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Ball Clay - Al2O3 2SiO2 2H2O

T Ball clay is an extremely rare mineral found in very few places around the world. Its name dates back to the early methods of mining when specialized hand tools were used to extract the clay in rough cube shapes of about 30 cm. As the corners were knocked through handling and storage these cubes became rounded and ball shaped. It also is sometimes referred to as plastic clay. Ball 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 area they mixed with other clay minerals, sands, gravels and vegetation before settling in low-lying basins to form overlaying seams of ball clay. Ball clays usually contain three dominant minerals: from 20-80% kaolinite, 10-25% mica, and 6-65% 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 Ball clay is a secondary or transported type of clay that is found in stratified layers, often alternating with layers of coal and with other types of clay. It is highly plastic. Although not so pure as kaolin, ball clay is relatively free from iron and other mineral impurities and burns to a light gray or light buff color. It tightens into a dense structure when fired to about 1800 degree, Different ball clays vary considerably in composition. Ball clays are impossible to use alone in pottery because of their excessive shrinkage, which may be as high as 20 percent when fired to maturity. They are usually used as an admixture to other clays to gain increased plasticity and workability. In manufacturing whitewares, ball clay is indispensable as an addition to the body to overcome the nonplastic properties of kaolin. However, if whiteness is desired, not more than about 15 percent of ball clay can be added to a clay body; more than this amount in a whiteware body results in a gray, off-white, or buff color. The presence of ball clay in a porcelain body decreases its translucence.
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