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Q&A on Pyrometric Cones
Pyrometric cones are very important for successful, repeatable firings. But we get many questions about cones and how they work. So here is a summary of almost everything I know about cones
Pyrometric cones are slender pyramids made from about 100 carefully controlled compositions. Each cone number is unique in measuring temperature within a small temperature range (less than 30°C). Cones measure the amount of heat absorbed. As the cone nears its maturing range, it softens and the tip begins to bend, drawn down by the influence of gravity or the weight of the sensing rod for cones used in the Kiln-Sitter. It typically takes about 20 minutes for the cone to bend fully. Each higher cone number requires more heat to bend. Faster heating rates require the cone to be heated to a higher temperature.
Cone bending is measured in degrees. Straight up is 0 degrees. When the tip is completely bent over so that the tip just barely touches the kiln shelf, that is 90 degrees. In general, a cone is considered "bent" when it is at about 90 degrees. But different potters have different thoughts about this. The most important thing is for you to be consistent with your own firings, in order to maintain predictable results. Cones bend faster the further along they are, so watch closely toward the end!
This is a common question, with no real answer. It depends on the rate of temperature increase. Remember that cones measure total heat over the whole firing. So if you fire quickly, your maximum temperature will have to be hotter than if you fire slowly (to reach the same Cone). The following link contains a chart of temperatures assuming a certain temperature rise. The rate of increase during the last 300 degrees is the most important. In other words, during the first 1500 degrees of firing (Fahrenheit), how fast you fire won't affect the end temperature very much. It is primarily the higher temperatures that will affect the end temperature.
- I have an old box of cones. Is it still good?
Yes. Cones do not change with age. According to Orton who manufactures the cones: "We have Pyrometric cones over 25 years old that we use for comparison testing and standardization. We do keep improving cones, though. Today’s cones are more uniform and perform better and more consistently than those we made 5, 10, or 50 years ago. Also, some changes occur when the supplier of our raw materials goes out of business.
Cones are not normally affected by usual amounts of moisture such as humidity in the air. If the cones are damp it is best to dry them out before use. If the cones have been extremely wet they should not be used. An example of this might be cones that were in a flooded basement or studio. These cones may have retained their shape but they may have lost their strength and can possibly break off in the Kiln-Sitter if they are small cones. The large cones that have been wet may not deform with the proper accuracy.
If you have a manual kiln, the only way to determine accurately when it is time to turn the kiln off is with Cones. If you have a Kiln-Sitter, the Cone will trip the Kiln-Sitter when the Cone bends, turning off the kiln. If you don't have a Kiln-Sitter you will manually watch the Cone through a spy hole or peep hole, and turn the kiln off when the Cone is bent.
If you have an electronic kiln, you don't need Cones. But it is still a very good idea to put Cones in. You can ensure that your kiln temperature is calibrated correctly. By putting Cones on every shelf, you can determine if there are hot and cold spots in the kiln. If the power goes out, you will be able to continue firing if you have Cones in. And finally, if you have any problems with your firing such as glazes not coming out the right color, or bubbling, you will know for sure what temperature was reached in that part of the kiln, which will be useful for troubleshooting. At the very least, with an electronic kiln, you should put Cones in every few firings to make sure the kiln doesn't drift and continues to fire at the right temperature as the elements age.
It is best to use the Cone number your glaze matures at, plus 1 Cone above and 1 Cone below. When the lowest Cone bends, you can start getting ready to turn off the kiln, or start slowing it down (especially with gas kilns). You will be at the right temperature to turn off when the Cone above is very slightly bent, the Cone below is extremely bent over (almost melted looking), and the middle Cone is bent to 90 degrees.
Orton Jr Cones are used in Kiln-Sitters.
Orton Standard Cones are used in Cone packs. A Cone pack is usually made by taking 3 cones, imbedding them in some clay, and allowing the clay to dry before firing. The 3 Cones include one at the firing temperature called the Firing Cone, 1 above called the Guard Cone and 1 below called the Guide Cone. For example, if you are firing at Cone 6, you use a #5, #6 and #7 Cone.
Orton Self Supporting Cones are used for placement in the kiln when you aren't using a clay pack. They stand up by themselves.
The last two are often called "witness cones" because you are "witnessing" or "watching" to see how far they bend, rather than using the cone to turn off the kiln sitter.
It is possible to use Orton Jr Cones as "witness cones" but these cones require slightly higher temperatures to bend, so you won't be as accurate.
The Kiln Sitter shuts off the kiln when a small cone placed under the sensing rod receives enough heat for it to fully bend. Bending is caused by the weight of the sensing rod. Note that because the cone in the Kiln Sitter is located at the kiln wall (closer to the heating elements), it frequently receives more heat than witness cones, causing the kiln to shut off early. Using the next hotter cone may be necessary. You can use a witness cone (or the 3-cone pack) to determine whether your kiln is shutting off early. Put the witness cones on a shelf near the Kiln Sitter to determine if a difference exists between the shelf and Kiln Sitter cones.