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Why not curved lentil as oven opening?

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  • Why not curved lentil as oven opening?

    I am in the process of building my second 42" Pompeii style with half bricks after a 13 year hiatus. Enjoying the planning process and finding this forum invaluable again. I am half wat through the stand on the current build. Photos on Flickr at (if interested original build photos available at

    My question is why do most opt to use a wooden mould to build over opening arch rather than use curved stell to define and hold shape. See my photo below from original build. As for the second layer of floor bricks over fire bricks, that is another story.

    Steve from South Australia
    Click image for larger version

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  • #2
    Steve, welcome back.

    I'd venture that for a wood form versus steel, it's mostly a matter of which material is easier to work with.

    I think the typical DIY builder has the tools to form wood, as well as easy access to wood.

    While I'm comfortable with both materials, in my case my arch is not a perfect arc/circle. There were a few specific cooking items I wanted to be able to fit in to my oven, thus the ellipse-ish shape versus the circle.

    Even though I'm comfortable working with metal, and I have the tools to work it, it was much easier and faster to create a wood form than to try and bend a half-ellipse to my desired dimensions.

    Perhaps the only other thing I MIGHT address is considering the possible effect of thermal expansion. A typically-sized steel arch for a 42" oven like I have could expand just under 1/4" in length when seeing a 900F temperature change. It's possible that the movement of the steel caused by thermal cycling could cause issues, it depends on the detailing.

    Nothing wrong with either method, an acquaintance of mine used a chunk of metal from an old propane cylinder to form his arches.

    My Build: Mongo's 42" CT Stone Dome Build


    • #3
      Appreciate the informative response Mongo. Will see how things play out by the time I get to building the dome. Thinking through various options is all part of the planning and building process! My build is a weekend by weekend job so plenty of time to think things through.

      One other thing that I can't settle on is the reason why people seem to prefer cutting fire brick base to a round shape before building the dome. Interested in the advantages, because it is quite a hassle compared to setting these bricks without cutting to cover beyond dome edge. Interested in others thoughts.
      Last edited by KISS4me; 08-26-2020, 05:24 AM.


      • #4
        Hi Kiss4me: the brick floor is cut to outer dome edge as to facilitate insulation afterwards. Preferably, you want to wrap floor edge and dome in one go.

        some would even cut the brick floor to fit INSIDE the dome so that the floor can be replaced after years of heavy use. But that is a whole different story.


        • #5
          Thanks for explaining why you would go to the trouble to curve cut bricks. Wrapping wall and floor in insulation makes sense. Cant see ours getting enough use to justify floor replacement. Could always lay replacemnt bricks on top of base if that was the case anyway. Because the floor of my first build did not absorb enough heat a I simply laid better suited bricks on top and this does the job.
          Lots of ways to skin a cat!


          • #6
            Slow progress. Stand construction coming along as per the photos here


            • #7
              Visited a pizza oven builder/supplier here in Adelaide (Traditional Brick Ovens) and pleased to see that I can source ceramic fire blanket, floor tiles and flue from them. Although they do kits, not redbrick dome that I have in mind, they use a very simple hearth structure - no perlite, but Hebel blocks (about 75mm or 3") then one layer of CalSil (or ceramic heat board) of 25mm thickness and 50mm floor tiles. Although I found a few useful forum threads about this I have a few questions:
              1. Is CalSil over Hebel blocks adequate insulation compared to perlite?
              2. Do you need to mortar or sand fill between Hebel and Concrete and Hebel and CalSil?

              Any ideas/links appreciated.

              PS: Hebel is a lightweight concrete construction block. Probably called something else in other places


              • #8
                50 mm calsil board (usual thickness) over 75mm Hebel is adequate underfloor insulation.If your question relates to that compared to 50 mm loose perlite over the 75mm Hebel, then no, the loose perlite has higher insulation value than calsil board, however most builders prefer to have the underfloor insulation layer far more solid. Usually recommended is a 5:1perlcrete or vermicrete which has a poorer insulation value than calsil board.It’s big advantage is its price over calsil board. It’s disadvantage is correct water addition, mixing, placement and then water removal.It is normal to have your highest quality insulation beneath the cooking surface so floor tiles, calsil, Perlcrete, Hebel would be the normal order of layers.
                2. Usually dry placement is sufficient.
                Kindled with zeal and fired with passion.


                • #9
                  Excellent information. Thanks David. My conclusion:
                  As a base I plan going with 90mm reo concrete and for the next 100mm - either pericrete or a combo of Hebel then pericrete. Then undercooking surface ceramic heat board (the one I was quoted on is 25mm thick and 1000 x 500 for $A35 a sheet) before the floor tiles.
                  Sound OK? Any comment on all pericrete vs Hebel then pericrete in the 'mid section'?


                  • #10
                    If you do all perlcrete rather than Hebel /perlcrete combo you’ll be using way more perlcrete which means way more water. The proportion is 3 L water for every 10L perlite. I’ve found a mixture of 50/50 vermiculite, perlite makes a better mix than either alone. You will probably use it for the over dome insulation anyhow. The vermiculite does not contain the irritating dust that perlite contains. Don’t be too misled thinking the perlite is better because it has higher insulation value. Once you add some cement to it the insulation value of perlcrete and vermicrete is identical.
                    Last edited by david s; 09-13-2020, 12:21 AM.
                    Kindled with zeal and fired with passion.


                    • #11
                      Well I never. Until you said this after reading the forum I had assumed that perlite was just a different name for vermiculture! Will go with the 50:50 mix as you recommend. Great advice about the water and hence the advantage of Hebel base before verm+perl crete layer. I now have a plan to work towards. Thanks again.


                      • #12
                        Cast some weep holes in the supporting slab when placing the mix in the form. This allows a pathway for steam to escape both during the water elimination phase as well as later if there’s water ingress from
                        Kindled with zeal and fired with passion.


                        • #13
                          Weep holes. Good point. What diameter and how do you create - pvc or metal pipe? leave in or remove when near set?


                          • #14
                            Just to be clear on David's comments, it is a 5 to 1 ratio of either perlite or vemiculite for floor insulation (btw, they are not the same materials, just similar insulation values when mixed with portland) or a 50/50 blend of vermiculite and perlite. It typically requires twice the thickness of p/v crete to equal a CaSi board.
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                            • #15
                              A member used cheap 1/2" pvc pipe, install just below surface of the top of the concrete hearth (say a 1/2") so he could screed the heath nice and flat. Then took a piece of rebar and from the bottom of the pvc and knocked out the thing piece of concrete. 2 or 3 should be good.
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