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THE PACIFIC MINERALOGIST 3

CERAMIC CLAYS
Formation-History-Deposits
By HAROLD J. HUECKEL
Formation. The forces of nature hand sorted and subjected to either
are . very active in the formation of a washing or grinding process.
our various rocks and minerals. Some few sedimentary deposits
Water, combined with oxygen, carb- may be used in the same form as
onic acid, and temperature changes, mined. Kaolin is a highly refractory
are the slow but sure levelling clay and burns to a beautiful color.
agents whereby rocks are broken
down, and moved, to form new Ball clay is a sedimentary clay
strata and mineral deposits. of high plasticity. The original ball
clay works were in England, whence
Feldspathic rocks, which have the clay was recovered from open
undergone this proces of weathering pits and mined in the form of huge
and breaking down, form a larger balls weighing approximately 33
part of the clay deposits. Through pounds; thus deriving its name, ball
the influence of water, and the carb- clay. This clay generally contains
onic acid dissolved in it, part of the a larger amount of carbonaceous
alkalines, limes, and silicas are matter than ather clays.
carried away. So, a pure potassium
feldspar would suffer a loss of about Fire clay is generally listed as a
50 % on complete transformation clay whose fusion point is not below
into clay. This percentage is much 3,000 degrees fahrenheit, and may
greater if besides weathering, a be divided into two main types,
washing takes place; such as in the plastic and flint clay. The flint clays
sedimentary clay beds. are of a harder structure than the
plastic and require · grinding to
Clays may be divided into two assure a better ·workability. The
classifications according to their plastic clays are readily broken
ongm. The first, residual or primary down in water and require no prep-
clays, are in place, and formed aration for use.
above the parent rock. The second,
sedimentary clays, have been trans- Chemical analysis alone does not
ported in quiet pools of water where give sufficient knowledge of the
the particles of clay are held in properties of clay. It serves only to
suspension · by the water and de- recognize constituents which may
posited according to the finess of prove beneficial or harmful. "Ratio-
the particles. Due to this suspension nal analysis" detects the amount of
and slow settling, clays of differ- quartz, clay substance, and feldspar,
ent character, and distinguishable so serves to show the workability or
by color or texture, frequently lie plasticity of clay.
one above the other.
History. As early as 5,000 B.C. the
There are three main groups of clay deposited by the overflow of the
clay used today by the Cewmic Nile River in Egypt, was used in the
trade; Kaolin, Ball clay, and fire manufacture of rough brick. This
clay.
t is perhaps the earliest known use of
Kaolin,·sometimes called china clay clay. Some 2,000 years later vases
is mainly a residual or primary clay, of terra-cotta were first made, and
found in lump form in the midst of on the walls of tombs of that period
pegmatites and granites. Before this were depicted scenes of the ancient
clay is ready for use it must be potters plying their trade.