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Cob mix ratios - Forno Bravo Forum: The Wood-Fired Oven Community

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Cob mix ratios

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  • Cob mix ratios

    Hello everyone!
    I'm new to the forum and wanted to say hello and ask a question or two.
    I'm building a cob oven based on Kiko Denzer's book.
    I've decided to use fire clay as my clay soil. The book indicates this can be done but gives no ratios for fire clay to sand ratios.
    I'm hoping some builders here have used fire clay and can give me some insights.
    I could use ratios for the sub floor as well as the inner sand/clay and outer straw, clay & sand layers.

  • #2
    Re: Cob mix ratios

    Hi,

    Unfortunately, I don't know anyone who has done that. Why would the ratios be different, though?
    "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

    "Success isn't permanent and failure isn't fatal." -Mike Ditka
    [/CENTER]

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    • #3
      Re: Cob mix ratios

      Thank you for your reply. We ended up going with a 3 sand to 1 fire clay. It looks to be a good mix. We formed the thermal layer this weekend.

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      • #4
        Re: Cob mix ratios

        Cool. Hope it goes well!
        "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

        "Success isn't permanent and failure isn't fatal." -Mike Ditka
        [/CENTER]

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        • #5
          Re: Cob mix ratios

          The outer straw insulating layer can be straw with only just enough slip (clay water) to stick the straw together. This can then be rendered over with a cement render once the inner oven and insulation are completely dry, which will take a dozen or so fires to do.
          Kindled with zeal and fired with passion.

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          • #6
            Re: Cob mix ratios

            I had planned to wait until the oven was completely dry then insulate with straw bats and cover those with cob. Let that dry, put up an enclosure and fill it with insulation (something fireproof so I can run wires or whatever else through the structure if need be).

            Assuming I ever get to buy a house... :sigh:
            "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

            "Success isn't permanent and failure isn't fatal." -Mike Ditka
            [/CENTER]

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            • #7
              Re: Cob mix ratios

              Ok that would work. If you make the enclosure a little bigger you can live in it beside your oven.
              Kindled with zeal and fired with passion.

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              • #8
                Re: Cob mix ratios

                Hi David,
                We're done with our inner layer and preparing for the outer insulation layer. I'm planing on using wood chips or bark chips for the outer layer mixed with clay. We've been using fire clay as our clay source for the whole project. Would you suggest adding som sand to the clay mixture for strength? The fire clay is a powder.

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                • #9
                  Re: Cob mix ratios

                  What you want is air in that layer. If all the spaces between the material you use for insulating are filled, then it won't insulate too well. You only need enough clay to hold those bits together. Use as little clay as you can. Sand will strengthen it but it will also reduce the insulating value.if you are building an enclosure you can place the stuff in dry. Peanut shell may be better because it is less dense.
                  Kindled with zeal and fired with passion.

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