The term "clay" is applied to various earthy materials composed dominantly of hydrous aluminum magnesium silicate minerals. The most familiar characteristic of clay is plasticity or the ability of moist clay to be fashioned into a desired shape. The physical properties of a clay are plasticity, strength, and refractoriness. Plasticity enables the clay to be molded; strength permits it to be handled during the forming, drying, and burning processes; and refractoriness permits it to be burned into a hard body of permanent form. Many types of clay are found in Kentucky. The most important ones are ball, bentonite, flint, fuller's earth, and halloysite. Mineralogy of the clays is described in Minerals.
Ball clays consist chiefly of the clay mineral kaolinite, with a minor amount of montmorillonite and organic material. Ball clay is lightweight, chalk-like in appearance, and used in china, whiteware, and various ceramic products.
Bentonite is a soft, low-specific-gravity, expandable clay. It is altered volcanic ash and is found in central Kentucky in beds up to 3 feet thick near the top of the Tyrone Limestone. Drillers have labeled these bentonite beds the Mud Cave and Pencil Cave. Because of its peculiar property of expanding when wet, bentonite is effective as a water sealer, especially to prevent pond leakage, and is also used in rotary drilling muds to prevent contaminating formations with drilling fluid.
Flint clay is medium gray to grayish brown, occasionally dark gray to black or red; harder than ball clay or fuller's earth; and has an earthy luster. The clay is a hard, tough, refractory clay consisting chiefly of the clay mineral kaolinite. Flint clay will not mix with water, and lacks the chief characteristic of clay, namely plasticity. It is compact, dense, breaks with a conchoidal fracture, and is resistant to erosion. The plant-root fossil Stigmaria is commonly preserved in the clay as a carbonaceous film. The Olive Hill Clay Bed in northeastern Kentucky is the principal source of flint clay, where it was used as firebrick. A special type of flint clay called tonstein occurs in the Eastern Kentucky Coal Field and is the product of volcanic ash falls.
Fuller's earth is a variety of montmorillonitic clay that is a natural bleach. It was originally used by fullers to "full" or remove grease from cloth; hence its name. Fuller's earth clay is found in the Jackson Purchase Region in the Porters Creek Clay of Paleocene age. The Porters Creek is very fine grained and contains scattered flakes of mica (muscovite). Fuller's earth is used as an absorbent.
Halloysite is a type of kaolinitic clay that occurs near erosional unconformities and is distinctive because of its tubular structure.