All clay can be classified as either residual or sedimentary. Residual clays remain more or less at the site of the original rock formation. They tend to be less plastic than sedimentary clays and contain larger-sized particles. Sedimentary clays have been transported far away from the parent rock. This type of clay has finer particles and therefore is the more plastic of the two types.
It is very uncommon for a single type of clay to be used as a clay body. This body might be made up any combination of the following clays. The ratio of these types of clay determine the plasticity, firing temperature, color and texture.
Kaolin is a very pure form of clay, it is near to the ideal formula of Al2 O3. 2SiO2.2H2O . It is never used alone as a body, but can be used to compare other clays. It can be fired to very high temperature, and can also be used in glazes. It is usually white. The major area where you can find this clay in the USA is the Southeast.
Ball clay is similar to kaolin after firing, however, in its unfired state, the color is a dark grey, due to the presence of organic materials. This clay can be found in swampy areas. Gasses and organic acids break down the clay into smaller particles than the sedimentary kaolins. Ball clay gives increased plasticity and strength when used in the clay body. Thus its throwing qualities are improved. Similar to kaolin, ball clay must be fired at a very high temperature. Tennessee and Kentucky have the largest deposites of ball clay in the US.
Stoneware clays are used quite often by the potter. These clays are generally plastic and are fired from 1222° C, to 1305° C. The color differs from buff to grey. When compared to kaolin, stoneware clays have many impurities such as calcium, feldspar, and iron. These impurities lower the maturing temperatures and give color to the clay. Stoneware clays can be found from New York to Illinois, and also on the Pacific Coast. When fired, stoneware clays become very strong.
Fire clay is a high-firing clay. It is usually used for insulating bricks, firebricks, and kiln furniture. Their physical characteristics vary from a fine plastic quality, to coarse and granular quality. They generally have an iron impurity. Fire clays can be found near coal veins. They are common to most states. However, they are rare in mountainous regions and the east coast.
Earthenware clays are a group of low-firing clays that mature at temperatures ranging from 955° C to 1120° C. Because of the low temperature, these clays are generally porous and fragile after firing. Some can not even hold water. They contain a high percent of iron oxide, which is a flux. Because of this flux, Earthenware clays can not be fired at a higher temperature than 1150° C or else it deforms and blisters. It can be found in almost all of the US. They can be shale.
Slip Clay are clays that have sufficient fluxes to be used as a glaze without any additions. While blue and white slip clays do exist, the most common are red tan, or brown-black. The best-know commercial slip clay is mined in small pits near Albany, NY. Slip glazes are easy to apply, and they have a long firing range.
Bentonite is a clay that is used in small amounts as a plasticizer. Deposits are found in most of the western mountain states. It has the finest particle size of any clay known. It is composed mainly of silica. It should be mixed dry, as it becomes quite gummy when mixed alone with water.
Clays that are completely satisfactory to fire seldom happen in nature. You can change the plasticity, color, and texture by mixing different types of clay. To make a clay body, you start with a fairly inexpensive clay that has no major faults. It should also be moderately plastic. Although, with the addition of Bentonite or ball clay, the plasticity can be improved. In some cases, the clay may be too fine to dry without cracking. You can remedy this problem by adding grog or a silica sand. (Grog is coarse, crushed, fired clay) When this is added, it opens up the clay body and makes it more porous so it will dry uniformly.
Fluxes such as feldspar, talc, dolomite, nepheline, syenite and bone ash can be added to a clay body if the clay lacks sufficient fluxes to fire hard enough at the desired temperature.
It is much cheaper to make your own clay. Homemade clay bodies cost about 10 cents per pound to make. While factory made clays tend to cost around 60 cents per pound. With homemade clay bodies you can also choose what color, plasticity, and texture you want much easier.