128 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
vania
is first burned in kilns, which serves to fracture it and thus to
facilitate grinding. Most feldspar, however, is fed just as it comes
from the quarry into a chaser mill consisting of two buhrstone wheels,
3 to 5 feet in diameter and 1 to 1-1/2 feet thick, attached to
each other by a horizontal axle, as are the wheels of a cart. The
horizontal axle is attached at its center to a rotating vertical shaft,
which causes the buhrstone wheels to travel over a buhrstone bed, the
feldspar being crushed between the wheels and the bed. In a few mills
the spar before going to the chaser mills is crushed in a jaw crusher.
The
material as it comes from the chasers is screened, the tailings being
returned to the chaser mills for recrushing, while the fines go to tube
mills for final grinding. The tube mills consist of steel cylinders
revolving on a horizontal axis. The cylinders are generÂally lined
either with hard-wood blocks or with blocks made of natural or
artificial siliceous brick and are charged with Norway or French flint
pebbles 2 to 3 inches across. The type of tube mill used by most
feldspar grinders is 6 to 7 feet long and grinds from 2 to 3 tons of
spar at one charging. Certain millers, however, claim to effect a
considerable saving in power by the use of larger mills, which grind
from 4 to 6 tons at one charge.
Feldspar
for pottery purposes is usually ground four to six hours, and in that
time most of it is reduced to a fineness of less than 200 mesh. Screen
tests made by the writer on four samples of comÂmercial ground pottery
spar collected personally from the bins at three feldspar mills showed
that from 99.3 to 99.8 per cent of the material would pass through a
100-mesh screen and from 96.7 to 98.2 per cent would pass through a
200-mesh screen. A sample of No. 3 spar, used only in making glass and
enamel ware, was notably coarser, 94 per cent passing through a
100-mesh screen, and 74 per cent through a 200-mesh screen. This grade
is ground only for two to three hours. Some feldspar prepared for use
in abrasive soaps is ground for ten hours.
After
grinding, the spar is ready for shipment either in bulk or in bags. The
red spars from Bedford, N. Y., and Bedford, Ontario, have a faint
pinkish tint when ground, but the cream-colored and white spars grind
to a pure white. In a few mills the ground spar is allowed to settle
slowly in water, so as to separate the finer from the coarser material,
but this method is now rarely used.
In
mills for grinding feldspar for poultry grit and roofing purposes the
spar is first crushed in jaw or rotary crushers and then between steel
rolls. It is then screened over vibrating screens, usually of the
Newago or Jeffrey type, to the various sizes desired.
