The Natural History of Clay

by Alfred B. Searle

Clays are moderately soft, solid bodies, particularly when moistened, and can usually be cut with a knife, though some indurated clays and shales are almost as hard as felspar. Their apparent specific gravity varies greatly, some clays being much more porous than others, but the true specific gravity is usually between 2·5 and 2·65; it is similar to that of quartz and slightly lower than that of felspar and mica. Many clays appear to be devoid of structure, but those obtained from a considerable depth below the surface are frequently laminated and have a structure not unlike that of mica. This will be discussed later.

Examined under a microscope, clays are seen to consist of grains of a variety of sizes, the largest of which will usually be found to be composed of adventitious materials such as sand, quartz, felspar, mica, chalk and limestone. The smallest particles—to which clays owe their chief characteristics—are so minute as to make any examination of their shape very difficult, but they are usually composed of minute crystalline plates together with a much larger proportion of apparently amorphous material. The exact nature of both the crystals and the amorphous material is still unknown in spite of many investigations; in the purer clays both forms of substance appear to have the same chemical composition, viz. that of kaolinite (H₄Al₂Si₂O₉), which the crystalline portion closely resembles.

Clays emit a characteristic yet indefinable odour when moist; the cause of this is very imperfectly understood, though it is not improbably due to decomposing organic matter, as this occurs in most clays.

The colours of freshly-dug clays are extremely varied and range from an almost pure white through all shades of yellow, red and brown to black. The predominating colours are grey or greyish brown and a peculiar yellow characteristic of some surface clays. The natural colour of a clay is no criterion as to its purity, for some of the darkest ball clays produce perfectly white ware on burning, whilst some of the paler clays are useless to the potter on account of the intensity of their colour when they come out of the kiln. The colour of raw clays is largely due to the carbonaceous matter they contain, and as this burns away in the kiln, the final colour of the ware bears no relation whatever to that of the original clay.

The colour of burned ware depends upon the iron compounds in the clay—these producing buff, red, brown or black (usually termed 'blue') articles—on the presence of finely divided calcium carbonate (chalk) which can destroy the colouring power of iron compounds and produce white ware, and on the treatment the clay has received in the kiln. A clay which is white when underfired will usually darken in colour if heated to vitrification, and one which burns red in an oxidizing atmosphere may turn blue-grey or black under reducing conditions. The extent to which the carbonaceous matter is burned out also determines the colour of the fired ware.

The presence of adventitious minerals in the clay may also affect its colour, particularly when fired.

The most obvious feature in a piece of moist clay is its plasticity[5] or ability to alter its shape when kneaded or put under slight pressure and to retain its new shape after the pressure has been removed. It is this property which enables the production of ornaments, vessels of various kinds, and the many other articles which are the result of the application of modelling tools, of moulding or of the action of a potter's wheel. So long as clay contains a suitable proportion of moisture it is plastic and may be made into articles of any desired shape, but if the amount of moisture in it is reduced or removed completely, the material is no longer plastic. It may become so, however, on adding a further suitable quantity of water and mixing, provided that it has not been excessively heated. If, in the removal of the moisture, the clay has been heated to 600° C. or more, it loses its power of becoming plastic and is converted into a material more closely resembling stone.

[5] A plastic substance is one with the characteristics of 'a fluid of so great a viscosity that it does not lose its shape under the influence of gravitation.'

The causes of plasticity appear to be somewhat numerous, though there is no generally accepted explanation of this remarkable quality which distinguishes clays from most other substances. It is true that wet sand, soap, wax, lead and some other materials possess a certain amount of plasticity, but not to anything like the same extent as clay.

So far as clays are concerned, their plasticity appears to be connected with the presence of combined water as well as of mechanically mixed water, for if either of these are removed, plasticity—both actual and potential—is destroyed. The part played by water is not, however, completely known, for the many theories which have been advanced only cover some of the conditions and facts.