Pyrometric Cone Chart

In short A pyrometric cone does not measure temperature. It measures HEAT WORK — the combination of how hot and for how long — which is why potters trust a bent cone over a digital readout. A cone that bends at 1,222°C on a slow final ramp bends noticeably higher when the kiln is climbing fast, because it has had less time to absorb the heat. Every cone table therefore comes with a ramp rate attached, and the one here is Orton self-supporting cones at roughly 60°C per hour. Fire faster and the bending point moves up by something in the region of 10 to 20°C. For a specific ramp, use your cone supplier’s own table; this page will not invent a second one. THE NUMBERING RUNS BACKWARDS AND IT IS THE MOST EXPENSIVE TYPO IN CERAMICS. Cones with a leading zero count DOWN as they get hotter: 022 is the coolest at 586°C, then 021, 020 and so on up to 01 — and then it flips, and 1, 2, 3 count UP. So cone 06 is 991°C and cone 6 is 1,222°C, a difference of 231 degrees and the difference between a bisque and a fully vitrified stoneware. Writing the zero matters. CONES ARE ALSO NOWHERE NEAR EVENLY SPACED. The gaps between neighbours in this table run from 5°C to 52°C. Cone 1 and cone 2 are only five degrees apart, while 020 and 019 are fifty-two. That bunching in the low single digits is exactly why kilns overshoot there: a controller drifting twenty degrees skips several cones at one end of the scale and none at the other. In practice: bisque is commonly 06 to 04, earthenware glaze around 06 to 03, mid-fire stoneware at 5 to 6, and high-fire stoneware and porcelain at 9 to 10 — all conventions rather than rules, and every clay body has its own range on the data sheet. And use three cones rather than one: a guide below, the target, and a guard above. One cone tells you it bent. Three tell you whether you crept up to it or shot past.