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

PREPAEING THE MATERIALS FOR ENAMEL-MAKING. 95
it is not advisable to use millstones of any other material than
quartz, such stones being usually much softer than the latter,
-and mostly containing ferruginous minerals detrimental to
the quality of the enamel. It is therefore best, in large
works, to proceed
with the reduction of the materials in a
systematic manner, somewhat as follows : The
material is
calcined, then quenched, and broken down in a stamping mill,
the resulting coarse powder being sifted and the fine sittings
passed on for further treatment, whilst the remainder is
returned to another department in the stamping mill.
EDGE-RUNNER MILLS.
The further treatment of the coarse powder is best per-
formed by dry grinding in edge-runner mills. These consist
of a horizontal plate, in the centre of which revolves a vertical
shaft, supporting a horizontal crosspiece
which serves as the
axis of a pair of circular millstones. As the vertical shaft
revolves, the millstones, rotating on their axes, describe a
circle on the horizontal bottom-plate, and pulverise the material
lying thereon, scrapers being employed to bring the material
continually into the path of the rollers. Figs. 6 and 7 show
the arrangement of a recognised excellent edge-runner mill
fitted with a revolving bottom-plate, G, which takes the form
of an annular trough, actuated by a bevel wheel mounted on
the vertical shaft J and receiving motion from the driving
pulley N through the cogwheel O on the horizontal shaft C.
The shaft J is attached at the bottom to the trough, and runs
on bearings M and L. The rollers or runners, H, revolve on a
fixed horizontal shaft, their movement resulting solely from
the friction produced by the material under treatment
between the runners and the bottom of the trough. The
material is kept in the path of the rollers by means of
.scrapers provided on the bottom -plate.
The material delivered, in the state of coarse sand, from