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CLAY AND CLAY BODIES

PRIMARY CLAYS

Primary, or residual clays, are those clays which have been formed on the site of their parent rocks and have not been transported. Primary clays are large in particle size, non-plastic, low in impurities, refractory, and low in shrinkage. Kaolin is a primary clay.

SECONDARY CLAYS

Secondary clays have been transported from the site of the original rock. In the process of transportation, the clay particles are ground to a much finer state, and are therefore much more plastic. They also acquire a range of impurities, including iron. Secondary clays are much more common than primary clays.

TYPES OF SECONDARY CLAYS

Ball Clays

Particles of ball clay are very small in size and are, therefore, very plastic. They also have a very high shrinkage rate. Due to impurities, they are less refractory than kaolin. Ball clays are used to increase plasticity in clay bodies. A plastic clay body may contain 15-25% ball clay. Too much ball clay in a clay body will cause excessive shrinkage and cracking and grayness in the fired color. The fired color of ball clays is gray to buff. We commonly use Tennessee Ball Clay and Kentucky Ball Clay in our clay bodies and glazes.

Fire Clays

Fireclays are so named because they are refractory (resistant to heat). Fireclays fuse at about 2732° F., have a range of plasticity and particle size, and a buff to brown fired color due to the presence of some iron. Fireclays make a good foundation or addition to a stoneware clay body. When the fireclay is course it provides "tooth" to a clay body. They are also excellent for making refractories such as firebrick. We commonly use Lincoln, Missouri and Greenstripe Fireclays.

Stoneware Clays

Stoneware clays are plastic clays that vitrify at 2192-2372° F. With the addition of grog, stoneware clays make usable stoneware clay bodies. They were often used by early American country potters. Kentucky Stone clay is an example of a stoneware clay.

Earthenware Clay

Earthenware clays are very common. Most of the pottery made worldwide is made from earthenware clay. Earthenware clays are very impure, with a high iron content and mature between 1706-2012° F. Because of the low maturing temperature, earthenware clays can be used as high temperature glazes. Bricks, pipes, roofing tiles, and planters are made from earthenware clays. Red Art Clay is the earthenware clay we are currently using.

Bentonite

Bentonite is a clay of volcanic origin which is used to add plasticity to clay bodies and to keep glazes in suspension. An addition of up to 2% is usually sufficient for these purposes.

CLAY BODIES

A clay body is a mixture of clay or clays and other minerals, blended together with water, for a specific purpose, such as throwing, sculpture, slip casting, tile-making, etc.

Necessary Properties

1. Plasticity- the ability to be shaped and hold its' shape.
2. Porosity- in the green state, the ability to dry evenly without warping and/or cracking. Fillers in the clay body allow the water to escape through open pores.
3. Vitrifiability- the clay body should become dense, hard and strong, or vitrified (glass-like) at the desired temperature. During vitrification, parts of the clay body fuse together.

Composition

1. Plastic elements- clays
2. Fillers- grog, silica, sand. Provides porosity to green clay. Provides plastic strength to clay bodies. Adds refractory components. The addition of silica affects the glaze fit.
3. Fluxes- glass forming elements. Lowers the melting point, promotes vitrification and provides the proper density. Feldspar is the primary flux for stoneware clays. Iron bearing clays are another way to provide flux in a clay body.
4. Color- usually provided by iron.

Common Clay Bodies

1. Porcelain- dense, non-porous white clay body fired above 2282° F.
2. Stoneware- dense, hard, non-porous clay body fired above 2012° F.
3. Earthenware- strong but porous clay body, fired below 2012°F.
4. Raku- heavily grogged clay body, designed to withstand heat shock. Fired between 1382-1652° F.

Formulating a Clay Body

Unlike glaze calculation, creating a clay body is an empirical process, achieved through trial and error. For example, to formulate a Cone 10 stoneware throwing body, you would first find a clay which has some of the characteristics that you desire. A desirable stoneware clay would fire to density, with 3% fired absorption at 2282° F. and shrink 13% at most. Then, make the necessary adjustments, through trial and error. Plasticity can be improved by the addition of plastic clays like ball clays. The color can be altered by the addition of iron or iron-bearing clays. The fired density can be adjusted by the addition of a flux such as feldspar. Texture and shrinkage can both be affected by the addition of fillers, such as grog and sand. Finally, the glaze fit can be influenced by the addition of silica.

Some examples of clay bodies

Cone 10 Chouinard Stoneware

 

Cone 10 Porcelain

 

Cone 5 Red

 

Cone 5 White

 

Raku Clay

 

Source: Clay and Glazes for the Potter, Daniel Rhodes, Revised Edition, Chilton Book Company, 1973, ISBN 0-8019-5633-1