Pyrometric device

Pyrometric devices monitor the combined effect of both time and temperature on which the properties of fired ceramic body and glaze are dependent. It is importnat to note that they do not measure temperature but the combined effect of time and temperature. Care should be taken with their interpretation as some naively assume the are a measure of temperature alone. Simplistically, and within limits the same result may be achieved by firing to a high temperature for a short time as to a low temperature for a long time. This is known as heat work, care should be taken with this concept as it has no units and is not a precise measure of anything. It is however easy to understand and works well in production environments.

In principle pyrometric devices relate the amount of heat work experienced to the measured shrinkage or deformation of regular shapes that are produced from standard ceramic compositions. Three types are commonly encountered:

1. Pyrometric cones. These are slender, three sided pyramids made from a range of compositions each with a reference number corresponding to a certain softening or melting temperature, although again it is heat work that is being reflected. During the firing of ware an appropriate range of cones is placed in the kiln with the magnitude of the firing being described as the reference number of the first pyramid whose apex bends to the same level as its base. 2. Holdcroft or Bullers’ bars Similar to the above except each piece is a square sectioned bar held horizontally across two fixed distance supports. During firing the softening of the material results in sagging at the centre. 3. Bullers' rings Are flat hollow centered rings whose contraction is proportional to the heat work experienced. After a brass gauge measures firing the contraction, the graduations being arbitrary numbers that are used to describe the firing regime experienced. Various grades of ring, each of slightly different compositions, are available to cover all firing conditions likely to be encountered.

Pyrometric devives were invented by the English potter Josiah Wedwood in 1782 that lead him to be elected a fellow of the Royal Society