Understanding Kiln Firing Temperatures: A Guide to Cone Numbers and Pyrometric Cones

There is a moment in every firing when the kiln stops being a box of heat and starts talking back. You open the peephole, the glow is almost too bright to look at, and a small pyramid of clay is beginning to lean. When its tip drops to meet the base, the firing is done. No dial and no digital readout tells you that quite as clearly.

Heat Work: Why Temperature Tells You Half the Story

A kiln held at 1200°C for twenty minutes and a kiln held at 1200°C for two hours are doing very different things to the pots inside. Clay bodies and glazes mature through heat work, which is the combination of how hot they get and how long they stay there. Push either factor far enough and the results change: a glaze that would dry to a smooth, even surface after a short soak can run down the side of a pot after a long one, and a stoneware body that is perfectly tight at cone 6 can begin to bloat and slump when you crawl to the top temperature over many hours.

That is why potters talk about soaking a kiln — holding it near the top temperature for the final twenty or thirty minutes — and why two firings that peak at exactly the same number on a digital controller can still produce completely different pots. The controller measures air temperature at one point in the chamber. It knows nothing about the chemistry happening in the clay. Heat work has to be measured by something that sits inside the kiln and suffers the same conditions as the pots themselves.

What a Pyrometric Cone Actually Measures

A pyrometric cone is a small, carefully formulated stick of ceramic material, shaped like a slightly flattened pyramid and engineered to soften and bend at a known amount of heat work. It is not a thermometer. It is a miniature clay body with a documented melting point, and it behaves exactly the way your pots do — only faster and far more visibly.

Cones are set at a slight angle in a pat of clay, with the flat face turned toward the direction you want them to bend. As the firing progresses, the tip softens and gravity takes over. The moment that tip drops to touch the shelf or the base of the pat, potters say the cone has "gone down" or "fallen." A cone bent into a full arc, tip level with its base, is your signal. A cone melted into a puddle means you blew well past it — and you probably want to check the kiln shelf.

Which Style of Cone Should You Use?

Cones come in several formats, and choosing the right one matters:

• Large self-supporting cones stand on their own base and are the standard choice for a witness cone pack you plan to watch through the peephole.
• Small cones fit into tight spots and older kiln sitter mechanisms.
• Junior cones are made specifically for kiln sitters and are not interchangeable with the larger sizes.
• Specialty shapes exist for industrial furnaces and lab work, but a studio potter rarely needs anything beyond the first two.

Reading the Numbers: The Cone Scale

Here is where nearly every beginner stumbles. Cone numbers are not straightforward, because the scale grew historically and was later extended downward. It runs from 022 at the coolest end to 42 at the hottest — and the leading zero is critical. Cone 6 and cone 06 are not the same cone. They sit roughly 220°C apart, and mixing them up is one of the fastest ways to ruin a load.

The logic is simple once you see it. Cones with a leading zero (022, 06, 04) count down toward the cool end as the second digit grows, while cones written without a zero count up toward the hot end. So cone 06 is cooler than cone 04, which is cooler than cone 1, which is cooler than cone 10.

Approximate deformation temperatures, assuming a moderately slow firing rate, look like this:

• Cone 022 — about 585°C (1085°F): lusters, china paints, and overglaze enamels.
• Cone 06 — about 1000°C (1832°F): the classic bisque firing for most stoneware and porcelain.
• Cone 04 — about 1060°C (1940°F): earthenware, low-fire glazes, and many commercial decals.
• Cone 5–6 — about 1200–1222°C (2194–2232°F): mid fire, now the most popular range for studio work in electric kilns.
• Cone 9–10 — about 1280–1305°C (2336–2381°F): high fire, the traditional range for reduction-fired stoneware and porcelain.

Notice those temperatures are approximate, and deliberately so. Published cone tables assume a specific firing rate — roughly 108°C (194°F) per hour — during the final stretch of the climb. Fire faster and the cone bends at a

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