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Cracks in Firebricks


Cracks in Firebricks

Reader Response: a Christmas note

Recent Q&As: Program Review, cooling segments, glass slumping


A customer recently asked, “I have had my kiln for only a few months and I am starting to see hairline cracks in the firebricks. What is happening? Should I be concerned?”

Insulating firebricks are a miracle of physics. They routinely withstand temperatures over 2,000 degrees F. That is hot enough to melt most metals: copper, bronze, brass, and aluminum.

We are accustomed to using products that stay new looking for years—cars, furniture, cameras. But kilns are different. The high temperatures they reach generate tremendous stresses. Since the insulating firebricks expand and contract with each firing, hairline cracks will appear in the bricks while the kiln is cold--even in a new kiln. Do not be concerned with these. They are normal. The cracks close tightly when the heated bricks expand. The cracks function as expansion joints.

The same applies to ceramic fiber insulation.


Thank you for the good wishes that many of you sent at Christmas. They were very touching.

Charlean Wilson wrote, “Opening the kiln is always a surprise--good, bad, or WOW, it is always a surprise. My husband and I have really gotten into glass. I have entered a craft show, and I have made many Christmas presents in my kilns. Everyone enjoyed their gifts. (We were still firing on Christmas morning!) Not bad for beginners. The QuikFire and Fusion-8 Paragon kilns were last year's Christmas gifts.”


Q. When I used Program Review, my digital controller showed a target temperature of 964 that I don’t want. How do I get rid of it?

A. Program Review shows the program that you last fired. If you have entered another program since the last firing, then that is the program that appears in Review.

It is possible that a target temperature of 964 was inadvertently part of the last program you entered. I wouldn't worry about the 964. I would simply reprogram the kiln. When you enter a new program, the old one is automatically removed from active memory.

Q. How do you program a cooling segment that will go from the fusing temperature, 1450 degrees F, to the annealing temperature, 970 degrees F?

A. If you want a fast temperature drop, program a rate of 9999 in the Sentry 12-key controller (1799 F rate for the 3-key Sentry Xpress controller). The kiln will cool down at an uncontrolled rate, meaning as fast as it is capable of cooling with the elements turned off.

Q. What is glass slumping? Is it like melting glass?

A. Glass slumping, or sagging, is the process of heating glass over a mold until the glass softens and sags into the mold. For example, if you heat a round glass shape over a bowl, the glass will take the shape of the bowl at high temperatures.

You can also lay a sheet of glass over a large hole in a kiln shelf. As the glass heats, it will sag down into the hole, forming a shape similar to a vase.

Q. I would like to melt stained glass into jewelry. Which is the best kiln for this and how much electricity do the models use?

I suggest a small kiln such as the Paragon Caldera for melting stained glass into jewelry. If you combine different colors of glass, they must be fusing compatible. This means that the glass expands and contracts at the same rate. If the glasses are incompatible, they will break apart after the piece cools. A small kiln uses about $1.00 worth of electricity per firing.

Thank you for reading the Kiln Pointers. I wish you a successful, creative 2007.

Thank you,

With best wishes,

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

Copyright 2007, by Paragon Industries, L.P.

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