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How should I fix a few cob cracks before insulating? - Forno Bravo Forum: The Wood-Fired Oven Community

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How should I fix a few cob cracks before insulating?

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  • How should I fix a few cob cracks before insulating?

    During the curing process, I got a few cracks on the outside of my cob oven. The cob was made from Hawthorne Bond clay (fireclay) and fine, white pottery sand. It was a 2:1 ratio by volume. The interior is looking pretty good as far as I can tell. The outside can be scraped at pretty easily though. I just assume the outside doesn't cure as well, and I imagine after I do all the insulation that should not be an issue. But I figured I should address the cracks while I have some clay and sand to spare.

    My thought is to lightly chisel at the cracks to expose them more, wet the region, and fill it in with a little bit of a wetter cob mix. The cracks aren't huge so I can't imagine injecting anything into them, and I imagine they'd stop a certain depth in. Yet they bother me.

  • #2
    Re: How should I fix a few cob cracks before insulating?

    Rocko, sorry, dude. Can offer any advice, but... I hope you get it.


    One question though, do you have any pictures of your build?

    A fellow Austin area dude looking for some inspiration here.

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    • #3
      Re: How should I fix a few cob cracks before insulating?

      Generally, we advise people to ignore surface cracking. Most cracks are hairline cracks and don't effect the integrity of the build. Cob is mud, and should be easier to patch than air-dry mortar.

      Speaking of cob, I'm no expert, but isn't cob made with straw, to prevent shrinkage cracks? Did Pharaoh force you to make bricks without straw?

      My geodesic oven project: part 1, part 2

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      • #4
        Re: How should I fix a few cob cracks before insulating?

        I am still calling it cob but it's not really in the classic organic sense. Online I saw people do cob in a similar method and figured nobody was talking about it falling in on them, so it must be okay. The straw would insulate where I wouldn't want it to right in the middle. And I didn't want to make a big pile of insulating cob on the outside, since it looks like it does just as well contributing thermal mass as it would do insulating.

        I guess you can say I did something between cob and refractory cast. It's not quite either but shares some from both.

        Before I cover this up in more conventional insulation I was hoping to patch it up in advance. I think the top of the dome is a little thin too so I was going to screw with it one way or another.

        Also to make things really stupid I threw some leftover firebricks I busted in half around the lower rungs of the dome. I thinks some of the cracks form there, and I have a rather longer--but not wider--crack running around where I stopped embedding bricks since I feared them falling down.

        I don't have too many pictures of my build in progress since I might be taking it for granted people could figure it out if I wrote it out.

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        • #5
          Re: How should I fix a few cob cracks before insulating?

          I guess you can say I did something between cob and refractory cast. It's not quite either but shares some from both.
          Let us know how that works. We like to hear about new approaches.

          Again, don't obsess about hairline cracks. We almost all have them.
          My geodesic oven project: part 1, part 2

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