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Refractory Confusion - Forno Bravo Forum: The Wood-Fired Oven Community


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Refractory Confusion

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  • Refractory Confusion

    Hey everybody, sorry for the list of questions, but I am at my wits end.

    I am having some trouble down here in Guatemala locating the right parts for my oven, we intend to fire every day and supply the local area with bread, and cook pizza in the evening so I am looking for Refractory concrete.

    I want to build an oven with thicker cladding and made out of refractory concrete, but so far I have only found 2 distributors who sell applicable refractory concrete in Guatemala, one is premixed, very expensive and the other has a Aluminium content of 50%

    Now I understand that the Alu content should be 35-45% so is that other 5% really that much of a danger? Can I use that and avoid cracking.

    Also I understand that you should not use rebar in the Refractory concrete, so where would by 5/8 Rebar for the hearth slab go, if it was made from refractory concrete, also the mesh in the dome cladding again, refractory concrete.

    And if the concrete has a Temperature range up to 1250 Degrees, does that mean I have to fire to that temp so the concrete sets.

    The major problem I am finding is that most of the refractory products are made locally and therfore I have to deal with representatives in Spanish, not such a bad thing but when you explain it is for a traditional oven they glaze over a bit and contradict what is said in The Bread Builders book.

    The insulation slab and the hearth slab should be poured on the same day to help them bond, but refractory products and portland could cause Flash setting of the concrete, no? so I can't mix them either, right?

    I cannot get Vermiculite or Pearlite except shipping it from the USA. I was informed that I can use Volcanic ash, about the size of a pea or bigger for my insulation, is this true?, I hope so, lots of volcanoes round here!

  • #2
    Re: Refractory Confusion

    Here's a long thread on volcanic pumice as an insulator: there are others:


    Search for "pumice". It seems it would work fine, "R" value similar to vermiculite. I don't know if we've heard from anyone who has actually used it.

    I know nothing about refractory concrete. Someone else can perhaps help.

    As far as the re-bar reinforced support slab, no need for that to be refractory at all: it's underneath the insulation.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Refractory Confusion

      Maybe I was not being clear but the oven I'm building is from scratch, firebricks and mortar. The refractory concrete would be in the hearth slab, as I understood the way the book read.

      Both the hearth slab (which should have rebar in it) and the cladding (has mesh) are to be refractory concrete, and the book clearly say that rebar in this type of concrete should be avoided, due to the high temperature cycles.

      The insulation slab is to be volcanic pummice and cement, also it is to be capped with the volcanic pummice and cement.


      • #4
        Re: Refractory Confusion

        If you have firebricks and ash then your are in business.

        The hearth slab is just standard concrete with rebar in it.

        Then there is an insulating slab poured on top of that (ash and cement)

        Then the floor of the oven is laid with fire bricks.

        And the dome is built using high-heat mortar and more fire bricks.

        Then most people clad the dome with a half inch to an inch of high-heat mortar.

        Then the oven needs about six inches of insulation. I assume your ash would work, just mix it with this formula - 5 parts ash to 1 part cement(not concrete) and spread it over the whole dome.

        To make high-heat mortar, just mix 3 parts sand to 1 part cement to 1 part fireclay to 1 part lime.


        And you should be just fine.

        I think most were assuming you wanted to cast the whole dome and floor, I know I did.

        I hope this helps.

        And keep asking questions, until it all makes sense!
        My thread:
        My costs:
        My pics:


        • #5
          Re: Refractory Confusion

          Ok thanks so I'll keep asking questions.

          I am building a pizza oven from scratch using The Bread Builders book as a guide, using low duty firebricks, using a refractory mortar as the glue, and then clad it all in refractory mortar so I can retain more heat, and insulate with volacnic ash.

          I believe it base goes.... Foundation, block, insulation layer, hearth slab, firebricks, refractory concrete cladding, followed by big insulation layer of volcanic pummice.

          But I am still worried that the cladding would have to reach 1250C to fully cure, as I have been informed by the manufacturers down here, although I could be having translation from Spanish problems.


          • #6
            Re: Refractory Confusion

            I also have the Bread Builders book. The oven as built in this book is a bread oven for sure. I have my doubts that is will easily get to 700+ pizza temperature in a timely fashion.

            Before you commit to a design go to this url:

            Download the Pompeiii Oven instruction book. The instructions are much clearer in the FB manual although the oven design criteria are not the same.

            Read first, build enlightened.
            Pesky spelling...

            The real art of conversation is not only to say the correct thing at the right time, but also to leave
            unsaid the wrong thing at the tempting moment.


            • #7
              Re: Refractory Confusion

              I think you should look at how they build their ovens in Guatemala and use the materials available there making adjustments based on common sense
              lava , clay I am sure it's plenty , when I was in panama I had a oven built for a restaurant and I used a local guy they use a very rich clay for their ovens it contains lava naturally ..they make their own bricks, the same clay is used for insulation of the dome.
              I guarantee you the oven is as hot as you need it and more and it will stay hot until the next day I was impressed for the simplicity and efficency of these ovens