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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.
|Jennifer Mathes, Ph.D.|