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Clays and Pastes

“It is the Art which gives the value, and not the material.”

—Dresser.

Clay being the chief material used by the potter, it demands a description which, without being too technical, will give from the start a clear idea of the nature of clays or pastes in general use.

It would be a needless complication to enter here into a discussion of the chemical analysis of bodies and materials. Whilst the chemist can and does determine with exactitude the relative quantities of each component, he cannot yet, let us perhaps be thankful, lay down with the same certainty the structural and molecular changes all these compounds will undergo in the fire. The old potters’ rule “of thumb” or, rather, common sense and experience, still count for something.

Clay is the word generally applied to the natural article when used without preparation, or after picking and washing. Paste is the term used for all composite bodies that have been through a complicated process of washing, grinding, mixing, and sieving, or even fritting, according to the desired quality of the ware for which it is required. Natural clays range from the pure white and very infusible kaolin, containing only alumina and silica with a very small percentage of alkalies, to the impure grey, red, or brown clays, containing, along with alumina and silica, magnesia, potash, soda, iron, lime, and carbon. Kaolin is used with China stone (a combination of felspar and quartz) to make porcelain, the finest and hardest paste known to potters. It has a very hard white translucent body, only slightly vitreous at the highest fire (around 1700° Centigrade).

From this, the highest grade, we have almost insensible gradations to common earthenware. Old English and French porcelain were compounded of clay, sand, and alkalies ground together to make a frit, re-ground and mixed with a stiffening material (in English porcelain, bone-ash), to support the vitreous matter in the intense heat. The finest earthenware does not differ greatly in its formula from soft porcelain, but it is not so hard or transparent. From this the scale descends to where the presence of lime or iron in the body colour it and render it easily fusible, so that at any great heat it turns black and collapses to a slag.

Clay on being dug up is usually weathered in the open, and dried and broken up and the greater impurities picked out. It is then thoroughly mixed with water in a blunger and passed through a succession of sieves until all foreign matter and impurities are left behind and it is the consistency of cream. This was formerly done by hand, the clay being raked into a thick “slub” and washed through a series of tanks until all impurities had settled, leaving only the fine clay in suspension. It is at this stage that any additions are made to form a paste. The modifying ingredients, ground and sieved to the requisite degree, are thoroughly incorporated with the slip, which is allowed to settle. The clear water on top is siphoned off and the paste dried sufficiently to handle. The modern method of preparation is to force the slip through a series of straining bags which remove most of the water and leave it stiff enough to work. Some of the hard pastes are so stiff or short that they require soap water to give plasticity, but usually after a thorough wedging it is now ready for the thrower.

Generally speaking, kaolin, China clay, ball clay, pipe clay, China stone, felspar, flint, quartz, sand, lime, chalk, and calcined bone are the ingredients of most modern pastes. These supply the alumina, silica, lime, potash, and soda, with traces of iron and magnesia, that are found in all clays when analyzed. Carbon is only present in impure bodies fired at a low heat.

Of these materials the clays rich in alumina and silica, such as kaolin or China clay, form the body-giving substance. The felspar or China stone furnish the fluxing ingredients for fusing and binding. The flint or bone supplies the stiffening matter for supporting and retaining the shape of the object in the fire.

Porcelain, though differing so widely in appearance and texture from the coarse mediæval earthenware or the pottery of the ancients, is found to have a distinct relationship when all these bodies are submitted to analysis. Much of the difference in bodies, apart from the impurities, lies in the temperature of the fire to which it has been submitted. At a low temperature such constituents as lime and iron are not much affected, but at a greater heat they act as fluxing agents.

To generalize upon a complex and difficult subject one might say that porcelain, both hard and soft (pâte dure and pâte tendre), is characterized by its pure white colour and by extreme hardness of body and glaze with transparency; fine stoneware by a very hard, opaque, and heavy body which may be white, buff, or grey, and salt-glazed or with a fine hard transparent glaze. Earthenware is softer and mostly opaque. It may range from something a little softer than soft porcelain to the coarse “Majolica” with a tin glaze, differing widely in colour of body and hardness of glaze.

With porcelain and the finest high-fired wares a purity of materials and uniformity of mass is absolutely necessary. It is here that one may well call in the aid of the chemist and manufacturer. In any case it is advisable to call in the chemist and the manufacturer when working on a large scale. With a small output, as with all good craftsmen, the fabrication of a good, reliable stoneware or earthenware paste is only a matter of patience and hard work. Before commencing to produce finished work on any scale, repeated experiments with different clays should be carried out. Notes of all trials, with and without glaze, are invaluable to the potter.

Rich clays can be stiffened, short clays enriched, and colour modified without a mass of expensive machinery. Rich, easily fusible clays tend to stunt or buckle at a high fire. Hard refractory clays often remain porous and are a fruitful source of crazing and breaking. The addition of flint or fine washed sand, finely powdered grog, or pitchers, or even refractory China clays, in quantities varying from about 5 per cent to 20 per cent, but settled only by repeated trials, will stiffen up or open out rich clays inclined to warp or burst. Rich fusible clays added to hard clays may stop the crazing, or the fusing point may be lowered by the addition of spar. Stiff gravelly clay will require finer sieving or repeated washing to rid it of some of the grit or sand. Rich greasy clays are better when not too finely sieved, but this point is of course dependent on the class of work to be undertaken. Slip can be settled in tubs, the water siphoned off, and then put to dry on plaster bats, or dry clay, powdered and sieved, may be stirred in until the mass is stiff enough to wedge thoroughly by hand.

In mixing or modifying without machinery it is sometimes advisable to do it in the dry state, otherwise some of the heavier materials are likely to sink and are thus not thoroughly incorporated with the body. The dry mass when well mixed is wetted enough to be wedged. When the body is coloured and a white ground is indispensable, an “engobe,” or dip, of white clay slip must be resorted to.

These processes are certainly tedious, but that will not deter a craftsman searching for the right clay in which best to shape his ideas. To the craftsman working alone it is the only way by which he can accommodate his clays to the various necessities of throwing, casting, or modelling. Each process will require a slightly different nature in the clay if the finest results are to be obtained.

Although in this craft book we shall not approach porcelain, a good, hard, true-ringing body with a tough well-fitting glaze should be a sine qua non with all craftsmen; and it is only in the fire that any true idea can be gained of the important influences clays and pastes have on the finished work. Here we have tried to indicate the chief characteristics of clays and to make clear the inevitable tendency of all potters who seek an imperishable medium for their craft towards a purer body and a higher fire.