Pottery Clay has previously introduced you to one of the most classic tools for monitoring ceramic kiln temperature: Pyrometric Cones.
This time, we’re taking a closer look at how to read pyrometric cones accurately. It may seem complicated at first, but once you understand how cone bending works, it’s actually much easier than you think—even if you’re new to ceramics.
Let’s take a look!
How to Read Pyrometric Cones Accurately
When reading pyrometric cones during a ceramic firing, imagine the cone’s bending position in relation to the face of a clock. The position of the cone can be described using the 1, 2, 3, 4, 5, and 6 o’clock positions.
As the kiln heats up, watch the tip of the cone as it gradually bends downward.
When the cone reaches approximately the 3 o’clock position, bending to around a 90-degree angle, it has reached the ideal endpoint for that specific cone.
Each pyrometric cone is made from a specific combination of ceramic materials and chemical compositions. As a result, different cone numbers soften and bend at different heatwork levels.
Cone temperatures are commonly identified using the SK (Seger Kegel) system, which indicates the cone number and its corresponding heatwork range.
Factors That Affect the Softening Point of Pyrometric Cones
When using pyrometric cones to monitor a ceramic firing, it’s important to remember that the cone’s bending point is influenced by more than just the kiln’s displayed temperature.
Several factors can affect how and when a cone bends, including:
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The rate of temperature increase during firing
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The kiln atmosphere, such as oxidation or reduction
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Gas movement and circulation inside the kiln
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The temperature of the kiln walls and surrounding areas
Pottery Clay Tip
During ceramic firing, it is generally recommended to keep the lower part of the kiln slightly hotter than the upper section.
If you are firing glazed ware, consider placing glazes that are less prone to running in areas where the temperature is higher.
Remember that every kiln has its own temperature profile, so using witness cones in multiple positions can help you better understand the actual heatwork throughout your kiln.
Important Safety Precautions When Checking Pyrometric Cones
Safety should always come first when working with a high-temperature kiln.
Never Look Directly Into a Hot Kiln with the Naked Eye
When a kiln reaches high temperatures, the interior can glow an intense yellow or nearly white. This bright light can make it difficult to see the cones clearly and may cause serious damage to your eyes.
Always use appropriate heat-resistant safety glasses or a protective face shield when viewing cones through a kiln peep hole.
What to Do If the Cone Is Difficult to See
If you have trouble seeing the cone clearly through the peep hole, gently blow a small amount of air through the opening.
This can help clear the hot air and make the tip of the cone easier to see as it bends.
Always Clean the Peep Hole
Before loading your work into the kiln, make sure the peep holes are clean.
Dust, ash, and other debris can collect around the openings and may be blown into the kiln when you use air to improve visibility. This could potentially land on glazed pieces and affect the final surface.
A clean peep hole ensures better visibility and helps reduce the risk of unwanted debris entering the kiln.
Keep Firing Records for More Consistent Results
One of the biggest advantages of using pyrometric cones is that they give you a visual record of the heatwork your kiln has actually delivered.
After each firing, once the kiln has been safely turned off and cooled, keep the used cones and record how far each cone bent.
It’s especially useful to document firings that produced the results you wanted—whether that means the perfect glaze color, surface texture, or overall finish.
Over time, these records will become a valuable reference for future firings.
The next time you fire the same type of work, you can use the previous cone position as a reference point to help determine when to end the firing.
With consistent records and careful observation, pyrometric cones can help you achieve more predictable and repeatable ceramic firing results.