quarrying districts to Trenton, the principal feldspar milling center, ace given below:
Freight rates per hundredweight on feldspar for carloads having a minimum weight of 40,000
pounds, May, 1909.
Bath, Me., to Trenton, N.J.................................... $0.15
Cathance, Me., to Trenton, N.J.................................17
Auburn, Me., to Trenton, N.J...................................16
The
requirements of the potter's trade demand that in general the
percentage of free quartz associated with the feldspar used shall not
exceed 20 per cent in the ground product, and certain potters demand a
spar which is nearly pure, containing probably less than 5 per cent of
free quartz. In order to be profitably worked, in most feldspar mines
between one-fourth and one-half of the total material excavated should
contain less than 20 per cent of free quartz. Freshness of the feldspar
is not essential.
A
factor of the utmost importance in the mining of pottery spar is the
quantity of iron-bearing minerals (black mica, hornblende, garnet, or
black tourmaline) which is present and the manner in which these
minerals are associated with the feldspar. The requirements of the
pottery trade demand that the spar be nearly free from these minerals,
which if present produce, upon firing, brown discolorations in white
wares. In order that a deposit may be profitably worked, these
minerals, if present in any appreciable quantity, must be so segregated
in certain portions of the deposit that they can be separated from the
spar without much more hand sorting and cobbing than is necessary in
the separation of the highly feldspathic material from that which is
highly quartzose or rich in muscovite. A number of pegmatite deposits
of coarse grain are rendered worthless for pottery purposes by the
abundance of one or more of these iron-bearing minerals. The presence
here and there of minute flakes of white mica (muscovite) is
characteristic even of the highest grades of commercial feldspar, and
chemically this mineral is not injurious. It is, however, exceedingly
difficult to pulverize the thin, flexible mica plates to a fineness
equal to that attained by the feldspar, and it is therefore necessary
in mining to separate carefully as much of the muscovite as possible
from the spar.
Operation
on a large scale with the aid of modern machinery reduces the mining
cost. Favorable topographic position—a situation, for instance, that
will permit the material to be excavated from a hillside opening
instead of being hoisted from a pit—also reduces the cost.
MILLING.
The
methods used for grinding feldspar for pottery, enamel ware, etc., are
similar in a general way in all of the Eastern States and are very
simple. The soda spar quarried in southeastern Pennsyl-
