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External crack - Forno Bravo Forum: The Wood-Fired Oven Community



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External crack

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  • External crack

    Oh no!

    Fired the oven last night (see pizza photos in photo bragging section) but just as the fire was finished and burning down the oven render cracked. The crack is only in the final waterproofing render that i put on last weekend (7 days ago). So I guess I fired too hot - too soon

    Also thought that maybe the render was too thin, only about 1-1.5".

    The crack started at the flue and then slowly worked its way all the way down to the arch. It is only hairline down by the arch but about 1/16" up by the flue.

    Any advice on how to repair?
    Do I need to render the entire oven again or just the front section?\
    If I leave the crack and tile the front would that work?


    Attached Files

  • #2
    Re: External crack

    I might suggest using the oven for a while...maybe crack will stabilize...mark it with a pencil to see if there is any movement and watch it when the oven is firing and when it gets hot to see what it does...any widening and where etc...then you will have a better idea of what to do.
    "Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity. " Charles Mingus
    "Build at least two brick ovens...one to make all the mistakes on and the other to be just like you dreamed of!" Dutch


    • #3
      Re: External crack

      I agree with Dutch. Did you add stain to the rendering material or did you paint afterwards?

      If it doesn't get worse, I might be tempted to take a grinder and open the crack up and refill it again. I forget. What's the insulation like below this layer?
      Everyone makes mistakes. The trick is to make mistakes when nobody is looking.



      • #4
        Re: External crack

        Those pesky cracks

        I think we're still looking for a dome that has not cracked. I would also be interested in hearing of someone has had good success with grinding out a crack and filling it.

        With the continued expansion and contraction of these ovens and fireplaces, are cracks just a part of life?

        I had an indoor fireplace and never was able to successfully fill an exterior crack on that one. (never ground it out though) And I do have two exterior cracks on my oven now. I've filled them a few times and painted over. Recently I tried an arcrylic caulk (more flexible).

        Reminds me of a song....

        Oh, the crack came back
        It wouldn't stay away
        The crack came back
        The very next day

        Dutch has it right too, give it time to grow (if it's going to) and stabilize
        sigpicTiempo para guzarlos..... ...enjoy every sandwich!


        • #5
          Re: External crack

          Then I read Frances post on her Ostrich......

          No bacon today,
          the bacon's gone away,
          sad as it is to say,
          the bacon's gone away

          ...sorry, I was out on the oven today and seem to to have got stuck in singing mode. Let me know what you think if you try it!


          Frances, this is too funny.....

          I think we were both humming the same tune!!

          .....another FB cosmic connection
          sigpicTiempo para guzarlos..... ...enjoy every sandwich!


          • #6
            Re: External crack

            Thats so funny, Jim! Great minds and all that, eh?

            About the crack: this subject interests me greatly, as my render will be covered with mosaics which means I probably won't see the cracks if/when they form...

            So is a crack in the outside render very serious? If some of you say you left the cracks as is? I thought that even a small crack could wick up enormous amounts of water every time it rains. What about tiling over the top, does that help at all?

            Great pizzas btw, talisker!
            "Building a Brick oven is the most fun anyone can have by themselves." (Terry Pratchett... slightly amended)



            • #7
              Re: External crack

              Thanks appreciate your support in these difficult times

              The insulation material is vermiculite mixed with sand/cement over a blanket.

              The top coat is tinted with oxide.

              I think the top coat is too thin over the arch, this bit was hard to do and I may have skimped a bit on the thickness

              I am having a big pizza party next weekend for Easter. Will report after that.



              • #8
                Re: External crack


                I don't think hairline cracks are much of an issue. BUT, exterior cracks on an igloo will allow moisture infiltration, and that is not a good idea. Wet insulators do not insulate. As well, if you've ever seen the face of a house brick that's gotten wet, then frozen, you'll know that the force of the freeze-thaw cycle will kick the front of the brick right off. It's called spalling. Once the cycle begins, in a brick wall or an oven dome, it's virtually impossible to halt. I'd recommend that significant exterior cracks be ground out in a V and filled for just that reason. XJs on the right track with a flexible material of some sort.

                "Made are tools, and born are hands"--William Blake, 1757-1827


                • #9
                  Re: External crack

                  There is a material used to caulk the joints in log homes...my brother usedit is his...supposed to stretch up to 4 inches I think he said...if it isn't hot it should work because I don't know how it holds up to heat
                  "Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity. " Charles Mingus
                  "Build at least two brick ovens...one to make all the mistakes on and the other to be just like you dreamed of!" Dutch


                  • #10
                    Re: External crack

                    Good to hear others DO have cracks in their render/stucco - I thought I was singularly inept in my cover-up :-)

                    I've noticed a crack about 5cm (2in) long starting in the two-coat render/stucco at the 'naked' arch near the top of the dome and pointing toward the apex.

                    I measured the outside temperature at that point when the oven was hot, and it registers 50-odd decrees Celsius (sorry about the missing F, I'm lousy at instant conversions). Perhaps I didn't put down the insulation properly at that point close to the masonry?

                    Whatever: it hasn't grown in subsequent fires but I agree that it will be best to push some more cement render into that hairline and paint over again.

                    BTW, the two render coats are covered with two acrylic paint undercoats and two acrylic top coats - I'd have though that stuff is elastic enough to accomodate minuscule movements?!


                    Last edited by carioca; 03-16-2008, 11:20 PM. Reason: spelling - why don't I spell check first?!
                    "I started out with nothing, and I've still got most of it"


                    • #11
                      Re: External crack

                      What about silicone sealers? I think they can handle high temps right?
                      sigpicTiempo para guzarlos..... ...enjoy every sandwich!


                      • #12
                        Re: External crack

                        make sure you use plenty of lime in that outer shell. Lime is much more elastic than cement. Did you use chicken wire mesh ?
                        For ferro cement to make it stronger you add more mesh not more cement. Ferro cement boats are incredibly strong but not very thick. I could be wrong but I think a thinner shell is better. I made mine only 10-15 mm thick
                        Kindled with zeal and fired with passion.


                        • #13
                          Re: External crack

                          Did you check the temp on the outside of the exterior? My guess is that since you have the exterior extends to the chimney which doesnt have the same amount of insulation, that the chimney expanded and caused pressure on the cooler section. If my theory holds true I would suggest somehow cutting a channel on the exterior around the chimney to allow expansion. If you have an IR thermometer it would be quite simple to measure the temperature differential.
                          Good luck.


                          • #14
                            Re: External crack

                            I just found this reference "The differential in thermal expansion between steel and concrete is so small that it almost never has to be taken into account. Much more
                            important, for a kiln or a chimney, is the differential in expansion
                            between the inside and outside resulting from a hot interior and cold
                            exterior. It is so serious a problem that kilns are made of bricks
                            and chimneys typically lined with space between layers to accomodate
                            thermal movement. That's why you leave a space between the ceramic
                            liners and the supporting brickwork in a conventional masonry

                            That first statement above surprised me...I always thought that using steel in a thermal masonry device was going to be an issue due to differential expansion of steel and masonry....I'm still learning!
                            sigpicTiempo para guzarlos..... ...enjoy every sandwich!


                            • #15
                              Re: External crack

                              This also tells me that having a thermal blanket and / or slip plane between the inner dome and outer dome cover is a very good idea. Almost like two separate structures! (makes me think of those russian dolls!)
                              sigpicTiempo para guzarlos..... ...enjoy every sandwich!