Although clay has numerous properties that make it valuable in a range of industrial processes, the plasticity of wet clay is perhaps unique. The origin of clays, whether primary or secondary, is important in understanding the properties in terms of particle size, plasticity, and the presence of iron and other contaminants. These topics are discussed in the following sections of this TechBrief. We supply clay products that exhibit a range of plasticity behavior to suit any application. Our English china clays are primary clays of exceptional purity and are the standard for white-firing bodies. Sedimentary ball clays in our product line have exceptional plasticity and are preferred in throwing and jiggering where high plasticity is required.
The property that is most important to traditional ceramics and the potter is the tremendous plasticity exhibited by many clays when mixed with the proper amount of water. This feature, unique among natural materials, permits the varied methods of fabrication that are well known to the potter and ceramist.
The plasticity of clay has been much studied by scientists but the exact mechanism is still not completely understood. To some degree plasticity can be attributed to fineness of grain. The smallness of the particles, each one of which is wet with water and lubricated on the surface with moisture, tends to make the particles cling to one another and thus cause the whole mass to retain a given shape. The fact that one can model with wet sand, which when dry has no plasticity at all, this principle. The finer the sand, the more successful will be the sandcastle. The plate-like shape of clay particles also contributes to plasticity. Plastic clay might be likened to a deck of playing cards wet with water. Each card is saturated with water, and a film of water between surfaces makes the cards slip and slide on one another and also causes them to stick together. Similarly, clay particles slip upon one another when a force is applied, but then hold to their new position. Chemical attraction between particles also contributes to plasticity. When an electrolyte is added to wet clay, the chemical affinity between particles is diminished and plasticity is drastically impaired.
The carbonaceous matter ordinarily present in clay also contributes to plasticity. Most common clays, which tend of to have a significant amount of carbon or in them, are rather plastic. The organic matter seems to act almost like a gum or glue in the behavior of the clay. While a small amount of organic matter in a clay may be helpful in developing plasticity, too much will make the clay excessively sticky, hard to work, and high in shrinkage.
Subtle differences in plasticity are hard to evaluate in any exact way. But a good idea of plasticity can be gained simply by pinching, rolling, or bending a small sample. The experienced thrower is perhaps the best judge of the plasticity of a clay, and shaping on the wheel might be thought of as the ultimate test for plasticity.
Clays vary a great deal in plasticity depending on their geologic history. Some coarse clays, even though quite nonplastic, are useful for making bricks and other heavy clay products such as tile and drainage pipes. Other clays are too plastic and sticky to be used by themselves and must be blended with other less plastic clays to be useful. Many clays, however, are usable just as they come from the earth and may be modeled or thrown on the potter’s wheel without any adjustments in composition. Raising the temperature of a clay mass will increase its plasticity.
Primary (residual) Clay
Clays may be classified in various ways, depending on what properties are of interest to the classifier. One might classify clays according to their color as they exist in nature. Or the classification might center on the idea of use, or of geologic origin. The potter is interested in what the clay will do for him in the making and firing of pots, and he will look at clay from that standpoint. He is interested in the plasticity or workability of the clay and in its reactions during drying and firing. The division of clays into two broad groups, primary clays and secondary clays, helps to classify clays for the potter and helps him to understand and make use of the peculiar working and firing properties of various clays.
Primary clays–or residual clays, as they are sometimes called–are those clays that have been formed on the site of their parent rocks and have not been transported, either by water, wind, or glacier. Primary clays are unusual, since normally the products of weathering are carried off down slopes by water into creeks and rivers, and eventually to lakes or to seas. But in some instances clays are left on the spot where they were formed by the disintegration of feldspathic rock.
Rock beds are broken down into clay largely by the action of ground water seeping through the rock and thus gradually leaching out the more soluble components. In some cases the percolation of steam or of gases from below may have contributed to the formation of clay. In typical deposits of primary clay, much unaltered rock remains, and the clay is found in irregular pockets. Since the clay has not been water-borne, there has been no opportunity for the selective sorting out of the various particle sizes, and large and small grains of clay are found mixed together. Deposits of primary clay are apt to be coarse-grained and relatively nonplastic. There has been little opportunity for fine grinding and sorting.
Primary clays, when they have been cleared of rock fragments, tend to be relatively pure and free from contamination with nonclay minerals. The reason for this is that most primary clays originate from beds of more or less pure feldspar, a rock that is relatively easily broken down in geologic time by the action of water alone. Another obvious reason is that since the clay is not carried by streams, there is much less chance for admixtures from other localities to alter its composition. We value primary clays, then, for their purity, their whiteness, and their freedom from objectionable mineral or organic contamination. Most kaolins are primary clays.
Secondary (sedimentary) Clays
Secondary clay is clay that has been transported from the site of the original parent rock. Although water is the most common agent of transportation, wind and glaciers may also carry clay. Secondary or transported clays are much more common than primary clays. In nature it is almost certain that eroded material will be carried to a new site.
Transportation by water has an important effect on clay. For one thing, the action of the water in streams tends to grind up the clay into smaller and smaller particle size. Then, when the water of the stream begins to slow down, some of the material that it carries will settle out. The coarse particles naturally settle first, leaving the fine particles still suspended in the water. When quiet water is reached, as in a lake or sea, the remaining very fine particles of clay sink to the bottom. This process of sedimentation tends to separate the coarse from the fine.
Transported clays are ordinarily composed of material from a variety of sources. In any one stream silts from numerous erosion sources are apt to be mixed together. This accounts for the more complex make-up of most secondary clays compared to pure kaolin. As would be expected, transported clays are apt to contain iron, quartz, mica, and other impurities. Carbonaceous matter, usually in the form of lignite, is also commonly found in secondary clays. Lignite, a form of coal, is formed with the clay as it is laid down by sedimentation in shallow marshes, lakes, or deltas where there is heavy vegetation.
Secondary clays vary widely in composition. Some are relatively pure and iron-free, such as the secondary kaolins, but these are quite rare. Ball clays are highly plastic secondary clays with little iron content. The great majority of secondary clays contain enough iron to make them fire to a buff, brown, or red color, and the maturing temperature is usually quite low. The clay may be found in small localized pockets, the sedimentation of one stream, or it may occur in vast deposits of millions of tons covering several square miles.