What are cones?
Cones owe their name to their shape. They are small cone-shaped pieces made from compressed glaze raw materials with a well-known composition. Today, they are manufactured by the American company Orton, which is why they are often referred to as Orton cones. Je kan ze kopen in de webshop.
You place the cones in a kiln, where they bend under the combined influence of temperature and time, a concept known as heatwork. The degree to which a cone bends indicates how much heatwork it has received. Cones allow you to measure and verify the heatwork inside your kiln. They also help you identify the hotter and cooler areas within the kiln.
Difficult? Perhaps a little. Let’s start at the beginning. After reading this article, you’ll never say “I fire to 1220°C” again. Instead, you’ll say “I fire to cone 7.”
Different types of cones
Numbering
Each cone is marked with a number. These numbers range from 022 to 14. The zero at the beginning of a number is very important. Cone 06 is not the same as cone 6!
De cijfers geven een temperatuurbereik weer. Voor een glazuurstook op steengoedtemperaturen gebruik je kegels tussen 5 en 12. Voor een biscuitbak heb je tussen kegel 08 en 04 nodig.
De kegel correspondeert met een temperatuur equivalent. Die kan je terugvinden op een “cone chart” of een kegeltabel. Dat is een tabel die de omzetting weergeeft tussen de kegel, de stookcurve en de corresponderende temperatuur.
Small, large, and self-supporting cones
Cones come in different types. You have small, large, and self-supporting cones. The small ones are used in ovens that are not computer-controlled. They make an oven with a “kiln-sitter” shut off when it has reached the correct temperature (cone).
Large cones and self-supporting cones have the same purpose. They provide an indication during or after firing of the heatwork that was generated in the kiln.
Heatwork? What is that?
The concept “heatwork” encompasses heat AND time.
Although you may not know the word "heatwork," the concept is certainly not unfamiliar to you.
A kitchen example to explain: You can cook a chicken in the oven at 220°C for 30-45 minutes. That same chicken will also be cooked after 4 to 6 hours at 80°C. We can debate the taste, but both versions will be perfectly edible.
The same applies to glazes: you can fire very quickly to a very high temperature or very slowly to a lower temperature and still achieve the same fired result. That is heatwork. Temperature as a function of time.
How to measure heatwork
Cones bend under the influence of heat and time, hence heatwork.
Cone 6 can be perfectly bent if fired to 1220°C at a rate of 60°C per hour for the last 100°C of the firing schedule. That same cone 6 will look the same if fired at 1241°C at a rate of 150°C per hour. Despite the 21°C difference between the two firing curves, one is slower than the other. This ultimately results in the same heatwork.
So never tell someone how high you fire. Immediately explain what your firing curve looks like and how long you possibly soak. Even better, state what cone you fire to. Then the other person will have all the information right away!
Cone selection
You choose cones based on your application. If you fire high-temperature stoneware glazes, you will most likely use cone 10. If you work with earthenware, cone 04 is more common. Using commercial glazes? Be sure to check the packaging, as the recommended cone is usually indicated there.
Some guidelines (for more details, please consult the cone chart):
- Earthenware: Cone 08 – Cone 05 (942°C – 1044°C)
- Mid-range stoneware: cone 5 1/2 – cone 7 (1203°C – 1257°C)
- High-temperature stoneware: cone 9 – cone 11 (1260°C – 1315°C)
Keep in mind that the final temperature depends on the firing rate during the last 100°C of the firing cycle. Cone 6 can therefore have a theoretical peak temperature of either 1220°C or 1241°C. It all depends on how quickly the kiln is heated.
The Orton cone chart in real life
My experience with Orton cones has shown that kilns in Flanders often seem to fire ‘hotter’ than indicated in the Orton cone chart. In addition to the heating rate, factors such as the cooling rate, soak time and the age of the elements also play a role. One possible explanation is that our European kilns are very well insulated and therefore cool down slowly. As a result, glazes and cones may continue to mature and bend slightly even after the kiln has stopped actively heating.
Programming your kiln strictly according to the temperatures listed in the Orton cone chart can therefore lead to unexpected results. Don’t focus too much on the temperatures associated with a particular cone. Every kiln is unique. You may need to set a different final temperature than the one theoretically recommended by Orton.
For example, to achieve a perfect bend on cone 7, I need to set my kiln to 1215°C and fire the last 100°C at a slow rate of 60°C per hour. This is a much lower final temperature than the one recommended by Orton.
Preparing cones for use
The self-supporting cones are the easiest to use. As the name suggests, they stand up on their own due to a wide base. This way, they are at the perfect angle, and you simply place them on the oven tray. Easy!
Large cones fall over when you try to stand them up straight. This is because they need to be tilted slightly. Therefore, you must mount them in a piece of clay. A piece of clay with cones like this is called a “boat” or a "cone pack.".
It is very important to press the cones into the clay at the correct angle. You can mimic that 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. Do not place the different cones too close together. Keep in mind that they will bend (to varying degrees). They should not overlap, otherwise you will not be able to read the result properly. You can see how to do it in the video below.
Another tip: let your boat dry completely before use. If you put it in a glaze firing immediately after assembly, you risk a clay explosion in the kiln. Not good!
How many cones?
If you are measuring your kiln for the first time, it is helpful to use several cones around your expected firing temperature. For example, if you set your controller to 1220°C and use a slow heating rate of 60°C per hour for the final 100°C of the firing, the Orton chart would suggest using cone 5½, cone 6 and cone 7. In theory, cone 6 should be perfectly bent at that temperature.
Where to put them?
Ideally, place a cone pack on every kiln shelf you use during a firing, especially when measuring for the first time. Most kilns have a temperature gradient, meaning some areas are hotter than others. By placing cones at different heights or locations, you can easily map these variations. In top-loading kilns, the middle section is often hotter than the top and bottom. Try to position the cones in roughly the same spot on each shelf level.
Interpreting results
A cone bends during firing when enough heatwork has been generated to reach that cone rating. If the cone remains upright, melting has not yet started and insufficient heatwork was produced. If the cone has melted down completely and lost its shape, the amount of heatwork has exceeded the cone rating. A correctly fired cone forms a smooth bend, with the tip reaching the same height as the base of the cone. In the photograph below, the third and fourth cones show this ideal bend.
Keep in mind that a slightly more or less bent cone only represents a difference of a few degrees. The exact curve of the cone is therefore not particularly important. However, if the cone is lying flat or still standing upright, it is a clear indication that too much or too little heatwork has been generated.
Hey, this isn't what I expected!
You fired the cones in the kiln and to your great surprise, your cones did not bend as you expected. The theoretical cone chart does not match your set temperature and heating rate. What is going on?
You may be generating more heatwork than intended, causing the cones to bend all the way over and lie flat. In that case, the temperature indicated by your kiln controller does not match what the cones are showing. As discussed earlier, this is actually quite common in modern, well-insulated kilns. However, there may be other causes as well.
The heating rate of your kiln may in reality be much slower than the rate you have programmed and expect. Many kilns only display an error message when the temperature rises by less than 20°C per hour. If you believe your kiln is heating at 150°C per hour, but it is actually only reaching 25°C per hour, this can have a major impact on the amount of heatwork generated. There can be several reasons for this, including:
- The kiln is too large for what the elements and/or power supply can handle
- The elements are worn out or (partially) broken
A possible solution is to lower the final temperature and/or reduce the soak time, or eliminate it altogether. A soak also contributes to the amount of heatwork generated. It is also a good idea to use a slow firing rate, for example 60°C per hour, for at least the final 100°C of the firing.
Perhaps the opposite of what is described above happened. The cones are not bent at all or barely bent. Your kiln is not reaching its temperature.
One solution could be to set the final temperature higher and/or extend the hold time.
If your kiln systematically fails to reach its peak temperature, it's a good idea to have it checked.
Please note: only adjust your firing schedule or final temperature if you are experiencing problems with underfired or runny glazes. If your glazes are coming out of the kiln beautifully, don’t change a thing. Do, however, make a note of the cone you are firing to and whether there are any differences in heatwork throughout the kiln. That information can be very valuable in the future.