Cone Equivalent Calculator

Pyrometric cones are how potters actually measure a firing, because a kiln's dial or digital readout reports air temperature at one point, not the cumulative heat-work the clay and glaze have absorbed. A cone is a small ceramic wedge formulated to soften and bend at a known combination of temperature and time, so when it slumps it has directly witnessed what the ware experienced. This calculator converts a Cone Number to the Fahrenheit and Celsius temperature that cone is rated to bend at, using the Edward Orton Jr. Ceramic Foundation's own published Temperature Equivalent Chart (Cone Numbers 022-12) for every cone in that range, extended with Orton's cone 13-14 figures beyond it, and rounding to the nearest one if you enter a fraction (cones are not evenly spaced -- cone 06 to cone 6 covers a much wider range than cone 6 to cone 10). Cone Number is the only input that changes which line of the table you land on, and the table is strictly increasing: cone -22 (about 1,087F) is the coolest listed cone and cone 14 (about 2,489F) is the hottest, with negative numbers used for low-fire work and positive numbers for mid- and high-fire stoneware and porcelain. Atmosphere does not move you along that table at all -- it switches which of Orton's two published cone series the temperature is read from.

Orton measured its Regular-cone equivalents in an electric kiln and an air atmosphere, and recommends its Iron-Free series instead for reduction firings between cones 010 and 3, because the red iron oxide in the regular cones can itself reduce and change how the cone behaves. The published Iron-Free equivalents differ from the Regular ones by anywhere from 30F below to 11F above depending on the cone -- not by a single uniform offset -- and above cone 3 Orton publishes no iron-free equivalent at all, so outside 010-3 both atmosphere settings return the same figure. What this calculator does not do is account for firing speed or hold time: heat-work depends on how long a kiln dwells near its peak, not only on the peak reading, so two kilns that both hit the calculated temperature can still under-fire or over-fire the same cone if one ramps much faster than the other. The figures here are Orton's Self-Supporting Regular equivalents at a 108F/hr (60C/hr) ramp over the last 180F of the firing -- the standard medium rate. Orton's own chart puts cone 6 at 2,165F at a slow 27F/hr ramp and 2,269F at a fast 270F/hr ramp, so a very slow or very fast final ramp can move the real number by 50 to 100F in either direction.