Man has used clay in various ways for many hundreds of years. From it he
made, and still makes, bricks to build his dwellings, pottery utensils
of many kinds, and other useful products.
Everyone knows what clay is, yet it is a substance difficult to define.
All clays are earth materials, most of them plastic or sticky when wet
but firm when dry. If heated sufficiently (fired) they become hard.
Clays are composed of various minerals. Of these, the so-called clay
minerals—complex substances composed mostly of alumina, silica, and
water—generally are the most important. They impart the property of
plasticity and also cause clays to become hard when fired.
Most clays are what geologists call unindurated (unhardened) rocks. Clay
that has been indurated and occurs in layered deposits is commonly
called shale. The layers may be from a fraction of an inch to several
inches thick. Most Illinois shales are not plastic when dug from freshly
exposed deposits, but they become plastic when crushed and kneaded with
water. The clays and shales of Illinois are the basis of a huge and
important industry.