About That Melt-Down

Learn from this!

Having repaired a few kilns I often am asked, “What happened”? A veteran teacher who KNOWS his kiln sent the following pictures to me. The kiln in question is a twenty one year old AH 30 that is well taken care of, is always fired according to “all the rules and regulations” and looked like it was five years old before this recent mishap.

The over-firing was a true mystery. Some pots became lava, while other pots in the same firing turned out just fine, yet others showed some sign of distress. “Wha’happened?”

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While at the school to repair the damage I talked at length to both the teacher and the students and observed the way the pots had been made. Students would use # 58 Amaco moist clay and formed an old-fashioned, simple thick-walled crock using a jiggering machine. There was nothing mysterious or particularly difficult about the process.

Once dry, the pots were sometimes bisque fired, some times glazed and then fired. As stated above some pots survived just fine, thank you, while others in the same firing were de-formed, misshapen and/ or showed some kind of internal stress. As visible in the center of the melted lava covered shelf above, that pot “survived”.

Albeit with a broken bottom. The two that had been sitting on the front part of that shelf also survived…

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Cross section view of one of the element holders – talk about flowing lava!

 More investigating yap-flap: “What do you do when you make a pot?” “Well, we take the clay from the carton, add some more water to make it softer so we can use it on the jiggering machine. “ To successfully make a pot on a jiggering machine very soft clay is required. “OK, so now you are making the pot, what do you do with the left over soft clay once the pot is made?”  “It goes in that plastic bag over there so the next person can use it.”

Eventually that plastic bag gets kind of full with soft clay… “OK. So now you have a dry pot. Gonna glaze it?” “Yes”. Meanwhile we’re still fixing the kiln and thinking. Chipping out that lava plus the remains of the six 245.00-dollar (that’s for each!) elements and holders is both tedious and dangerous. Chisel and hammer… all that flying glass… better wear a complete face mask!

Cleaning out the kiln took longer than re-installing the five floor elements and one side element. Now to fit the firebrick “ribs” in between…

Still thinking about those melting pots…

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Fully operational again, the twenty-one year old kiln is ready to serve for another twenty-nine or so. (Talk about kiln value!) Shortly afterwards, at the Amaco factory, the ceramic engineers, the kiln department foreman and yours truly examined the shelf  and the pots and discussed  the matter at hand. “What happened?”

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a surviving pot

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“weird” stuff in another

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That’s gotta be glaze!

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what glaze would do inside a wall

Several scenarios are possible and from the looks of the surviving pots very likely:

Scenario 1

Someone glazed a green pot and while doing so broke it. The remains were put in the above-mentioned “left-over” bag. The dry clay pot pieces,  with glaze on them, soaked up the excess water in that bag and became part of the left over slurry…. To be used by someone else to make a  jiggered crock… Undetectable at that point, the glaze that had been on the dry green ware had become part of the clay… during firing it became a flux with the disastrous results.

Scenario 2 

Someone cleaned up the glazing area with a wet sponge and decided to squeeze the sponge over the bag containing the left over jiggering clay.  Someone else then used that clay to make a crock …

And then, based on years of experience in matters such as these “Mr. Amaco” and the “people-in-the-know” unanimously concluded the following:

The clay had to have been contaminated with something that lowered the melting point of the clay and caused the meltdown, even though the final temperature was only 1900-some degrees. That “something” more than likely was glaze – where it came from or how it was mixed in with the clay will remain a mystery and the over-firing was ruled an unfortunate accident.

End of story.