china clay China clay, also known as kaolin, is a white commercial clay consisting predominantly of the mineral kaolinite, a hydrated aluminosilicate. The name ‘kaolin’ is derived from the village of Gaoling in Jiangxi province, China, where the white clay was mined. The nearby Jingdezhen potteries used the kaolin to create their fine white porcelain. The composition of Chinese procelain was identified by Europeans in the eighteenth century and deposits of kaolin were sought in Europe. In 1746 William Cookworthy, a Plymouth chemist, recognized the occurrence of kaolin in Cornwall, in south-west England, and subsequently manufactured porcelain from Cornish china clay. The deposits of china clay in Cornwall and Devon rapidly attracted the attention of the Staffordshire potters, who until the early nineteenth century used Cornish and Devonian kaolin almost exclusively in the production of fine ceramics.
China clay deposits have now been found throughout the world and new deposits are still being discovered. Kaolin deposits may be classified as primary or sedimentary. Primary, or residual, deposits were formed by the alteration
in situ of the parent rock, which may have been igneous, metamorphic, or sedimentary, by volcanic, hydrothermal, and weathering processes. Sedimentary, or secondary, kaolins are derived from the erosion of pre-existing deposits and the subsequent transport and deposition of the clay. In Cornwall and Devon the kaolinite is derived from the late-stage magmatic or hydrothermal decomposition of feldspars within granite. It is separated from the host granite by washing it out with high-pressure water hoses, a process known as monitoring. The kaolin content rarely exceeds 20 per cent of the altered granite, but the depth of kaolinization extends in many places down to 300 metres. The best-known sedimentary kaolin deposits are from Georgia, in the USA. These were formed from the erosion of deeply weathered crystalline rocks in the Piedmont Plateau and deposited along Georgia's Fall line during the Cretaceous and Tertiary periods. Here the kaolin is found in lenses, often up to 20 metres thick and with a high percentage of kaolinite, around 80–90 per cent.
The world market for high-quality kaolin is about 25 million tonnes per annum. The USA is the largest producer with 9 million tonnes, and south-west England the second with 3 million tonnes. Other important producers are Germany, France, Ukraine, China, Czech Republic, New Zealand, Brazil, Spain, Indonesia, and Australia.
Kaolin today is used in making paper, plastics, rubber, paints, fibreglass, ceramics, some foods, sunscreen lotion, and many other products. Almost 80 per cent of the kaolin produced in Georgia, Devon, and Cornwall is used in filling and coating paper. Filler clays are so called because they fill the gaps between wood fibres in the papermaking process. The addition of these fillers also improves the strength, smoothness, brightness, and opacity of the paper. Coating clays may be applied to this base paper to impart a glossy surface suitable for high-quality printing of illustrations. Additional uses of kaolin continue to be developed; they include a wide range of new applications in the paper industry, ranging from low-cost pulp extenders to high-opacity fillers and high-gloss and high-brightness coatings.
Ian Wilson
