Pyrometric Cone Values: The Complete Cone Chart & Everything a Potter Needs to Know

You program your kiln to 1220 °C, press start and trust that the controller does the right thing. But what really happens inside the kiln? Does the heat actually reach every corner evenly? And is “1220 °C displayed” the same as “1220 °C of heat work on your clay”? The answer is: not necessarily. This is exactly where pyrometric cones come in, a tool that many potters know but few truly understand.

In this article we explain everything an advanced potter should know about pyrometric cones: how they work, what the cone values mean, which systems exist and how to use cones sensibly in your firing practice.

 

This article was written by Formwerk Berlin.
- Your pottery supplies from Berlin with shipping across Europe -

 

Contents

• What are pyrometric cones?
• Why cones measure heat work, not temperature
• Orton cones, Seger cones and others: the different systems
• Understanding the cone chart: values from 022 to 13
• Heating rate: why it affects the cone value
• Reading cones correctly: what the bending tells you
• The three-cone set: guide, target and guard cones
• Placement in the kiln: where to set up your cones
• Standard cones, self-supporting cones, Kilnsitter cones: what's the difference?
• Practical tips for use in the studio

 

Why cones measure heat work, not temperature

This is the crucial difference that many underestimate. A thermocouple, the sensor in your kiln controller, measures the air temperature at one specific point in the kiln. What it does not capture: how long the kiln stays at that temperature, whether the heat is evenly distributed, and how the clay body and glaze actually react to the total heat exposure over time.

Cones capture exactly that. A kiln held at 1222 °C for three hours affects clay and glazes differently than a kiln that only briefly touches 1222 °C and cools down immediately. The controller shows the same number in both cases, but the firing result is different.

For ceramics, this is exactly what matters. Clay does not sinter because a thermometer shows a certain number, but because it has absorbed enough energy over a certain time. Glazes do not melt at a momentary value, but in response to the sum of thermal work. Cones reflect exactly this sum.

 

Orton cones, Seger cones and others: the different systems

Historically, there are three common cone systems you should know:

Seger cones (SK) are the oldest system. They were developed in 1886 by the ceramic chemist Hermann Seger in Berlin to check whether a firing had reached the desired point. In Germany they were the standard for a long time, but today they are hardly available anymore. If you read older technical literature or recipes from German-speaking countries, you will come across them there. For new purchases, you will almost always reach for the Orton cone.

Orton cones are today's standard in international and German pottery supplies. They are precise, well documented and available over a wide temperature range from around 590 °C (cone 022) to about 1370 °C (cone 14). The chart below is also based on this system.

Original cones from the US manufacturer Orton are available here: Orton cones

Harrison cones (HPC) come from Great Britain and play a minor role on the German market.

 

Understanding the cone chart: values from 022 to 13

The numbering of the Orton system follows an unusual but, once understood, logical sequence. The cones start at 022 (the lowest temperature, approx. 590 °C) and count backward to 01, then forward from 1 to 14. The “0” before the number signals the low-temperature range. It is not a mistake, it is the system.

Here is the chart according to the Orton standard for self-supporting cones. The two most common reference rates are given, 60 °C/h and 150 °C/h in the last 100 °C of the firing. Orton also lists a third, slow reference of 15 °C/h, more on that in the next section.

Cone no.	°C at 60 °C/h	°C at 150 °C/h	Range
022	586	590	Decal, luster and overglaze firing
021	600	617
020	626	638
019	678	695
018	715	734
017	738	763
016	772	796
015	791	818
014	807	838
013	837	861
012	861	882	Bisque firing Most common in German studios: 08 to 07
011	875	894
010	903	915
09	920	930
08	942	956
07	976	987
06	998	1013	Glaze firing for earthenware and whiteware many low-fire glazes fall at 06 to 04
05½	1015	1025
05	1031	1044
04	1063	1077
03	1086	1104
02	1102	1122
01	1119	1138	Transition range, hard earthenware
1	1137	1154
2	1142	1164
3	1152	1170
4	1162	1183	Stoneware, lower and mid range many stoneware clay bodies only start at cone 6
5	1186	1207
5½	1203	1225
6	1222	1243
7	1239	1257
8	1249	1271	High-fire stoneware, reduction firing
9	1260	1280
10	1285	1305
11	1294	1315	Porcelain and specialty clay bodies
12	1306	1326
13	1331	1346

Source: Orton, temperature equivalents for self-supporting cones at heating rates of 60 and 150 °C/h over the last 100 °C. Orton gives separate values for standard large cones.

Two notes on the ranges: The rightmost column is a guideline, not a rule. The firing recommendation from your clay and glaze manufacturer always takes priority, and the ranges overlap.

One more note for everyone working with porcelain: Classic hard-paste porcelain is fired at 1380 to 1420 °C, which puts it above this chart, from cone 14 up. The clay bodies sold as porcelain for studio use, on the other hand, are usually formulated for 1260 to 1300 °C and so sit in the range of cones 9 to 11. There are now also porcelain bodies for cone 6, which is interesting if you have a smaller kiln.

 

Heating rate: why it affects the cone value

This is one of the most underestimated aspects of working with cones. The Orton values refer to fixed heating rates: In the last 100 °C before the final temperature, the kiln is heated at 15, 60 or 150 °C per hour and then shut off without a hold. These three rates are the frame of reference for all cone values.

In practice this means: cone 6 drops at 1222 °C at 60 °C/h, but only at 1243 °C at 150 °C/h. It is the same cone and the same heatwork, just reached by different paths. The cone reacts mainly in the final phase of the firing, roughly in the last 100 °C.

Which column applies to you? A fully loaded electric kiln rarely manages more than 60 to 100 °C per hour in the final phase. For most studio firings, the 60 °C/h column is therefore the right guide, and the figures in our shop refer to these values as well. You need the 150 °C/h column if you ramp up very quickly at the end, for example with small, lightly loaded kilns or raku firings.

The third reference, 15 °C/h, is easy to overlook but very useful in practice. If you deliberately run the last 100 degrees slowly, the same cone goes down much earlier: cone 5 already at 1159 °C instead of 1186 °C, cone 6 already at 1185 °C instead of 1222 °C. Conversely, this means that if your kiln has a limited maximum temperature, you can still reach higher cones: you need time instead of temperature. And if you fire out slowly anyway, the 60 °C/h column is too high for you, and you may well fire past your target.

Hold times add to this. If you hold the final temperature, you keep putting heatwork into the ware without the controller showing a higher temperature. A hold can easily make a difference of half to a whole cone. So if you program 1222 °C at 60 °C/h, which already corresponds to cone 6, and then hold for another half hour, the heatwork is above cone 6 even though the controller never showed more than 1222 °C. The exact effect depends on the kiln, the load and the firing schedule, so it is best to measure it yourself once with cones.

 

Reading cones correctly: what the bending behavior tells you

After the firing, the shape of the cone tells you what really happened in the kiln. What matters is the final position of the tip relative to the base.

• Cone stands upright, no deformation: The kiln did not reach the heatwork of this cone, so it was underfired.
  
  
• Cone slightly tilted, about 45 degrees: The heatwork is approaching the target value but has not been fully reached yet.
  
  
• Cone bent about 90 degrees, tip level with the base: Ideal firing point. The heatwork of this cone was reached exactly.
  
  
• Tip touches the floor, cone lies flat or has melted: Overfiring, the kiln went beyond the target.

The most common reading mistake is assuming the cone has to touch the floor. That is already too much. The cone has reached its end point when the tip has sunk far enough to sit roughly level with the top edge of the base, without touching it.

For an even more precise evaluation, Orton offers a measuring gauge, a reference sheet on which you can read off the bending angle of the cone. This is especially helpful if you want to calibrate your controller or compare different firing schedules.

 

The three-cone set: guide, target and guard cones

Professional potters rarely use just a single cone. The so-called three-cone set gives you much more information about the state of your firing. All cones you place in the kiln to monitor a firing are called witness cones. The set consists of:

• Guide cone: The next lower cone in the temperature series, so cone 5 if your target cone is 6. It is the first to fall and tells you the firing is approaching its goal. Ideal if you control a gas or wood kiln manually.
  
  
• Target cone: The cone whose heatwork you are aiming for. When its tip is level with the base, the firing is complete.
  
  
• Guard cone: The next higher cone, so cone 7 if your target cone is 6. If it deforms noticeably, your kiln went too far, and that is your signal for overfiring.

Watch out in the 0-series: The numbering runs backward there. For target cone 04, the guide cone is 05 and the guard cone is 03, not the other way around. If you are unsure, simply go by the temperatures in the chart instead of the numbers.

It is worth repeating this trio in different areas of the kiln, that is top, middle, bottom or left, middle, right, to map the heat distribution. This makes particular sense in larger kilns or after replacing the heating elements.

 

Placement in the kiln: where to put cones

For a cone to give reliable information, it has to be placed correctly. A few important points:

• Visibility through the peephole: If you watch the firing manually, the cone must be visible through the peephole. Place it slightly behind the opening, not directly in front of it.
  
  
• Distance from the heating elements: Do not place cones directly next to the heating elements. Local temperature spikes there can lead to false readings.
  
  
• Stable footing: The cone must be set at exactly the right angle, typically 8 degrees of tilt toward the falling side for large cones. Self-supporting cones take care of this for you.
  
  
• Enough room to fall: Leave a few centimeters free in front of the falling side so the cone can bend unobstructed and does not get caught on ware or kiln posts.
  
  
• Multiple positions: To map the temperature distribution in the kiln, place cone sets at the top, in the middle and at the bottom. Differences of half a cone between kiln zones are not unusual, but anything beyond that should motivate you to adjust your loading.

 

Pyrometric cones stand upright during the firing; the heatwork has not yet reached its peak

 

Large cones, self-supporting cones, Kiln Sitter cones: what is the difference?

You will come across different cone formats on the market. Here is an overview:

Large cones (Large Orton Cones) are the classic cones, about 6 cm tall. They have to be set in a clay base or pushed into a refractory cone holder so that they stand at the right angle. The angle is crucial for a correct reading. Orton gives separate temperature values for them.

Self-supporting cones (Self Supporting Cones) have an integrated base and stand at the correct angle without any aids. Another advantage is the exact measurement of the end point, because the base serves as a fixed reference. For most studio firings they are the more practical choice, easier to set up and easier to read. The values in the chart above apply to this type.

Kiln Sitter cones (Small Cones and cone bars) are small cones or bars for mechanical shut-off systems, the so-called Kiln Sitter. Such systems are found almost exclusively on older American kilns. If you work in Germany with a kiln from Rohde, Nabertherm, Kittec or a comparable brand, you have an electronic controller instead and do not need this format. For firing control there, you use regular witness cones.

A related tool that works on the same principle is PTCR temperature measuring rings. They also capture heatwork rather than pure temperature, but deliver the result not as a visible end point but as a measurable number: The ring shrinks during the firing, and its final diameter lets you read off the nominal temperature from a table. For quick, visible firing control, the cone remains the first choice, while for precise measurement and mapping of the temperature field, the rings complement it well.

PTCR temperature measuring rings come in packs of 15, and each ring shrinks during the firing in proportion to the heatwork absorbed.

 

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 actually prevail in the kiln. A cone firing every few months helps you spot 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 will see where your kiln deviates systematically and can counteract it in a targeted way.

Always calibrate new kilns. No controller is set up perfectly from the factory. In the first firings, always include witness cones in several areas of the kiln, and then adjust your firing schedule accordingly.

Old cones can be reused. Cones have no expiration date. If they are in good condition, have been stored dry and do not break easily, you can keep using them. Cones that have already been fired, however, are spent.

Be careful with glaze recipes from English sources: English-language recipes use the Orton system with plain numbers like “Cone 6” or “Cone 10”. This corresponds exactly to Orton cones without a leading zero. So cone 6 is 1222 °C at 60 °C/h, or 1243 °C at 150 °C/h, and already 1185 °C with a slow final phase at 15 °C/h. If “SK” comes before the number instead, a Seger cone is meant, and the numbers are not transferable.

Electric and gas kilns are not the same, even for cones. In a reducing atmosphere (gas kiln, wood firing), the cone reacts differently than in an electric kiln with an oxidizing atmosphere, because the iron oxide it contains changes its melting behavior under reduction. That is why iron-free cones exist for reduction firings. If you switch between electric and gas kilns, you should always accompany the first firings with witness cones.

 

 

Frequently asked questions about pyrometric cones

What is the difference between Seger and Orton cones?
Both are pyrometric measuring systems, but with different number series and different end point temperatures. Seger cones are hardly available on the market anymore today. Orton cones are the current standard and are often still called Seger cones in stores. The numbers are not directly transferable, so the same number does not mean the same end point temperature.

How many cones do I need per firing?
At least one target cone. For more precise control, we recommend the three-cone set of guide, target and guard cones, ideally at several positions in the kiln (top, middle, bottom).

How can I tell that the cone has fallen exactly right?
When the tip has sunk far enough to sit roughly level with the top edge of the base. If it touches the floor, the firing was too hot or too long. If the cone is still almost upright, the heatwork was not reached.

Can I use pyrometric cones more than once?
No. Cones that have been fired once have already done their work and cannot be used reliably a second time. Unused cones, on the other hand, can be stored almost indefinitely as long as they are kept dry.

Which cone do I need for the bisque firing?
In German studios, the bisque firing usually falls between 900 and 1000 °C, which corresponds to cones 010 to 06, with most firings at 08 and 07. What matters most is what the manufacturer of your clay body specifies.

Why does my controller show a different temperature than the cone suggests?
This happens more often than you think. Thermocouples age, they only measure at a single point, and they do not capture hold times in their sum. The cone measures the actual heatwork in the firing chamber, which makes it the more reliable indicator of how far the firing has progressed.

At which cone should I fire stoneware?
Most German stoneware clay bodies are designed for 1220 to 1280 °C, which means cones 6 to 10. Bodies for the lower range start at cone 4, but they are rarer here. Studio porcelain usually matures at cones 9 to 11, although there are also porcelain bodies for cone 6. Classic hard porcelain is higher still, at 1380 to 1420 °C. Always check the specifications from your clay manufacturer and match them to your glaze.

Do I still need cones with a programmable digital controller?
Yes, especially then. A digital controller programs a target temperature, not heatwork. Cones tell you whether your kiln is delivering what you set, and they help you spot systematic deviations, for example from an aged thermocouple or an uneven load.

 

Have fun firing, the Formwerk Berlin Team