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Cracks in newly fired oven - Forno Bravo Forum: The Wood-Fired Oven Community


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Cracks in newly fired oven

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  • Cracks in newly fired oven

    Before today I had fired my new oven 6 times with fires increasing in size each time, but never to the point where the ceiling turned white. Today, after installing the chimney I put in the first good sized fire. Everything worked well with the top 4 or 5 courses of the dome turning white, but the other courses staying black. I don't yet have my infrared thermometer so I don't know what the temperature rose to, but I do know it was HOT. I tried the 1 Mississippi, 2 Mississippi approach but I didn't get past the 1 Miss....
    After about an hour and a half I noticed that there was some smoke coming out from some new cracks. Holy smoke!

    What should I do about this issue. I haven't yet closed in the dome so do have access for some repair which I presume I have to do. Should I just slather on some more mortar, or should I use a crack fixer, or what?

    Any suggestions would be much appreciated.


  • #2
    Cracks are common

    (M) I also had leakage of smoke through mortar cracks; many builders reported the same. Generally speaking that is not a serious structural problem but is a result of either incomplete coverage of the outside of your dome with mortar, or separation of mortar from your bricks due to differences in thermal expansion.

    (M) I covered the outside of my dome with aluminum foil, and then more Perlcrete. It was very difficult to get the Perlcrete to stay on the foil so I ended up wrapping the dome after covering it with foil, with chicken wire and finally the Perlcrete. If you use the same method, start from the bottom and overlap the foil as you would if you were laying roofing material .

    (M) The result was that I stopped smoke from exiting, stopped rain from entering, and perhaps reflected radiant heat back to the dome.

    (M) I believe that very few builders chose to use foil and I can't be sure that the result was worth the effort, but having done it, I feel it certainly will not worsen the oven's efficiency.

    (M) Here is one image of that work in progress:

    "Everything should be made as simple as possible, ...
    but no simpler!" (Albert Einstein)


    • #3
      Oven cracks

      Thanks Marcel for your suggestion and photo illustrating your solution to the problem. As you can see from the attached photo I already have my walls and roof framed in which will make the extra vermicrete difficult but not impossible to put in place. I'll have a look today and see what I can do.
      Attached Files


      • #4
        New perlcrete layer to prevent smoke escaping through dome

        Here are photos of my new layer of perlcrete as suggested by Marcel. I used chicken wire, sometimes 3 layers of it, over the dome then the perlcrete slathered on in layers in the hope of negating any cracking in the perlcrete.

        Contrary to Marcel's suggestion of starting at the bottom of the dome, I started at the top. I did this, not to be ornery, but because the bottom was much more difficult to get to with the sides of the housing in place. I put perlcrete on to half way down the dome walls. The little bit of smoke that had emanated from the dome all arose from the top of the dome where one would expect it to be. I figured the likelihood of smoke coming out the sides of the dome would be pretty minimal. Now I'll just have to fire the oven up again and see if the whole thing is satisfactory.

        Attached Files


        • #5
          Dry perlite or vermiculite can soon be poured via the roof

          (M) "A mans gotta do what a mans gotta do". So if you couldn't get to the bottom of your dome, your variation will still work fine.

          (M) After you close the front wall consider investing in lots of dry perlite or vermiculite. You can pour it over the roof joists before you do your actual roofing. "Col.Corn" &/or James gave me the idea of blocking the corners to keep from wasting the perlite. I used sheet metal but any fireproof material such as "Durock" would have satisfied my personal requirement to exclude any material that was flammable.

          (M) If you pour dry perlite, do some shopping for prices. The 4 Cu./Ft. bags where I live (Oregon) varied between $10 and $12 a bag. I used 8 bags but I feel the $80 spent was a relatively small amount in the total expenditure for so much insulation value.

          (M) I'm looking forward to images of your rock siding. It should look great!


          "Everything should be made as simple as possible, ...
          but no simpler!" (Albert Einstein)