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How to Estimate the Kiln’s Natural Cooling Rate

After last week’s kiln pointer, several people asked more questions about slow cooling.

Q. Why would one need slow cooling?

A. With some projects, it is necessary to control the kiln’s cooling rate. For instance, fusing thick glass pieces requires slow cooling to avoid cracking the glass. Certain ceramic glazes look better when cooled slowly after the kiln reaches maturity.

Q. I get an FTC message on my Sentry controller when I use a slow cooling rate.

A. Most digital controllers can slow down the cooling rate. But if you program a cooling rate that is faster than the kiln’s natural ability to cool, you may get an error message. For instance, Paragon’s Sentry 2.0 will flash FTC (Failed to Cool) if the programmed cooling rate is too fast. To avoid that error message, estimate your kiln’s natural cooling rate:

During a typical firing, write down the temperature and time that your kiln fired to maturity. Let the kiln cool naturally. Every two hours, write down the temperature shown on the digital controller. If you have a switch-operated kiln, you will need a pyrometer to read the temperature.

To figure cooling rate, subtract a temperature from the one that came two hours before. Then divide by two hours. That is the hourly rate.


The kiln shuts off at 2200°F.

After two hours, the temperature falls to 1800°F.

2200 minus 1800 equals 400

400 divided by 2 hours equals 200

In this example, the cooling rate is 200° per hour. The cooling rate may get slower the closer the kiln gets to room temperature.

To avoid the FTC (Failed to Cool) message, program a slow cooling segment at a slower rate than your kiln’s natural cooling rate.

Wishing you a great fall season,

Arnold Howard Paragon Industries, L.P. 2011 South Town East Blvd. Mesquite, TX 75149-1122 972-288-7557 / 800-876-4328 Fax: 972-222-0646 /

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