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MikeFaul

Fractured Kiln Shelves...

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Just wondering if anyone has encountered this issue. I have well tempered core-lite kiln shelves (26" rounds) I use at the base of my kilns. Just recently we started do fast low fire firings to set underglaze. The last two fires we lost the bottom shelf (bottom shelf only). I'm attaching photos. 

 

We've been using the same pack pattern for more than 2 years including a 3 post lift to the next shelf without issue.

 

None of the other kilns have manifested the issue, but this is the only one we've been running fast fires to ^05.

 

I'm thinking the speed of the fire has something to do with it, but that's only a theory.

 

We have run fast fires to the same ^05 on the same program in our 18" kiln with 18" full rounds without incident. We've been doing this for more than 2 years.

 

The first time it happened (yesterday), the shelf was older, about 3 years old. I would say approximately 200+ firings on it. My first suspicion was that some sort of fatigue had occurred. So, I replaced it with a kiln that was less than 6 months old and had less than 50 firings on it. On the very first firing the exact same fracture occurred. 

 

No shelves above the bottom shelf fractured or cracked. All had the same post layout, and all posts were directly above one another. The first time it happened there were only 2 full shelves above the bottom shelf plus one lightly loaded (3rd shelf). The second time it happened there was one single fully loaded shelf above bottom shelf.

 

The types of pots, loads, and weights per shelve were identical in the two firings.

 

After the first shelf fractured I added four (1) 1.5" x 1.0" posts around the center of the bottom shelf for additional support. 

 

If there are any clarifying questions I can respond here, thoughts and suggestions are welcome...

 

Mike

post-58862-0-13483200-1484418331_thumb.jpg

post-58862-0-46638200-1484418348_thumb.jpg

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wow Mike is back

Was that old shelve stored on a concrete floor? I'm thinking a tad of moisture in it coupled with a fast up cycle as the bottom tends to run cool until the kiln gets hot then WHAM it's hot. Store the shelves on wood off concrete.

any chance of moisture ?I want to know where they are stored?

One last note

I think you could put at least one more mug in as they are pretty far apart-really

I would have the non underglaze sides touching-one mug extra per shelve adds up.

Thats all I got.-Did you ever find any throwers?

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I agree with Mark unless maybe the shelf had been taken out and placed over something when it went back in. Are there any spacers between the shelf and the floor? Anything other than these : " I added four (1) 1.5" x 1.0" posts around the center of the bottom shelf for additional support. "

What about under the next level of posts? Were ALL the posts level?

 

 

Marcia.

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Hi Mike.. long time no PM... get the CNC/ 3D Cad running?

 

My answer: unequal COE. The bottom of this shelf was in direct contact with the bottom of the kiln; the coldest part in a firing. The top of the shelf was in direct contact with the heat of the kiln: probably 300-400 degrees difference between the top of the shelf, and the bottom. The problem was further complicated by a fast fire. I do the same thing, but I cut up an old kiln post into 1" slices and spread then around underneath the shelf. This allows for heat circulation.

 

Nerd

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Looking carefully at your loaded photo  two of the cracks go towards those posts-Makes me wonder if that shelve was warped or not on a flat surface???

I think some supports (say 3/4 inch )would help under the shelve-mine are chunks of kiln shelve under my bottom shelve under the posting spots.

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Are the 3 posts on the top of the shelf aligned with 3 corresponding posts underneath the shelf?

 

Additional posts generally don't help, they just cause the shelf to rock which creates a lot of stress points.

 

A shelf that large may not be able to handle the really fast heat up unless it's up off the floor to the point that the bottom element groove is under the shelf to throw some heat under there and even it out. A little preheat wouldn't hurt, either.

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If the only thing that has changed is the firing schedule then I would put it down to that. I would contact the manufacturer, Resco, send them some pictures then give them a call. If they agree it was from the firing schedule perhaps they would replace at least one of them if there isn't a caveat somewhere advising against this.  

 

Those pieces are really blown apart, again back to too fast a firing.

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so I think you said you added for posts under the bottom shelf around the center. Were there 3 outer edge posts to support the ones supporting the shelves as well?

If you had 7 posts under the bottom shelf, it could be some were slightly different in height and stressed that she enough to crack it.

 

Marcia

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Is the shelf always the bottom shelf? Could it have been warped before the fast firing? I'm thinking if there were additional supports under the shelf, which must be the case or the shelf would have dropped, then an uneven support would add to the stress on this shelf. Always under the assumption that the 3 posts were the most effective and efficient way to stack...

Are your posts needing replaced/ground.. remembering an article by Mark..

How are all these comments tracking in your brain Mike?

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wow Mike is back

Was that old shelve stored on a concrete floor? I'm thinking a tad of moisture in it coupled with a fast up cycle as the bottom tends to run cool until the kiln gets hot then WHAM it's hot. Store the shelves on wood off concrete.

any chance of moisture ?I want to know where they are stored?

One last note

I think you could put at least one more mug in as they are pretty far apart-really

I would have the non underglaze sides touching-one mug extra per shelve adds up.

Thats all I got.-Did you ever find any throwers?

 

Mark... 

 

It's good to be back... I had some serious health issues last year that bit into my time. It was all I could to stay afloat. Fortunately, they have all mended and I'm back to my old self. 

 

I was thinking if we inverted every other cup on the inner rings, side stacked, and double stacked we could increase our bisque yield by 75%! Why stop at just one cup!?!? I think we are on a quest to redefine "tight pack"... :-)

 

That was a bisque fire, ^05. No surfaces touched during the fire. Some pieces may have slid around when the shelves split though... 

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I agree with Mark unless maybe the shelf had been taken out and placed over something when it went back in. Are there any spacers between the shelf and the floor? Anything other than these : " I added four (1) 1.5" x 1.0" posts around the center of the bottom shelf for additional support. "

What about under the next level of posts? Were ALL the posts level?

 

 

Marcia.

 

Under the floor shelf, we place a total of 8 1.5" x 1.0" spacers. Two spacers about 5" apart on each of four sides in a cross pattern. So on pair opposes the other. 

 

The spacers are all original. I don't think they have been changed out since 2013. We recently changed the elements and thermocouple. At that time the entire kiln was vacuumed out before the spacers were returned and the shelf reloaded. I did not specifically check for level, so unless they lost their level over time there was no reason for them not to be level. Can they change shape over time? 

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Mike 

I would use some wadding on the spacers first time you put shelve in to level it all up.

Was this shelve stored on concrete at all for any time before use? or was it always on bottom and the fast schedule was just done and it cracked?

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Yes are there any spacers off the floor?mine full shelve  on bottom has some 3/4 inch spacers keeping it off the cold floor

 

Yes, when the first shelf fractured we had 8 spacers. They were all 1.5 x 1.0" in size. I described the pattern of placement in my response to Babs... On the second shelf I added four additional spacers thinking that warpage may be the problem. These we placed under the center of the shelf in a square patter around the venting holes. 

 

The floor is 18" square porcelain tile, not concrete. It was completely dry for the 48 hours before firing. We have a wall hook system for storage of shelves. Shelves go from the kiln onto the wall hooks, not the floor. I put this system in about 6 weeks ago when I tripped over some electrical cords and went flying into a pile of kiln shelves. I spent nearly a half hour on the floor with a ripped up knee and nothing to grab onto but hot kilns trying to figure out I could get to a standing position.

 

Moisture is a possibility, but probably not a probability given it was the floor shelf that's seldom if ever pulled from the kiln.

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Mike 

I would use some wadding on the spacers first time you put shelve in to level it all up.

Was this shelve stored on concrete at all for any time before use? or was it always on bottom and the fast schedule was just done and it cracked?

What's wadding? 

 

Never stored on concrete... 

 

Recently replaced a shelf that fractured in a firing two days earlier... That shelf was just on the bottom and the fast schedule was run.

 

This shelf was pulled from a wall rack and replaced the bottom / floor shelf.

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Are the 3 posts on the top of the shelf aligned with 3 corresponding posts underneath the shelf?

 

Additional posts generally don't help, they just cause the shelf to rock which creates a lot of stress points.

 

A shelf that large may not be able to handle the really fast heat up unless it's up off the floor to the point that the bottom element groove is under the shelf to throw some heat under there and even it out. A little preheat wouldn't hurt, either.

No, the shelves above are not aligned with the spacers below, but we've been using this post stack for 2 years without incident. If the cause is misaligned force vectors it must be an accumulated fatigue thing. And, if that's the case why would it have happened on the new shelf just as fast and old shelf? 

 

The fast fire could be causing the negative space above the shelf to heat way faster than the space below. Good Point! We might have to go to a custom fast fire with a hold at the low end of the ramp to allow the bottom to heat up before moving up to the top of the schedule. I would hate to raise the shelf. We fight for cubic inches, to increase our firing yield, every day...

 

I did not know that more posts brought nothing to the party. I'll try reducing the number of posts and align them with the rest of the posts in the stack. It's worth a test. Thanks!

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so I think you said you added for posts under the bottom shelf around the center. Were there 3 outer edge posts to support the ones supporting the shelves as well?

If you had 7 posts under the bottom shelf, it could be some were slightly different in height and stressed that she enough to crack it.

 

Marcia

 

Here is the configuration of spacers on the first fracture:

 

                                X           X

 

 

     X                                                           X

 

     X                                                           X

 

 

                               X           X

 

Keep in mind, that configuration was in place since 2013 without incident... After the first shelf broke, I modified the spacer patter as follows:

 

 

                               X           X

 

 

 

     X                         X        X                      X

 

     X                         X        X                      X

 

 

 

                               X           X

 
The second shelf split on the first fire of this config... It was a newer shelf, much newer. 

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Mike In ceramics often things work until (even thought not done right) until one pushes the limit -such as your fast fire.

It does matter that all posts are under one another from the floor to the ceiling. You may get away with this for years but sooner or later it will get you .

Wadding is EPK and alumina hydrate mixed 50% 50% with a little water.

next time make sure all pots including under floor all always lined up-the wadding as on under floor pots only to level. fire the 1st time a bit slower to dry wadding .

I will speak to your pots layout only after I know what you use above that full shelve ? all full shelves or some 1/2 shelves??

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Is the shelf always the bottom shelf? Could it have been warped before the fast firing? I'm thinking if there were additional supports under the shelf, which must be the case or the shelf would have dropped, then an uneven support would add to the stress on this shelf. Always under the assumption that the 3 posts were the most effective and efficient way to stack...

Are your posts needing replaced/ground.. remembering an article by Mark..

How are all these comments tracking in your brain Mike?

 

 

I loaded it myself, the only reason I recently loaded it was because the first and much older shelf fractured. That shelf had been the floor shelf since 2013. It was only removed to replace the elements and vacuum the kiln. 

 

I suppose the stresses building up over time would explain the first fracture, but what about the second? It happened on the first fire.

 

There was no signs of warpage when I loaded it, and I did check to see if there was a wobble, and there was none. Now, our posts have some miles on them, and they have been dropped and knocked around. So, it is entirely possible we had a post that was no perfectly level. Plus we had some pots blow from too much moisture a few days ago, so there could have been some debris that wasn't completely cleaned out from the kiln. I did find some bisque shards, but nothing under the spacers, when I cleaned out the shelf.

 

It's possible the posts need to be replaced. I'll do that as precaution, and check them over to see if they need to be ground. Good suggestion...

 

This good stuff, I'm learning a lot...  No one ever told me how post the spacers when setting up a kiln. If I can get by with just 3 that would be nice.

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Mike In ceramics often things work until (even thought not done right) until one pushes the limit -such as your fast fire.

It does matter that all posts are under one another from the floor to the ceiling. You may get away with this for years but sooner or later it will get you .

Wadding is EPK and alumina hydrate mixed 50% 50% with a little water.

next time make sure all pots including under floor all always lined up-the wadding as on under floor pots only to level. fire the 1st time a bit slower to dry wadding .

I will speak to your pots layout only after I know what you use above that full shelve ? all full shelves or some 1/2 shelves??

 

We use full rounds all the way up the stack. We use to use halves, but when I'm firing a full load of Rocks cups, I need every single cubic inch I can get. We use a 3 post arrangement between shelves. In bisque I allow a 1/2" space between the rim and the bottom of the next shelf, except when I get to the thermocouple. I make sure there we have enough vertical space to provide 2" of clearance on all sides of the thermocouple. 

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Just post three spacers from the floor to first shelve and them post over this all the way up every time so load is straight down to floor. Just use 3 spacers under the 3 stilt pattern.-no need to use anymore than the three all the way.If you need to use 1/2 shelves just stilt as close to the 3 point stand as possible.

My car kiln has 3 contact points under my 12 x24 shelves and the load goes up over 5 feet.Just keep the stilts over one another-especially in the lower parts of load.If you crack another bottom shelve you are just going to fast is my guess.

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Just when you think you've nailed it ceramics bites you just to let you know you are not in control.

If the shelf is sitting at element level and the kiln is whooped up, I would add that as a factor to consider, edges of shelf subjected to rapid heating whilst the ware and inner shelf area because of density of pack getting there at a much slower pace. Just thinking here Mike, sorry to hear about your accident, occy health issues in the studio abound, prob glad it was not an employee who tripped.

B

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