J. Soc. Occup. Med. (1976) 26, 35-40

Manufacturing Processes Pottery Manufacture J. G. DATHAN Employment Medical Adviser, Health and Safety Executive, Stoke-on-Trent usually made from plaster-of-Paris. Still more items, such as figures and models, are cast by pouring liquid clay slip into plaster moulds. These are left for varying periods of time and then the surplus slip is poured away, leaving a hollow coating of clay lining the inside of the moulds. When sufficiently dry, the parts of the moulds are removed and the ware is fired at a temperature of about 1100°C (or some 200 °C higher for bone china), and this converts the soft, friable ware into a hard and slightly porous but insoluble material known as 'biscuit ware'. Biscuit ware is ready for glazing. Glaze is applied to the biscuit ware in order to cover the ware with a glass-like surface intimately fused with the biscuit; the resultant ware being impervious to liquids. Colour may be included in the initial slip, but is more usually added to the glaze. Colour decoration is generally applied to the biscuit ware either by means of transfers, or, in very high quality ware, by hand, and subsequently covered by a transparent glaze. Enamels are usually applied superficially to the glaze. The base of all pottery glazes is silica. Other metals are used in combination, in particular lead, boron, alumina and various alkalis. By many generations of potters lead was regarded as essential to the higher quality glazes, giving a higher gloss with less tendency to defects that may arise from the process of glazing; such as 'crazing' of the finished glaze which can result from thermal discrepancies between the glaze and the biscuit ware. Since 1947, however, as a result of greater awareness of the toxic effects of lead absorption, there have been regulations defining upper limits for the amount of lead normally used in glazes. Many firms now use 'leadless glazes', i.e. glazes containing not more than 1 per cent lead calculated as lead monoxide, others still use a 'low solubility glaze', i.e. one yielding not more than 5 per cent lead, again calculated as the monoxide, under the 35

Downloaded from http://occmed.oxfordjournals.org/ at University of Auckland on June 5, 2015

Pottery manufacture has always been both a creative and a profitable activity; and although techniques have advanced out of all recognition since the days of the master-potters of 200-300 years ago, the basic principles have continued since potteries were established 400-500 years ago, being attracted to North Staffordshire by the availability of both clay and coal. Neither commodity is of local importance in recent years, as coal-fired kilns are extinct, being replaced by gas and electricity; whilst clay is now mostly transported by sea from Cornwall, which explains why so many potteries have been sited on the banks of the Trent and Mersey Canal, which joins the Mersey near Runcorn. Even this method, in its turn, has been superseded, giving way to road transport, although quite recently some slight local use of the canal is again being made owing to increased costs of road haulage. The first form of pottery to be manufactured was earthenware. Bone china was a much later development, dating really from the early part of the nineteenth century. Earthenware basically consists of ball clay, china-clay, flint and Cornish stone, and is quite opaque. Bone china is made from china-clay, Cornish stone and calcined bone, and can be almost transparent. This ware is usually of a much finer texture than earthenware, which is fired at a considerably lower temperature than china. Both forms are, however, treated in a fairly similar manner. The basic ingredients are mixed together in appropriate proportions and the resulting liquid clay is known as the 'slip'. This is then pumped through a filter where excess fluid is removed, leaving a semi-plastic clay which is then put through a 'pug' to further mix it, de-aerate it and make it more plastic and ready for use. Various items are still made on the potters' wheel by a very skilled man known as a 'thrower'; these are mainly vases and erect hollow-ware. The vast majority of ware, however, is made on moulds

36

OCCUPATIONAL MEDICINE

conditions of a standard test. Customarily the lead, in the form of the carbonate or red lead oxide, is mixed dry with silica and heated to melting point to produce a glass-like combination of lead silicate known as the 'frit'. The prepared frit is ground with the addition of china-clay and water to produce a 'glaze-slip'. The biscuit ware is then dipped into the glaze-slip, when the water tends to be absorbed by the biscuit leaving a coating of the glaze on the surface of the ware. Sometimes the glaze is applied by spraying instead of dipping. The glazed article is then fired again, but at lower temperature, around 900-

1000 C, or 700-800 °C in the case of enamels, which intimately fuses the glaze to the body of the ware and gives a fine, glossy finish. The ware then goes to a polisher who grinds off any minor irregularities, and in the case of a figure he sees that it stands steadily on its feet or base. The article is now ready for offer to the public. These processes are not without their unfortunate side-effects: lead poisoning still exists to a very limited extent, and silicosis, now more often referred to as pneumoconiosis, still features regularly in the Coroner's Court; but these would be the basis of another story.

The Brighton Congress The 18th International Congress on Occupational Health was held at Brighton from 14 to 19 September, 1975, with more than 1500 delegates attending. The biggest contingent —over 90—came from Brazil in a chartered aircraft, and of the other non-European countries the USA, Japan and Australia were also well represented.

A cold, wet September weekend greeted the delegates but thereafter, apart from a few showers, Brighton was seen at its best in sunshine with a refreshing sea breeze. The talk on Brighton at the end of the first day's programme encouraged many delegates to explore the town, and while wives peered through the shop windows, their husbands looked aloft to admire the architecture. The 'Lanes' were thoroughly

of Occupational Medicine

The president, Dr Morris Cooke, talking to assistant editor. Dr Peter Pelmear, on the

stand.

Downloaded from http://occmed.oxfordjournals.org/ at University of Auckland on June 5, 2015

Notes and News

Manufacturing processes. Pottery manufacture.

J. Soc. Occup. Med. (1976) 26, 35-40 Manufacturing Processes Pottery Manufacture J. G. DATHAN Employment Medical Adviser, Health and Safety Executive...
1MB Sizes 0 Downloads 0 Views