There are a great many different kinds of clay as described in numerous textbooks devoted to the subject. All clays are the products of weathering feldspar rock. The conditions occurring during the weather process determine the type and composition of the clay. Several of the principal types have been summarized in this Technical Brief to assist you in choosing the right type for your needs. Please refer to the product section of the website to compare the many types of clays we stock!
Kaolin
Kaolin, or china clay, though relatively scarce in nature, is of particular interest to the potter. It is indispensable in the making of pure white porcelain or china. Its scarcity is indicated by the fact that the deposits of kaolin in Europe were largely unnoticed until early in the eighteenth century. Significant deposits of kaolin occur in Europe, England, and North America, as well as in Asia, but they are considerably less common as other types of clay.
In China, wares made from white clays were fashioned at least from the beginning of the Han Dynasty, 200 B.C., or earlier. The management of kiln temperatures up to about 1200°, and the manufacture of vitrified white ware, using kaolin as the chief clay, dates, in China, to as far back as A.D. 600. This antedates the manufacture of porcelain in Europe by 1000 years. In China, china clay or white burning kaolins are more commonly found than elsewhere; furthermore they are more plastic and workable than the white clays of other regions. Early Chinese potters at first made a soft white earthenware from kaolin. Gradually, over a period of development lasting several hundred years, they learned to reach higher temperatures in their kilns and to make the proper additions to their clays to achieve the hardness, whiteness, and translucency of true porcelain. This discovery of porcelain was a technical triumph in the development of ceramics.
The whitest-burning kaolin clays, and hence the most pure, are primary clays that were weathered at the site of the feldspar. They are coarse in particle size and are therefore less plastic compared to most sedimentary clays.
In chemical composition kaolins approach the formula of the mineral kaolinite (Al2O3.2SiO2.2H2O). Kaolin is a highly refractory clay and has a melting point above 1800°. Used by itself, kaolin is difficult to shape into objects because of its poor plasticity, and also, because of its refractoriness, it is difficult to mature by firing to a hard, dense object. In practice, therefore, kaolin is seldom used by itself; other materials are added to it to increase its workability and to lower the kiln temperature necessary to produce a hard; dense product. As would be expected, the shrinkage of kaolin is low because of its relatively coarse grain structure, and it has little dry strength.
Ball Clays
Ball clays are somewhat the opposite; of kaolin in their properties. They are higher in iron content, more fusible, much more plastic, and fine in particle size. Ball clays and kaolin are really complementary in character and are often combined in clay bodies to adjust the mixture toward a practical, workable clay. Ball clays are said to have been so named because of the practice in England of forming the damp clay in the mines into large balls that could be rolled up onto wagons for transport.
Ball clay is a secondary or transported type of clay that is found in stratified layers, often alternating with layers of coal and with other types of clay. It is highly plastic. Although not so pure as kaolin, ball clay is relatively free from iron and other mineral impurities and burns to a light gray or light buff color. It tightens into a dense structure when fired to about 1800°. Different ball clays vary considerably in composition.
Ball clays are impossible to use alone in pottery because of their excessive shrinkage, which may be as high as 20 percent when fired to maturity. They are usually used as an admixture to other clays to gain increased plasticity and workability. In manufacturing whitewares, ball clay is indispensable as an addition to the body to overcome the nonplastic properties of kaolin. However, if whiteness is desired, not more than about 15 percent of ball clay can be added to a clay body; more than this amount in a whiteware body results in a gray, off-white, or buff color. The presence of ball clay in a porcelain body decreases its translucence.
In the raw, ball clays are usually dark gray because of the presence of carbonaceous material. This carbon burns off in the firing and does not affect the final fired color of the clay. The more carbon a ball clay contains, the more plastic it is apt to be. Some ball clays, however, such as those from certain districts of Tennessee, contain little carbon and are quite white in their raw state. Ball clays from England, which are valued for their high plasticity and freedom from iron, often have a great deal of carbon in them, which gives to the raw clay its dark brown or almost black color. Ball clays that contain a large amount of carbon, particularly if this carbon is in the form of bits of lignite or coal, must be carefully screened before they are used.
Ball clays are useful in a great variety of ceramic products and are mined in large quantities from extensive deposits in Kentucky and Tennessee. Various producers market them under different trade names, but the name "ball clay" always indicates a light-burning clay of high plasticity.
Fire Clays
Fire clay is not so well defined a type of clay as either ball clay or kaolin. The term "fire clay" refers to refractoriness or resistance to heat, and clays which vary widely in other properties may be called fire clays if they are refractory. Some fire clays are very plastic and some lack plasticity, and the fired color may vary. Any clay which resists fusion or deformation up to about) 1500° may be called fire clay. Such refractoriness or resistance to heat means that the clay is relatively pure and free from iron, although most fire clays burn to a buff or brownish color, sometimes with darker splotches due to concentrations of iron-bearing minerals.
Fire clays are useful for a great variety of products, principally in the manufacture of fire brick and other refractory parts for kilns, furnaces, boilers, and melting pots. Industries such as steel, copper, and other metallurgical industries could not operate without fire brick furnaces in which high-temperature smelting is done.
Fire clays are also used as additions to stoneware bodies or to bodies for saggers and other kiln furniture where an increase in refractoriness is desired. In stoneware bodies fire clay may furnish a desirable roughness or "tooth" to the body and may also give texture to the body because of the presence of dark iron spots. Fire clay is also useful in mudding in kiln doors, making clay pats for pyrometric cones, and for wadding under kiln shelves and sagger lids.
In bodies for large terra cotta pieces or sculptures, the open, coarse texture of some fire clays makes them an ideal addition.
Stoneware Clay
Stoneware clays are plastic clays which mature or become vitreous at 1200° to 1300°. Their fired color ranges from a very light gray or buff to a darker gray or brown. Stoneware clays are secondary, or sedimentary, clays. They vary widely in color, plasticity, and firing range, and there is no sharp distinction between what might be called a fire clay, a sagger clay, or a stoneware clay. The classification really hinges upon the possible use of the clay in ceramics, rather than upon the actual chemical or physical nature of the clay or its geologic origin. One clay, for instance, might be successfully used both as a fire clay in the making of bricks and refractories and as a stoneware clay in the making of high-fired stoneware. Many clays are quite suitable for making stoneware without any additions. Such clays may have just the right plasticity for wheel work and may have desirable drying and firing characteristics. The small country potteries of the last century, which produced utilitarian wares such as crocks, jugs, and churns, usually employed a single stoneware clay which was dug in the neighborhood and pugged ready for use without the addition of any other clay. Such a natural clay body may burn to very pleasing colors and textures and may take salt glazes, slip glazes, or high-fired stoneware glazes.
Earthenware Clays
Most of the usable clay found in nature might be called "earthenware" clay or common clay. These clays contain iron and other mineral impurities in sufficient quantity to cause the clay to become tight and hard-fired at about 950o 1100° C. In the raw such clay is red, brown, greenish, or gray, as a result of the presence of iron oxide. Fired, the clay may vary in color from pink to buff to tan, red, brown, or black, depending on the clay and the condition of the firing. Most of the pottery the world over has been made of earthenware clay, and it is also the common raw material for brick, tile, drain tile, roof tile, and other heavy clay products.
Common red clay may be highly plastic--in fact, too plastic and too sticky to be used by itself; or, on the other hand, it may be quite nonplastic because of the presence of sand or other rocky fragments. The potter will look for smooth plastic earthenware clay that he may modify by the addition of some sand or some nonplastic clay. The brickmaker will look for an earthenware clay that is naturally coarse and contains considerable sand or other non-plastic fragments, and with such a clay he will be able to press, dry, and fire his bricks without having them warp, crack, or shrink excessively.
References:
Rhodes, Daniel, Clay and Glazes for the Potter, Chilton Book Company, Radnor, PA, pp 19-24 (1996).
Van Olphen, H., Clay Colloid Chemistry, John Wiley, New York, (1974)