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Can You Reuse Underfired Cones?

The cones in the back row were underfired the first time and refired the second time with the fresh cones in the front row.


Can You Reuse Underfired Cones?

Reader Response: Kilns in a field, crop circles

Recent Q&As: When to turn off the Orton vent; relay life in a pottery kiln compared to a glass kiln

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Pyrometric cones are small pyramids of clay that measure heat work in ceramic firings. If the kiln shuts off too soon due to a power failure, can you reuse the underfired cones? Or should you partially unload the kiln and insert fresh cones?

Yes, you can reuse the cones if the kiln shuts off 100F / 56C or more below the point of maturity. It is okay to use the partially fired witness cones, even if they cooled back down to room temperature. The fired cones will bend at a slightly lower temperature than new cones, but this is okay, because the ware in the kiln has already received the same amount of heat work as the cones.

If the kiln shuts off within 100F / 56C of the cone’s maturity, and the kiln temperature drops 50F / 28C, do not reuse the cones. They will not bend properly. This is because a hard shell will form on the cones. If you are not sure how hot the kiln fired before it turned off, use fresh cones in the next firing.

The photo above shows self-supporting 015 cones. The cones in the back row were underfired to 1300F / 704C and then fired again with the fresh 015 cones shown in the front row. The sample pairs are from three different firings. As you can see from the pictures, the pre-fired cones bent slightly more than the fresh cones.



Two Kiln Pointers ago I wrote about the time we photographed kilns in a grassy field. John Troy of Baldwinsville, New York sent a scanned image of a Paragon catalog cover with the picture of kilns in a field. You can see it here:

Mario Miguel Echevarria of Longmont, Colorado wrote, “John Troy's scanned photos from the old Paragon catalog made me chuckle. The Facebook page close-up picture of kilns resting together in a field brought to mind a great idea for a prank. Anyone who is familiar with crop circles would appreciate that this would be a pretty frightening sight for a farmer who has never seen a kiln and just finished watching the Mel Gibson’s flick ‘Signs.’ I have a farmer friend who would be ripe for such a feat. But I don't think I could convince any of my ceramic artist friends to unplug and lug their kilns to his picturesque fields.”



Q. At what point in the firing should you turn off the Orton kiln vent?

A. It depends on what you are firing. Generally, you can leave the vent on throughout the firing and cooling cycle. The vent will help remove moisture during “candling” at the beginning of the firing. Though moisture will get into the fan and vent duct, it will dry out later as the air heats up. Turn the vent off when the kiln is cool enough to unload barehanded.

Another factor: The cooling rate can affect the appearance of glazes. If you want to slow the cooling, turn off the vent 30 minutes after the kiln has fired the ware to completion.

Also to be considered is quartz inversion. If your ware is breaking during cooling, turn off the vent before quartz inversion, which is around 1063F / 573C.

Q. Do the relays in pottery kilns outlast the relays in glass kilns?

A. (Relays send power to the elements of a digital kiln.) Pottery kilns fire much hotter than glass kilns. Nevertheless, the relays in pottery kilns usually outlast the relays in glass kilns. This is because glass firings typically include long holds and controlled coolings, where the relays cycle frequently. An exception to this would be a high ambient kiln switch box temperature caused by poor ventilation. That shortens relay life. Hotter firings can contribute to that.



"Every child is an artist. The problem is how to remain an artist once we grow up." --Pablo Picasso


On September 12, I visited the Rainbow Art Glass Open House. The Sunday in Farmingdale, New Jersey was overcast and tranquil. Plywood sheets across upright glass crates formed exhibitor tables. Artists wandered through the warehouse past bins of colorful sheet glass.

One artist told me, “Every time I open my kiln, it is like Christmas.” She used to keep the kiln in a 100-year-old carriage house behind the main house. “I would go out in my jammies in the snow to open the kiln,” she said. “It is always exciting.” A glass, ceramic, or enameling studio is a haven of joy in a sometimes troubled world.

You can see pictures from the trip at

Thank you,

With best wishes,

Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd., Mesquite, Texas 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 / / /

PRIVACY NOTICE: Under no circumstance do we share or sell your email address.

Copyright 2010, by Paragon Industries, L.P.

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