What Is a Pottery Cone? Your Guide to Pyrometric Cones
If you've spent any time around kilns, you've probably heard someone mention pottery cones and wondered what they were talking about. Pyrometric cones are small, pyramid-shaped ceramic devices that measure the heat your clay and glazes experience during a firing. They've been around for over a century, and they're still one of the most reliable tools in any potter's toolkit.
This guide covers everything you need to know about pyrometric cones, from how they work and how to read them to why they still matter, digital controller or not.
What Is a Pottery Cone?
A pottery cone (often called a pyrometric cone, kiln cone, or witness cone) is a small pyramid made from carefully calibrated ceramic materials. You place it inside your kiln before firing, and it bends as the kiln heats up. The amount it bends tells you how much heatwork your kiln delivered.
Heatwork is the combined effect of both temperature and time on your clay and glazes. A pottery kiln that reaches 1,222°C (2,232°F) in six hours delivers different heatwork than one that reaches the same temperature in 10.
Your thermocouple reads air temperature. A cone measures what your work experienced.
That distinction matters more than most beginners realize. According to the Edward Orton Jr. Ceramic Foundation, cones respond to the same conditions as you ceramic ware because they're made from similar materials.
Two kilns could both read the same thermocouple temperature, but if the firing schedules differed, the cones would show it. They give you a picture of what happened inside the kiln that a temperature reading alone cannot.
How Do Pyrometric Cones Work?
Cones are made from a blend of ceramic materials, similar in composition to the clays and glazes being fired. As the kiln heats up, the glass-forming compounds in the cone soften. Gravity takes over and pulls the tip downward in a slow, predictable bend.
The exact temperature at which a given cone bends depends on how fast the kiln is heating. A faster firing pushes the bend point higher. A slower firing brings it lower.
This is what makes cones more useful than a thermocouple alone. They respond to the full thermal history of the firing, not a snapshot of air temperature at one moment.
Each cone number on a pyrometric cone chart corresponds to a specific level of heatwork. When that cone bends to the proper angle (tip level with its base), you know your firing reached the conditions that particular clay body or glaze needs to mature. That's why potters who fire the same cone number get consistent results even when their kilns and schedules look different.
A Brief History of Pyrometric Cones
Potters have been trying to measure kiln heat for centuries. Josiah Wedgwood built one of the earliest pyrometric devices in 1782, publishing his findings in the Philosophical Transactions of the Royal Society of London. His system used small clay cylinders that shrank at high temperatures.
The modern cone system came along about 100 years later. Hermann Seger developed standardized pyrometric cones in 1886 at the Royal Porcelain Factory in Berlin.
In 1896, Edward Orton Jr. founded the Standard Pyrometric Cone Company in Columbus, Ohio. Orton cones became the standard across North America, and the company still produces them today.
Understanding the Cone Numbering System
Cone numbers range from 022 (the lowest temperature) all the way up to 42 (the highest). Most studio potters work within the 022 to 12 range, which covers everything from low-fire earthenware to high-fire porcelain.
Here's where many beginners make expensive mistakes. Cone 04 is not the same as cone 4. That "0" prefix means a lower temperature.
Cone 04 fires at roughly 1,060°C (1,940°F). Cone 4 fires at roughly 1,168°C (2,134°F). Mixing these up can ruin an entire kiln load, so pay close attention to that zero.
The numbering runs in two directions. From 022 down to 01, the numbers decrease as temperature goes up. Then from 1 upward, the numbers increase with temperature.
It trips up nearly everyone the first time, but once you get used to it, the system clicks. Rather than memorizing exact degrees, most potters learn which cone range matches their clay and glazes, then let the cones do the measuring.
Most studio work falls into one of three kiln temperature ranges.
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Low-fire (cone 06 to 02), roughly 999°C (1,830°F) to 1,101°C (2,014°F). Used for earthenware, most commercial glazes, and decorative work.
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Mid-fire (cone 2 to 6), roughly 1,142°C (2,088°F) to 1,222°C (2,232°F). Popular for functional stoneware and a go-to range for many studio potters.
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High-fire (cone 8 to 12), roughly 1,263°C (2,305°F) to 1,326°C (2,419°F). Used for porcelain and durable stoneware with tighter vitrification.
Types of Pyrometric Cones
Not all cones look the same or serve the same purpose. You'll run into four main types, each built for a different setup.
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Self-supporting cones come with a built-in base, so they stand upright on their own. Set them on a kiln shelf and they're ready. These are the most convenient option for hobby potters and smaller kilns.
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Large cones lack a built-in base. You press them into a small clay pad (called a cone pack) at a slight angle before placing them in the kiln. This is the standard format for studio use and gives you the most accurate readings.
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Small cones fit kiln sitter devices. These mechanical shut-off systems use the cone's bending action to trigger a switch that turns the kiln off. You place the kiln sitter cone in the tube, and when it bends far enough, the kiln shuts down.
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Pyrometric bars work with kiln sitters, too. They function the same way but have a different shape.
One thing to keep in mind. Different manufacturers (Orton, Seger, Staffordshire) use similar numbering, but their cones are not interchangeable. Test before swapping brands. A cone 6 from one maker may behave slightly differently than a cone 6 from another.
The Three-Cone System
A single cone tells you something. A set of three cones side by side tells you much more.The three-cone system places cones in a cone pack, each one number apart.
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Your guide cone sits one number below your target. It bends first and signals that you're getting close. If you're firing to cone 6, your guide cone would be cone 5.
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Your firing cone is the actual target. When this cone bends until its tip is level with its base, your firing is complete. Potters sometimes call this the "6 o'clock" position.
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Your guard cone sits one number above your target. It should remain standing after the firing. If your guard cone bends significantly, you overfired.
This setup gives you a clear read on how your kiln performed. It's especially useful for spotting hot and cold zones. Place cone packs at different shelf levels, and you'll quickly see whether your kiln heats evenly from top to bottom.
How to Make and Place a Cone Pack
Building a cone pack takes a few minutes and saves you from guesswork.
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Roll a small piece of clay into a flat base, roughly the size of a golf ball pressed flat.
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Let it dry before use. Wet clay can pop in the kiln.
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Poke a few small holes in the base so trapped moisture has a way out.
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Press your cones into the clay in ascending order (guide, firing, guard), spaced about half an inch apart.
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Tilt each cone at a slight angle, about eight degrees off vertical, leaning in the direction you want it to bend.
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Leave about two inches of cone exposed above the clay.
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Place your finished cone pack where you can see it through the peephole.
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If you're testing kiln uniformity, set up packs at the top, middle, and bottom shelves to compare results across different zones.
Why Cones Still Matter With Electronic Controllers
Modern kilns with digital controllers are impressive. You program a firing schedule, press start, and the controller handles ramp rates, soak times, and shut-off automatically. So why bother with cones at all?
Thermocouples and cones measure different things. Your thermocouple reads the air temperature inside the kiln. A cone measures actual heatwork, the same way your clay and glazes experience it.
Air temperature and heatwork aren't always the same, especially in a kiln with uneven heat distribution.
Thermocouples drift over time, too. After hundreds of firings, they gradually lose accuracy and start reading a few degrees off. You won't notice until your glazes stop turning out right or pieces come out inconsistent from load to load.
Cones stay accurate. A cone 6 today behaves exactly like a cone 6 five years from now.
Placing witness cones in your kiln every few firings gives you a reality check on your controller's performance. If your cones show underfiring but your controller says everything completed normally, it's time to check your kiln calibration or replace that thermocouple. Even when things seem fine, running cones once every 10 or 15 firings builds a record of your kiln's behavior that becomes invaluable when something eventually shifts.
A kiln with a quality programmable controller gives you the best starting point. Brands like Paragon, Olympic, and Evenheat build controllers that let you set precise ramp rates and hold times for each firing stage, so you get repeatable schedules from one load to the next. Cones then confirm that what the controller promised is what your work received.
Reading Your Cones After a Firing
Once your kiln cools and you open the door, your cones tell the story of the firing.
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Your guide cone should be fully bent, touching or nearly touching the shelf surface. This confirms the kiln reached at least one cone level below your target.
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Your firing cone should be bent with its tip level with the base (the "6 o'clock" position). That confirms your firing reached the target heatwork.
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Your guard cone should be standing straight or showing only a slight tip. If it's still upright, everything went as planned.
If the firing cone didn't bend enough, you underfired. The kiln may not have reached temperature, or the heating rate was too fast for the cone to respond fully. Check your controller settings and thermocouple accuracy.
If the guard cone bent significantly, you overfired. This often means your thermocouple is reading low and needs replacement, or your controller needs recalibration.
In gas kilns, pay attention to color changes on the cones. Discoloration or bloating can point to atmosphere issues that affect glaze results.
Keep your fired cones for reference. Labeling them with the date and firing schedule helps you track kiln performance over time and gives you a baseline to compare against if results start shifting.
Fire With Confidence Using Pyrometric Cones
Pyrometric cones give you a deeper connection to what's happening inside your kiln. They're one of the simplest tools in ceramics. Whether you are new to cone firing or a seasoned potter, every pottery cone you place in your kiln gives you information no digital readout can match, making them one of the most useful tools for getting consistent, repeatable results firing after firing.
If you're shopping for a kiln that pairs well with a solid cone routine, we carry models from Paragon, Olympic, Evenheat, Jen-Ken, and more. Every kiln ships with the delivered price listed upfront, no hidden fees or surprise charges at checkout. Our team fires these same kilns in our own studios, so we can help you match the right one to your clay body, your firing range, and your space. Browse our kilns with electronic controllers, or give us a call and we'll walk through your options.
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