644 MINERALS YEARBOOK

mentation. The varying proportions of the base materials and the different
impurities determine the suitability of clay for different purposes and set
its value in commerce.
 Clays range from low-grade widely distributed varieties suited only for
brick, tile block, and sewer pipe, to high-grade clays such as kaolin, ball
clay, highly refractory fire clay, etc., which occur only in certain localities
and are suited by their peculiar properties or characteristics to specific
purposes. Although the high-grade clays usmally are sold by the producers
as unfinished materials and appear in commerce as raw materials, low-grade
clays ordinarily do not warrant shipment in the raw state, as a rule being
manufactured into finished articles by the producers at their point of origin.
 In this discussion clay is considered under the following headings:
(1) Kaolin, including china clay and paper clay; (2) ball clay; (3) fire
clay; (4) stoneware clay; and (5) miscellaneous clay, which includes all
clay of which a comparatively small tonnage was sold by producers and clay
which cannot properly or definitely be classified under the other general
headings.
 Kaolin, ball clay, stoneware clay, and fire clay are adapted to the finer
ceramic industry by their properties of being readily molded, retaining their
shape when dried, and firing at comparatively low temperatures. They are
all used in the pottery and finer ceramic industries in the manufacture of
china and porcelain tableware and sanitary ware, floor and wall tile, electrical
porcelain fittings, etc. These clays have diflerent characteristics as to
bond, ease of molding, shrinkage, and firing temperatures, and two or more
clays usually are blended to obtain the desired features in manufacture and
appearance and texture of the fired product.
 Kaolin, to a large extent, and ball clay, to a rather small extent, find
use in the paper, rubber, textile, and paint industries as fillers or extenders
and coatings because they grind readily to extreme fineness and are soft
in texture.
 Fire clays are characterized by their resistance to high temperatures without
fluxing or fusing. Although a high degree of plasticity is desirable it is
not necessary, as deficiency in this property can be overcome by the addition
of some other plastic, low-fusing clay as a binder. The bulk of the production
of fire clay appearing in commerce is used for the manufacture of fire brick;
as fire-clay mortar for furnace, retort, crucible, and ladle linings; and
for similar purposes where high temperatures must be withstood. Fire clay
marketed unwrought constitutes over half the total commercial production
of clay, but it is oniy a fraction of the total production of fire clay,
as most firebrick plants operate their own fire-clay mines and purchase only
a small part of their requirements from other producers.
 Fire clay includes cliaspore and burley clay, also special fire clays high
in alumina and capable of withstanding extremely high temperatures such as
are encountered in cement and lime kilns, hightemperature boiler settings,
and nonferrous metallurgy.
 Miscellaneous clays include clays having special properties or uses of which
comparatively small quantities are marketed. They include such clays as slip
clay, a low-melting-point clay used as a glaze and as a binder for artificial
abrasives; bentonite, a peculiar clay which can absorb several times its
volume of water and swells greatly when wet, and is used for medical purposes,
as a purifying agent in oil refining,