Hands In Clay

Pottery Gallery and Teaching Studio

What is Clay?

The basic substance of all pottery is clay.  It is an abundant natural material making up about three fifths of the Earth's crust.   It can be divided into two basic groups: primary clays, which are mined precisely where they were originally formed; and secondary clays, which have been carried off by a variety of methods:  erosion, rain run-off, suspension in rivers, etc.,  and deposited far from the original site.

Clay in its simplest form is nothing more than powdered rock - decomposed granite - found all around us in gardens, fields, riverbeds, along roadsides and in the swamp. It originates from feldspathic rock that was molten when the Earth was formed but was transformed over millions of years by decomposition and weathering. It's the feldspathic origin of clay that makes it possible to fire to a dense and permanent hardness and cover it with an impermeable coating of glasslike material called glaze. The two main ingredients are silica and alumina but it also contains small quantities of other materials that alter the clays color and texture. To be capable of producing a pot or ceramic, a clay must also contain a flux and a heat resistant material, or refractory.  The amount of flux a clay contains in relation to the refractory material is one factor that controls the point at which a clay reaches its optimum density or maturity.

We classify the pots we make according to this maturity point, or to the temperature the pot was fired.  The three categories are:

  • Earthenware
    A low fire pot or ceramic, usually fired between cone 06 and cone 03, 1830F-2010F, usually red or tan in color.  The ceramic body remains porous and  unvitrified - below it�s maturity point  with 5-20 percent absorbency.

  • Stoneware
    A high fire glazed pot or ceramic that has reached its maturity point, where both body and glaze are fused together in a non-porous, vitrified state.  Fired to a temperature above Cone 6, 2190F, with little or no absorbency, it is usually gray, tan, or reddish.  Stoneware is similar to porcelain, the chief difference being increased plasticity and the color, which is the result of iron and other impurities.

  • Porcelain
    A hard, vitreous, non-porous and non-absorbent clay body that is white and translucent.  Made from clay prepared from feldspar, china clay, flint, and whiting.  It is generally high fired between Cone 6-10 but can go as high as Cone 14-16.

We also classify each individual clay according to its own unique throwing properties - plasticity, color, and ingredients - sticky clay, smooth clay, white clay, brown clay, red clay, clay with grog, Raku clay, and handbuilding or sculpture clay.  A clay that is a joy to shape on the wheel may not be a successful clay in the handbuilding class.  Also a smooth white clay may not work in a Raku firing or be successful in holding its shape for larger projects. Thus the clay worker must take into account the properties of each clay body in relation to each project and the desired result.

In today's modern world the clay bodies we use are actually blended together from a number of different ingredients to match that of clay actually dug out of the ground. The preparation of clay, therefore, has become a highly specialized industry supported by all the resources of a modern chemical laboratory. Here at Hands In Clay we offer a variety of clay bodies that we feel will accommodate the needs of every potter.