China clay is a rock that is rich in kaolinite and belongs to the group of aluminum silicates. It is often referred to as kaolin or white clay. Kaolin derives its name from the location where it was first discovered, in Kao-Lin, China. It is found in abundance in soils from hot, moist climates, particularly in tropical rainforests. China clay is an industrial mineral that primarily is used in the porcelain industry but also in combination with other raw materials in a wide range of applications.
Kaolin is a soft white clay mainly composed of fine particles combined in the form of platelets. The clay is a result of the alterations of aluminum silicates found in rocks that are rich in feldspars, such as granite, under the action of weathering and hydrothermal processes. The process by which granite converts into a soft matrix that is found in kaolin deposits is called kaolinization.
Various aspects influence the end use of china clay. The degree of crystallinity influences its brightness, whiteness, opacity, resistance and viscosity, all of which determine its commercial interest. Uses of china clay include coated paper, ceramics, medicine, cosmetics and food additive in toothpaste. It is the main component used in porcelain.
The Chinese were the first to develop techniques for making porcelain from china clay, during the seventh and eighth centuries A.D. By the 14th century, they were exporting it to Europe. The English developed techniques for obtaining porcelain from china clay during the 18th century.
China clay has poor plasticity, so it generally is used in combination with a range of additives, including bentonite and ball clay. Its value for the industry of porcelain is given by the fact that it maintains its whiteness after being fired. When fired at temperatures of about 2,372 degrees Fahrenheit (1300 degrees Celsius), this clay has a shrinkage of about 12 percent.
Other uses of china clays include the production of smoking pipes in Asia and Europe. Kaolin is used in the production of some grades of paper to ensure brightness. It contains traces of uranium, so china clay is a useful material in radiological dating. In organic farming, it is used in sprays that are intended to prevent insect damage.