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Lessons from a Kiln Repairman

Dave Coggins, a friend in Queensland, Australia, kindly shares kiln stories. Each one has a lesson that you might one day remember and find useful.


Hearing your story about carbon blackening the interior of a kiln reminded me of incidents that happened to me during my kiln repair days.

I have had quite a few frantic calls from customers after flammable organic material was accidentally introduced into the kiln. When smoke started pouring out, they shut down the firing in a panic. After everything cooled off and they opened the kiln to be greeted by a sooty blackened interior, they called me saying their kiln was burned to a crisp, everything was destroyed, a total disaster!

If only they had just ignored the smoke and kept firing, there would have been no problem at all. By the time the kiln reaches red heat, all the organic materials are burned away.

There were lots of phone calls from customers about the dreaded overfiring. It didn't matter how many times I stressed that kilns should never be left unattended, especially at the end of the firing, people still completely trusted the Kiln Sitter or automatic controller. I must say that electronic controllers were usually quite reliable; the major cause of overfirings in ceramic kilns was the failure to carry out any sort of maintenance on the Kiln Sitter.

I have seen kilns where the Kiln Sitter rod was shaped like a banana and the cone supports were almost burned away. The customer said, "It still works OK. Don't replace anything!" Needless to say, there was wailing and gnashing of teeth when the kiln overfired on the most vital load of the entire year.

The Moral - ALWAYS stick to the recommended Kiln Sitter maintenance schedule.

One of my favourite stories is about the doll artist whose kiln overfired, and when we opened the kiln, we were greeted with all the melted and flattened little doll faces peering accusingly up at us from the kiln shelf, as though they were saying, "Look what you have done to us. We're all melted!" It was like the Wicked Witch of the West melting, with just the eyes looking out, and it was really a sad and pitiful sight. But we couldn't help laughing. I wish I had taken a picture.

It's amazing how runny ceramic becomes at a high enough temperature. I saw a few meltdowns in ceramic firings where the entire load ended up as a solid 3"-thick slab on the kiln floor. Of course the kilns were complete write-offs. Many people don't realise that overheated liquid ceramic or glazes actually dissolve the firebrick used in most kilns, like hot water on honeycomb candy. Once the ceramic melts enough to run down the bricks, the kiln is usually beyond repair.

Even pottery clay can melt. When we were teaching pottery, one time a thrown pot melted completely into a bubbly mess on the shelf at stoneware temperatures. Of course none of our students owned the pot. We didn't find out who the owner was until years later when we were looking at a bit of the melted pot and saw someone's initials!

There were also meltdowns in glass kilns as well. One of my customers had a very successful business screen-printing drinking glasses with company logos for breweries, hotels, etc. He fired the printed glasses in a modified front-loading, ceramic-fibre-lined pottery kiln. He had automatic controllers to switch the kiln off, but one day one of the controllers failed and the whole load of several hundred glasses melted down to a large slab of solid glass in the bottom of the kiln. Fortunately, the glass didn't stick to the fibre too badly, so we were able to rescue the kiln by removing the slab in several pieces. But he had to explain to his customer where several hundred of their glasses had gone! After that, he had me install a back-up system to shut down the kilns in case of an overfire. You can't have too many back-up devices.

Even with the best control system, I still recommend that every kiln should be checked when it is due to shut down. We could never sleep with a kiln firing. Many times we got out of bed in the middle of the night to make sure the kiln had switched off.

I used to build a few kilns as well as repair them, and one of my good customers had two of my 7 cubic foot top loading, brick lined kilns. She used to fire pottery to high stoneware--1300 deg C or 2370 deg F--using a programmable controller. One day the controller went a bit crazy and overfired the load, soaking at maximum temperature for many hours. The result was a solid block of pottery, props and shelves.

The kiln itself wasn't in too bad condition, but we had to get the load out without further damaging the bricks. Nothing for it but to lift the whole lot in one go. You just can't imagine how heavy 7 cubic feet of pottery, props and shelves really is. I just managed to get it out, but my back has never been quite the same. We dragged the heavy lump outside, and the customer used it as a decoration in her back yard. It certainly was an unbreakable sculpture!

Many of my customers were schools and colleges. Among these were a few Art Colleges that could be relied on to destroy kilns at regular intervals with their "experimental" firings. Many foreign substances were introduced into pottery just to "see what happens," the results of which was usually a call to me to remove molten material from the brick or ceramic fibre floor or wall, or worse still, from among the elements.

Many strange items were introduced, including rocks and minerals of all types, various types of wire, and in one case a kitchen tap (faucet)--it melted! But the best of all would have to be the student who decided to make some sort of peculiar artistic ceramic statement using a bicycle. Yes, a full-sized bicycle complete with all fittings, and in quite good condition as well. She coated the entire bicycle with clay slabs held on with wire, then placed the entire mess--sorry, artistic creation--into a kiln and fired it to stoneware temperature. The kiln was a large gas trolley kiln lined with bricks, so luckily there was no damage to the kiln. But the bicycle was a different story. By all reports, the firing of the bicycle was accompanied by a considerable amount of smoke and a horrible smell. I saw the result of the firing, and it was a sad and sorry looking artistic statement as well as a waste of a perfectly good bicycle. I can now state that a bicycle frame will not melt at 1300 degrees C!

I have another story from our own painful experience. Some people approached us to test fire a new product that they were developing from recycled materials. We were very suspicious, of course, and tested small samples in a test kiln. The results were quite OK, so we agreed to do a larger quantity in our big kiln. Unfortunately we didn't allow for the fact that a large mass of the material would not heat sufficiently quickly and get enough oxygen to burn properly, and the kiln produced very large volumes of foul smelling smoke, much to the annoyance of our neighbours. Even after we shut down the kiln, it continued to smoke for several hours due to the material's own self-combustion, and as a result several very expensive kiln shelves were cracked. Needless to say, NO further experiments were carried out!

Of course, the moral of all these stories is NEVER FIRE ANY FOREIGN MATERIALS IN A CERAMIC KILN.

I have a bad story against myself as well. It was many years ago when I was learning to build and repair electric pottery kilns by re-assembling an antique front-loading brick pottery kiln for my wife. The kiln's elements were mounted in two large cast ceramic plates, one on each side of the kiln. Each plate had many grooves cast into its surface to hold the elements. The original plates were broken, and I needed to make new ones. I made a very fancy casting box to reproduce them, and purchased some castable cement.

I cast two plates and cured them slowly as per the directions. When they were dry, we fired them in our gas kiln over a 48-hour period, and the result was two beautiful cast element plates. I assembled the kiln, fitted the plates and the elements, and did a test firing. Great--the kiln worked perfectly. Another firing, this time to a higher temperature. Everything went really well until we got to about 1100 degrees C (2012 degrees F). Then some strange noises started coming from inside the kiln. Very peculiar--more noises--then the lights in the kiln room started dimming. There was obviously an electrical storm going on inside the kiln; time to shut down.

When the whole glowing mass cooled down, I discovered that large chunks of my beautiful new element plates were melted along with several of the elements. What had gone wrong? This material was rated to over 1200 degrees C--plenty of margin.

After a few enquiries the truth was revealed: The components of the castable included iron, and at that high temperature the iron in the element plates allowed electrical current to flow between the elements THROUGH the castable material. This was a large kiln, with beefy wiring, so the results would have been similar to an electric arc welder: The castable had melted and run like water!

Needless to say, the next castable material I used for the element plates had ZERO iron content, and the kiln lasted for many years’ service. ------------------------------- 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 /

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