Because the pyrometric cones are numbered, kiln operators do not refer to temperature in the kiln, but the cone numbering between 022 and 14. A zero at the beginning of a number refers to the lowest temperatures. Cones from different manufacturing companies are comparatively similar and remain reliable over a long period of time. Cones do not age but must be kept in an area free of moisture.
Pyrometric cones determine when a firing is complete or if the kiln provided enough heat. They are affected by time and temperature. A kiln operator can position a cone on each shelf of the kiln by placing it on a small piece of clay or a plaque. He then observes the cones through the kiln peep holes. Once a cone has begun to bend, it bends quickly over a period of about 20 minutes. A bent cone means the kiln has gradually reached a certain temperature over an allotted amount of time.
A kiln sitter is a weighted trigger on the inside kiln wall. A cone is placed in the trigger resting on two prongs and supporting a small sensing rod. When the cone bends, the sensing rod drops and the kiln automatically turns off. A quality cone matures at a rate of 150 degrees Celsius per hour. The pyrometric cone provides predictable results.
When glazes from the kiln show bubbling or come out the wrong color after firing, a pyrometric cone can help to determine if the kiln is not reaching the proper temperature. This information is helpful in troubleshooting a kiln. Electronic kilns are available and do not require a cone. They shut off automatically according to their programming; however, use of a cone is still an accurate way to determine if the electronic kiln is displaying the correct temperature.