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Kiln firing unevenly.


brettwulc
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I have a skutt 231 (a bit old) with newer elements and sitter has been calibrated, sensing rod looks fine. The whole kiln received maintenance before I purchased. 

Everything seems to operate well, the bars seem to melt properly, and shut off the kiln each time. 

But the kiln seems to fire hottest on top and gradually gets cooler as you move down. With my last cone 6 firing the top cone was bent a little past cone 6 where the bottom cone was not bent at all. I have a multimeter coming in the mail to test the elements, and i've been playing with the density of pots throughout the kiln - but keep getting the same results. 

Should I be getting bars a cone or two higher of my desired temperature?  

All input greatly appreciated, thank you!!

 

(This cone was from a recent 06 fire, this being the top shelf.)

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Edited by brettwulc
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1 hour ago, Sorcery said:

What kind of switches and when are you using them?

I had to fire 4 hours with the top cracked to get mine even. About 16 hours total for single fire to cone 6.

Sorce

They're low, med, high switches. 

I've been doing all on low for 2 hours, then medium for 2 hours, and then high for 2 hours. 

I don't have a downdraft, which I know would improve the uneveness - but don't have the $600 to invest right now. 

Would turning my bottom and middle section to med/high before the top make a nice difference? 

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A downdraft vent will only help to a degree. It's not the magic bullet Skutt makes it out to be. Do you have a kiln shelf at the bottom of the kiln, up on 1/2" posts? That will help insulate the bottom. Adjusting the load is the best way to even out the firing. Put tall pieces or wide bowls, things with large volume but low mass, at the bottom. Pack the top with small cylindrical pieces to get a lot of mass up there. It would be worth investing in a pyrometer that has two or more probes so you can see what's happening in the kiln. Running the top cooler till the end may help. It's definitely worth a try.

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1 hour ago, neilestrick said:

A downdraft vent will only help to a degree. It's not the magic bullet Skutt makes it out to be. Do you have a kiln shelf at the bottom of the kiln, up on 1/2" posts? That will help insulate the bottom. Adjusting the load is the best way to even out the firing. Put tall pieces or wide bowls, things with large volume but low mass, at the bottom. Pack the top with small cylindrical pieces to get a lot of mass up there. It would be worth investing in a pyrometer that has two or more probes so you can see what's happening in the kiln. Running the top cooler till the end may help. It's definitely worth a try.

Thanks Neil - I haven't dove into a vent so far because that's my fear.. And also feels like something I can hopefully solve without one. 

I figure it just takes tinkering with pots in the kiln and getting to know this kiln better.  I've had shelves propped up off the floor since I got it. And a pyrometer has been on my list, I will definitely get one. 

Also, I figure i'll try firing to cone 7, or 04-05 and see where that gets me. 

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16 hours ago, brettwulc said:

Would turning my bottom and middle section to med/high before the top make a nice difference? 

I reckon. The way mine worked was 5 on off only switches that controlled the 5 elements. So the lowest was on longest. 

With as many possible configurations you have with 3, 3 position switches, I think you can drive yourself insane trying different combinations.

I would do the bottom on low for an hour, then the middle on low for an hour. 2hrs

Then bottom to medium for an hour, then middle to medium for an hour, then top to low for an hour. 5hrstotal.

Then bottom to high for an hour, middle on high for an hour, top to medium for an hour. 8hrs. Maybe the top stays on low here.

Then medium the top, high if necessary.

You kinda have to be aware of the different types of heat you are creating, convection, radiation, etc. The top will be heating through out, then when the switches come on near it, it gets hotter faster.

Knowing the wiring of these switches also helps know where each setting is heating where what etc. I believe there are a couple different ways these 3 3 switches are actually wired.

What was the total time till your sitter fell?

I don't think you'll have to leave the top open. You should be able to dial yours in better. Evenness tends to take time though, I don't think you'll get it even under 12hours.

Sorce

 

 

 

 

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You can work with your glazes and the kiln to get  maximum  results from the kiln.  You can put C5 on the bottom and C6 on top,  some glazes are happier in the middle.  My Skutt is the same way,  my vent system didn't make that much difference.    I couldn't reach the bottom of my kiln  (getting old)  so I filled it with old kiln bricks and then put a split shelf on them.   I was careful not to block my vent holes,  my kiln fires very evenly now.   I know it waste some space but I have never been happier with my old Skutt.       Denice

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3 hours ago, Sorcery said:

I would do the bottom on low for an hour, then the middle on low for an hour. 2hrs

Then bottom to medium for an hour, then middle to medium for an hour, then top to low for an hour. 5hrstotal.

Then bottom to high for an hour, middle on high for an hour, top to medium for an hour. 8hrs. Maybe the top stays on low here.

Then medium the top, high if necessary.

You kinda have to be aware of the different types of heat you are creating, convection, radiation, etc. The top will be heating through out, then when the switches come on near it, it gets hotter faster.

Knowing the wiring of these switches also helps know where each setting is heating where what etc. I believe there are a couple different ways these 3 3 switches are actually wired.

What was the total time till your sitter fell?

I don't think you'll have to leave the top open. You should be able to dial yours in better. Evenness tends to take time though, I don't think you'll get it even under 12hours.

Sorce

 

 

 

 

Thanks for the info! This sounds like a nice method, I have some underfired pots from a previous firing and I think I'll take a crack at this today.

I know on high it's the top two elements, and medium is every other bottom element? I've been toying with them a bit. But all seem to glow properly/were placed properly.

For an 06 the sitter has been falling around 6 hours, and for a 6 it's around 7 hours. Seemed too fast, ill be taking my time and seeing how the temps rise, placing larger pieces on the bottom and mugs on top. Is going a a kiln above my desired temp a good idea? I know people tend to do this with sitters. I'll post my results. 

I'm getting my hands on a pyrometer as well so I can actually know whats going on in there. 

Fingers crossed!

Thanks everyone. 

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30 minutes ago, brettwulc said:

Thanks for the info! This sounds like a nice method, I have some underfired pots from a previous firing and I think I'll take a crack at this today.

I know on high it's the top two elements, and medium is every other bottom element? I've been toying with them a bit. But all seem to glow properly/were placed properly.

For an 06 the sitter has been falling around 6 hours, and for a 6 it's around 7 hours. Seemed too fast, ill be taking my time and seeing how the temps rise, placing larger pieces on the bottom and mugs on top. Is going a a kiln above my desired temp a good idea? I know people tend to do this with sitters. I'll post my results. 

I'm getting my hands on a pyrometer as well so I can actually know whats going on in there. 

Fingers crossed!

Thanks everyone. 

Dude posted a 4 zone pyro for cheap, that's the only other way to really know when what is doing what. Since you already did THE RIGHT THING by having cones! 

In an attempt to "slow cool" mine once by flipping switches on and off, I found that where elements were on, the heat was almost exactly noticably different. Meaning that direct heat things "see" from working elements is much greater at higher temperatures.

So you should only need the top "on" (whatever that means) for a little while before the sitter falls.

I think a 6 in the sitter, or the exact cone, is best at first, sit with it, have witness cones visible, and turn it back on till they fall evenly.

I feel once you're slower and more even, you'll end up at the right sitter cone choice.

It's a little important to remember those 2 cones melt at different end rates. (See the chart)

This may make a slight difference in your witness to sitter cone numbers.

Sorce

 

 

 

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Happy to say my last cone 6 fire was a huge success! Thanks for all the input from everyone.

Not sure if one or all of the recommendations did the trick, but all of my cones fell and everything came out great. Even this faux ash glaze I've been playing with turned out. ( I'll include a pic) 

I followed something similar to the schedule Sorc proposed and it ended up being 11 hours for everything to drop. Keeping the top cooler throughout the fire, putting a few larger pieces at the bottom, packing the top and overall taking more time. 

I also hard wired my boxes and got rid of the plugs. :)

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congrats, brett!   i just read this whole thread and see that you said you turned the low, medium and high know for 2 hours each.     there have been a lots of posts lately from folks who think their firing should have finished in very few hours.   i wonder if they are reading "turn it on high for two hours"  and thinking "then turn it off"  when it should be "turn it on high and let the sitter turn it off when it has reached the right amount of time and temperature to bend the cone in the sitter"?   i see your sitter cone is bent properly so you can't be turning the kiln off yourself.  odd............

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