You program your kiln to 1,220 °C, press start – and trust that the controller will do the right thing. But what actually happens inside the kiln? Does the heat really reach all corners evenly? And is "1,220 °C displayed" the same as "1,220 °C heatwork on your clay"? The answer is: not necessarily. This is exactly where pyrometric cones come into play – a tool that many potters know, but few truly understand.
In this article, we explain everything you, as an advanced potter, should know about pyrometric cones: how they work, what the cone values mean, which systems exist, and how to use cones effectively in your firing practice.
This article was written by Formwerk Berlin.
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Table of Contents
- What are pyrometric cones?
- Why cones don't measure temperature – but rather heatwork
- Orton cones, Seger cones & Co.: The different systems
- Understanding the cone chart: Values from 022 to 14
- Heating rate: Why it influences the cone value
- Reading cones correctly: What bending behavior reveals
- The three-cone set: Guide, witness, and guard cones
- Placement in the kiln: Where to set cones
- Normal cones, self-supporting cones, kiln sitter cones: What's the difference?
- Practical tips for use in the studio
👉 In Brief: What are pyrometric cones?
Pyrometric cones are small, slender pyramids made of ceramic materials, formulated to deform at a defined temperature and firing duration. They do not measure temperature in the classic sense, but rather what is known as heatwork (also called thermal effect) – i.e., the combined effect of temperature and time on the ceramic material. This makes them one of the most reliable tools for controlling your firing process, especially if you are aiming for reproducible results.
Why cones don't measure temperature – but rather heatwork
This is the crucial difference that many underestimate. A thermocouple – the sensor in your kiln controller – measures the air temperature at a specific point in the kiln. What it doesn't capture: how long the kiln stays at that temperature, whether the heat is evenly distributed, and how the clay material or glaze actually reacts to the total heat load over time.
Unlike a thermometer, which measures instantaneous temperature, cones measure heatwork – how long the kiln was at temperature, not just what temperature it reached. A kiln held at 1,222 °C for three hours affects clay and glazes differently than a kiln that briefly touches 1,222 °C and cools immediately.
For ceramics, this is precisely what is relevant. Clay does not sinter because a thermometer shows a certain number – it sinters because it has absorbed enough energy over a certain period. Glazes do not flow at an instantaneous value, but rather in response to the sum of thermal work. Cones precisely represent this sum.
Orton cones, Seger cones & Co.: The different systems
Historically, there are three common cone systems you should be aware of:
Seger cones (SK) are the oldest system. They were developed in 1885 by ceramist Hermann Seger for his research into the refractoriness of various materials. In Germany, they were the standard for a long time – today they are hardly produced anymore and are only rarely available. If you read older technical literature or recipes from German-speaking countries, you will encounter them there. For new purchases, you will practically always opt for Orton cones.
Orton cones are the current standard in international and German pottery supplies. Therefore, Orton cones are now used in many areas. They are precise, well-documented, and available in a wide temperature range from approx. 600 °C (cone 022) to over 1,380 °C (cone 14). The table you see in the book image is also based on this system.
Harrison cones (HPC) are mainly used in Anglo-American countries and play a subordinate role in the German market.
For your practice in the studio: If you buy cones today, you will almost always buy Orton cones. When reading recipes, pay attention to which system is specified – especially older books still use Seger cone numbers, which are not identical to Orton numbers.
Understanding the cone chart: Values from 022 to 14
The numbering of the Orton system follows an unusual but, once understood, logical sequence. Cones start at 022 (the lowest temperature, approx. 600 °C) and count backwards to 01, then forwards from 1 to 14. The "0" before the number indicates the low-temperature range. It's not a mistake, it's the system.
Here is the complete table according to the Orton standard, as also shown in the image – with firing temperatures at an assumed temperature increase of 150 °C per hour:
| Cone No. | °C | °F | Range |
| 022 | 600 | 1112 | |
| 021 | 614 | 1137 | |
| 020 | 635 | 1175 | |
| 019 | 683 | 1261 | |
| 018 | 717 | 1322 | |
| 017 | 747 | 1376 | |
| 016 | 792 | 1457 | |
| 015 | 804 | 1479 | |
| 014 | 838 | 1540 | |
| 013 | 852 | 1565 | |
| 012 | 884 | 1623 | |
| 011 | 894 | 1641 | |
| 010 | 900 | 1652 | |
| 09 | 923 | 1693 | |
| 08 | 955 | 1751 | Low fire / Earthenware |
| 07 | 984 | 1803 | |
| 06 | 999 | 1830 | |
| 05 | 1046 | 1914 | |
| 04 | 1060 | 1940 | |
| 03½ | 1080 | 1976 | |
| 03 | 1101 | 2014 | |
| 02 | 1120 | 2048 | |
| 01 | 1137 | 2079 | |
| 1 | 1154 | 2109 | |
| 2 | 1162 | 2124 | |
| 3 | 1168 | 2134 | |
| 4 | 1186 | 2167 | Mid-Range Temperatures / Stoneware |
| 5 | 1196 | 2185 | |
| 6 | 1222 | 2232 | |
| 7 | 1240 | 2264 | |
| 8 | 1263 | 2305 | High Temperatures |
| 9 | 1280 | 2336 | |
| 10 | 1305 | 2381 | |
| 11 | 1315 | 2399 | |
| 12 | 1326 | 2419 | Porcelain Clay / Porcelain |
| 13 | 1346 | 2455 | |
| 14 | 1366 | 2491 |
Source: Orton Standard, temperature values at a heating rate of 150 °C/h in the last 100 °C
An important note on reading this table: The temperature specifications apply to a specific heating rate. If you heat up more slowly, you achieve the same heatwork at a lower peak temperature – more on this in the next section.
Heating rate: Why it influences the cone value
This is one of the most frequently underestimated aspects when working with cones. The kiln is heated in the last 100 °C before the measurement temperature at 60 or 150 °C/hour and switched off without a holding time. These two standard speeds are the reference framework to which all cone values refer.
In practice, this means that a Cone No. 6, with a heating rate of 60 °C/h, will show the same heat work at a lower peak temperature as it would at 150 °C/h. The cone reacts particularly in the final phase of firing, approximately in the last 100 °C. Therefore, anyone who works with a hold time or heats up very slowly must factor this into their cone selection.
For daily practice: If you program your controller to a fixed end temperature and do not use a hold time, you can refer well to the 150 °C/h column. As soon as you incorporate hold times or heat up slowly – which makes sense for many glaze recipes – you should choose a cone with a slightly lower nominal value, as the heat work will be higher than the thermometer reading suggests. As a rule of thumb: a hold time of 20 minutes can correspond to half to a whole cone value of additional heat work, depending on the temperature range, firing curve, and kiln. So, if you fire at Cone 6 with a 20-minute hold time, you achieve heat work that might correspond more to Cone 7 – even if the controller never displayed more than 1,222 °C.
Reading cones correctly: What the bending behavior tells you
After firing, the shape of the cone reveals what truly happened in the kiln. If the cone tip touches the base, there was overfiring. If the cone is still almost upright, there was underfiring. Only when the cone tip points slightly towards the base has the fall temperature been reached.
In detail, the following states exist:
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Cone stands upright, no deformation: The kiln did not reach the fall temperature of this cone – underfiring.
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Cone slightly inclined, tip pointing towards the base: The heat work is approaching the target value but has not yet been fully achieved.
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Cone at approximately 90°: Tip touches the base: Ideal firing point. The heat work of this cone has been fully achieved.
- Cone lies flat or has disintegrated: Overfiring – the kiln has gone beyond the target.
If the firing is too short, the cone remains too vertical. If fired correctly, the tip of the cone touches the base. If over-fired, the cone falls completely or melts.
For even more precise evaluation, there is the so-called Orton measuring gauge – a reference sheet on which you can read the bending angle of the cone. A cone bent at 90 degrees is considered correctly fired. This is particularly helpful if you want to calibrate your controller or compare different firing curves.
The three-cone set: Guide, target, and guard cones
Professionally working potters rarely use just a single cone. The so-called three-cone set gives you significantly more information about the state of your firing. In the Orton standard, all three cones you place in the kiln for monitoring are called "Witness Cones" – they collectively attest to what truly happened in the kiln. The set consists of:
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Guide Cone: A cone with a lower nominal value than your target cone. It falls first and signals that the firing is approaching its target. Ideal if you are manually controlling a gas or wood-fired kiln.
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Firing Cone: The cone whose heat work you are aiming for. If it lies perfectly at 90° on its base, the firing is complete.
- Guard Cone: A cone with a higher nominal value than your target cone. If it deforms, your kiln has gone too far – this is your signal for overfiring.
Orton recommends repeating this trio in different areas of the kiln (top-middle-bottom or left-middle-right) to map heat distribution. This makes particular sense for larger kilns or after a heating element replacement.
Positioning in the kiln: Where to place cones
For the cone to provide reliable information, it must be correctly placed. Some important points:
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Visibility through the peephole: If you are manually observing the firing, the cone must be visible through the peephole. Place it slightly behind the peephole, not directly in front.
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Distance to heating elements: Do not place cones directly next to heating coils or rods – local temperature spikes occur there, leading to false readings.
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Stable stand: The cone must be set up at the exact correct angle – typically an 8° incline towards the bending side for standard cones. Self-supporting cones relieve you of this task.
- Multiple positions: To map the temperature distribution in the kiln, place cone sets at the top, middle, and bottom. Differences of half a cone value between kiln areas are not uncommon – anything beyond that should motivate you to adjust the loading.
Pyrometric cones standing upright during firing – the heat work has not yet reached its peak
Standard cones, self-supporting cones, Kiln Sitter cones: What's the difference?
You will encounter different cone types in stores. Here's an overview:
Standard cones (Large Orton Cones) are the classic, approx. 5–6 cm high cones. They must be inserted into a clay holder or placed in a refractory cone support to stand at the correct angle. The angle is crucial for accurate reading.
Self-supporting cones have an integrated base and stand at the correct angle without additional support. Another advantage is the precise measurement of the fall point using the foot as a reference. They are the more practical choice for most studio firings – easier to set up, easier to read.
Kiln Sitter cones (Small Cones / Bars) are small cones or rods used for automatic kiln shut-off systems (Kiln Sitter). They mechanically trigger the kiln's shut-off when the fall temperature is reached. A functioning Kiln Sitter with the correct cone is a reliable safety solution, but it does not replace reading Witness Cone sets if you want to understand the heat distribution in the kiln.
Practical tips for studio use
A few practical points you should know:
Calibrate your controller regularly. Thermocouples age and over time show lower temperatures than are actually present in the kiln. A cone firing every few months helps you detect the drift – before it ruins your glazes.
Document every firing. Note the cone number, heating rate, hold time, and cone result. After a few firings, you'll recognize where your kiln systematically deviates and can take targeted countermeasures.
Absolutely calibrate new kilns. No controller is perfectly set ex-factory. Always use Witness Cone sets in the first firings – in several kiln areas – and then adjust your programming accordingly.
Old cones can be reused. Cones do not have an expiration date. If they are in good condition, have been stored in a dry place, and do not break easily, they can still be used. However, once used cones cannot be used a second time.
Be careful with glaze recipes from English sources: English-language recipes usually use the Orton system with simple numbers like "Cone 6" or "Cone 10." This corresponds exactly to Orton cones without a leading zero – so Cone 6 is 1,222 °C at 150 °C/h.
Electric and gas kilns are not the same – even for cones. In a reducing atmosphere (gas kiln, wood firing), the cone may react slightly differently than in an electric kiln with an oxidizing atmosphere. The reason: Reduction affects the melting properties of glazes and can slightly shift the perceived cone value. If you switch between electric and gas kilns or want to apply recipes from one system to the other, you should keep this in mind and always accompany the first firings with Witness Cone sets.
👉 Conclusion
Pyrometric cones are not a tool from the past – they are a precise, indispensable instrument for anyone who wants to fire reproducibly and consciously. Anyone who understands that cones measure heat work and not mere temperature thinks about their firing fundamentally differently. Whether for calibrating the kiln controller, mapping the heat distribution in the kiln, or for assurance with a new glaze recipe – regularly used Witness Cone sets provide information that no thermocouple alone can.
You can find Orton cones in various temperature ranges in our shop: Formwerk Berlin.
Frequently asked questions about pyrometric cones
What is the difference between Seger and Orton cones?
Both are pyrometric measuring systems, but with different number ranges and slightly differing fall temperatures. Seger cones were produced in Germany until 2005 and are now rarely available. Orton cones are the current standard. When reading older recipes or technical literature, it is worth looking at a comparison table, as the numbers are not 1:1 transferable.
How many cones do I need per firing?
At least one target cone. For more precise control, the three-cone set of guard, target, and guide cones is recommended – ideally at several positions in the kiln (top, middle, bottom).
Can I reuse pyrometric cones?
No. Once fired, cones have already completed their heat work and cannot be reliably used a second time. Unused cones, however, have an almost unlimited shelf life, provided they are stored dry.
Why does my controller show a different temperature than the cone suggests?
This is more common than you think. Thermocouples age, measure only at one point, and do not capture hold times in their entirety. The cone measures the actual heat work in the kiln chamber – making it the more reliable indicator of the actual firing state.
At what cone should I fire stoneware?
Most stoneware bodies mature in the range between Cone 4 (approx. 1,186 °C) and Cone 7 (approx. 1,240 °C). Porcelain, depending on the body, ranges between Cone 8 and Cone 12. Always check your clay manufacturer's specifications and match them with your glaze.
Do I need to use cones even with a programmable digital controller?
Yes – especially then. A digital controller programs a target temperature, not heat work. Cones tell you whether your kiln performs as you have set it and help you detect systematic deviations (e.g., due to an aged thermocouple or uneven loading).
Happy firing from the Formwerk Berlin Team