Pyrometric Cone

What it is

A thermocouple measures temperature at one point. A pyrometric cone measures heat work — the combined effect of how hot and for how long — which is what actually matures clay and glaze. Two firings that both reach 1222 °C will produce different results if one soaked for an hour and the other did not; the cones will show the difference.

Each cone has a number and a corresponding bending temperature at a defined heating rate. Cone 06 bends around 999 °C, cone 6 around 1222 °C, cone 10 around 1300 °C. The numbering is confusing by design: the low numbers with a leading zero (06, 05) are the lowest temperatures, and they count upward to 01, after which the sequence restarts at 1 and runs to 10 and beyond.

In use, three cones are placed in a plaque visible through a peephole: a guide cone one number cooler, a target cone, and a guard cone one number hotter. The firing is complete when the target cone bends to about 90°, the guide is already down and the guard is still standing. Watching cones is how you verify what the controller actually did.

How to use it well

• Use a three-cone set — guide, target and guard — to bracket the firing.
• Place cones where they are visible through the peephole from the start of the firing.
• Record the cone number, the time it bent and the controller reading in your firing log.
• Replace the cone set for every firing; a bent cone cannot be reused.

Safety notes

• Cones are fragile — handle with tweezers and store in a rigid box.
• Looking through a peephole at high temperature requires IR-filtering safety glasses.
• Do not use a cone plaque as a shelf support; it will deform.

Compatible materials

Clay bodies

• All bodies — cones measure the firing, not the clay

Glazes & surface

• Cones confirm glaze maturity

Where it sits in the process

Bisque, glaze and specialty firings in the kiln.

Firing  Kiln & Firing  Entry tier

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