferable for fine white or good coloured enamels to pick
out the best colourless quartz and use this by itself.
In preparing enamels for technical purposes the chief
point to be regarded is the power of withstanding
the action
of chemicals and rapid changes of temperature, colour being
a subordinate consideration ; consequently, in such cases, the
selection of the quartz need not be looked after so strictly,
since a slight yellow tinge will do no harm, and will, moreover,
be for the most part masked by the white components in the
matrix. For enamels for coating the inside of cooking utensils,
where a pure white colour is more particularly required, no
trouble should be spared in sorting out a very pure grade of
quartz or even in procuring the same from a distance, as the
better colour of the enamel and higher value of the ware will
more than compensate for the greater initial outlay.
Sedimentation.
A very useful method of freeing quartz sand from ex-
traneous admixtures is that of washing by sedimentation,
which is particularly advisable when, as not infrequently
happens, the chief impurity is clay. It may be stated here
that the silicates of alumina form glasses of exceedingly high
to be
fusing point, and consequently these bodies require
eliminated from quartz sand intended for the production of
enamels.
The washing is effected in a very simple arrangement of
wooden tubs provided with tap holes at different heights.
One of these tubs is filled about three parts full with clean
water, which is stirred round and round while the sand is run
in. As soon as this is done the stirring is discontinued, and,
after waiting a few minutes until it seems probable that the
sandy particles have settled down, the uppermost tap hole
is opened and the water run off.
When the sand contains clay, the particles of which float