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.