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Kiln Pointers

Taking the Confusion Out of Pyrometric Cones

To make up for the many weeks that I haven't sent out kiln pointers, I am sending three in one day. If you want information on a particular subject, just let me know.

Pyrometric Cones Ceramists of the past judged when the firing was completed by the color of the heat and length of firing. In 1886, a German ceramist named Seger made clay cones that bent when the ware received the proper heat work. He positioned the cones on a shelf inside the kiln. By looking through a view port in the kiln, he could see the cones bend and knew when to turn off the kiln. His cones took the guesswork out of firing.

Today we still use Seger’s cones. They are small pyramids of clay and mineral oxide that soften and bend when exposed to heat. When consulting your dealer with a glaze problem, you should have a bent cone from that firing. The cone will help trouble shoot the problem. The cone lets you compare one firing to the next. On a digital kiln, a cone that bends less and less with each firing indicates that the thermocouple is wearing and will soon need replacement. Without cones it would be difficult to know that.

Cones are manufactured by the Orton Ceramic Foundation and available from your distributor.

Cone Numbers The most confusing thing about cones is the way they are numbered. But once you understand the reason behind the numbers, the system becomes clear. Pyrometric cones are numbered from 022 through 01 and 1 through 10. Cone 022 matures at the lowest temperature, and 10 matures at the highest.

Seger numbered his original cones from 1 to 20, with 1 being the lowest temperature. Later, cones of even lower temperatures were needed, so new numbers were added. The new numbers started with “0” and went from 01 to 022, with higher numbers getting progressively cooler.

To avoid confusion, think of the “0” as minus. Numbers without the “0” are positive. The higher the positive number, the higher the temperature. “0” numbers are negative. The higher the negative number, the lower the temperature. With this understanding, you can quickly see that cone 5 is hotter than 05.

The number is stamped on the base of the cone. The cone number for each material is usually stated on the label by the clay or glaze manufacturer. Your supplier can also give you the cone number.

Heat Work Cones are rated by temperature. But it is more accurate to think of them as measuring heat work, not temperature alone. Heat work is the combined effect of time, temperature, and the atmosphere inside the kiln. All these factors affect the maturity of your ware and not just temperature. For instance, firing to a lower temperature for a longer time will produce the same maturity as firing to a higher temperature for a shorter time.

Consult your supplier for recommended firing rate. (Rapid firing is like cooking: the turkey may be done on the outside but not on the inside.)

Using Cones Place the cone on a kiln shelf with the ware. Kiln wash shelves before placing cones on them. (I have had cones stick permanently to bare kiln 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. Cones come in either standard or self-supporting. Standard large cones must be mounted in a clay or wire plaque with 2" of the cone exposed above the cone holder. Self-supporting cones stand upright without holders. We recommend self-supporting cones; they are easier to use than standard large cones.

Handle cones carefully. If dropped, they may develop cracks that could affect their performance. Age and normal humidity do not affect the accuracy of cones. However, do not use them if they become wet.

I hope you found this information helpful!

With best wishes,

Arnold Howard Paragon Industries, Inc.

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