USES.
The
principal consumers of feldspar are the potter}', enamel-ware,
enamel-brick, and electrical-ware manufacturers, its most important use
being as a constituent part of both body and glaze in true porcelain,
white ware, and vitrified sanitary ware, and as a constituent of the
slip (underglaze) and glaze in so-called "porcelain" sanitary wares and
enameled brick. The proportion of feldspar in the body of vitrified
wares usually falls between 10 and 35 per cent. Its melting point being
lower than that of the other constituents, it serves as a flux to bind
the particles of clay and quartz together. In glazes the percentage of
feldspar usually lies between 30 and 50. The trade demands that
feldspar for pottery purposes be nearly free from iron-bearing minerals
(biotite, garnet, hornblende, tourmaline, etc.), and that it contain
little if any muscovite. The requirements in regard to the percentage
of free quartz vary with different potters. A few manufacturers of the
finer grades of pottery demand less than 5 per cent of free quartz and
may even grind the spar themselves so as to be sure of its quality,
preferring to insure a constant product even at higher cost by
themselves mixing the requisite quantity of quartz with the spar. Most
potters get satisfactory results with standard ground spar carrying 15
to 20 per cent of free quartz, and in some acceptable spars the
percentage runs even higher. In the finely ground mixture as it comes
from the mills it is difficult to separate the quartz from the feldspar
by plrysical methods on account of the extreme fineness of the
material. Chemical analysis seems to be the readiest means of
determining whether its percentage is high or low.
Feldspar is also used in the manufacture of emery and carborundum wheels as a flux to bind the abrading particles together.
Small
quantities of feldspar are used in the manufacture of opalescent
glass. The feldspar used for this purpose is ranked as No. 3 by the
miners. This generally contains more free quartz and muscovite than
that used for pottery purposes, and most of it contains also fragments
of iron-bearing minerals. Most of the spars known to the writer which
are used for opalescent glass are rich in soda. They are not ground so
fine as the pottery spars (p. 128).
Small
quantities of carefully selected pure feldspar are used in the
manufacture of artificial teeth. Some is used in the manufacture of
scouring soaps and window washes, the fact that feldspar is slightly
softer than glass rendering these soaps less liable than soaps which
contain quartz to scratch windows or glassware. Two firms in New York
State and one in Connecticut crush feldspar for poultry grit and for
the manufacture of ready roofing. 63096°—Bull. 445—11-----9
