A pyrometric cone is a small, slender cone of ceramic material made to soften and bend at a known amount of heatwork. Potters use cones to measure firings, and to name firing ranges, such as cone 06 or cone 6.
What Is Pyrometric Cone?
Cones measure heatwork, the combined effect of temperature and time, rather than temperature alone. That makes them more meaningful than a thermometer reading, because glazes and clays also respond to heatwork. A cone fired slowly bends at a lower temperature than the same cone fired fast.
Orton is the best-known maker, and its charts list the temperature each cone bends at for different heating rates. The numbering runs from cone 022, the coolest, up to 01, then from cone 1 upwards, so cone 06 is much cooler than cone 6.
Cones are used three ways: as witness cones placed in the load to see what happened, as small cones in a kiln sitter that switches older kilns off, and as a shorthand for firing ranges on clay and glaze labels.
Kiln controllers use thermocouples to measure temperature, but many potters still put witness cones in every firing to check that the controller and the heatwork agree.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.