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Re: [teacherartexchange] kiln failure

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From: Marvin Bartel (marvinpb_at_TeacherArtExchange)
Date: Sat Apr 28 2007 - 06:08:21 PDT


>I . . . put in a fresh cone and refired it. To my great dissappointment when I opended up the kiln it had melted all my students work. The cone no longerexhisted.
>
>Did this happen because my sitter or temp gauge failed? I have never had this happen to me. I am so upset and I know my students will be upset as well.

I hate to learn this way, but it has worked for me and some of my best students.

CONE SITTER PROBLEMS
Generally, this happens because the previous firings have produced some fumes that have been escaping through the small openings around the rod that lays on top of the cone. As the fumes exit, there is some condensation of the volatile chemicals that are in the fumes that escape from the kiln. As the vapors condense, they eventually gum up the mechanism and the rod gets stuck. Hence, it does not move as the cone melts. It does not release the weight that has to drop down to turn off the kiln.

Some variation in firing can happen if the cone is not centered in the sitter, but it generally does turn off eventually. There is a little plate on the outside that can be adjusted (two screws) to make sure the trigger hook operates without friction (unless it gums up). There is a metal trigger piece in the weight that slides up and down (one small set screw tightens it) to adjust the amount of movement needed to release the hook as the cone melts. You can set to require less cone movement.

ROUTINES THAT HELP
Every time I insert the cone, I first wiggle the rod to be sure it feels free and easy. I also check that is centered between the cone supports. It generally takes quite a few firings before the mechanism gets gummed up.

PREVENTING THE GUM UP
If the kiln produces fumes, installing a bottom mounted vent will help. This creates a vacuum in the kiln so fumes do not exit though the mechanism.

In some kilns, I have to place the sitter lower in the kiln. This helps keep it clear. The chimney effect tends to make any small openings in the top portion exits for fumes. The bottom openings tend to be entrances for room air.

SAFE PRACTICES
Computerized kiln controls are better, but a person must still manually check the kiln shortly after it is expected to switch off. I do not fully trust any un-redundant system or person. When I fire, I place a small piece of masking tape on my wristwatch with the letter K on it. When students fire, they have to phone me when the kiln is off so I can remove my piece of tape. It saves the kiln while allowing others to learn how to do it.

Always include at least one witness cone (a cone you can visually check in front of one of the peeps). I set it in clay that includes a spoon shape to catch the bent cone. Keep the cone clay quite thin so it does not explode in a glaze firing. If the clay is wet going in, I poke many pin holes in it to vent the steam from the thick parts.

Marvin

Marvin Bartel, Ed.D., Professor of Art Emeritus
Goshen College, 1700 South Main, Goshen IN 46526
studio phone: 574-533-0171??
http://www.bartelart.com
http://www.goshen.edu/art/ed/art-ed-links.html
"Art is me when I am myself." ... a kindergarten girl when asked, "What is art?"
"You can't never know how to do it before you never did it before." ... a kindergarten boy working with clay for the first time.

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