Forest Preserve District of Cook County, Illinois
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Clay
Nature Bulletin No. 435-A   November 27, 1971
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

CLAY
When Nebuchadnezzar was king of Babylon in the 6th century B.C., messages were sent and records kept by writing on little pillow-shaped tablets of soft wet clay. They were baked in the sun. Those clay tablets are still found by the thousands in ruins of ancient cities excavated in that dry climate. The Babylonians also sun-dried bricks of clay and straw for building, as the Israelites did during their captivity in Egypt. This process spread to Spain and Mexico and our American Southwest. Called "adobe", in regions with little rainfall it has the advantages of being cheap, fireproof, cool in summer and warm in winter.

The American Indians, as well as most other primitive peoples, used clay to make pottery utensils and art objects of many kinds. Fragments of this earthenware can be picked up in almost every farmer's field in our region. The ancient Chinese artists carried the use of clay to a high degree of perfection in their fine pottery and porcelains. From prehistoric times right down to the present, clay has served man longer and in more ways than any other mineral dug from the earth.

Clay, which is mostly aluminum and silica, usually originated from weathering of such rocks as granite in the earth's original crust. It is found beneath old swamps, as mud on the bottom of the sea, and as the ground-up rock flour left by melting glaciers. In addition, clay is also defined as those soil particles below a certain size -- usually 1/25,000th of an inch. Indeed, some clays are so fine that one pound of them, spread in a single complete layer, would cover several acres. A bit of pure clay in your mouth feels soapy, not gritty. The individual clay particles are tiny flakes or rods -- some with clean-cut edges, others frayed. These shapes may account for the slipperiness of wet clay and its ability to cling together and be molded.

Depending mainly on the kind and amount of impurities present, there are many varieties of clay, each with its own special uses. Fire clay, which will not melt or crumble at extremely high temperatures, is used to make the brick linings for industrial furnaces and those huge ladles that carry molten metal. Another kind goes into stoneware, such as bean pots, casseroles, and hard clay products like roofing tile, sewer pipe and face brick. Ordinary brick, drail tile, building tile and pottery are made from some of our most common clays. Porcelain is baked from a highly purified form of white clay called kaolin. The "earth colors" -- ocher, sienna and umber -- are clays with iron and manganese impurities, and are used for pigments in paint. Surprisingly, clay also is used in phonograph records, medicines, cosmetics, crayons, pencils, tooth powder, soaps, paper, textiles and water softeners.

Illinois is rich in underground wealth. As a mineral producer it ranks first among the states of the Upper Mississippi Valley, fifth in the nation, and exceeds all but a half dozen countries of the world. Of course, in money value, coal and oil are most important but, in our state, clay and clay products come third with an annual value of about fifty million dollars, More important than any money value of clay products is the clay in Illinois soils. The right amount of clay, mixed with coarser particles and organic matter forms the loams that make this the bread basket of the country.

The best men, or so runneth the proverb, are but common clay.


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