Porcelain
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Porcelain Pottery has been made since Neolithic rimes, when people in agricultural communities fashioned vessels for cooking and storage. Wet clay is plastic and can be molded into any desired shape, but when baked, the clay is remarkably résistant to weathering and other agents that corrode métal or destroy organic materials. Early potters found techniques to make their art even more durable, by using différent types of clays, adding other materials to the mixtures, and developing new methods for firing. One of the most sophisticated forms of pottery is porcelain.
Porcelain is one of the hardest ceramics. Porcelain is a vitrified pottery with a white, fine-grained body. Other earthenware is porous, opaque, and coarse, but porcelain is often translucent. This translucency, as defined by Western practice, is what classifies a material as a porcelain. The Chinese, however, define porcelain as any pottery that is résonant when struck. Most porcelain is white, although some has a gray or dusky appearance. Because it is one of the hardest ceramics, porcelain is now used for laboratory equipment and electrical insulators Grade porcelain was first made during the Tang dynasty in China at about the 7th century A.D. Known as true, or hardpaste, porcelain, it was fashioned out of kaolin (a pure white clay formed when the minerai feldspar breaks down), mixed with petunse (a feldspathic rock found in granité), and ground into powder. The mixture was fired at an extremely high température of 1450°C (2650°F), which caused the petunse to vitrify; the refractory kaolin, however, allowed whatever figure was fashioned to retain its shape. Because it was extremely difficult to maintain an even heating distribution throughout the entire kiln, the température rose so high in many initial attempts that even the clay vitrified. The early discovery of porcelain was probably an accident, an offshoot of Chinese stoneware manufacture with the serendipitous addition of ground white rock (petunse) and white clay (kaolin). The proportions of kaolin and petunse vary. "Sévère" porcelain has a higher percentage of kaolin, making it appear harsh and cold,
MRS BULLETIN/JULY1990
while "mild" porcelain has a lower percentage of kaolin, resulting in a mellow, satiny appearance. During the Sung dynasty (960 - 1279) royal factories began producing porcelain for the Impérial palaces. Since the 14th century, most Chinese porcelain has been made in the city of Ching-te-chen. Although porcelain manufacture had spread to Korea by the 1100s and to Japan by the 1500s, the Chinese guarded their secret for many centuries. A Persian travêler to Egypt wrote an account describing the Chinese porcelain he had seen; his report circulated in Italy and caused much interest. Marco Polo also described Chinese porcelain, and items of exported pottery made their way along the trade routes. In 1171 the Egyptian Sultan Saladin presented the Sultan of Babylon with a gift of 40 pièces of "China ware," which was described by returning Crusaders. Italian princes, fascinated by
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