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ball clay
Ball clays are kaolinitic sedimentary clays that commonly consist of 20-80% kaolinite, 10-25% mica, 6-65% quartz. Localized seams in the same deposit have variations in composition, including the quantity of the major minerals, accessory minerals and carbonaceous materials such as lignite.〔 They are fine-grained and plastic in nature, and, unlike most earthenware clays, produce a fine quality white-coloured pottery body when fired, which is the key to their popularity with potters.
Ball clays are relatively scarce deposits due to the combination of geological factors needed for their formation and preservation. They are mined in parts of the Eastern United States and from three sites〔The Bovey Basin in South Devon, the Petrockstowe Basin in North Devon and the Wareham Basin in South Dorset.〕 in Devon and Dorset in South West England. They are commonly used in the construction of many ceramic articles, where their primary role, apart from their white colour, is to either to impart plasticity or to aid rheological stability during the shaping processes.
==History==
The name "ball clay" is believed to derive from the time when the clay was mined by hand. It was cut into 15 to 17-kilogram cubes and during transport the corners of the cubes became rounded off leaving "balls".〔(【引用サイトリンク】 publisher = Industrial Minerals Association - North America )〕
The ceramic use of ball clays in Britain dates back to at least the Roman era. More recent trade began when a clay was needed to construct tobacco pipes in the 16th and 17th century. In 1771 Josiah Wedgwood signed a contract for 1400 tons a year of ball clay with Thomas Hyde of Purbeck,〔See Purbeck Ball Clay〕 enabling him to fire thinner walled ceramics.〔(【引用サイトリンク】 publisher = The Purbeck Mineral & Mining Museum )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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