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Is there a size limit for home brew cast over sand?

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  • Is there a size limit for home brew cast over sand?

    I have been planning on building a 40” ID oven. Base is completed. I was planning on casting over sand with 3:1:1:1 mix with added poly fibers. Now that I’m getting closer to that step I’m getting nervous. I was going to do 2.5” thick and then 4” with perlite. Most of thee builds I see at smaller , should I change my plans or move forward? Any suggestions, tips appreciated. I did most of my research before finding this amazing forum.

  • #2
    I have to admit that I’ve not heard of anyone building a 40” oven using homebrew castable. Generally that size is a commercial sized oven, you won’t be firing it up to cook 3 pizzas for the family on a Friday night. I can’t think of a reason why it would be a problem though. Where I think you may have trouble is building the 4” thick perlcrete layer. By not having a ceramic fibre blanket layer you will not get its benefit of acting as an expansion joint. Placing wet perlcrete against the inner homebrew casting can create problems (swelling and cracking of the perlcrete) when the temperature suddenly hits around 200C and the moisture in that location suddenly turns to steam. Remember that to achieve the correct consistency the insulation mix requires3 parts water for every 10 parts perlite by volume, so there’s tons of free water to eliminate.There is a technique to overcome this problem and that is to apply layers of the perlcrete no thicker than around 35 mm and no richer than 10:1 While on the mix a mixture of 50/50 perlite and vermiculite makes a more workable mix than either of them alone. To make the mix more workable adds handful of powdered clay for every litre of cement added. Going richer than 10:1 drastically reduces the insulation value of the layer. Once your first layer has been applied it requires at least a week of drying before applying a second layer over it. Then the same week of drying before applying a third layer. If this all seems arduous the better solution is to do at least one layer of 25mm ceramic fibre blanket over the casting before any Perl/vermicrete layers.
    Hope you find this advice logical. Oh, and make sure you wear gloves when applying the Perl/vermicrete mixture.
    Kindled with zeal and fired with passion.

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    • #3
      FYI,

      David S is the Forum's cast expert so he has a lot of experience with cast ovens and does them commercially.
      Russell
      Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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      • #4
        Thanks! Maybe I need to consider downsizing a little. I will do some more homework before I get in too deep. May reconsider and do brick.

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        • #5
          So i decided to build the dome with brick, I still think I* will use perlite for at least some of the insulation. David S, will i have the same issue with ". By not having a ceramic fibre blanket layer you will not get its benefit of acting as an expansion joint. Placing wet perlcrete against the inner homebrew casting can create problems (swelling and cracking of the perlcrete) when the temperature suddenly hits around 200C and the moisture in that location suddenly turns to steam. "?

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          • #6
            Yes, whether you build in brick or cast, if you place wet vermicrete against the inner dome the high proportion of moisture can create steam problems. If you lay up the vermicrete in layers of around 35mm and allow each layer a week of drying (in ideal drying conditions) before proceeding, then the problem is considerably reduced. Then the water should be driven out with fire slowly before covering the vermicrete with a hard shell.
            Last edited by david s; 11-02-2020, 12:14 PM.
            Kindled with zeal and fired with passion.

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            • #7
              david s quote While on the mix a mixture of 50/50 perlite and vermiculite makes a more workable mix than either of them alone. To make the mix more workable adds handful of powdered clay for every litre of cement added.... aloha seeing this again
              My build:: https://community.fornobravo.com/for...nch-wfo-hawaii

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