Carolina Environmental Diversity Explorations
Clays of the Piedmont · By Dirk Frankenberg
Primary and secondary clays
Figure 1. Mining for primary clays near Hillsborough. (Photograph by the author. More about the photograph)
The old photograph on the introductory page and Figure 1 show secondary and primary clays being recovered from the earth’s crust in North Carolina’s Piedmont. Most of the clays used in pottery are secondary, but much brick-making clay and some specialized industrial clays are recovered directly from their geologic sources. The first photograph showed early Piedmont potters digging stoneware clay from a secondary clay deposit in Chatham County around 1900. The photograph shows a ditch beside the stream that carried the clays from their upstream source to this deposition site in the Piedmont.
Primary clays in the Piedmont began to be used much later than secondary clays because their recovery required mining and/or moving large quantities of material. Figure 1 shows a modern mining operation for recovery of pyrophillite, a hydrated form of silica and alumina, from a primary deposit near Hillsborough. This deposit began forming over 500 million years ago, when heat and pressure within the earth’s crust created a molten mass of alumina that was forced upwards through fissures in the overlying sandstone. Note the steeply tilted bedding planes of the talc-like layers of pyrophillite. This vertically-oriented mass of primary clay was altered by repeatedly being dissolved and recrystallized by hot water flowing upwards through it.
Ultimately, almost all of the clay’s original impurities were dissolved away, leaving a pure deposit of pyrophillite, a chemical that expands very little even when heated to 2000 degrees. This property makes it a valuable stabilizer in industrial ceramics such as wall and floor tile, sinks and toilets, electrical insulators, and commercial clays for making earthenware and stoneware.
When mixed with other clays, pyrophillite reduces their tendency to expand and contract when heated and cooled — a big advantage if you don’t want your ceramic object to crack or fracture when it is fired or when it is exposed to changes in temperature when in service.
