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uneven heating in kiln and a multitude of funny glaze problems....

Hi all,

I just finished a glaze fire yesterday and the results were decidedly mixed. It is an electric kiln and I am firing to cone6, it also has a downdraft vent installed and uses a kiln sitter.

The top peep hole is the only one available as the bottom one is sealed up. The kiln load had 4 shelves, one on the bottom, one 4 inches above that, one 8 inches above that and the last one 6 inches above that.

I put a cone plaque with a 5,6 and 7 cone on each shelf. But because I'm limited to the top peep hole I can only see the cones on the very top.

The kiln sitter went off at around 2237 but at that point the cone 6 on the top shelf hadn't even moved yet.... so I kept the kiln going - roughly holding that temperature. I watched the cone5 go completely over and kept watching, at a certain point the cone6 was at about 3 o'clock when I shut it off. It proceeded to go to about 5 o'clock as it cooled then stopped moving.

When I unloaded the kiln I was disappointed to see that the shelf immediately below the top shelf had actually fired to cone 7, The shelf below that had cone 7 at 3 o'clock and the bottom shelf was cone 6 at about

3 o'clock. So the heating is very uneven. I'm not sure what to do about that.

Then there is the work itself. One big bowl with a store bought hunter green glaze has pit marks all over it. A celandine (spelling) glaze that should be blue is almost a transparent light green color. Another store bought glaze looks great but is blistered almost as if it was blowing bubbles but only inside the foot ring on the bottom. Other bowls look great ones with brushed on clear crackle (even though it didn't crackle) and others.

I would like to refire and save the pitted green bowl, can I do that?

Any other tips or condolences would be appreciated!!! (really only marginally bummed out)

Zander

Reply to
Zander
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Hi again Zander,

The temperature distribution you describe is quite common for your style of kiln. The top and bottom zones are usually cooler than the middle, because the lid and floor are absorbing heat rather than radiating it.

Many kilns have hotter elements in the top and bottom to overcome this problem - some have three controls for top/middle/bottom so you can adjust the middle one to slightly less than full to compensate. Or maybe you just have to live with it.

The effects you describe are variously underfired and overfired - when you know your glazes better, you can put them in the part of the kiln which gets the best results for each glaze. It's all part of the learning process - you will always get a few duds.

Pitting generally means underfiring, bubbling and cratering might be overfiring - but there is a whole range of problems and issues which can only really be diagnosed by seeing the pieces.

You can refire a glazed pot - it may come out better, the same, or worse - usually about 50% chance of improvement - nothing to lose anyway!

Don't be discouraged - pottery is always uncertain - there will always be successes and failures - but the occasional excellent pieces make up for all the failures.

Dave Coggins

Reply to
Coggo

Are you stacking with half shelves or full shelves? Make sure that you leave good space around the edges and the center to allow the air to flow easily through the kiln. With that vent you have, that should actually help you to fire more evenly. Is this a new kiln? What kin and model is it? Check your elements to make sure they're all working properly and that there are not bits of brick clogging the elements or the area where the elements are seated. Follow your instruction manual for proper shelf positiong. And make sure those middle space and edge space don't have handles and other parts of pots going into them. Make sure the pots stay within the confines of the shelf area itself, so as not to imped the flow of heat. Fire slowly. Often uneven firings like yours, when all other possibilities have been excluded, are caused by too rapid firings. Try firing much slower and you will probably see a big difference in your results.

Reply to
ShantiP1

It also helps to have taller stilts for the bottom and top spaces. I use whole shelves, usually use 6 inches or more on the bottom, stack the plates and shorter pieces in the middle, and leave about 6 inches on the top again. Your results could also be explained if an element burned out in the firing, near the top peephole, but I'm guessing you haven't fired enough to expect that yet. Brad Sondahl

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Reply to
Brad Sondahl

I have 3 full shelves and 2 half shelves, so I am usually creating 4 full layers, I've tried staggering the half shelves for better flow but it never seems to work out with the size pieces we are firing.

I fired faster this time than before, due to my turning down the flow on my downdraft vent which I though was drawing too much the temperature climbed at a faster rate, so instead of 8 hours to cone 6 I was around 7 hours.

I think I will try firing much slower next time, is it possible to fire too slowly?

Also, I've ordered a bartlett vc-6f controller for this kiln which I'm imagining will help a lot. Thanks to everyone for all the great advice.

Zander

Reply to
Zander

i also find filling up the kiln with extra furniture if you don't have that much product helps. i try to cap off the last layer with another kiln shelf.

...but don't use granite rocks....

steve

Reply to
Slgraber

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