POTTERY

Standard pottery is built from lumps of clay moulded on a table turned by a kick-wheel. Handles are made from strips or rods of clay added after turning. Kilns were usually rectangular and brick-built. The pottery is stacked on lime or clay bars or arches over the fire. The spaces between the arches are loosely filled broken shards of pottery. A typical kiln will hold around 200 pots.

The fires are lit by stock-holes at both ends of the kiln. The temperature is raised slowly and timed with an hour-glass. When the mortar and arches begin to drip, the maximum temperature has been reached, and the fires are damped down and the kiln sealed. It has to be dismantled to remove the pots once they have cooled down sufficiently.

Stoneware is a special type of pottery devised around the tenth century and is made by firing the clay to an especially high temperature. It is naturally impervious to water and so does not require glazing.

Slip is made from especially fine clay, and is paler in colour than normal pottery. It may be applied as an outer coating and then etched away in a decorative pattern to reveal the darker interior.

The standard medieval glaze consists of a lead-gum mixture. It can either be applied to leather-hard pots before firing, or alternatively the pot can be fired twice, with the glazing added in between. Traditionally, glazes can be coloured red by adding rust or lead compounds, yellow by adding litharge (lead monoxide) or green by adding copper or brass filings.

By the thirteenth century, blue and purple glazes could also be produced, and white pottery could be produced using a mixture of potash glass, litharge, albis and salt. Also by this time, a metallic sheen could be imparted by 'lustre painting' with glaze containing fine ochre followed by low temperature firing in small kilns.