Types of Natural Clay
Kaolin
 
Whitemud Clay formation in southern Saskatchewan.
 
 

There are a great number of very different clays in nature. This variety is explained by the various geological conditions at the origin of the formation of clay beds. Different clays can be mixed one with another. From this wide range, the ceramist distinguishes certain categories, based on the similarity of plasticity, texture, composition and use.

Kaolin is of particular interest to the potter. It is essential to manufacture real porcelain. Kaolin beds exist in Asia, Africa, North America as well as in Europe, but they are much more rare than layers of other types of clays. In China, white burning kaolin is more common than everywhere else on Earth. It is more plastic and easier to work with than those of other countries.

The manufacture of white pottery began in China at the beginning of the Han dynasty, 200 years before our era. Originally, Chinese potters made soft and white clay bodies from kaolin. The control of firing up to approximately 2300 oF and the first white vitrified pottery using clay bodies made mainly from kaolin exist in China since at least the 6th century of our era. Slowly they learned how to reach higher firing temperatures and how to modify their clay bodies to obtain the hardness, whiteness and Tran lucidity of genuine porcelain. The discovery of porcelain was a real technical triumph in the field of ceramics.

Kaolins are primary clays formed by local decomposition of feldspar. They are mainly found in small depressions rather than in vast stratified layers and they are relatively free from mineral impurities like iron. Their grain is coarse and they are not plastic, compared to most sedimentary clays. Pure kaolins are very refractory and their melting point exceeds 3200 oF. Alone they are very difficult to use because of their low plasticity and high melting point. Consequently kaolin is rarely employed alone, other materials are added to make it more plastic and to lower its melting point to obtain vitrified pottery. Kaolin modified in this way is called porcelain .

Ball clays have, so to speak, contrary properties than kaolins. They are part of secondary or alluvial clays. They contain more iron, are more fusible, much more plastic and their particles are smaller. Ball clays and kaolins are really complementary, and are often mixed together to obtain clay bodies that are easier to work. Although less pure than kaolins, they contain relatively little iron and other impurities, and fired at approximately 2400 oF, they take on a gray or brown hue and are dense and tight.

It is impossible to use them alone in pottery because of their excessive shrinkage, which can reach up to 20 % when fired to their maturation temperature. They are used in addition with other clays to increase their plasticity. In preparing porcelain bodies, their addition to the paste is essential to correct the lack of plasticity of kaolin. However, in higher quantity than 15 % there is the risk of appearance of gray or brown.

Unfired, these clays are generally dark gray because of the presence of carbonic matter. This carbon, entirely burned in the kiln does not affect the color of the fired body. Such clay is more plastic since it contains more carbon. However, certain ball clays contain little carbon and are completely white in their natural state.

Ball clays are used to manufacture a large variety of products. In the United States, they are extracted from important layers in Tennessee and Kentucky.