Clay
Why It Acts The Way It Does
by F.H. Norton
In this article an attempt is made to describe, in terms understandable by the average potter, the structure of clay and the reasons why it acts in such a unique way when used in a pottery body.
The purest type of clay is kaolin, found in great quantities in our southern states. There are several kinds of kaolin, the most plentiful are the sedimentary kaolins which may be used as mined or put through a washing process to remove unwanted minerals such as quartz, feldspar or iron oxide. Then there are residual kaolins which must always be washed as they contain large amounts of non-clay minerals. When properly treated, however, they become our whitest kind of clay. One other kaolin of interest, found in northern Florida, has high plastic properties.
Kaolins are almost never used as the complete pottery body because of low plasticity, high drying shrinkage and very high firing temperatures to vitrify. They are white burning, however, and serve as the basis for white earthenware and porcelain after an additive of quartz and feldspar.
Another class of clays is the ball clays found in Kentucky and Tennessee. These are fine grained clays containing organic matter. They are not used as the complete body, but rather in additions of 10% to 20% to give added plasticity, better green strength and improved casting properties. These clays are not so white burning as the kaolins, but when used in small amounts have little adverse effect on the fired color.
Another class of clays used by the potter is stoneware clays. These clays burn buff or gray and usually contain enough nonplastic material to give good working and firing properties. In some cases, however, these clays are washed and some additions made to give enhanced properties.
The early pottery used in this country was made from red-burning glacial clays found in nearly all villages in the eastern United States - the same clay as was used for making bricks. These clays are quite plastic and are still used for some types of pottery, but it is difficult to fire them into a water-tight body.
Another clay similar to the last one, known as slip clay, is used as a glaze for stoneware and electric insulators. The traditional source is Albany, New York.
Lastly there is an extremely fine grained clay called bentonite that is used as very small additions to bodies for enhancing their plasticity.
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