Help! Cracks in Oven!

[stanbalder]

I am building a pizza oven for a couple in Glasgow. Having looked at all the websites and books I could we decided the best method would be to follow Russell Jeavons and construct in brick.

Dome went up a treat in Engineering brick, rendered with 'No Crack Concrete' and left to dry in the baking Scottish sun for a week.

Trouble is Mr Jeavons is rather enthusiastic about getting on and using the oven, describing cracks as inevitable, and basically saying it can be pretty much fully fired the day after construction. And so it has cracked. I have (hopefully) attached a photo from todays firing showing the cracks all over.

Any suggestions? Start again? Seal with fire cement? Another layer of concrete and slower firing? Any help would be very much appreciated.

Cheers,

stan

[Dutchoven]

Stan,
It is quite true that cracks are. I think what is going on is firstly that you used concrete as a render...probably without any layer of material to allow slippage(aluminum foil works well). Firing it fully so soon did not help either because the concrete holds lots of water and heating it so high creates steam which has to find a place to go.

The best method would have been to insulate first using ceramic fiber blanket and then vermiculite cement(vermiculite and Type N mortar) before coating with a layer of render. How thick is the current concrete render? It will become part of the thermal mass of this oven so you could now insulate with both the insulators mentioned above or just the vemiculite cement with a slip layer in between and then coat the whole thing in another layer of render. You would both have a more efficient oven and have less of a likelyhood of the render cracking(although it still happens sometimes).
Hope this helps!
Dutch

[Acoma]

I would like to see if those cracks go through to the inside.

I had cracks like that all over my outside dome. Mine went through in 3 locations that are pretty narly. I used furnace cement for all the outside cracks. For the inside, I am leaving alone for now. I will use my oven for a while to see what other cracks may possibly develop. Afterwards, I will cover with perminant insulation, and fix the cracks inside the dome.

I was not trying to talk about myself, but was trying to share similar crack scares and what I have done to address mine. I would like to see a photo of cracks inside yours if you have them.

[stanbalder]

Thanks for both of those comments

In your case Acoma it seems as if you did everything pretty much by the book and still you got cracking so there must be plenty of others out there who had this too. Very nice job on the oven too I checked out the pics.

I don't know whether refractory cement and firebricks would have cracked just as much. Does this still need the 7 step firing?

I think the initial casing with no crack cement should indeed have had a slip layer as suggested by Dutchoven, but I've ordered the blanket as suggested.

I installed a small oven kit recently that had needles set in to the cement, I wondered if anyone has tried this instead of fibres to prevent cracking?

Cheers

stan

[Dutchoven]

Thanks for both of those comments
Your welcome!

In your case Acoma it seems as if you did everything pretty much by the book and still you got cracking so there must be plenty of others out there who had this too. Very nice job on the oven too I checked out the pics.
I agree nice job Acoma!
I don't know whether refractory cement and firebricks would have cracked just as much. Does this still need the 7 step firing? IMHO all ovens should get slowly ramped up to temperature primarily because of the steam issue...a good refractory cement is calcium alumina based and withstands the heat and cool cycles better so...it sounds like in the beginning most mortars will react the same...but I have heard it said "if it's brick it will crack
I think the initial casing with no crack cement should indeed have had a slip layer as suggested by Dutchoven, but I've ordered the blanket as suggested. I think it will be OK anyway...physics and the fact that you burn the oven so hot and complete even if the crack is visible inside there should not be any issues...this oven will have a bit more mass to heat up but with good insulation it will perform very well...you should take a few pictures
I installed a small oven kit recently that had needles set in to the cement, I wondered if anyone has tried this instead of fibres to prevent cracking? Stainless steel needles are used as reinforcement much like rebar in reinforced concrete...ovens like that will usually not crack because they are made from fewer pieces and each piece is more homogeneous(sp?)

All the best!
Dutch