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Pyrometric Cones

Pyrometric cones are very important for successful and repeatable firings.
We can read all kinds of books about them but there is always still many questions about cones that are never included.
Here is a collection of everyday questions people want to know about cones.

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How do cones work?


Pyrometric cones are slender pyramids made from about one hundred carefully controlled compositions.
Each cone number is unique in measuring temperature within a small temperature range less than 1382 degrees F.
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 twenty 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.


When is my cone considered bent?


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.
Usually, a cone is considered bent when it is at about 90 degrees.
But different potters have different ideas 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!


. What temperature does a particular Cone correspond to?


Everybody wants to know this, with no real answer.
It depends on how fast the temperature is increasing.
Remember that cones measure total heat over the whole firing.
If you fire quickly, then your maximum temperature will have to be hotter than if you fire slowly to reach the same Cone temperature.

The Pyrometric Cones Temperature Chart on this site will show you a chart of temperatures assuming a certain temperature rise.
The rate of increase during the last three hundred degrees is the most important. During the first fifteen hundred degrees fahrenheit of firing and 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 cones, they have pyrometric cones over 25 years old that they use for comparison testing and standardization.
They say that they do keep improving cones though.
Today’s cones are more uniform and perform better and more consistently than those we made five, ten, or fifty years ago.
Some changes do occur when the main supplier of our raw materials goes out of business.


My cones got wet. Can they still be used?


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 though.
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.


Why should I use Cones?


If you have a manual kiln, the only way to determine 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, because that is the only way you can ensure that your kiln temperature is showing correctly.
By putting Cones on every shelf, you can determine if there are hot and cold spots in the kiln, and if the power goes out, you will be able to continue firing if you have Cones in.
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.
Even with an electronic kiln, you should put Cones in every few firings to make sure the kiln is heating evenly and continues to fire at the right temperature as the elements age.


What Cone number should I use?


It is best to use the Cone number your glaze matures at, plus one Cone temperature above and one Cone temperature 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 below is very slightly bent, the Cone above is extremely bent over almost melted looking, and the middle Cone is bent to ninety degrees.


Which type of Cone do I want?


Cones called Jr. Cones are used in kiln sitters.
Standard Cones are used in Cone packs.
A Cone pack is usually made by taking three cones, imbedding them in some clay, and allowing the clay to dry before firing or use cone plaques which can be bought at your ceramic supply shop.
The three Cones include one at the firing temperature called the Firing Cone, one above called the Guard Cone and one below called the Guide Cone.
If you are firing at Cone 6, you would use a #5, #6 and #7 Cone.
Self Supporting Cones are used for placement in the kiln when you aren't using a clay pack or cone plaque.
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.
It is possible to use Jr Cones as witness cones but these cones require slightly higher temperatures to bend, so you won't be as accurate.


How exactly does a kiln sitter work?


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 three 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.
There is a page called Kiln Sitter that has further info about the subject.

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