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Types of Clays

Clay is basically a mixture of rock powder, that has been broken down by nature over millions of years, and water. There are various types of clay. These can be examined from two main perspectives: the geological origins of the clay and the type of clay body.

Geological Origins

  • Primary Clays
    Primary clays are clays that are found in the same spot as the parent rock (various types of granite), from which the clay originated. This means that the clay hasn't been moved by water, glaciers or other forces of nature. Most primary clays can be classified as kaolins. Kaolin is fairly non-plastic (difficult to shape) so it is never used on its own. It is also highly refractory, which means that it doesn't melt or fuse until a high temperature range, notably around 3,200o F (1760o C). Because kaolin is white, is usually used in white clays, such as porcelain.
  • Secondary Clays
    Secondary clays have been moved about by the forces of weather, rivers and glaciers over millions of years. In the process, the simple composition of the primary clay becomes more complex, as other ground up materials, such as iron, ash or quartz are mixed in. Secondary clays can further be broken down into
    1. Ball Clay -- has some iron content; high content of organic matter (carbon); is more plastic; high shrinkage rate; melts around 2300o F (1260o C)
    2. Bentonite -- highly plastic; high volcanic ash content; good binding qualities
    3. Fire Clay -- has some iron content; melts around 2800o F (1400o C); varying organic content and plasticity
    4. Earthenware -- high iron and mineral content; naturally plastic; maturing range between 1700o F (926o C) - 2100o F (1150o C);
    5. Stoneware -- maturing range between 2,200o F (1205o C) - 2300o F (1260o C)

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