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

X.
PEEPARING THE MATERIALS FOR ENAMEL-MAKING.
The purification and reduction of the principal materials
have already been dealt with, and but little remains to be said
on this matter. Hard substances such as quartz and felspar are
difficult to pulverise in their ordinary state, but when quenched
may be easily reduced in stamping mills, the resulting coarse
powder being afterwards brought into an extremely tine state
of division by grinding in suitable mills, and, if necessary,
washed by sedimentation.
GLAZE MILLS.
For reducing the hard materials, the so-called glaze mills
are used. These are of various patterns, but one chief point
has to be borne in mind in their construction, namely that
the substances to be ground must be kept out of contact with
metal, on account of the coloration produced by the particles
of the latter worn away by attrition.
In small works use may be made of an ordinary grinding
mill, in which the materials are ground in a wet state. The
composition of the grinding rollers or millstones may be a
source of inconvenience. A stone very rich in quartz, preferably
of the
a porous quartz rock, is generally used ; but, though
seventh degree of hardness, such stones wear away very
quickly, since the felspar to be ground is of the sixth degree,
and the quartz also of the seventh degree of hardness ; and
on this account the stones of glaze mills require very frequent
renewal an expensive and tedious operation. Nevertheless