Ball Clays Dave Finkelnburg

Instructions

Ball clays are very plastic so they are widely used in clay bodies. However, like many ceramic raw materials, not all ball clays are created equal. Understanding what controls their properties aids in using ball clays successfully.

Defining the Terms

Facts and Formation

Because of its unique properties, it is logical to assume that ball clay is a particular clay mineral. That isn’t true. Technically speaking, all ball clays are mostly kaolinite, the same clay mineral that makes up kaolins. The majority of ceramic clays used around the world are kaolins. If ball clay is mainly kaolinite, what makes ball clay different than ordinary kaolin? The answer is particle size. While clay mineralogy is complex, and kaolinite can also form in other ways, it is useful to think of kaolinite simply as altered feldspar. Most of the clay on earth began life as molten rock that solidified slowly into feldspar and mica. In cases where those minerals are exposed to water for long periods of time, the flux atoms are slowly leached out and the relatively pure aluminum-silicate crystal structure is left behind. In this way, feldspar is altered into kaolinite clay. The leaching or alteration leaves flat, plate-like kaolinite crystals that, under very high magnification, are seen to be in stacks. When this kaolinite (weathered feldspar) is found right where it was formed, the clay deposit is kaolin. Besides kaolinite, the kaolin also contains quartz, feldspar, mica and other minerals found in the parent rock. Sometimes kaolin is very near or at the surface of the earth or becomes exposed over time. Surface deposits of kaolin are quite soft and may be eroded by wind or, more often, by water. Erosion moves the stacks of kaolinite crystals downhill or downwind. In this process of transportation, the stacks literally break down. The stacks split apart into shorter stacks or individual plates and the plates break into smaller pieces. Significant accumulations of kaolinite transported by erosion are what we call ball clay. Very importantly, ball clays have a very wide mix of particle sizes, including very fine clay particles. Ball clays settle in deposits that are relatively uncontaminated except for plant matter. A common misconception is that more contaminated deposits are often earthenware. Typically, the parent mineral of earthenware is mica rather than feldspar. Thus earthenware is high in iron and the clay minerals tend to be mostly illite and chlorite rather than kaolinite.

Developing a Ball Clay Body 

Ball clays share four common characteristics—particle size, shrinkage, variable quartz, and organic content. All influence how ball clays perform in a particular clay body or glaze. Because of the wide range of particle sizes in ball clays, they are especially plastic and thus make a clay body easier to work with. The fine particles present also increase green strength of the dry clay body so dry ware is sturdier. However, fine particle size also means ball clays require more water of plasticity. Having to add more water to make a particular clay plastic means more water will evaporate on drying. This makes drying shrinkage greater. Because of the increased shrinkage, ball clays are almost never used alone in a clay body, but rather are blended with kaolins and non-plastic materials like quartz sand. When ball clay deposits form, there may be significant fine quartz sand transported in the same way and to the same location as the ball clay. As a result, ball clays are highly variable in their quartz content, which can range from a few percent to 15% or more. In cases where glaze fit is important, it is essential to adjust the quartz added to a clay-body recipe if the ball clay is replaced with a substitute. This is because increasing quartz content reduces fired-body expansion. In the process of transport and deposition, ball clays can become contaminated with plant matter (organic materials). Although the organics burn out in firing, in excessive quantities, organic materials can deflocculate a ball clay. That can dramatically reduce plasticity of the body. Testing any ball clay substitution for deflocculation is important when making plastic clay.

This article was written by Dave Finkelnburg and excerpted from the May 2013 issue of Ceramics Monthly.