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went cob crazy; how do I insulate with less space? - Forno Bravo Forum: The Wood-Fired Oven Community



I'm Peter Reinhart! Ask Me Anything! Monday, February 15, 2016 7:00-8:00 pm EST

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went cob crazy; how do I insulate with less space?

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  • went cob crazy; how do I insulate with less space?

    I had bought plenty of clay and sand for the cob oven I was scheming to make, and today was the big day. It was a hard afternoon that turned into a harder evening, but we got it slapped together. The only problem is that we all got carried away and make the oven a little bit bigger. I suspect the inner size has gone up from the original 36" to take up the whole space of the firebrick base that I had cut out. On top of that though, the walls are much thicker. I had reserved a 48" pad on which to rest all of this, and I see I only have a few inches to spare going around at the extremes!

    My original plan was to mix up some perlcrete at 6:1 or even more steep and cover it all up in it, but with this amount of room I wish to consider other options like an insulating blanket. Or maybe I really should cut back on the cob? I am thinking I could comfortable carve out half an inch without a fuss; I have some bricks in the bottom parts of the wall that I am afraid wouldn't be as strongly supported if I knock back too much. I also wonder if I could do a trick like expand out the pad some. This could create a concrete pad that has a lip, and it would have been poured after the initial pad, so I am thinking this is a tricky proposition.

    I want to consider all the possibilities, except for blowing it up and starting over.

  • #2
    Re: went cob crazy; how do I insulate with less space?

    Hi. I don't know how to answer your question--I just have a new question for you. I live a little bit north of Austin and plan to build a cob oven, myself, this spring/summer. Did you find a good source for the right kind of fire bricks somewhere in central Tex? That's been a struggle. Also, I plan to get powdered Hawthorne fire clay from Armadillo Clay (in Austin) and mix it up with a sand slurry since I don't have a lot of land for sourcing clay. Do you think that's advisable or did you figure out a better way? Thanks! and sorry I can't answer your question (I know the insulating layer is important, so maybe look for a way to extend your base around the perimeter of the oven floor?)


    • #3
      Re: went cob crazy; how do I insulate with less space?

      For fire bricks in Austin: Mpi-austin.com | MASONRY SUPPLY COMPANY | Austin & Laredo Texas

      I ran into Tscarborough at one of their sites so at least one of them hangs out here.

      I pretty much did exactly as you seem to be scheming. I mixed Hawthorne Bond with pottery sand to form my cob. I had a lot of fire bricks left over that I inserted into the lower walls. My oven is shaped a bit taller because I intend for it to serve double duty as an oven in the hot months. So I needed to be able to fit stuff after sliding extra firebricks inside for more thermal mass for baking or other more sustained work. Now I'm more concerned I've gone bonkers on the thermal mass. There's a chance I will be reliant on my wife and her uncanny ability to start fire that melt off your face in order to do pizza and naan well.

      I am also tempting not extending the base and just having the top mushroom out a little bit. I am also going to wait until I've fired it up a few times before I determine my insulation needs for certain. Right now it's all still pretty wet. It's hard to imagine that maybe by this weekend it should be dry enough that I have to dig out the sand or else it'll crack as it contracts.


      • #4
        Re: went cob crazy; how do I insulate with less space?

        Sounds like a cool project you've got going there. Here in Temple it's pretty muggy this morning outside--hopefully your oven will stay moist enough until the weekend.

        Thanks a bunch for the tip on procuring fire bricks. I also heard Elgin-butler has them cheap, but I can't get good enough info from their representative to know I'd be getting the right kind.

        So, I plan on trying to use glass bottles and cob to build up my base, and I was thinking about your quandry. Maybe you could affix empty bottles around the top perimeter of your base using cob or something, and using the bottles as a structural framework, make that mushroom top that you mentioned by filling in with cob. Very bad diagram http://i.imgur.com/8qyVm.png

        Oh, by the way--how big is your oven and how many pounds of hawthorne clay (I assume powdered) did you need?


        • #5
          Re: went cob crazy; how do I insulate with less space?

          I've heard of that bottle method before. I don't know what came of it though. In terms of weather and all, I have had it tarped today to keep a potential thunderstorm from having its way. I was hoping it would actually start to dry by the weekend so it would be more robust when I dug out the sand form. I need to look that up a little bit more but I vaguely recall one should dig that out ASAP or else the cob will contract around the form and crack.

          I have to double check on the amount of clay I used, but I think it came down to three 50# bags. This was for an oven of diameter around 36". Also I built the oven taller. Imagine then the walls go up around six inches before tapering off in a circle with an 18" radius. I can't recall how much sand I blended into it. I think it was 5 100# bags. The methods I read up used volume so weight doesn't necessarily correlate there. It was two pails of sand to one pail of clay per mix. Water was all relative.

          talking with my wife, I think aesthetically we like the idea of attaching a flat cinder block on the sides affected (not the front) right where it would lip over. I'm thinking of trying to drill into the slab and the block and ramming through some rebar with some mortar to help reinforce.


          • #6
            Re: went cob crazy; how do I insulate with less space?

            The Elgin brick are the right kind but the no 2 cheapos will be a little beat up. Fine for the oven.


            • #7
              Re: went cob crazy; how do I insulate with less space?

              Originally posted by Rocko Bonaparte View Post
              There's an Acme Brick in Round Rock, as well. I got mine for my LBE there.