Clay

Natural Clay
While clay is a loosely used term for a very large variety of modeling materials, it is a natural substance. Clay is actually soft, disintegrating rock. During its development, it is exposed to weather, and is ground and washed. It is a mineral with a crystal structure, but the individual crystals are too tiny to be seen with the naked eye.

Primary clay is found close to the parent rock, and includes china clay, desired for its whiteness. It has a larger particle size than secondary clay. Secondary or sedimentary clay has been moved from its origin by water or ice. The particles of rock are broken up and ground during its movement, resulting in a finer material. These clays tend to contain impurities such as iron that gives it a gray, cream or brown color, and have collected organic matter that may darken the color and cause the clay to smell. The category contains many subcategories of clays called stoneware and earthenware.

Ready-To-Use Clay
While some artists may choose to mine their own clay from a natural source, most prefer to purchase clay after it has been cleaned and processed commercially. Clays are formulated for particular qualities, such as color, texture, or porosity. Clay may be best suited for throwing, hand building or sculpting. It may fire at a very high temperature, it may be used for a special process like raku, or it may be cured in a home oven or left to air-dry. It may be provided in dry form allowing the artist to mix batches as needed. Very large quantities are mixed in a machine called a pug mill. Moist, ready-to-use clay is convenient and practical. It is available in a variety of colors from white to gray or terra cotta, a reddish earth brown

Low fire clays tend to be reddish or white. After firing, the object may not be watertight unless it is glazed to seal the surface. Glazes are commonly used by the hobbyist and in educational programs. Cone 06-04 on the box denotes “low fire.”

High fire clays include stoneware or formulas that fire from Cone 4-10. Often buff to gray in color, these clays are used to make usable things such as plates, cups, vases, bakeware and tiles, or for sculpture.

Porcelain is the purest form of clay and is desired for its whiteness and translucency. This very fine textured clay is preferred for small, delicate items, for the melodic tones of wind chimes, and for items such as tiles or dinnerware. It fires from Cone 3-10.

Polymer Clay
Developed in Germany in 1930, it is made from PVC, or polyvinyl chloride. The PVC is mixed with a plasticizer and colorant, but the formula differs greatly between manufacturers. Uncured clay in one brand may be soft and pliable, another may be fairly tough and almost dry-like, yet another brand may be something in between. After it is cured (baked), the object is durable and rigid, though these characteristics vary a little depending on the brand. A color shift or darkening may be more noticeable with some brands compared to others.

Unlike earth clay, polymer clay does not shrink, and it doesn’t crack. Since it does not contain water, it will not dry out if left uncovered. It may be stored in any airtight container. Some artists prefer to wrap it in wax paper and/or store it in a plastic food bag.