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Uneven Kiln Firing


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#1 LauraT

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Posted 11 October 2010 - 09:30 PM

My electric kiln fires an entire cone higher in the bottom of the kiln than the middle or top. It is a Paragon J-27 & has only fired 20 times. Firing to cone 5 with a hold of 20 min. makes for beautiful pots, but the middle & top shelves don't vitrify. The pyrometric cone shows the bottom fires to cone 6. Today I fired a Cone 6 program w/ a 5 min. hold. I didn't pack the bottom of the kiln tightly but it still it fired to cone 7 (and ruined my pots). :( What to do? What to do? Thanks!



#2 Lucille Oka

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Posted 12 October 2010 - 03:56 AM

There are several things you didn’t mention-- did this happen in your previous firings or is this the first time this has occurred? If so, what have you done differently? Is your kiln a zoned kiln? Check your manual. And do you have the correct size kiln shelves recommended by Paragon? It sounds like the heat is being trapped in the bottom of the kiln. Improper stacking can be a cause for the uneven firing. To help resolve this try using half shelves that when put together they are at least one and a half inches smaller than the interior diameter of your kiln to allow the heat to move up and allow for a gap between the two shelves when they are stacked at the same level. Stagger some of the shelves and try not to cover up the gap so heat can rise. Also this is very important be sure you leave a 1” space between the kiln floor and the first shelf. Do not cover up your elements with the shelves. Make every attempt to keep shelves between the elements. Be sure your peephole plugs are in place except for the top one. It allows heat and fumes to escape. Keep a log of every firing by date, time and program. List which pots are in the kiln on each shelf and be sure to note the clays, glazes and underglazes in your log, so that you can repeat your successful firings. Be sure to place the correct cones at the correct angle if not using self-supporting cones. Remember junior, senior, and self supporting cones all have different firing temperatures and firing speeds. Be sure to test new clays and glazes before you commit to firing your fully decorated load. *Most importantly read your kiln manual for trouble shooting.*
Lucille Oka

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#3 LauraT

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Posted 12 October 2010 - 04:39 AM

This is not the first time this has happened. I am using 1/2 shelves, & have been staggering heights. Shelves are off the bottom of kiln & I always take care not to block a heating element. I could however, probably add more space between shelves & wall of kiln. And I was told by kiln repairman, not to remove top peephole. Kiln is vented w/ Orton vent. Not sure about zoned kiln... I'll investigate. Thanks, Lucille.



#4 Deb Evans

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Posted 18 October 2010 - 11:38 PM

Hi Laura, I looked up your kiln - it's a glass and clay kiln, has floor and lid elements. I'd reverse load = tall stuff on the bottom going to small stuff on the top.
Is this computer controled or switches? How is it vented?
An attached vent/exhaust system will probably even it out.

might also try increasing 1st shelf distance to 2" off floor.
And of course, ck in w/ paragon - they know their equipment.
Good luck




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