(the combined effect of both time and temperature) when firing materials inside a kiln
. It is important to note pyrometric devices do not measure temperature, but can report temperature equivalents
. The intention of the device is to visually communicate any activity within the ware. In principle, a pyrometric device relates the amount of heat work performed on a ware to a measurable shrinkage or deformation of a regular shape.
Care should be taken with the interpretation, as some naively assume they are a measure of temperature alone.
- Are flat, hollow centered rings whose contraction is proportional to the heat work experienced. A micrometer or gauge measures the fired ring, with the difference being an arbitrary number that is used to describe the firing regime experienced. Various grades of ring, each of slightly different compositions, are available to cover all firing conditions and temperature equivalents likely to be encountered. Examples of pyrometric rings include Bullers Rings, PTCR Rings, and TempCHEKs. Bars
- Are square, sectioned bars held horizontally across two fixed distance supports. During firing the softening of the material results in sagging at the center. Pyrometric Bars have found popularity in Kiln Sitters, which uses the described deformation to act as a triggering element, thus turning off the kiln at a desired point of maturity. Cones
- Are slender, three sided pyramids made from a range of compositions each with a reference number corresponding to a certain heat work. Rather than shrink as rings do, a cone's tip will bend forward to the same level as the base at the time of maturity. Other deformation of a cone, such as bloating, cracking, or bending backward, can be appropriately interpreted to troubleshoot activity inside the kiln.
Archaeologists working at Northern Song
period (960 to 1127 AD) kiln sites in the Chinese provinces of Shaanxi
have reported that pyrometric cones about five centimetres tall and made from loess
were used to help control the firing of the kilns.
In 1782, English potter and industrialist Josiah Wedgwood created accurately scaled pyrometric device, with details published in the Philosophical Transactions of the Royal Society of London in 1782 (Vol. LXXII, part 2). This led him to be elected a fellow of the Royal Society.
The modern form of the pyrometric cone was developed by the German ceramics technologist Hermann Seger and first used to control the firing of porcelain wares at the Königliche Porzellanmanufaktur (Royal Porcelain Works) in Berlin, in 1886. Seger cones are to this day made by a small number of companies and the term is often used in Europe as a synonym for pyrometric cones.
The Standard Pyrometric Cone Company was founded by Edward J. Orton, Jr. in 1896 to provide a calibrated, visual device to measure the amount of heat delivered to ceramic wares during firing. Following the death of Dr Orton in 1932, a charitable trust was established to ensure the continued operation of the company, for the benefit of the ceramic arts and industry.
References and Further Reading
- The Bullers’ ring. Reid J.L.Interceram 35, (4), 44, 1986
- Role of August Hermann Seger in the development of silicate technology.Lange P.Ceram.Forum Int./Ber.DKG 68,No.1/2,1991
- The Seger Cone: 100 years old. Osterr. Keram. Rundsch. 23, (9/10), 9
- 100 years ‘Seger Cone’. Joger A. Silikattechnik 36, (12), 400, 1985