Pyrometric Cones and Heatwork Explained

Heatwork, Not Just Heat

In ceramics, success in the kiln isn't determined by temperature alone it’s achieved by reaching the correct heatwork, which combines temperature + time to fully mature clay and glaze.

That’s why we fire to a Cone, not just a number on a digital controller.

Pyrometric cones are trusted tools that respond to both time and temperature. They show what your ware actually experienced not just what the kiln reported. Firing to temperature alone can lead to:

• Underfired glazes (dull or matte finishes)
• Overfired ware (glaze runs or distortion)
• Inconsistent results due to load size, element wear, or ramp rate

Firing to the correct cone number helps ensure consistent, reliable, and professional quality results.

What Is a Pyrometric Cone?

A pyrometric cone is a small ceramic indicator formulated to bend at a specific level of heatwork (the combined effect of temperature and firing time). These cones are used to assess whether a firing has delivered the correct amount of heat energy to mature glazes and clay bodies.

There are two main types of cones: bar cones and witness cones.

Bar cones are designed to work with mechanical kiln shut-off systems such as the Kiln Sitter. As the kiln heats and the bar cone reaches its designated cone point, it softens and bends. This bending action triggers the Kiln Sitter’s mechanical arm, which shuts the kiln off at the precise moment. Bar cones are especially useful in manual kilns, providing a reliable and automatic way to end the firing when the correct level of heatwork has been achieved.

Witness cones, on the other hand, are triangular-shaped cones that are placed inside the kiln. Unlike bar cones, witness cones do not control the kiln; instead, they provide a visual indication of whether the target cone was actually reached during the firing. Often used in sets of three (a guide cone, target cone, and guard cone), witness cones allow you to confirm whether your firing was underfired, on target, or overfired. They are an essential diagnostic tool for both manual and digitally controlled kilns, helping to ensure consistent results firing after firing.

Why Use Cones Instead of Just a Temperature?

Cones give us a more accurate measurement of what the ware experiences inside the kiln. Digital controllers display temperature but they can’t measure heatwork. They don’t account for:

• Worn or aging elements
• Inconsistent loading
• Thermocouple drift
• Power fluctuations

Witness cones reveal what actually happened, making them essential for:

• Verifying your controller’s accuracy
• Detecting cold spots in your kiln
• Ensuring 
  • The glaze has fully matured
  • The clay body has vitrified properly
  • The ware absorbed the necessary heatwork

Time + Temperature = Cone

The point at which a cone bends depends not just on temperature, but also on how quickly that temperature was reached. This is known as the rate of fire.

For example:

• A Cone 6 firing at a slow rate may bend at around 1185 °C.
• The same Cone 6, fired quickly, might not bend until 1243 °C.

This is why cone charts list three different temperatures for each cone, depending on the ramp rate.

Pyrometric Cone Firing Chart

The chart below shows the target temperatures for each cone number, based on different firing rates.

(Actual values may vary by manufacturer refer to your cone supplier)