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Top shelf of kiln underfiring


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Hi!

I have run my used kiln several times now (cone 04 bisque and cone 6 glaze) with orton cones on each shelf and have noticed that my top shelf appears to be underfiring slightly.  I haven't completely filled the kiln yet and only had work on the lower two shelves which seem to be firing up to temperature. I suspect that this is because I am leaving the top peep hole open.  I have read that it is important to leave the top peep hole open but if that is causing the top shelf to underfire it seems like that will be an issue in the future when I want to fire a full load. Any advice on how to solve this? 

Thanks!

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Leave it out for the first 1000F degrees, then close it up. See if that solves the problem. If not, leave it open as it's good to have oxygen in the kiln. Are you running a downdraft vent? If so, keep it plugged for the entire firing so it doesn't spoil the draft.

If you're loading the kiln and it's not a full load, spread the load out more or less evenly throughout the kiln. Having an empty section will affect how evenly it heats.

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9 hours ago, neilestrick said:

Leave it out for the first 1000F degrees, then close it up. See if that solves the problem. If not, leave it open as it's good to have oxygen in the kiln. Are you running a downdraft vent? If so, keep it plugged for the entire firing so it doesn't spoil the draft.

If you're loading the kiln and it's not a full load, spread the load out more or less evenly throughout the kiln. Having an empty section will affect how evenly it heats.

Just curious, why the first 1000 degrees? Moisture leaves long before that. Just looking to learn here. TY

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6 hours ago, rox54 said:

Just curious, why the first 1000 degrees? Moisture leaves long before that. Just looking to learn here. TY

Chemically bound water leaves at about 1100 degrees. More oxygen better for all firings to oxidize as intended so whether bisque and helping remove contaminants or even in glaze firing, generally 1000 degrees is good practice and not overkill so as to waste a bunch of energy. 

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6 hours ago, rox54 said:

Just curious, why the first 1000 degrees? Moisture leaves long before that. Just looking to learn here. TY

There are also organic materials in the glaze that burn out. The longer you can get air flow in the kiln the better. We've seen lots of examples of how down draft vents improve color development in glazes since they pull fresh air into the kiln.

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On 5/13/2021 at 3:28 PM, neilestrick said:

There are also organic materials in the glaze that burn out. The longer you can get air flow in the kiln the better. We've seen lots of examples of how down draft vents improve color development in glazes since they pull fresh air into the kiln.

Thank you for the replies. Just to be clear, with an Envirovent, I should keep the peeps in, but if you are not using a downdraft vent, keep the peep out til 1000 degrees?

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You might have a element that is worn out.   Did you replace the wires when you bought the used kiln?   The top ring of a firing is usually hotter than the middle and bottom rings.  Denice

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