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Breakage with a sitter?


ronfire

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I fired 5 large pieces in my Skutt 1027-3 with a sitter, 3 sinks and 2 platters. All the pieces where on 1/4" clay rollers to help with even cooling by isolating them from the shelves. The concerning part is only 2 of the 5 pieces survived the bisque firing, both of the pieces where at the top levels of the kiln, 1 sink and 1 platter.

Could this be caused by the firing method of the sitter, starting the bottom on low for 1hr then the middle , and then the top. The same 1 hour intervals for medium and then all to high after 6 hours of working the system. I was thinking the top of the kiln heated slower than the bottom and centre.  Would a wall mount kiln controller help solve this issue?

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You've been around for a while...is there something that you're doing that was different from your previous firings? Is there something new in the mix here...a different firing schedule...different clay,,,different anything? 

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12 minutes ago, ronfire said:

I fired 5 large pieces in my Skutt 1027-3 with a sitter, 3 sinks and 2 platters. All the pieces where on 1/4" clay rollers to help with even cooling by isolating them from the shelves. The concerning part is only 2 of the 5 pieces survived the bisque firing, both of the pieces where at the top levels of the kiln, 1 sink and 1 platter.

Could this be caused by the firing method of the sitter, starting the bottom on low for 1hr then the middle , and then the top. The same 1 hour intervals for medium and then all to high after 6 hours of working the system. I was thinking the top of the kiln heated slower than the bottom and centre.  Would a wall mount kiln controller help solve this issue?

How did they fail? Crack, explode, warp....?

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The pieces cracked from rims. Wondering if they heated faster than the ones that did not fail.I have been having good luck with these pieces but they are usually near the top of the kiln with other smaller pieces below. This firing there was only the 5 and used the same kiln firing sequence as usual. The sink and platter that survived are built the same as those that did not.

 

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1105947593_ScreenShot2020-06-21at8_12_13AM.png.aa3e85838ef56e0201f689fccfb6ca67.png

 

 

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I always  considered my electric top of kiln to be the hottest the soonest in a bisque. How about sheilding the sink rims with other work around the rims to slow down the rims temp wise.Or just fire small stuff up top.

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The Sinks were made on a wheel with maybe 15lbs of plainsman m340 and are 16" across. The platter are slab rolled with reclaimed clay and I do get failures with them but has been better for the last few I made. Looking at the cross sections of the sinks I think it was the difference in the thickness of the piece. I felt the one that did make it and feels like I did a better trimming job.

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Look a lot like cooling cracks, even though unglazed, have seen it happen on a top shelf. In the last few years I have been firing with a thicker lid, and it does seem to help in the cool down on the top allowing it not to cool as quickly. The one piece looks like trimming problem, but is it enough to cause a crack?

 

 

best,

Pres

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My studio has the same kiln with a sitter,  I have never fired my bisque that fast.   I also do a slow cool,  I have a smaller kiln that I can start filling while the Skutt cools.  I use a dual Skutt pyrometer in my glaze firings to keep track of it and make any adjustments  I need to keep a even firing.    My smaller Paragon finally died several years ago and I thought I could get along without it.   I was wrong,   I could produce more pots having two kilns to work from and keep the work flow I was use to.    Denice

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The only change from previous firings was the kiln was loaded with only 5 large pieces. Usually there is a kiln full of mugs and smaller items with the larger pieces near the top. The 2 pieces that did not break where at the top of the Kiln, A large platter on a 1/2 shelf moved toward the centre of the kiln instead of 1 side and the Sink below that.

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When I enlarge the image of the sink in the last photo the crack appears wider at the top than the bottom, this would mean the crack started at the rim and travelled downwards. If the rims on the pieces with the thick bottoms dried long before the bases that would set up stresses that can cause those type of cracks. I'm seeing 2 more cracks in the base of that sink, when did you cut the hole in the bottom? Until you get a feel for the thickness you could try using some pushpins or tacks to measure the thickness of the walls and base. Push a pushpin or two through the pot on the lower wall plus another one or two in the base. Trim until you hit the pins then remove them and blend clay over the holes (snip off the ends of the pins if they are too long).

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