“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.