It is almost 180 years since Josiah Wedgwood reported to the Royal Society some of the first results of systematic research into ceramics ever to be conducted. The art of ceramics has been described (Bradstreet 1958) as a system in which ‘minerals of inconstant composition and doubtful purity were exposed to immeasurable heat long enough to carry unknown reactions partly to completion, forming the heterogeneous non-stoichiometric materials known as ceramics’ and it has also been suggested that this situation has been little changed by a more formal scientific approach. It is certain that Wedgwood conducted his remarkable systematic series of experiments in this context—his notebooks record nearly 7000 trials—and, as J. W. Mellor (1915) has pointed out, ‘it is difficult to realize the state of chemistry at the time of Josiah Wedgwood. There was no Dalton’s Atomic Theory. . . and chemistry was autocratically dominated by old King Phlogiston’. Nevertheless, Wedgwood’s work is one of the great foundation stones of the English ceramic industry, and within a hundred years the basic formulae for most indigenous whiteware bodies were established. The introduction of the white burning Devonshire clays, and calcined flint (cryptocrystalline quartz) is generally attributed to Thomas Astbury, who is also credited with some more sophisticated experiments on the use of lead ores in glazes. The age-old technique of ‘potting’ or shaping the plastic clay was supplemented about 1730 by the introduction of slip casting, in which a ‘slip’ or aqueous suspension of clay and minerals is cast into a porous mould, now plaster of paris but originally prefired porous ware. These processes of potting and slip-casting remain a challenge to the applied physicist even today. Another remarkable development of this period was the production of a body containing calcined animal bone—the forerunner of the quite characteristic English bone china, which is one of the strongest of all traditional ceramic materials. English earthenware now consists of china clay, ball clay, silica (calcined flint) and Cornish stone, the last being a felspathic mineral (generally containing some fluorspar) which serves as a flux. The starting materials may include 50 to 60% of clay, 30 to 35% flint, the remainder being Cornish stone. When fired, it is a microporous material, consisting of crystalline phases bonded by a complex glassy phase: the pore and grain size may be of the order of 0·1
μ
or less. Bone china contains about 50% of animal bone, 25 to 30% clay and the remainder Cornish stone. Porcelain in the Continental tradition, in which potash felspar is used as a flux, is not a major feature of the English domestic ware industry, but electrical porcelain, as used in the power distribution industry, and some chemical ware, are essentially based on clay, quartz and felspar. Bone china and most porcelains are fully vitrified and effectively non-porous. The production of ware in all these media constitutes the largest branch, in the economic sense, of the ceramic industry of this country, and the articles produced are strikingly characteristic and in some ways unique.
Selection justification of methods for obtaining glass-ceramic materials
