Ball Clay - Rarer And More Important Than You Think
Industrial mineral by Harold Hough
Mention Tennessee No. 10 or Kentucky No. 4 and someone may think you are talking about a handcrafted whisky or bourbon. Unfortunately, it is nothing so interesting. It’s a type of ball clay. Yet, you probably have more use for Tennessee ball clay than you would a Tennessee sipping whisky.
Ball clay is a critical part of our everyday life. It is a major ingredient in our commodes and the insulators of our spark plugs. It is also a critical ingredient for the fine quality china that graces your table.
Ball clay is actually quite rare and most of it is mined in Britain or the Eastern United States. The oldest deposits are in England and clay from these deposits have been used by manufacturers of white-bodied pottery since the days of the famous 18th century potters such as Josiah Wedgwood, Astbury and Spode. The American ball clay industry started in 1860 in Tennessee.
Ball clays consist of ultra fine clay particles. This helps to make them sticky (or 'plastic') and easily shaped when damp (the word 'clay' is derived from the Old English 'claeg', meaning sticky). Some also have fluid properties that are valuable in the casting of large ceramic pieces such as toilet bowls. Many ceramics contain both ball clay and china clay. The ball clay helps to shape the piece, adding 'green' strength to the body before firing, while china clay provides extra whiteness.
Ball clay is a sedimentary material that comes from decomposed granite. Since each deposit is a product of the unique mineral nature of the region, ball clays from different regions have different characteristics and colors. That is the reason for the number of names like Tennessee No. 10. Tennessee No. 10, for instance fires white, while Tennessee No. 5 fires grey white. Generally Tennessee clays fire the whitest. It is the amount of iron and other oxides that determine the final color.
Ball clay has been mined since the Roman times. It got its name because in earlier days, it was cut by workmen into cubes. However, as they were handled, the corners wore off and they soon turned into balls.
Today most of the ball clay is mined with small earth moving equipment like backhoes or smaller excavators, which allow the mine to better differentiate the clay grades. Deposits range from 18 to 40 feet in depth and there is usually 25 to 80 feet of overburden on top of the deposit. Since the characteristics of the deposit vary from place to place, the excavators work several benches and the clays are carefully blended to provide a consistency of color, shrinkage, and composition. Usually the top part of the deposit has a clay with the largest amount of organic material in it. The middle part of the deposit is the purest and the lower part of the deposit has more silicious clay in it.
The key to ball clay production is constantly monitoring what is being mined. Every hour (of more often if required) the clay is tested for quality. The mined clay is then sent to a primary drying facility. There it goes through several processes of shredding, drying, air floating, and slurrying. Then each type of clay is separately stored until it is needed. These end point clays are then used to blend the final product.
One ball clay mining company is Old Hickory Clay Company of Tennessee. They are the only remaining private American clay company and operate 13 mines in both Tennessee and Kentucky. They provide a myriad of ball clays for a number of industries – each specifically tailored for the needs of the customer. Electrical insulation clays must be able to be turned and need a high fired strength. Pottery clays vary in color. Ceramic clays differ according to the firing process. Sanitary products require a very high temperature firing for vitrification. Many characteristics like firing temperature and the type of glaze have an impact on the type of ball clay required.
Although ball clay mining isn’t as glamorous as gold mining, or as obviously critical to our economy as coal mining, it’s one mineral that we are all glad to use.