Pyrometric cone

Heat work, not just temperature

A cone softens and bends from the combined effect of temperature and time, which Orton calls heat work. A kiln that heats slowly bends a cone at a lower temperature than one that heats fast. That’s why clay and glazes are rated by cone instead of by degrees.

Common cones

Orton’s chart gives temperatures for three heating rates. These are for self-supporting cones (1¾ inches above the base) heated at the medium rate of 108 °F (60 °C) an hour over the last 180 °F (100 °C) of the firing. °C values are converted from Orton’s °F figures.

Read the zero

A leading zero means a cooler cone. Cone 06 is about 400 °F cooler than cone 6. Among the zero cones, the bigger the number the cooler it is: cone 022 is cooler than cone 06.

Reading the bend

A cone starts at about the 1 o’clock position. Orton counts it as at its end point when it has bent 90°, with the tip at about 5 o’clock. Going on to touch the shelf (6 o’clock) takes only a few more degrees. Faster firing pushes the bending temperature up; at 270 °F an hour, cone 6 bends at about 2269 °F instead of 2232 °F.