HELP - Cracks in Dome

[nodison]

Hey All,
Well I finally fired up the 42" oven on Saturday night, after a full week of curing, and was surprised how much wood I had to keep putting on to get the dome to temperature where it started turning white, but we did eventually get there and it was awesome.
But to my horror cracks developed in the outer layer of High Temperature Mortar (HTM). I only have a rather thin coating of HTM on the outside so wasn't to concerned, but then after looking inside up around the top of the dome, after it had turned white, I saw more cracks in there, one of the cracks seems to go from up near the top all the way down to the floor, since the oven has cooled down I have had a good look and found all the cracking is in the mortar joints only.
First question to all is, Have the cracks appeared because of the thin layer of HTM on the outside, allowing to much movement when hot?
What can I do to fix it? Can I get in the oven and chip the mortar out that is cracked and refill with poor mans mortar? Should I put a thick coat over the outside of the dome?
What to do!!!!!!!!!!!
Any suggestions welcome.....
Thanks in advance
Keith

[brickie in oz]

Re: HELP - Cracks in Dome

What to do!!!!!!!!!!!

Nothing, it wont collapse.

[Moley]

Re: HELP - Cracks in Dome

Hi blokes,
I'm finishing an enclosed oven and just about to start on a gabled roof. I have a couple of questions before i roof it in however. I finished the dome with a skin of mortar all around the dome and plan to fill the cavity between the dome and the brick walls with loose vermiculite. Is it worth putting another coat of mortar before the loose fill? There are a few cracks in the first mortar coat.
Secondly, the gable wall at the front of the oven will be the front of my brick chimney, is there anything i should do to stop the mortar joints cracking on the chimney? This is my first attempt so I want to do it right first up.
Thanks,
Geoff.
PS how do i post a new thread instead of replying to existing threads?

[dmun]

Re: HELP - Cracks in Dome

Is it worth putting another coat of mortar before the loose fill? There are a few cracks in the first mortar coat.

I wouldn't. The only reason to parge the exterior of your dome is to add thermal mass, which isn't usually necessary.

Secondly, the gable wall at the front of the oven will be the front of my brick chimney, is there anything i should do to stop the mortar joints cracking on the chimney?

It's usual practice to leave a half inch gap between the flue tile liner, and the brick surround. There shouldn't be excess heat on the masonry if you build in this gap.

[nodison]

Re: HELP - Cracks in Dome

Nothing, it wont collapse.

That's what I like about you, nice short answers no messing around.
Thanks makes me feel better.
Keith

[Moley]

Re: HELP - Cracks in Dome

Yeah, thanks for the help.

[nodison]

Re: HELP - Cracks in Dome

Hi all I first posted this help message in November 2010, since then we have had a heap of pizza nights, all exceptional I might add, but the cracks that appeared originally on the inside of the dome opened up rather heaps, 2 of them down at floor level opened up to about 7 to 8 mil. going up the dome and getting narrower as they went up the dome, there was 2 major cracks like this sort of on each side around the middle from the opening and the other not so bad one at the back opposite the opening. What I have found out about the cracks is they have been caused by the joints in the floor bricks opening up which I guess forces the dome out and causing the cracks. Does anybody know why the floor brick joints would open up like this? I sort of figure that there might be ash or flour getting between the joints and expanding with heat causing the floor bricks apart and in turn causing the dome cracks.
In saying this though it would appear the cracks haven't increased in size in the last 3 firings. I have just spent the last couple of hours inside my oven filling cracks, ain't that fun.....
Has anyone got any thoughts on this?
Thanks
Keith

[david s]

Re: HELP - Cracks in Dome

Kilns are often made with dry joints. ie there is no mortar.This is to allow for thermal expansion. Because we fire our ovens very rapidly there is a lot of uneven thermal expansion. The mortar joins should always be weaker than the refractory materials used so that any cracking will take place along the mortar joins. The dome is a self supporting structure and will (should) not collapse. cracks in the outer shell are more problematic because they will lead to etnry of water. don't worry about cracks on the inner dome. Regarding the floor, most ovens have dry joints to allow for this thermal expansion and ease of brick replacement. Just keep cooking and if you want to be kind to the refractory materials heat the oven slowly, so the thermal expansion has a chance to even out.
Dave

[nodison]

Re: HELP - Cracks in Dome

Thanks for that advice Dave, I haven't been to worried about the large cracks as they haven't increased in size in the last few firings, they just looked a bit unsightly, but have now repaired them (hopefully). The thing I couldn't understand was why the floor bricks are moving, and it's not all of them. They were laid as per the plans on top of the high temp board, and only used fireclay to lay them and get them level, I didn't use any mortar or fireclay between them. I just thought there might be some reason for the way they have moved apart, not all have moved just a few and as I said it looks like they have pushed the dome out causing the cracks. My hope now is that they have finished moving which should stop the dome cracks as well.

Question: Will sand expand under extreme temperature?
I did sweep some really fine sand between some of the floor bricks where there was a bit of a gap after laying them, the gaps were caused by the bricks not being the exact same size, something to do with the way they were made. Are firebricks normally the exact same size or is it just the lot I brought?
Thanks again
Keith

[david s]

Re: HELP - Cracks in Dome

Yeah, what you describe is normal. All bricks vary in size slightly. Just keep firing and cooking and you'll lose your worries about cracks and gaps. The floor bricks will fill up with wood ash anyway.
Dave

[nodison]

Re: HELP - Cracks in Dome

Yea thanks Dave as I said I haven't been to worried about them but just couldn't figure out why the floor bricks moved. Anyhow thanks for the advice and I will just keep on firing and cooking.
Cheers
Keith