The difficulty of this classification is due to the different aspects in which their chemical composition, physical characteristics, and geological origin may be considered.

Authors generally follow the classification suggested by Brongniart about 1840; by it clays are thus divided: -

1. Kaolin Clays

White, yellowish or greyish, refractory, thin to the touch, and not easily forming a paste with water.

2. Plastic Clays

Refractory, dense and greasy to the touch, forming with water a tenacious, supple, and dough-like paste.

3. Smectic Clays Or Fuller's Earths

Full and greasy to the touch, frothing when beaten up with water, of variable colour, fusible in the porcelain kiln, absorbing oils easily, hence used in cloth-fulling.

4. Figuline Clays Or Potter's Earths

Same physical properties as the plastic clays, but coloured and much more fusible.

5. Marls Or Effervescent Clays

Not very stable, but very fusible, forming with water a brittle paste which effervesces under acids.

6. Ochreous Clays

These contain a large quantity of oxides of iron, yellow and red, and are used as colouring-matter.

Several objections may be made to this nomenclature: the kaolins are badly defined; to make a special class of plastic clays is to infer that plasticity exists little, if at all, in other clays; whereas, as a matter of fact, potter's earths possess this property to as great a degree as the plastic clays; also there is no mention in the list of a clay, or clay-like earth, which is very extensively found on the surface of the Globe, and, wherever it exists, is used in the manufacture of bricks; this is the "lehm" or tableland slime, a mixture of clay and very fine quartz, coloured with an oxide of iron. Another classification has been proposed by M. le Chatelier. This savant has affirmed that, when clays are heated, they undergo molecular changes; one of these, dehydratation, manifests itself by an absorption of heat which takes place at different temperatures according to the nature of the clay. Moreover, some of them show, at about 1000° C, a peculiar phenomenon attended by a sudden elevation of temperature. Combining these two facts, M. le Chatelier has found that the great majority of clays can be assigned to five very distinct classes which do not generally overlap one another. These are: -

However interesting these facts may be scientifically, it seems to us difficult to adapt them to the technical point of view from which we are looking. We shall content ourselves with modifying Brongniart's classification as follows: -

1. Kaolin Clays

White, generally crystallised, refractory, undergoing no contraction in firing.

We must not confuse with these very characteristic substances certain white refractory clays, wrongly called kaolins, which may in the last resort be used for making china, but which have not the well-defined properties of the true kaolin.

2. Refractory Clays

Yellowish or greyish white, usually soft and greasy to the touch, and forming with water a paste which is often tenacious, dough-like, and supple.

3. Figuline Clays Or Potter's Earths

Plastic like the foregoing, but coloured, becoming generally red in baking, and much more fusible than the preceding classes. Some contain no limestone, the rest a small quantity which does not at the most exceed 2 or 3 per cent.

4. Clayey Marls Or Effervescent Clays

Of variable plasticity, very fusible, and characterised by the presence of a large quantity of limestone; according to the amount of this limestone, they are called clay marls or calcareous marls; it is to it that they owe their property of effervescing under the action of acids.

5. Lehm, Tableland Slime, Or Brick-Earth

An intimate mixture of clay and very fine quartz, coloured yellow or red by oxides of iron. When the lehm is calcareous, it takes the name of loess.

6. Smectic Clays or Fuller's Earths.

7. Ochreous or Ferruginous Clays.

The latter two kinds are not used in pottery.

Geological Origin Of Clays

Clays are usually found in fairly regular seams in nearly all stratified soil which has been formed in the midst of fresh or sea water; for there are frequently found in them the fossil remains of sea or fresh water shells, as well as other fossil organic debris. The clays are formed by the deposit of substances formerly in suspension within a liquid in motion.

These substances come from the destruction of primitive volcanic rocks by the combined action of the atmospheric agents: wind, water, air, heat, cold, etc. Nevertheless, we shall see that other more complex agencies, of a chemical nature, must have had a share in the formation of certain clays, especially of the enormous deposits of kaolin like those of Limousin in France and Cornwall in England.

The majority of volcanic rocks contain a common ingredient called felspar, which is a combination of silica with different metals: aluminium, potassium, sodium, calcium, etc. Such are the pegmatites (Haute-Vienne, Pyrenees, Cornwall), formed of quartz (crystallised silica) and felspar, the granites and gneiss, which are both composed of quartz, mica, and felspar.

1 The following description and figures are borrowed from that excellent work by M. de Lapparent, Trait*š de G*šologie, 3rd ed., Paris, 1893, Masson et Cie. The eminent geologist has been kind enough to revise this summary, and I thank him heartily for doing so. - L. L.

The typical felspar contains -

Under the influence of water charged with carbonic acid, the alkaline silicates existing in felspar are decomposed into alkaline carbonates or earthy alkalines, and give up silica, which remains in veins in the dry rock, where the current of filtering water is too slow to carry it off in suspension. The aluminium silicate which remains engenders a product resembling kaolin, hence the name of kaolinisation has been given to this mode of disintegration of felspar rocks. When the rock contains grains of quartz, these are not affected, and as the water draws from them the clay element formed by the kaolinisation of the felspar, it follows that, under the sole influence of the filtering water, the majority of granitic rocks are changed into sand. In our climates, this change can be effected down to fifteen or twenty metres below the surface. What proves conclusively that it is the work of the moist air assisted by variations of temperature, is that it does not occur to an appreciable extent in Egypt, where the equable and dry climate leaves granite almost intact after the lapse of centuries.

If the kaolinisation of felspar rocks, caused by meteoric water, has yielded here and there some veins of clay pure enough to be called kaolin, this superficial action has not been capable of forming great layers of kaolin like those of Limousin, and we must therefore find another explanation of their existence.