Different Type of Clay Used for Ceramic
by: rhusain
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Rarely is clay found completely free of impurities. The most common of these are sand, lime carbonate, iron sulphide and organic matter. These impurities cause variations in the color and texture of the clay. Most pure clays lack many desirable properties which have to be supplied by adding ingredients to them. If you found pure clay plastic enough to shape into a piece of pottery, you would probably be dismayed to find it shrink excessively under fire, causing the model to warp and crack.
Two minerals which must be added to pure clay are flint and feldspar. Flint, which is silica, is employed to control shrinkage and to open the clay body when fired, permitting moisture and gases to escape. Feldspar makes the clay fuse properly at the desired temperature.
Talc is also added to many clays, especially if they are to be used for casting figurines. This is done to prevent shrinkage and also to lessen the tendency to craze. (Crazing refers to the tiny cracks which appear in a glaze when it does not fit the clay.)
To lower the maturing point of clay, ground glass is sometimes mixed into the body. To strengthen clay, or to reduce its tendency to crack, grog may be added. Grog is composed of fired clay which has been ground into small particles. Sand or powdered quartz can also be utilized effectively as clay strengtheners.
Nowadays the hobbyist need not be bothered with the complicated details of preparing his own clays. Most dealers stock a large variety, either ready-mixed to a plastic consistency or ready for the hobbyist to mix with water and wedge to the consistency he requires.
Best known of the commercial grades of clay are ball clay, china clay and fire clay. Ball clay is highly plastic and can easily be shaped by hand. It fuses at a fairly high temperature and is used in the manufacture of all kinds of pottery, chinaware and stoneware. England, Kentucky, Tennessee and Florida supply some of the finest ball clays. China clay, otherwise known as kaolin, is the purest known form of clay. High-quality china clay is abundant in England and is exported extensively. In this country, Georgia is the prime source of supply. Kaolin is the only type of raw, pure clay the hobbyist should purchase for starting. This is because it has so many uses. To mention only a few, kaolin is utilized in making most glazes; it can be used in making under glazes; it is an ingredient of kiln-wash.
Fire clay has a very high verification, or hardening, point. This clay is valuable in the construction of kilns and the manufacture of refractory tiles, bricks, and other objects which must be able to withstand extremely high temperatures.
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About the Author
Mitch Johnson is a regular writer for http://www.curtains-n-drapes.com/ , http://www.ezceramicsresources.info/ , http://www.ceramicsforme.info/
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