motor
to spark. Much of the sheet mica used in electrical apparatus is first
made up into large sheets of mica board or micanite. In this form it is
available for use in most of the purposes for which ordinary sheet mica
can be used. It can be bent, rolled, cut, punched, etc. Bending is
accomplished during baking, or by heating to soften the shellac used in
the manufacture of the mica board. Insulation for commutators is
generally cut from "amber" mica board.
Scrap
mica, or mica too small to cut into sheets, and the waste from the
manufacture of sheet mica are used in large quantities commercially.
The greater part is ground for the manufacture of wall papers,
lubricants, fancy paints, molded mica for electrical insulation, etc.
Ground mica applied to wall papers gives them a silvery luster. When
mixed with grease or oils mica forms an excellent lubricant for.axles
and bearings. Mixed with shellac or special compositions, ground mica
can be molded into desired forms, and is used in insulators for wires
carrying high potential currents. Ground mica for use in molded mica
for insulation purposes should be free of metallic minerals. For
lubrication purposes it is necessary that gritty matter be eliminated,
either after grinding or by using only pure mica for grinding. For wall
papers and brocade paints a ground mica with a high luster is required.
This is best obtained by using a clean light-colored mica and grinding
under water.
Coarsely
ground or bran mica is used to coat the surface of composition roofing
material, especially that manufactured by the Western Elaterite Roofing
Company, of Denver, Colo. The mica serves the purpose of keeping the
material from sticking when rolled for shipping or storage.
In
the Western States the dry process is the common practice in-grinding
mica, but in the mica regions of the Eastern States the greater part of
the mica is ground under water. In dry-grinding machines the mica is
pulverized by the beating action of teeth or bais on cylinders
revolving at a high rate of speed. In wet-grinding machines the mica is
beaten and torn under water by teeth or spikes mounted in wheels or
cylinders revolving at a comparatively slow rate of speed. The
capacity of the dry-grinding machines or pulverizers is considerably
greater than that of the wet-grinding machines. The dust of fine mica
scales from the pulverizers is often a cause of annoyance to workmen
around the mills, as it is very irritating to the throat and lungs when
breathed. It is claimed that mica-ground under water is better than
that ground dry. Some consumers demand the wet-ground mica, claiming a
greater purity and more brilliant luster. It is possible that the same
effect could be obtained by thoroughly washing dry-ground mica and
floating the product.
PRICES AND PRODUCTION.
The following statements in regard to the price of mica are also quoted from Sterrett's report:
The
average price of sheet mica in the United States during 1908, as
deduced from the total production, was 24.1 cents per pound, as
compared with 33 cents per pound in 1907 and with 17.7 cents in 1906.
The average prices per pound of sheet mica as reported in the
production from several States were as follows: Virginia, 44.2 cents;
South Carolina, 35.7 cents; South Dakota, 33.3 cents; Alabama, 24
cents; North Carolina, 19.1 cents. These average values vary greatly
from year to year, a result caused in part by variation between the
proportion of rough and trimmed sheet mica sold by the producers and in
part by variation in the size of sheet produced.
The
prices of several sizes of selected mica quoted in the price list of a
large mica company of New York during 1908 were as follows:
