What is clay? Clay is a confusing term in geology. On the one hand, clay is defined by a particle size-a sedimentary rock with particles less than 1/256 millimeter in diameter. However, Clay is also a family of sedimentary minerals, the aluminum silicates. Their average hardness is 2-3. Their average specific gravity averages 2.6. Clay's most notable characteristic is its ability to absorb water. Clay can be formed from the weathering of silicate rocks (feldspars, micas, iron-magnesium, etc), from the erosion of the sedimentary rocks or volcanic ash.
Ball Clay from west Tennessee
When was it deposited? Clay has been deposited in Tennessee at several different times. Bentonite dates from the volcanic eruptions of the middle Ordovician. The ball clay deposits date from the Tertiary period.
| Quaternary |
| Tertiary |
| Cretaceous |
| Jurassic |
| Triassic |
| Permian |
| Pennsylvanian |
| Mississippian |
| Devonian |
| Silurian |
| Ordovician |
| Cambrian |
| Proterozoic |
What are its uses? Different clays are used for different purposes.
- Ball Clay - is a rare, very fine-grained white clay used to make fine china
- Bentonite - is clay formed from volcanic ash which has the property of swelling when wet. It is used for cat litter, and in mining to plug boreholes to prevent contamination of groundwater and to produce impermeable layers for ponds, dams, etc.
- Common Clay - used to make bricks
(photo by J.K. Roberts)
Kiln yard of the Dixie Brick & Tile Company, Puryear
from The Clays of West Tennessee,
TN Division of Geology Bulletin 49, 1940
Clay has been used by native people for pottery making for thousands of years, and this use has continued. Starting in the early 19th century common clay was used to make bricks. While people had been using the ball clay of west Tennessee to make fine china, its widespread use did not start until World War I when imports from England were cut off.