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Should You Use Witness Cones in Ceramic Firings?

A pyrometric 7 cone photographed as it bent. The photo was shot through green #3 safety glasses.


Should You Use Witness Cones in Ceramic Firings?

Reader Response: 220 volt appliances on 208 or 240 volt circuits

Recent Q&As: Firing rate compared to driving a car

Memorable Quote



“I received a frantic phone call from my great granddaughter,” wrote Bonnie Staffel. “The controller on her kiln shut off with an error message, and she didn't know how to tell if her pots needed to be refired.

“I am distressed to find so many potters relying on the controller to do everything when in reality it only turns the kiln up in stages and off when the temperature is reached or there is an error,” Bonnie continued. “I reminded her to always put three cones in the kiln--one below, one at, and one above the desired result in heat work. Especially newbies feel they can work without cones. I can't emphasize this habit enough and I am sure you do also. This along with knowing the color of the heat in the kiln will give potters great control over their firing practices.”

Through the distant centuries, ceramists sensed when to turn off the kiln by observing the color of the light in the kiln and noting the firing time. Eventually cones were developed to bring greater accuracy to firing. Cones are small pyramids of clay and mineral oxide that bend when exposed to heat.

Ceramists are not the only ones who use cones. Cones can be used to check the heat distribution in glass or heat treating kilns.

Even if you fire a digital kiln, you should still use cones, as Bonnie Staffel suggested. Bent cones help to trouble shoot problems. Even when there are no problems, cones are still useful, because they help you systematically to compare one firing to the next. On a digital kiln, a cone that bends less with each firing indicates that the thermocouple is wearing and will soon need replacement. Without cones it would be difficult to know that.

Kiln wash shelves before placing cones on them. Otherwise the cones may stick to the bare shelves. The cone slants 8 degrees from vertical and bends in the direction of the slant. Place the cone so that it will not touch nearby ware as it bends.



In last week’s Kiln Pointer, “The Kiln That Wouldn’t Reach Temperature,” I wrote, “While most of Europe uses 220 volts, it is not standard in the U.S.”

Rick Shimek, a retired electrical vocational instructor, wrote, “UL requires a listed appliance to behave normally within 10% of its rated voltage. This would allow a 220 rated appliance to work within the range of 208v to 242v. Also, in Florida, and probably elsewhere, the power company is required to deliver no less than 7 1/2% of rated voltage. (193v for 208v; 222v for 240v). I hope Paragon isn’t rating kilns at 220v. I fire mine daily and love it.”

We don’t rate kilns at 220 volts. The models that come in 240 and 208 volt versions use different elements so that with either voltage, the wattage is close to the same.



Q. I don't understand why a temperature controller would overshoot the program on the ramp up.

A. If you're driving 80 miles per hour and slam on your breaks, the car will slide some distance before it finally stops. The same concept goes for heating a digital kiln. If you program the kiln with a full rate (as fast as it can go) to a low temperature, it will have a hard time "slamming the breaks" and stopping at exactly 135F. It will bypass this temperature, but eventually level back out within about 10 - 15 degrees of 135F.



“One of the main findings from my past research is that warm, close relationships are vital to the overall health and well-being of human beings.” Robert Waldinger, Director of the Harvard Study of Adult Development


Richard Lackey passed away a few weeks ago. Richard attended more Paragon kiln maintenance seminars than anyone else including the seminar teachers and our customer service representatives. We will miss him. The seminars will not be the same without him.

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 2017, by Paragon Industries, L.P.

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