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Hearth and Dome construction questions - Forno Bravo Forum: The Wood-Fired Oven Community



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Hearth and Dome construction questions

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  • Hearth and Dome construction questions

    Hello all,

    This is my first post.

    I am currently making a brick oven which will be a igloo style. I have already built the foundation slab and cinder block walls. I am at the hearth construction stage and I have a few questions.

    - Why can't the hearth be constructed with reinforced rebar portland cement layed with firebrick ontop? Does the cement itself absolutley need to be insulated?

    Once I complete that stage, I am on to the dome construction. Now I love the look of the brick style, but I love even more the convenience of modular precast refactory concrete molds. I intend to precast 6 section molds of the dome, including flu and mortar them together. Then I will cover the dome with a ceramic blanket and lay normal red brick ontop.

    Is this a crazy idea? Am I nuts to think that this is more convenient? Can this idea work?

  • #2
    insulate the oven

    You can make the oven without insulation under, but you'll find that the concrete under the oven is a huge heatsink. The firebricks fairly efficiently transfer the heat, but concrete is not an insulator and it will slowly soak up heat from the firebricks. My father's igloo style oven is made with concrete covered by firebricks with a thin layer of sand between the firebricks and the concrete. He also has the flue rising from the top of the dome (which loses heat faster than the pompeii design), so it is not likely to be just the insulation that's the problem, but it takes his oven about 3 hours to heat up to good pizza temperatures while mine of similar dimensions takes 1 hour.

    Others have have talked about a homecast refractory dome but I don't think I've seen that any has tried this. I wonder what Drake has to say about that idea, as I believe he cast his vent. There are different refractory mortars, some air setting and some heat setting. It's certainly an interesting idea but may not be simpler than brick construction once you deal with preparing a mold, the cost of refractory and curing the segments. I bet I assembled my brick dome in less time than it would take to cast a dome, unless you have a separate mold for each piece - but I will agree with you that it was not simple (at times mindless, but not simple).


    • #3
      I think Maver has the right term. Firebrick and concrete are heat sinks, that basically suck heat away from the inside of the oven chamber -- which is where you want it. Doing a good job of under-oven insultion is not difficult, or expensive, and we highly recommend it.

      This is more than just theoretical. There are ovens out there build on a standard concrete slab, and they do not hold heat well. I even did a temporary installtion of an FB oven on straight concrete slab a few years ago, to see what would happen. The bottom of the concrete got very hot, very fast. I took the oven apart.

      On whether to cast your oven, or build it from bricks; casting a dome sounds tricky. You have to get the shape right, and you have to worry about the casting material. Good stuff is expensive, and it would be a shame to build an other with a material that doesn't heat up well, or last.

      Firebricks are a mass produced item, made from good materials and they are filn fired -- and they are not too expensive. They are mostly made for fireplaces and industrial applications, and we get to ride on those production runs. Just like our high-tech insulators.

      Hope this is helpful.
      Pizza Ovens
      Outdoor Fireplaces


      • #4
        I think I have my answers then. Insulate the hearth, and go for the brick.

        A few other questions....

        -Can we buy precast ovens in montreal,canada? Do you know a distributor?

        - Is the perfect ratio 6:1? 6 being regular garden vermiculate. 1 being standard portland cement?

        - What do I use to bond the firebrick onto the hearth? Just normal high temp mortor?

        Cheers gents!


        • #5
          Of course Forno Bravo would be thrilled for you to use one of our precast ovens. We ship all around North America from a central warehouse, and we have our Canadian export doc's and dealer/installers in BC, Ontaria and Alberta. We can talk about that off line -- I am on james@fornobravo.com.

          Yes, the 6:1 (garden vermiculite to pure portland cement) works well.

          How you set the floor depends on which type you are building. Bricks and the Artigiano oven work best in either sand, or a sand/fireclay mixture, where they are levels and rest rightly together. The Casa and Premio floors are cast as pie shaped pieces and work best when they are set with refractory mortar and the joints are closed with refractory mortar.

          You can check out the Pompeii Oven plans here:


          Pizza Ovens
          Outdoor Fireplaces


          • #6
            Is there a special technique in mixing the cement and vermiculate? Example mix the portland and water first, then add vermiculate?

            Also for a 30k bag of portland, does that mean i need 6x30k bag of vermiculate? That sounds like an awful lot. Please excuse my ignorance...


            • #7
              There is a description of vermiculite and how the hearth works here:


              It goes over mixing, dimension, forms, etc. That should help. In general, you mix the portland and vermiculite by volume, not weight, using shovel-fulls, or bucket-fulls. My them dry to coat the vermiculite, and add water until you get something like oatmeal. It sets nicely.

              Pizza Ovens
              Outdoor Fireplaces


              • #8
                Is there anywere I can purchase refrax in montreal,canada? Any canadians out there?



                • #9
                  Originally posted by asparapani
                  Is there anywere I can purchase refrax in montreal,canada? Any canadians out there?
                  Refractory dealers sell insulating blankets for on top, and blocks or sheets for underneath the oven. They go by different brand names in different areas. They are rated for temperature: a wood fired oven will rarely get above 1000 f. so the lowest rated material should be fine.

                  An ordinary brick yard or mason supply will be a better bet for your firebricks and refractory mortar. Refractory dealers sell a higher grade (and price) of firebrick than we need.
                  My geodesic oven project: part 1, part 2


                  • #10
                    Refrax is our product. It's made in Italy by our oven producer, and we stock it in NorCal. I like it, it's made specifically for brick ovens, and we recommend it. It's the "Kleenex" of Refractory Mortar here -- it's sold by just about every building supply store in the country.

                    That said, and as David notes, there are other refractory mortar products that you can find locally. Check your local suppliers and see what you find.

                    You can also make your own, either with Portland cement or calcium aluminate.

                    The recipes are here:


                    Lot's of good choices.
                    Pizza Ovens
                    Outdoor Fireplaces


                    • #11
                      Vermiculite cement mixing


                      In some of my old answers I had explained how to work with vermiculite/cement mixture and percents between them.

                      If is your intention to use the vermiculite/cement to isolate your dome, your could go as high as 8:1, that is better as isolation.

                      To do the mix, it will better to mix the vermiculite with the cement when dry, this permit each particle of vermiculite to be surrounded by cement before be wet.

                      Whit the mixture perfectly homogeneous could be better to shower it with a light stream (more a smooth shower) and mixing at same time, to not “wash” the cement from the vermiculite particles.

                      Good work.



                      • #12
                        hello all,

                        Yet another novice question that needs proffesional answering...

                        Is it possible to use high temp mortor paste,(the one to bond firebricks with) and cast it into a mold instead of refractory concrete?

                        I plan on making my molds throughout the winter in my heated garage so it will have time to cure. I'm still stuck on the dome mold technology.



                        • #13
                          Originally posted by asparapani
                          Yet another novice question that needs proffesional answering...
                          You're right on this - You need a professional answer. Refractory mortar is not made be any wider than a brick joint. There are castable refractory materials that harden at room temperature, but other than a flue transition, no one here has much used them.

                          You need to talk to a refractory dealer. These places sell to people building serious things like kilns and boilers, and will have access to an applications engineer, who will be able to answer your questions. Refractory materials, even fireplace mortar, is expensive stuff, far more expensive than firebrick, and you don't want to cast up a big dome sector, and find that it's cracking on drying or firing.
                          My geodesic oven project: part 1, part 2


                          • #14
                            thanks for the profffesional suggestion....

                            I'll go speak to a refractory specialist


                            • #15
                              Re: Hearth and Dome construction questions

                              Hmmm. I am working on the dome, and have done 4 courses, and it looks as if the 5th will be the last that can be done without some sort of formwork. I have used old common red bricks, which vary in size quite a lot, firebricks too expensive around here. ($7 each). Have used mortar only, no shims, to place the bricks, and so far it has worked OK. Using formwork will be a pain, and I have been considering making a precast refractory concrete dome section to finish the dome off, rather than laying more bricks.

                              Sounds as if this is not really possible - too hard to get a mix of concrete which will stand the heat and which will set at air temperature.

                              Filling a perfectly good dome with sand to make a sand form seems like a desperate measure. Any other ideas?