Fire clay resembles shale in that it is also a clayey rock and becomes
muddy upon wetting and rubbing. It differs from shale at sight in that
it (fire clay) is not laminated like shale, but occurs instead in a
massive structure which is relatively uniform throughout. Fire clay
fractures naturally into blocky or irregular fragments ranging in size
from boulders to rough flakes, whereas shale weathers into layered,
platy chips.
Shales are commonly buff, yellow, reddish, greenish, or brown in
addition to gray in color, whereas good useable fire clay predominates
in white, cream, and gray to almost black (if much organic matter is
contained in it). Shale is ordinarily gritty with hard sand particles,
but most good Missouri fire clay contains only a small amount of sand.
Of course, fire clay may grade into sandstone through a sandy clay
phase, but this part would not be confused with a layered, gritty shale.
The really determining characteristic of fire clay is its resistance to
melting under high temperature. The most positive test for this property
is to heat the fire clay to a white heat in comparison with standard
preparations (Pyrometric Test Cones) whose fusion temperatures are
known. Most of Missouri fire clay will withstand a clean oxidizing heat
of over 3000° Fahrenheit without melting.
Clay minerals originate, in general, from the weathering of previously
existing silicate rocks and have therefore been called, on occasions,
“rotted rocks.” The writer has long insisted that clays, particularly
fire clays, should be thought of instead as purified or refined rocks.
The original silicate rocks and minerals, which were rich in
constituents melting at low temperatures, have been soaked, leached, and
washed by chemically active ground water and rain water until many of
the undesirable elements have been carried away, leaving a refined
material which we use and know as fire clay. Missouri possesses one of
the largest reserves of finest quality fire clay in the world. Special
bulletins on Missouri fire clay are published by the State Geologist,
Rolla, Missouri, and may be obtained from his office.
[Illustration: Plastic fire clay. Shows typical break and naturally
polished slicken-sided surfaces. From Mexico, Missouri.]
[Illustration: Typical break.]
Persons who have undeveloped fire clay deposits on their property
frequently ask advice on whom to contact and how to arrange for sale of
their fire clay, with the expectation of a fair return and fair
treatment. The writer recommends in such cases that the owner of the
clay dig into his deposit to obtain a fresh, clean, _representative_
specimen of his fire clay (about one pound) and send it to one or more
of the large substantial fire brick or refractories companies operating
in Missouri. Obviously the company located nearest the deposit, or with
the lowest-cost shipping facilities, will be in a favored position to
purchase the clay. If the individual is skeptical about the
trustworthiness of the company’s report, he may send opposite parts of
the sample lumps to competitive companies. Of course, the individual may
have his clay tested by an independent laboratory at his own expense,
but this is ordinarily a useless, costly experience because a company
will duplicate those tests in its own laboratory before purchasing the
clay. If the refractories companies find the clay useful to them they
will proceed with negotiations. If the clay is of inferior quality or if
it is not needed by the particular company at _that time_, even though
of acceptable quality, usually the company will return a truthful report
at no cost to the clay owner.
The same general advice is given in regard to the development of any
mineral deposit which the holder may have. The caution about obtaining a
representative sample is especially to be emphasized. It applies to the
metallic ores, mineral water, and common rock as well as to fire clay.