The generally accepted definition of porcelain is that of a white, vitrified, translucent ceramic, fired to a temperature of at least 1280 centigrades. The body of most Chinese porcelain is made from a mixture of white China clay (kaolin) and porcelain stone (dunzi, a feldspathic rock); the latter being ground to powder and mixed with the clay. The body and glaze are usually fired together in a reducing atmosphere at a temperature between 1200 and 1300 centigrades in a single firing, forming an integrated body/glaze layer. During the firing the petuntse vitrified, while the refractory clay ensured that the vessel retained its shape. The reason porcelain is translucent is because the silica atoms have started to form glass. The Chinese definition of porcelain (tzu) resembles the Western definition of "Stoneware", besides having as a key feature that it should ring when struck. White ceramic pices made from Kaolin clay and fired at a temperature of over 1000 degree and with a thin layer of green glaze on the surface had already appeared in the Shang Dynasty (16th century -11th century B.C.). White and translucent porcelain with underglaze blue and white decoration was not made before the Yuan dynasty (AD 1279-1368). The secret of true hard porcelain similar to that of China was not discovered in the West until about 1707 in Saxony by Ehrenfried Walter von Tschirnhaus, assisted by the alchemist Johann Friedrich Böttger.