Flint fire clay is very fine-grained, smooth or slick, and breaks with a
shell-like (conchoidal) fracture. It varies in color from white to
black, but most flint fire clay mined is near to white. It is relatively
non-plastic—that is, does not readily slake or form a sticky mass when
worked a little in water. In fact, flint fire clay has been used locally
as road surfacing because it does not become very muddy and sticky. Of
course, it is inferior to black-top or concrete road surfaces and has
too high a commercial value now to be used extensively as road metal.
A hard, white variety of flint fire clay which breaks with numerous
conchoidal fractures in appropriate shaped fragments has been called
locally “pop-corn flint.” This clay, and other sand-free flint clay,
when crushed between one’s teeth “goes to water” in the mouth. Many clay
miners use the chewing test to establish the freedom of their clays from
gritty sand, which renders flint clay inferior in quality.
[Illustration: Flint fire clay showing typical “slick break” and
conchoidal fracture, from near New Florence.]
Flint fire clays occur geologically in old land depressions and in
roughly funnel-shaped pits surrounded by an enclosing layer of
sandstone, the whole lying within limestone country rock. The most
prominent flint fire clay deposits are found in Callaway, Warren,
Lincoln, Osage, Gasconade, Maries, Franklin, and Phelps counties.