Pyrometric cone

The pyrometric cone is "A pyramid with a triangular base and of a defined shape and size; the "cone" is shaped from a carefully proportioned and uniformly mixed batch of ceramic raw materials so that when it is heated under stated conditions, it will bend due to softening, the tip of the cone becoming level with the base at a definitive temperature. Pyrometric cones are made in series, the temperature interval between the successive cones usually being 20 degrees Celsius. The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK)."^[1][2]

For some products, such as porcelain and lead-free glazes, it can be advantageous to fire within a two-cone range. The three-cone system can be used to determine temperature uniformity and to check the performance of an electronic controller. The three-cone system consists of three consecutively numbered cones:

• Guide cone – one cone number cooler than firing cone.
• Firing cone – the cone recommended by manufacturer of glaze, slip, etc.
• Guard cone – one cone number hotter than firing cone.

Most kilns exhibit inherent temperature gradients between the upper and lower chambers. The magnitude of this thermal variance is influenced by the kiln’s structural design, the depletion level of the heating elements, the density of the load distribution, and the specific pyrometric cone target. Typically, these disparities are more pronounced during lower-temperature firings.^[3] To accurately map these thermal profiles, pyrometric cones should be positioned on the lower, middle, and upper shelves. Analysing the results allows for strategic adjustments in loading patterns and firing schedules to achieve greater uniformity. Furthermore, the installation of a downdraft venting system can significantly mitigate temperature fluctuations by circulating air more effectively throughout the chamber.^[4]

The maturation of a pyrometric cone is determined by the integration of temperature, time (heat work), and atmospheric conditions, though temperature remains the primary variable. Because firing environments often deviate from the laboratory conditions used during initial calibration, results are expressed as an equivalent temperature.^[5]

Monitoring the deformation of these cones provides a visual confirmation that the kiln has achieved the desired state of maturation. Additionally, small cones or bars can be utilised to mechanically trigger kiln shut-off controls once the requisite thermal energy is reached.^[6]

Pyrometric cones can be used in a "kiln sitter", a device which senses the softening of a cone and produces a mechanical output through a trigger assembly, typically to switch off the kiln.^[7]

Pyrometric cones are sensitive measuring devices and it is important to users that they should remain consistent in the way that they react to heating. Cone manufacturers follow procedures to control variability (within batches and between batches) to ensure that cones of a given grade remain consistent in their properties over long periods. A number of national standards^[8][9][10] and an ISO standard ^[11] have been published regarding pyrometric cones.

Even though cones from different manufacturers can have relatively similar numbering systems, they are not identical in their characteristics. If a change is made from one manufacturer to another, then allowances for the differences can sometimes be necessary.

In 1782, 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.^{[12][13][14][15] [16]}

The modern form of the pyrometric cone was developed by Hermann Seger and first used to control the firing of porcelain wares at the Royal Porcelain Factory, Berlin (Königliche Porzellanmanufaktur, in 1886, where Seger was director.^[17] Seger cones are made by a small number of companies and the term is often used as a synonym for pyrometric cones.^[18][19] The Standard Pyrometric Cone Company was founded in Columbus, Ohio, by Edward J. Orton, Jr. in 1896 to manufacture pyrometric cones, and following his death a charitable trust established to operate the company, which is known Edward Orton Jr. Ceramic Foundation, or Orton Ceramic Foundation.^[20]

Pyrometric cones are often referred to as Orton Cones within the United States, but in his lifetime Orton preferred calling them Seger cones.^[21]

I hope no one will ever apply my name to the cone system in any way, because Dr. Seger deserves all the glory which there is in having brought us this most convenient system. My cones are labeled Standard pyrometric cones. I have hesitated to print the term Seger cones on my output because the German cone makers...might feel I was attempting to cut under or defraud them...The name Seger cone will then become a sort of monument to that prominent man, who must always be recognized as the first scientific ceramist of all history.

A biennial ceramic art exhibition for small work, the Orton Cone Box Show,^[22] took the Orton Cone company's pyrometric cone box as the size constraint for submissions.

The following temperature equivalents for pyrometric cones were retrieved from references in the External Links section.