Heat stroke

So, I had some other clever title in my mind when I went to sleep last night, but for the life of me I can’t remember what it was. It was something humorously ambiguous about the benefits (and losses) of getting really hot. This was not it….

Suffice it to say that I over fired my kiln. Again. I have just not started this run-in to the Holidays very well….

The first two “Oops!”s were me forgetting that I was even firing the kiln and suddenly realizing that I needed to check things. A bisque was okay after I came out and shut the lid about 400 degrees after I normally do, but my first glaze kiln of the season I completely forgot I was firing, and had to yank myself out of the kitchen when in the midst of pouring a cup of soymilk I realized I was supposed to be doing something. Doh! Turns out I had overshot my target temperature by about 50 degrees, and the much cooler bottom had not been given a chance to even out. Even with the lower melting glazes in the bottom only a handful came out alright, and the top three zones were just a bit hotter than I would have liked. Most survived okay. I use my least favorite pots in the first load anyway, so the risk was limited this firing….

The kiln I unloaded yesterday was not as lucky. This time I sat on it at all the crucial junctures of temp and got everything to where I thought it should be. “A perfect firing!” I thought. But it turns out the bottom was slow enough getting to temp that my stalling of the upper parts still dramatically over fired two glazes in particular. On the insides of pots one boiled. On the outside of pots they both ran…. Crap! That was about two dozen pots! A handful were able to get enough flattening with my bench grinder that I think they are alright (sort of), but many were not….

Not picture: two otherwise beautiful serving dishes that have the glaze boiling on the interior....

Not picture: two otherwise beautiful serving dishes that have the glaze boiling on the interior….

Its a good thing I use wads. Its a good thing I have drip edges along the bottom of the walls.

The blue glaze on those front cups is a new one for me and I haven’t figures out all its tolerances (obviously). In the previous round of firings I had left the bottom of the kiln unloaded and not worried too much about getting temp down there. My target temp (which I even played conservatively this time) had far less time spent hovering just below it. This time I was supposed to fire some pots for a friend in a glaze that could only be fired much cooler than the other glazes I use, so I had to try to get the bottom to at least a temp that those glazes would work. Turns out I sacrificed the rest of the kiln to do that….

Of course I could replace those bottom elements, but the ones in there are only about two dozen firings old. Way too early for them to crap out on me, I thought. They still look pretty good too. For the price of a new set of elements I could practically run the rest of my firings for the season without using that bottom zone to load pots in. I gotta think about this…..

At least my two test flasks (thrown as demos during class) were able to get resuscitated!

Sometimes demo pots can turn out worth keeping

Sometimes demo pots can turn out worth keeping

Gotta go check the kiln I’m firing now, so this is it for the present. Wish me luck!

Peace all!

Happy potting!





About Carter Gillies

I am an active potter and sometime pottery instructor who is fascinated by the philosophical side of making pots, teaching these skills, and issues of the artistic life in general. I seem to have a lot to say on this blog, but I don't insist that I'm right. I'm always trying to figure stuff out, and part of that involves admitting that I am almost always wrong in important ways. If you are up for it, please help me out by steering my thoughts in new and interesting directions. I always appreciate the challenge of learning what other people think.
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6 Responses to Heat stroke

  1. Even with a computer controlled kiln that evens out the temps in different sections you can still overfire! This last glaze load for the Guild had wine coolers that the Guild was donating for a fundraiser for African grandmothers looking after children whose parents had died of AIDS and some pieces for a member who was having her first big sale. Instead of programming a hold of 8 minutes somehow I put in 8 hours! Fortunately I went in to check the kiln after about 4 hours into the hold so the damage was not as bad as it could have been – and I was surprised how well some of the cone 6 glazes held up, though there will be lots of shelf grinding!

    • Glad I’m not the only one!

      But seriously, I hope most of those pots can be salvaged. I’m always sad to hear of the losses that other potters suffer, and it seems even worse knowing that many of these pots were allocated for a good cause. Potters always seem to struggle through. We are used to our own losses. Part of doing business, I guess. But that some of those pots were for a donation really is disappointing…..

      Hope all turns out well!

      Good luck!

  2. yvettedelacy says:

    I forgot a kiln not long ago as well! I was on the computer late at night when it dawned on me MY KILN!! it overfired by few hours and my work was blistered and handles droopy… I haven’t sold the bumpy pieces but have sold all the cups at markets as seconds!

    • Yikes!

      Glad you were able to salvage some of them. Some of my glazes have a real tendency to blister/boil if it gets even slightly above the zone of when its actually hot enough….. A few disasters like that once prompted me to start using glazes with a wider heat range. I have some glazes that go from below cone 6 to above cone 11. The problem is that most of these glazes are boring, and i eventually decided it was worth the risk of losing swaths of pots to end up with even a few that I really liked…..

      Ah, the life of a potter!

  3. Scott Cooper says:

    I wonder if this is particularly a hazard of electric kilns? I’ll occasionally forget I’m firing an electric bisk, but never a glaze firing in the gas kiln. And wood… I mean, zero chance, right?

    • Yeah, I think electric kilns are too related to toaster ovens for us to get freaked out by. I used to have nightmares about coming back to a gas kiln I was firing and discovering the door had blown off and flames were billowing out into the ceramics studio. That being said, the guy on the late shift of the woodkiln I used to help fire was notorious for dozing off in the wee hours. At least with the wood kiln you couldn’t really leave the site while on duty.

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