Kiln Lessons

Lesson #1

Observe the beauty of the over-fired mass…

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Anybody who’s ever fired a kiln knows the sickening feeling one gets when first opening a kiln and one is confronted with the above. While the molten lava often has beautiful colors and flow patterns it is a sad tale indeed to have to tell your students that their master pieces were reduced to a molten mass.

Most schools use earthenware clay that has a maturing temperature of around 1850 degrees and a melting point of around 2000 degrees. When pieces are glazed and the kiln is over-fired the glaze has a tendency to act as a flux and lowers the melting/flow point of the clay.

Most school kilns can attain a temperature of 2350 degrees; if the kiln load consists of earthenware and the kiln is left “on” beyond  the maturing temperature of the clay (around 1850) there will be a problem. As is obvious in the above pictures the kiln over-fired, but why, wha’happened? In this particular case, as near as I can tell, there was an explosion in the kiln.

Those things occur, but what a lot of people are not aware of is that when there is an explosion, all kinds of things can and will happen:  the explosion causes a zillion tiny clay/glaze particles and smithereens to float around in the kiln atmosphere. In time these particles and larger smithereens land on the shelves, on other pieces, in the element grooves and sometimes on the kiln sitter tube assembly. As the kiln gets hotter and hotter those small particles of clay and glaze melt.

Attached to something like the cone supports and or the kiln sitter tube these now molten particles fuse the cone, the cone supports and the kiln sitter rod in one big molten mass causing the kiln sitter to be inoperative. The timer on a kiln sitter is supposed to be a back up to turn the kiln off should the cone not do so. It is crucial that the timer is set to turn the kiln off minutes after the cone was supposed to.

Keeping a kiln log will show you that the interval between cone cut-off and timer cut-off can be reduced to as little as five minutes. During which time nothing serious in terms of an over-firing will happen.

Should the timer not have been set correctly and there is lots of time on the timer the kiln will just keep going until the timer runs out of time or the elements short out… In the above case the teacher was very fortunate that there was only a couple of hours left on the timer which shut the kiln off before it was totally ruined.

As it was a few wall sections had to be replaced as well as all the kiln furniture. By necessity all the elements and the kiln sitter tube assembly also had to be replaced, but when it was all over the repair was still a lot cheaper than a new kiln!

There’s a lesson here – you knew there’d be one, didn’t you?

Kilns are among the most expensive pieces of equipment in the art room. Should you not know how to properly operate the kiln or how to properly set the timer on the kiln sitter, I urge you to get a copy of my instruction manual. Please! Your administration will be grateful!

Click here to read “About That Melt-Down”.

Lesson # 2

Never, I repeat, never leave the kiln un-attended!

The above does not mean that you have to sit in front of the kiln for the duration of the firing. It does mean that once you figured out (via that kiln log!) when the firing is supposed to be done you be there! So you can turn off the kiln should for some reason the kiln sitter does not do its job.

It cannot be over-emphasized that keeping a kiln log and getting to know your kiln is of the utmost importance and really, when you come right down to it, it does not take very long to get to know the characteristics of your kiln. Firing a kiln is not rocket science and common sense goes a long way to understanding that toaster oven on steroids!. Should you, after reading the manual still have some concerns, don’t hesitate to contact me. I’ll be glad to help!

By now everyone knows that I’ve been around kilns for a while. It never ceases to amaze me how little those who are trained to teach the next generation know about firing kilns…  This being MY web site and being an opinionated sort of guy I can spout off a little. Again!  😉

The other day I was called upon to repair an Amaco AH 25SF. Smaller cousin to the AH 30SF, the “25” is on of the larger front loaders. Like most Amaco kilns this model too carries a five-year warranty and is one of the more popular school kilns. As is the case with the AH 30,  the “25” has elements in the floor. Highly recommended by the factory and “those in the know” (like yours truly…) operators are encouraged to place a false bottom on top of 1” shelf supports on the floor of the kiln to protect the elements from possible explosions and the resulting “smithereens”.

Now everyone knows that “explosions” happen, either because the kiln is fired too fast, or the ware in the kiln was not quite dry when the firing cycle began… It is one thing to have explosions, quite another thing to not realize that the resulting debris and smithereens need to be cleaned out…. The kiln pictured below suffered multiple explosions.

At no time were the smithereens vacuumed out. As a result  the bottom element grooves filled up with glaze and clay particles. So many explosions took place and so many smithereens were the result that the element grooves filled up to the point of embedding the element coil in lava… Each subsequent firing resulted in more melting and re-melting  of the lava and it should be considered a miracle that the heavy-duty elements lasted two years before giving up the ghost…

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Warranty does NOT cover this blatant kind of disregard of basic house keeping rules and operating guidelines!  Good thing Amaco is such a nice company to do business with! Amaco kilns are built to withstand abuse and they can be repaired to top notch “as new condition”. It would of course be better if the universities did a better job of training our future teachers in the correct operating procedures of these very expensive pieces of equipment! The AH 25 retails for “only” $ 5495.00 and like all AH or HF Amaco kilns has a lifespan of forty to fifty years IF PROPERLY TAKEN CARE OFF!

Ah well, as I mentioned earlier, things like this keep me in business… It took some doing, but once the floor brick and all the lava was chiseled out, and the floor was rebuilt with new brick and new bottom elements, were installed the kiln was like new again. As part of the repair, the operators also received a “mouthful” as well as a copy of the manual “Of Clay, Glazes and Kilns” in which basic operating procedures and a few other bits and pieces of information are discussed.  To purchase YOUR copy of “Of Clay, Glazes and Kilns” please click this link.