Plastic Clays

Plastic clay is an extremely rare mineral found in very few places around the world. It is also sometimes referred to as ‘ball’ clay, a name which dates back to the early methods of mining when specialised hand tools were used to extract the clay in rough cube shapes of about 30cm. As the corners were knocked through handling and storage these cubes became rounded and ‘ball’ shaped. Plastic clays are sedimentary in origin. Ancient rivers and streams washed kaolinite (formed from decomposed granite) from its parent rock. As the streams flowed from upland areas the kaolinite mixed with other clay minerals, sands, gravels and vegetation before settling in low-lying basins to form overlaying seams of plastic clay. Plastic clays usually contain three dominant minerals: kaolinite, mica and quartz. In addition, there are other ‘accessory’ minerals and some carbonaceous material (derived from ancient plants) present. The wide variation both in mineral composition and in the size of the clay particles results in different characteristics for individual clay seams within a deposit. Internationally, deposits of high quality plastic clay are much rarer than those of kaolin. The best deposits currently known and exploited commercially are found in South West England, the Westerwald area of Germany, several basins in France, eastern Ukraine around Donetsk and southern parts of the USA. Further important deposits have been identified in Thailand, Indonesia and China. Plastic clay is extracted using hydraulic ‘back-hoe’ excavators, working at ‘benches’ cut into the quarry to access the seams of clay. Individual raw clay selections are carefully blended according to pre-determined recipes to provide a product with a consistent and predictable range of characteristics and behaviour. The first stage in processing is then to shred (or ‘kibble’) the blended clay into smaller, more regular lumps about the size of a golf ball. Much blended clay is sold in this shredded form. Further processing through drying and grinding yields powdered plastic clays and treatment by calcination produces chamotte. Ceramics manufacturers (particularly in the sanitaryware sector) have also benefited from the development of refined plastic clays and chamottes which offer improved performance and reduced manufacturing process costs. Refined clays are available in ‘noodled’ and slurried form.