Medieval & Post-Medieval Kiln Technology

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RESEARCH INTO MEDIEVAL & POST MEDIEVAL KILN TECHNOLOGY

David Dawson

7th April 2008

David along with Oliver Kent has carried out twenty eight years of experiments and research into kiln technology. His research has taken him to many countries in the world. In his talk he told the story of kiln technology up to 1750 AD.

Pottery is the most common find in archaeology but it is not always fully appreciated. Close examination of the colours of the clay give indications to the firing conditions within the kiln whilst the style of decoration might indicate its provenance. To date very few kiln sites have been located and they can easily be missed when excavating. Where there is no surviving brickwork there is a problem as heat rises and doesn’t go sideways very far, so the signs of burning are only millimetres thick and very easy to trowel through. Where brickwork does survive, one is normally dealing with the bottom three to four courses. Some sites such as Plympton are identified through discovering the kiln waster dump.

There are three phases in the firing of pots, first the water needs to be dried out and this involves putting fires in usually overnight. The second phase is to bring the temperature up to 300ºC to fire the clay; one has to be very careful at this stage as the pots can explode. Thirdly, if they are to be glazed, the temperature is taken up to about 600-650ºC to melt the glaze. There are great variations in temperature within different parts of the kiln and the potters made use of this when carrying out mixed firings. During the firing there are changes to the oxygen and carbon dioxide levels which go from oxidation → reduction → re-oxidation and this affects the colour of the clay. Similarly, glazes can also be affected by this process.

The first glazed vessels during the medieval period were lead glazed and this was obtained from galena. Later we have tin-glazed earthenware known as Delftware. With the improvement of kiln technology we see salt-glazed stoneware which needed far higher firing temperatures. David passed around some samples of different types of vessels to help with the understanding of these processes.

David then explained the different types of kiln technology starting with the open top kilns which were common in the medieval period leading on to more sophisticated up-draught kilns, cross-draught kilns and domed-top kilns and eventually around the 18th century we see the introduction of coal-fired kilns. As technology changes we see different types of ware. Although this was a technical subject it was a far from boring talk. The interest was reflected in the large amount of questions that followed. David finally spoke a little on some of the Plympton pottery sherds that were displayed and commented on how this was an important site.

Shirley Ryan