Clay differs from earth or soil in that it possesses certain characteristics which these do not possess. Its distribution is very wide but for the most part it lies concealed from view. In certain parts of the country it is so abundant that it breaks through the surface or is exposed as an outcrop but usually it is covered by the soil which supports vegetation. Unless the subsoil consists of sand it is easy to expose a clay by plowing or digging with a spade. It usually appears as a greenish or bluish substance of close and uniform structure. The texture is sometimes smooth but more often numerous small stones are found imbedded in the mass. Such clays as are commonly found can be used for the manufacture of some kind of pottery but in the great majority of cases the ware will be red when fired because the clay contains a proportion of oxide of iron. A pure clay does not contain this and therefore becomes white or nearly white in the kiln.
Pure clay, known as clay base or clay substance forms a part of all natural clays though sometimes only a small part. It consists of silica, alumina and water in a state of combination and is thus known as a hydrous aluminium silicate. While this substance is very common as an ingredient of ordinary clay, it is rarely found alone or uncontaminated. Commercial or workable clays may be said to consist of clay base and sand, with or without other impurities such as lime and oxide of iron. For working purposes it may be granted that the potter has to deal with a mixture of clay and sand. But sand is not a definite expression. It may vary both physically and chemically within wide limits. The physical nature has to do with condition, the chemical with composition. Thus a sand may be almost as coarse as gravel or as fine as the clay itself. It may be a pure quartz sand or it may be a crushed rock of almost any composition. The former is known as quartz, the latter as feldspar or feldspathic sand because it approaches in composition the group of minerals known as feldspars. Each of these ingredients, clay, quartz and feldspar, has an important part to play in the transformation of clay into pottery. Few of the clays used in making white pottery possess these ingredients in the correct proportions so that it becomes necessary to make a mixture in which the necessary proportions will be found.
For successful pottery making three properties are demanded in a clay. First, plasticity. Without this, clay could not be shaped at all. It constitutes the obedience of a clay to the forming influence whether hand or mold. The necessity for this quality may be illustrated by the proverb "Making ropes of sand" as an example of the impossible. Sand, possessing no plasticity, cannot be shaped or made to hold together.
The second property is porosity. A clay which exhibits a high degree of plasticity can be easily shaped but it cannot be safely dried. The water of plasticity cannot escape and therefore the clay warps and cracks. The function of porosity is to prevent this. A porous clay permits the water to escape freely and the clay can be dried without damage. This condition is produced by the admixture of sand or by the presence of sand in a natural clay. A coarse sand is more effective than a fine sand but a sand that is too coarse will interfere with delicate working while a sand that is too fine approximates the action of the clay itself and produces a substance which is dense rather than porous. Porosity is therefore the reverse of plasticity and these two properties must be adjusted so as to balance each other.
The third necessary property is commonly known as vitrification but could be better named "densification" because complete vitrification is not attained in ordinary clay wares. This property may be defined as that which causes a clay to yield to the action of a high temperature so that the result is a ware, more or less dense, which is hard, durable and sonorous. With this there must be coupled a certain amount of resistance to heat treatment so that the pottery does not fuse or collapse during the firing. Here also is found the need for adjustment. The clay must yield to the fire but not completely. It must resist but not entirely.
Plasticity is due to the clay base. Not only to its quantity but to its quality also. Some forms of clay in which clay base predominates are not plastic because the clay base itself is coarse grained. Other forms with less clay base present are plastic because this ingredient is fine grained and tough. Pure clay base is also highly resistant to fire and therefore contributes to the refractoriness of the mass.
Porosity is caused by the sand in the clay. Any kind of sand will produce porosity but the effect differs with the condition of the sand. Coarse sand is more effective than fine sand. More sand will, of course, cause greater porosity.
Vitrification or densification is due to the feldspar or fusible sand. This also varies with the condition. A fine-grained feldspar will produce vitrification more easily than the same amount of coarse feldspar.
Certain substances are available for use in pottery mixtures, which possess one or other of the necessary properties in high degree so that they will impart these properties to a mass to which they are added.
Kaolin or china clay is usually fine, white, and refractory. Some kaolins are rather plastic but most of them are "short" in working and rather tender. For the production of a white ware kaolin is indispensable. No other ingredient will afford the pure white color which is sought after in porcelain and china.