Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

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Tunnel kilns are a continuous process that appeared first in 1751, but industrially in the 1920s for lower temperature (750-800 degrees C) decorative firings and in around 1950 for biscuit and glaze firing. Wares are conveyed slowly on trolleys from a comparatively cool region at the entrance to the full heat at the centre, and after firing they cool gradually as they move to the exit.

Tunnel kiln showing trucks - courtesy Worcester Porcelain Museum

Tunnel kiln showing trucks - courtesy
Worcester Porcelain Museum

The latest version (“roller hearth”) has continuously moving ceramic (mullite or silicon carbide) rollers that transport the wares without the need for trolleys, thereby reducing the heat required. The fixed temperature profile achieves a high quality and consistency. In early electric tunnel kilns moth balls were used to give a reducing atmosphere!

Cracking in the kiln is caused by uneven heating and cooling rather than rapid heating and cooling. Accordingly bone china cups can be biscuit fired in a special kiln in as little as 7 minutes without cracking, however, plates when stacked need significantly more time. Today fast firing kilns are very efficient to save energy and are in the form of a low thermal mass continuous tunnel that can be brought to temperature in one hour, run for an eight hour shift, then turned off.

11.7 Glazing

Glazes are glasses that completely melt at temperatures lower than the body receiving the glaze or any previous underlying glazes. To achieve this, lower temperatures fluxes are used, and since combinations of metal oxide fluxes are more effective than single ones, glazes become more complex than ordinary glass. Examples of glazes are: - for single-fired porcelain, 83% silica, 8% alumina, 6% calcia and 3% potash, melting around 1420 degrees C; and for stoneware, 72% silica, 8% alumina, 8% calcium oxide, 4% magnesia (magnesium oxide), 4% zinc oxide, 2% potash and 2% soda, melting around 1240 degrees C. To achieve lower temperatures, lead and/or boric oxide were invariably used, except that now lead is avoided on health grounds. For a glaze below 1000 degrees C a composition might be 56% silica, 20% lead oxide, 11% boric oxide, 5% alumina, 5% calcium oxide and 3% magnesium oxide. Glazes are also ground finer than bodies, typically 80% less than 10 microns, although if ground too fine the glaze becomes very viscous and is vulnerable to peeling. To make the glaze stick to the biscuit body it has to be fluid but not run off – in the past gums were added to help but now synthetic polymers are used. Glaze can be applied by painting, spraying or dipping. Sometimes hand dipping resulted in finger marks being left in the glaze. Nowadays the electrostatic application of glaze is used that is more efficient and achieves a good quality finish.

Dipping wares in glaze. Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent and Worcester Porcelain Museum

Dipping wares in glaze. Image courtesy of the
Potteries Museum and Art Gallery,
Stoke-on-Trent and Worcester Porcelain Museum

Finger marks from hand dipping in glaze, Chinese Jun ware - courtesy Glade Antiques)

Finger marks from hand dipping in glaze, Chinese
Jun ware - courtesy Glade Antiques

The opacity of tin oxide is due to its fine (0.4 micron) crystals reflecting the light; nowadays zirconium oxide is used. Matt glazes are achieved by allowing slow recrystallisation of the glaze on cooling, producing very small crystals, but glazes with large crystals are also used giving attractive effects.

The problem of choosing a glaze for any ceramic is to achieve a match between the expansion coefficients of body and glaze. As the glaze is weaker than the body, it is put reasonably in compression when cooled to avoid it crazing under tension. This entails a glaze having less expansion than the body so on cooling it shrinks less than the body and is compressed. Glaze has had a major impact on the development of pottery bodies as it is easier to adjust the expansion coefficient of the body by changing its composition, firing regime or fineness of the raw materials than the glaze. Bodies having the very low expansion required for cooking are difficult to glaze and special glazes based on Lithia, zinc oxide and alumina have been devised.

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