Why does the Piedmont have so much clay and how is it used?

Potters digging stoneware clay from a secondary clay deposit in Chatham County around 1900. (Photograph courtesy of Mr. and Mrs. Gail Welch, in whose family album it was found, and of Charles G. Zug III, in whose book Turners and Burners: The Folk Potters of North Carolina (UNC Press, 1986) it was first published. More about the photograph)

North Carolina’s Piedmont has so much clay because clay is, quite literally, “common as dirt.” Seventy-five percent of the earth’s surface is composed of silica (SiO₂) and aluminia (Al₂O₃), the primary ingredients of what is commonly called clay. Geologists use the word clay in two different ways. It is used at the microscopic level to designate all sediment particles less than 4 microns in diameter. It is also used at a macroscopic level to designate a natural, earthy, fine-grained material that develops an ability to be deformed under pressure (plasticity) when mixed with water.

These two aspects of clay, combined with another — the capacity for clay particles to fuse together into a hard, non-porous form when heated — determine the major commercial use of Piedmont clays by humans. Clay is used to make bricks for construction, and North Carolina is one of the major brick-producing states in the nation. Clay is also used to make containers such as jugs, jars, bowls, and vases, and North Carolina has long been famous for its pots and for the skilled artisans that make them.

There are many types of clays. They differ in their geologic setting, their chemical composition, and the types of pots that can be made from them. Clays are termed primary or secondary depending upon whether they are recovered from the place where they were formed geologically (primary clays) or from a place where they were carried by currents of wind or water (secondary clays).

Although all clays include silica and alumina, clays differ in composition, both in the percentages of these chemicals present as well as in the percentages of other chemicals such as oxides of iron, magnesium, calcium, sodium, potassium, and titanium. Kaolin is a clay mineral composed almost exclusively of silica and alumina and used in making translucent ceramics known as porcelains. Oxides of elements other than silica and aluminum are thought of as impurities in clay. Kaolin is so pure and translucent that it can be thought of as opaque glass.

Stoneware has fewer oxides than earthenware, and it makes pots that are not porous to water and that will ring like a bell when struck. Earthenware has the most impurities; it is essentially baked earth. Its somewhat “open” structure is often porous to water and can withstand thermal shock, and so earthenware is good for flower pots and cooking vessels.

The fewer impurities in clay, the higher its melting point. As a result, earthenware containers are fired at temperatures of about 1800 degrees Fahrenheit, whereas stoneware requires temperatures of 2300 degrees, and porcelain more than 2500 degrees.

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