Clay    page 1 of 5  next

Chemical Properties


Clay is a natural substance occurring in great abundance in nature. It is constantly being formed on the earth's surface as a result of the weathering of a very common form of rock called Feldspar.

The great bulk of material found on the earth's surface is of a small number of substances that are relatively light in weight and "Float" to the earth's surface. The heavier materials such as metals occupy the earth's core. Of the materials found of the surface of the earth Silica is the most abundant (60% of all material on the earth's surface). The second most abundant is Alumina (15%). These materials are chemically referred to as compounds - Meaning that they are a made of two or more elements that are chemically bonded together. In Nature very few materials exist as pure elements; most have formed chemical bonds with other elements - usually oxygen. Silica, for instance is the mineral (or compound) name for the element Silicon that has combined with the element oxygen. Alumina is the mineral name for the material that results from the bonding of Aluminum and oxygen.

Feldspar, from which clay is formed, is the mineral name for a family of compounds that results from a chemical bond between Silica, Alumina, and one of three different metals (Potassium, Sodium or Lithium). This rocky substance occurs in great abundance and it's exposure to air and water causes it to change very slowly over vast periods of time into clay. This weathering process results in water, (a compound of Hydrogen and oxygen) replacing the metal in the Feldspar and changing the Feldspar into a new substance we call clay. The chemical formula for pure clay (mineral name Kaolinite) is Al2O3 2SiO2 2H2O.

The weathering process of clay formation results in a number of variations in clay types that are important to the potter. These clay types are the result in variations in the particle size of a particular deposit and/or the quantity of impurities (usually iron oxide ) that has been mixed with the clay during weathering.
 Common name  Impurities  Particle size
 Kaolin  none  large
 Stoneware  1% Iron oxide  mixed (large & small)
 Fire clay  1% iron oxide  large
 Ball clay  1% Iron oxide  small
 Earthenware  7% Iron oxide  mixed

Clays are generally classified as either primary or secondary according to their geologic history. A primary clay (kaolin) is almost always found in the same place as the feldspar from which they were formed. They are relatively uncontaminated by other minerals and remain unsorted by particle size. Kaolin is relatively rare in nature and difficult to work with because of it's low plasticity. The vast majority of clays are called secondary because they have been transported by weathering action of wind and water from their original site of formation. The weathering process tends to mix in other minerals (notably iron) and to sort the particles by size - clay carried by a fast moving stream will tend to settle to the bottom by particle size according to how turbulent the water is.

If clay is heated to a sufficiently high temperature as in a pottery kiln, it is chemically transformed into a new substance. The principle difference between this new substance and the original clay is that the water portion of the clay molecules have been driven off leaving a compound of Silica and Alumina. This new substance is monolithic; the individual particles are fused and will longer dissociate into a plastic mud when wet.

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