The two terms are used interchangeably to describe a type of clay that fires to a white color. The name kaolin comes from the two Chinese words kao-ling, meaning high ridge, and was originally a local term used to describe the region from which the clay was obtained.
Kaolin (Al2O32SiO22 H2O) usually contains less than 2% alkalies and smaller quantities of iron, lime, magnesia, and titanium. Because of its purity, kaolin has a high fusion point and is the most refractory of all clays. Lone kaolins are widely used in casting sani-taryware, ceramics, and refractories.
Georgia china clay is one of the most uniform kaolins to be found. Generally speaking, there are two types of Georgia-sourced kaolin, both of which are widely used for casting and other processes. One type imparts unusually high strength and plasticity, and is used for both casting and jiggering where a high degree of workability is required. The other type typically is a fractionated, controlled particle size clay that also behaves well in casting, dries uniformly, and reduces cracking of ware.
Calcined Kaolin
This commercial product is made from a specially prepared kaolin that is low in iron and alkalies. Analyses show the calcined product to be principally mullite (3Al2O32SiO2) in association with an amorphous siliceous material. In fact, of the alumina present, 96% is converted to mullite. Iron content is not only low but in such a state of oxidation as to facilitate solid solution with the alumina.
Outstanding properties of refractories containing this calcined kaolin are high refractoriness and retention of shape under load; high resistance to corrosion by slags, glasses, and glaze or enamel frits; resistance to thermal shock; and high mechanical strength. It is being used in thermal shock bodies, refractories subjected to reducing atmospheres, kiln furniture compositions, thermal insulation bodies, low expansion bodies, permeable ceramic compositions, high-temperature castables, investment molds for precision casting, as a placing medium, as a kiln wash, as gripping sand for high-tension insulators, and in many other special refractory applications.
