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Kiln Bottom Crack Repair?

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#1 Ray Bright

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Posted 17 October 2012 - 07:20 AM

My last post, Hairline Cracks..., has generated a good deal of interest in fixing cracked kiln bottoms, and for GOOD REASON. So I hope to further this discussion, because there are unanswered questions, and a lot of dangerous kilns. Here's the last post @ my last post:

If it's not impolite, I'll jump on this thread with a similar question. I am a self-taught potter that has never fired. I recently moved from Oregon back home to Costa Rica, and took an electric kiln with me (it was tested before I shipped it, and passed with plying colors). During the move the metal legs buckled and folded, resulting in some decent cracks in the bottom of the kiln. more than hairline, see the attached pictures.

someone recommended using a kiln-repair paste on those bottom cracks, so I had a friend bring some down, but haven't had a chance to use it or test the kiln yet.
My question to you guys, is for tips for a Newbie, on how to baby my kiln to compensate for those cracks. Being in Costa Rica, I can't easily order specialized items, and I can't easily replace that kiln, I need it to last me many years.

I will need to come up with a different base since the metal one is busted. I had never considered that the base needed to be perfectly level. what other things should I consider for the base support? I'll need to MacGyver it, and I had originally thought that I could sit the kiln directly on some cement blocks. bad idea?

thankfully I brought some extra kiln shelves with me, so I have both full and half shelves. should I just plop one on the bottom of the kiln? should I sit one under the kiln to help distribute the load?

thank you all for the amazing resource, this is incredible support.

Since this is such an important safety issue, Can we get a definitive answer? Just asking . . .


#2 Marcia Selsor

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Posted 17 October 2012 - 07:40 AM

yes the base should be level or you can get cracks in the base and /or the lid.
Costa Rica should have refractory suppliers where you could get some repair material.
Check with some local masons who may build wood ovens of fire places or boilers.
Marcia Selsor, Professor Emerita,Montana State University-Billings

#3 neilestrick


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Posted 17 October 2012 - 08:14 AM

For starters, a kiln should never be transported on the stand. And if the floor slab is separate from the walls, it should never be transported while sitting on the floor slab. The wall sections can be stacked during transport, but the floor should be moved separately or set on top. If the floor slab is attached, is should be moved sitting directly on the floor, preferably on a flat piece of plywood and a layer of construction foam (the pink or blue stuff).
If you have a crack that goes all the way through the floor or lid and goes all the way across, essentially breaking the slab in half, brick cement WILL NOT hold it together. The slightest little flex in the bricks will cause it to crack again. The slab should be replaced.

When a kiln is set up, the floor slab should be set on the stand and then check for unevenness. It doesn't necessarily have to be perfectly level, but it should not rock at all. Metal shims should be placed under the fet of the stand until the whole thing sits solidly on all four feet and the slab does not rock. As each ring of the kiln is placed on the slab it should be rechecked.

Hairline cracks are normal. A big solid slab expands and contracts during the heating and cooling cycle, and it has to give somewhere. Those small cracks are not necessarily indicative of future big cracks. They can stay small for years and years if the slab is firmly supported by the stand.

Neil Estrick
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC

[email protected]

#4 Jime



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Posted 18 October 2012 - 07:57 AM

thanks guys, this is great info
Ray, sorry to hijack your thread!

I'm attaching the picture of the kiln floor here, in case anyone wants to take a look

Neil, thanks for the great information. I tried to get information on how to transport the kiln, and your answer is much more thorough than anything I found.

I do still have the repair paste, that I'll try to use, unfortunately I won't be able to replace that kiln bottom, or the specific kiln stand.

as a workaround does it make sense to put a kiln shelf on the bottom? or would that be likely to damage the kiln more?

is it important to have empty space under the kiln, like the original metal stand, or would there be any problem with using a more solid base like cement blocks?

thanks for all your ideas!
Attached File  11 11 Jimena broken kiln (3).jpg   286.07KB   20 downloads

#5 TJR


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Posted 18 October 2012 - 09:19 AM

Here are a couple of answers, as you have a lot of questions.
1. There is a kiln repair compound called Sairset. It is a ceramic cement, and will fire up to temperature. Do not use regular cement, as it will melt and run all over.
2. You can sit your kiln on cinder blocks, so long as your kiln does not wobble.
3. You can put a kiln shelf in the bottom on the inside of the kiln. If you wish, you can raise this up on 1 inch posts, or it can rest directly on the bottom of the kiln.
I think you should have some serious questions for your shipping agent. Why would this metal stand be crushed? Sounds like they stacked something on top of your kiln, that was very heavy!
Lots of people put another, older kin bottom or lid under their kiln for extra insulation. Don't put a kiln shelf under your kiln on the outside. The kiln shelf will just crack. Do not put anything like plywood under your kiln, either, as it will just BURN!

#6 Jime



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Posted 21 October 2012 - 04:50 AM

TJR thanks for the great answers!

I'm glad to know that the cement bricks could work. The seem like the most stable solution I can come up with, so I'll go with that, and try to make sure they are nice and level.

Sadly, absolutely nobody to blame but myself, I really wish I could put it on someone else. Out of the entire container full of housewares, this was the ONLY item that even had a scratch. Full-length mirrors and tons of glassware came out perfectly, but my precious kiln (new to me) was tortured...

the repair product I got is called Kiln Patch, and it was recommended by a ceramic store, so I'm hoping it doesn't end up crumbling away

the idea of the shelf at the bottom of the kiln would be in order to prevent heat loss, right? not as a way of extending the life of the kiln, right?

would it make sense to put pieces directly on that bottom shelf, or should I leave that one empty and load the one above it?

thanks to everyone for the help, I'm a total newbie to firing, and I'm out of my element. The cracked kiln just makes the whole process more nerve-wracking!

#7 oldlady


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Posted 30 October 2012 - 05:34 PM

[would it make sense to put pieces directly on that bottom shelf, or should I leave that one empty and load the one above it?

yes! the reason why you want a shelf inside the kiln at the bottom is to allow you to put ware on it for firing. if you have a messy melting glaze that runs, only the shelf is ruined or damaged instead of the very valuable bottom. if you put the shelf on a trio of half inch posts you lose less space.

your kiln looks very small, if i am counting the number of full size bricks that i see in the pictures correctly, it appears to be about 15 inches or so wall to wall. i cannot tell its depth but it does not look as though you have much space to spare. if you mark the top surface of your kiln wall with a triangle at the places where you have put those first three half inch posts and keep using posts for additional shelves at the same place for each firing, your kiln will be happier than if you just put posts wherever you want when you load additional shelves. put the posts in place first, then the ware. a thick sharpie mark will last many firings before the heat makes it fade.

if this sounds complicated it is because i cannot think of a better way to say something so simple.
"putting you down does not raise me up."

#8 Jime



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Posted 30 October 2012 - 10:16 PM

that is a great idea!
I'll try to make sure that I put the posts in the same spots to help distribute the load evenly.
yes, it's a Cress kiln, and I think it's about 17" diameter, if I remember correctly

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