Pyrometric Cone Chart for Orton Cones 022 to 14

How the pyrometric cone chart works

The pyrometric cone chart doesn’t calculate a temperature, it looks one up. Orton tested each cone in controlled electric kilns and published the temperature at which it bends at three heating rates, and the tool shows those figures for the cone you pick.

The rates are slow, medium and fast: 15, 60 and 150 °C per hour over the last 100 °C of the firing, or 27, 108 and 270 °F per hour in Fahrenheit. We show Orton’s own Fahrenheit figures rather than converting, so the numbers match the chart in a box of cones. To check a conversion yourself, use °F = °C × 9 ÷ 5 + 32.

The headline range runs from the lowest published figure to the highest, so cone 6 reads 1185 to 1243 °C. Cones 022 to 020 have no slow-rate figure, so their range covers only the medium and fast rates. In compare mode, the difference is the second cone’s medium-rate temperature minus the first cone’s, and the step count is how many cones apart they sit, half cones included. Cone 6 against cone 10 is 1285 minus 1222, which gives 63 °C over four steps.

What temperature is cone 6, cone 5, cone 04 and cone 06?

Cone 6 bends at 1185 to 1243 °C, cone 5 at 1159 to 1207 °C, cone 04 at 1046 to 1077 °C and cone 06 at 981 to 1013 °C, depending on heating rate. These four are the cones most studio potters meet first, because they cover the usual bisque firing and the two most common glaze ranges.

At the medium rate of 60 °C an hour, the figure most charts quote on its own, cone 6 is 1222 °C, cone 5 is 1186 °C, cone 04 is 1063 °C and cone 06 is 998 °C. In Fahrenheit that’s 2232, 2167, 1945 and 1828 °F. Glaze catalogues usually quote that medium figure, so if your kiln finishes slowly, the cone may already be down before the controller reads 1222.

The zero in front of a cone number matters more than beginners expect. Cone 06 and cone 6 are about 220 °C apart, because the numbers starting with 0 get hotter as they count down from 022 to 01, and then the plain numbers climb from 1 to 14. Firing an earthenware clay rated for cone 06 to cone 6 can leave it slumped or melted onto the shelf, so it’s worth reading the label twice.

Why does heating rate change the cone temperature?

Heating rate changes the bending temperature because cones measure heatwork, the combined effect of time and temperature, not temperature alone. A slow finish gives the cone longer to soften, so it bends at a lower reading, while a fast finish needs a higher reading to do the same work.

That’s why the tool shows a range rather than a single number, and why a firing log that says cone 6 at 1205 °C isn’t necessarily wrong. Your glaze responds to heatwork in much the same way, so the cone is often a better guide to how a glaze will look than the number on the controller.

Holding the kiln at temperature, called a soak, adds heatwork too. Orton’s guidance is that a one to two hour soak near a cone’s temperature may bend the next cone up, and four to six hours can bend two cones higher. Many potters find a short hold smooths their glazes, but a long soak quietly pushes the firing hotter in cone terms.

How do you read a witness cone after a firing?

A cone counts as properly fired when its tip has bent down level with its base, roughly the five o’clock position, and Orton’s charts are based on that point. A cone still standing upright hasn’t had enough heatwork, and one slumped flat and puddled has had well past it.

Orton’s figures assume self-supporting cones at their built-in mounting height of 1¾ inches, about 44 mm. Many potters fire a set of three: a guide cone one below the target, the target cone, and a guard cone one above. A bent guide cone, a target at five o’clock and a guard still mostly upright means the kiln hit its mark. Orton notes a cone typically takes 15 to 25 minutes to go from starting to bend to its end point, so the bend tells you about the last stretch of the firing.

Placement matters as much as the reading. Kiln makers such as Skutt suggest a witness cone a short distance from the wall and near the thermocouple, where you can see it through a peephole. Cones on more than one shelf, compared over a few firings, are one of the cheapest ways to learn your kiln’s hot and cool spots.

Which cone should you bisque and glaze fire to?

Most studios bisque fire to around cone 06 to 04 and glaze fire to the cone your clay body and glaze are both rated for. The clay is the limit, so check the bag first, because a glaze can’t make an earthenware body survive a stoneware firing.

A bisque firing only needs to turn clay into hard, porous ceramic that still soaks up glaze, so it sits low on the pyrometric cone chart. Our own firing service in Mordialloc bisques at cone 06, glaze fires earthenware at cone 03 and runs mid-fire glaze firings at cone 6, all in electric kilns.

For glaze firing, earthenware and terracotta usually mature somewhere between cone 06 and cone 1, mid-range stoneware and porcelain between cone 5 and 7, and high-fire bodies at cone 8 to 10. If a glaze and clay disagree, fire to the lower of the two and test first. Our guide to kiln firing temperatures walks through each stage, and the comparison of cone 6 and cone 10 firing helps if you’re choosing between ranges. If you’re newer to clay, our workshops and classes are a friendly way to learn how firing fits into the whole process.

Common mistakes when reading a cone chart

The most common mistake is treating one number as exact, when every cone really has a range that depends on heating rate. The others are mixing up cone types, using the wrong cones in reduction, and forgetting that a soak adds heatwork.

Small cones, the ones that sit in a kiln sitter, typically bend 7 to 10 °C earlier than self-supporting cones, according to Orton, which suggests putting the next cone up in the sitter and confirming with a self-supporting cone on a shelf. Large cones mounted at the same height use the self-supporting temperatures shown here.

Reduction is the other trap. The red cones from 010 to 3 contain iron oxide, which can change in a reducing gas firing and bend unreliably, so Orton recommends its iron-free cones there. If you only fire electric, as our studio kilns do, regular cones are fine.

Finally, thermocouples drift as they age. A cone that bends earlier or later than the controller expects, firing after firing, usually means the reading has wandered, not that the chart is wrong.