How do Orton cones work?
Cones. You've probably seen them at the ceramics store but aren’t sure what they do. How do Orton cones work? Do you really need them? You might think you don’t, especially if you have a programmable kiln - just set it to 1240°C and you're done. Simple, right? In this article, I'll explain why cones are useful, even with a computer-controlled kiln.
What are cones?
Cones get their name from their shape. They are small, cone-shaped compressed blocks of glaze materials with a well-defined composition. Today, they are produced by the American company Orton, hence the name Orton cones. You can buy them at ceramics stores, such as Colpaert.
You place them in a kiln where they bend under the influence of heat and time, a concept known as "heatwork." The degree to which the cone bends indicates how much heatwork it received. By using cones, you can measure and check the heatwork in your kiln, and identify the hotter and cooler spots in your kiln.
Sounds complicated? A bit, indeed. But don't worry, we'll start from the basics. After reading this article, you'll no longer say "I fire at 1250°C" but will say "I fire to cone 7."
Different kind of cones
Numbering
Each cone has a stamped number ranging from 022 to 42. The leading zero is crucial. For example, cone 06 is not the same as cone 6!
The numbers indicate a temperature range. For a stoneware glaze firing, you use cones between 5 and 12. For a bisque firing, cones between 08 and 04 are needed.
The cone corresponds to an equivalent temperature, which you can find on a "cone chart" or cone table. This table shows the conversion between the cone number, the firing curve, and the corresponding temperature.
Small, large en self-supporting cones
Cones come in various types. You have small, large, and self-supporting cones.
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Small Cones: Used in kilns that are not computer-controlled. They work with a kiln sitter to shut off the kiln when the correct temperature (cone) is reached.
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Large Cones and Self-Supporting Cones: Serve the same purpose of indicating the heatwork generated in the kiln during or after firing.
Heatwork? What's that?
The concept of "heatwork" contains both heat AND time. Even if you’re not familiar with the word heatwork, the concept is likely not foreign to you.
Here’s a kitchen example to illustrate it: You can cook a chicken in the oven at 220°C for 30 to 45 minutes, and it will be cooked through. Alternatively, you can cook the same chicken at 80°C for 4 to 6 hours. While the flavor may vary, both methods will result in a perfectly edible chicken.
The same principle applies to glazes: You can fire quickly to a very high temperature or very slowly to a lower temperature and still achieve the same final result. That’s heatwork—temperature in relation to time.
Cones measure heatwork
Cones bend under the influence of heat and time, which is known as heatwork.
Cone 6 can be perfectly bent if it is fired to 1220°C at a rate of 60°C per hour for the last 100°C of the firing cycle. The same cone 6 will look the same if you fire it to 1241°C at a rate of 150°C per hour. Despite the 21°C difference between the two firing curves, the slower one achieves the same amount of heatwork as the faster one.
So, never just tell someone how high you fire. Instead, explain what your firing curve looks like and how long you potentially hold the temperature. Even better, specify the cone number you fire to. This provides the other person with all the information they need!
Cone choice
You choose cones based on the application. If you’re firing glazes at high stoneware temperatures, you might opt for cone 10. If you're working with earthenware, you might use cone 03. If you're using commercial glazes, be sure to check the packaging, as the cone is usually indicated there.
Here are some guidelines (for more details, you can refer to the cone chart):
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Earthenware: cone 08 - cone 05 (942°C - 1044°C)
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Mid-range stoneware: cone 5 1/2 - cone 7 (1203°C - 1257°C)
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High-fire stoneware: cone 9 - cone 11 (1260°C - 1315°C)
Note that depending on your firing speed, the final temperature can vary in the last 100°C of the firing. For example, cone 6 can correspond to a theoretical top temperature ranging from 1220°C to 1241°C! Everything depends on the heating rate.
Preparing Cones for Use
The self-supporting cones are the easiest to use. As the name suggests, they stand upright on their own thanks to a wide base. This allows them to maintain the perfect angle, and you simply place them directly on the kiln shelf. Easy!
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Large cones, however, can fall over if you try to stand them upright on their own, because they need to be slightly angled. Therefore, you should mount them in a piece of clay. This piece of clay with the cones is called a "cone pack" or "boat."
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It’s crucial to press the cones into the clay at the correct angle. You can mimic this angle by placing a loose cone behind your clay boat on a flat surface. Sit in front of your boat and press the cone into the clay at the same angle. Don’t place the cones too close to each other, as they will bend to varying degrees. If they fall on top of each other, you won’t be able to read the results accurately. Check out the video below to see how it’s done.
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Another tip: Allow your cone pack to dry completely before use. If you place it in the kiln right after assembling it, there’s a risk of a clay explosion inside the kiln. Not good!
How many cones?
When conducting an initial measurement, it’s useful to use several cones around your estimated firing temperature. If you set the kiln to 1220°C with a slow ramp rate of 60°C/hour for the last 100°C of the firing, you would theoretically choose cones 5 1/2, 6, and 7. Cone 6 should ideally be perfectly bent at that temperature.
However, in reality, many kilns tend to be "warmer" and generate more heatwork than anticipated. Therefore, I recommend including cones 8 and 9 in your initial test as well. This way, you can better understand how your kiln performs and adjust accordingly.
Where to place them?
Ideally, you should place a cone pack on each kiln shelf used in your firing, especially for the first measurement. Kilns typically have a temperature gradient, meaning some areas are hotter than others. By placing cones at different heights or locations, you can easily map this out. In top-loading kilns, the center of the kiln shelf is usually cooler than the sides. Try to place the cones at roughly the same position on each level.
Interpreting results
You've completed a firing, and your cones have been fired and bent. Now what? Interpreting the results is not too difficult.
You look at the bending of the cones. Imagine that the cone is projecting onto a clock face:
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If the clock shows 1 o’clock, the cone is not bent (see the left cone in the photo below).
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If the clock shows 5 or 6 o’clock, the tip of the cone touches the kiln shelf or the base of the self-supporting cone (see cones 3 and 4 in the photo below). The cone itself forms a nice arc without drooping.
The firing is equivalent to the cone number if the cone forms a smooth arc without flattening. Cones that are completely flat indicate that the kiln was too hot, while cones still standing at the 1 o’clock position haven’t reached enough heat. Thus, the cone number has not been achieved.
Oops, this is not what I expected!
You fired the cones and to your great surprise your cones do not bend as you expected. The theoretical cone table does not match your set temperature and firing speed. What is going on?
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You may generate more heatwork causing the cones to melt. The kiln temperature does not match what the cones indicate. This can have several causes but the most common is this:
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The heating rate of your kiln is actually much slower than you set and think. Many kilns only give error messages when they heat up less than 20°C per hour. If you think your kiln is heating up at 150°C per hour but in reality it is only heating up at 25°C per hour, this has a huge effect on the heatwork generated! This can have several reasons, including:
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the kiln is too big for what the kiln elements and/or power supply can handle
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the kiln elements are worn or (partly) broken
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you have a well insulated kiln that maintains the heat very well, also during cool down. This gives extra heatwork. The cones bend, even during the slow cool down of your kiln. This is what happens very frequently in good quality kilns such as Nabertherm, Rohde, .... These kilns often need to be programmed to an end temperature way lower than what the Orton cone chart suggests.
A solution could be to lower your end temperature and/or not to soak or to soak for a shorter time. The latter also affects the heatwork. It is also a good idea to use a slow heating rate (60°C per hour for example) for at least that last 100°C. I use a slow heating rate for the last 200°C of the firing.
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Perhaps the opposite of what is described above happened. The cones are not or barely bent. Your kiln does not reach its temperature.
A solution could be to set the end temperature higher and/or to soak longer.
If your kiln consistently fails to reach its temperature, it is a good idea to have it checked.
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Note: Only make adjustments to the firing curve or end temperature if you have problems with unmelted or dripping glazes. If your glazes come out of the kiln beautifully, do NOTHING . Do note which cone you fire to and if there are any differences in heatwork throughout the kiln. This is important information for the future.
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Do you still have questions and would you like advice? Buy a kiln kit in which I really explain everything about ceramic kilns, cones, heatwork and much more. In such a kiln kit there is also a 15-minute advice moment in which we discuss your results one-on-one.