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Refractory mortar queries - Forno Bravo Forum: The Wood-Fired Oven Community



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Refractory mortar queries

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  • Refractory mortar queries

    I'm looking at options to insulate the outside of my oven dome (see post on "Redgum Roarer"). The dome is outside and not built under an enclosure (another story). I was thinking of coating it with a dry mix refractory mortar and then probably add a coat of paint to seal it from the rain etc. I've notice the Forno Bravo site has "Refmix" but it is fairly costly to get it down to Oz and especially Adelaide.......although I'm open to suggestions.

    At a local commercial kiln/boiler/oven/furnace business I have found this dry mix product made by Shinagawa called “Shiralite 120MW” (used to be called MORAL COOLCAST 120 MW). I can get it for about $44(AUS)/20kg bag. The spec sheet says:
    -medium weight insulating castable
    -based on selected lightweight aggregates and calcium aluminate cement.
    -1200C/2192F max service temperature
    Can get more details if needed……

    I've seen this applied to the inside of commercial applications in parts where they can't apply firebrick - stuck to the boilers steel door. Its strong, high abrasion resistant stuff. These ovens/boilers run at 1000C/1832F and you can put your hand on the outside shell when it’s running (without burning of course).Talking to the local guy, he knows of installations where this has effectively been used to coa the exterior of pizza ovens. Sounds like I would need about a 40 - 50mm thick layer.

    This seems like it might be the product to use but I wanted to hear feedback from those on this forum becuase its helped me out a great deal. Am I headed in the right direction? Is this the right sort of product?

    Should this or something else similar be the right thing - I've also got to work out approximately how many bags I need. Any point me in the right direction to calculate the cubic area, please?

    Oven (outside measurements):
    Circumference = 3800mm/149inches
    Width = 1350mm/53inches
    Height = 640mm/25inches


    Build #1

    Build #2 (Current)

  • #2
    Re: Refractory mortar queries

    The insulating castable sounds effective, but expensive. Do you lack access to vermiculite or perlite, from which you could mix with cement to obtain an insulating layer? You could then stucco over it for appearance.
    Last edited by maver; 05-16-2007, 11:07 PM.


    • #3
      Re: Refractory mortar queries

      thanks Maver. Yes they have vermiculite. I'm asking about pricing now.
      Since my original post I had an interesting phone call.....I got onto a couple guys further up the supply chain. It turns out they can get me this and other supplies cheaper.

      It sounds like an insulating blanket might be a better option. Originally I thought I couldn't use it on my exposed dome but one guy explained about putting a wire mesh over the blanket and then applying a render (think you guys call it stucco) on top of that. He said this will give a better insulative value.

      Has anyone done this? and does the final layer of render tend to crack when its put on top of the blanket (with mesh)?

      sorry about the change in direction .....

      Build #1

      Build #2 (Current)


      • #4
        Re: Refractory mortar queries

        See pictures below

        Attached Files


        • #5
          Re: Refractory mortar queries

          Volume of sphere calculator:

          Remember, it's radius, not diameter. Subtract inner sphere volume from outer sphere volume, and divide by two (it's half a sphere). The entry should be about the same volume as the missing door hole.
          My geodesic oven project: part 1, part 2


          • #6
            Re: Refractory mortar queries

            Damon (Bacterium)

            I constructed my oven just as you describe - firebrick dome and entry which I used Heatstop 50 refractory mortar, then skim coated (less than 1/2") the asssembled dome with the refractory mortar, then 2" ceramic blacket, wire mesh, then 3" of insulating concrete (perlite & portland cement mix). The wire mesh gave the perlite mix something to hold on to until it set up. I then covered the entire oven with Type S and Type N mortar (stucco) - Type S and Type N are both weather tight (the type N is much 'stickier' - good for vertical surfaces). Kept covered for 7 days with damp towels to allow it all to cure; then began my fires.
            I guess I am one of the lucky ones.....I have done nearly 20 firings and have not found a single crack in the dome. I did have one hairline crack develope last weekend just above the entry (I only used 1" of blanket and 2" of perlite in this area). I now plan to cover the entire oven with mosaic tile, not really worried after this many high temp fires that it will crack.

            To summarize, nearly everyone experiences some form of cracking. I think two main factors have lead to my having zero dome cracks:

            I cut each brick to fit...using minimal mortar in my joints (was a very tedious and sometimes frustrating task).

            Well insulated (I think the 2" blanket is the key). With an internal dome temp of 1000+ degrees F, the exterior of the dome has never risen over 140 (the sun actually heats it to 125).

            Hope this helps


            • #7
              Re: Refractory mortar queries

              thanks RT, that's given me confidence now. Thats great work where the sun makes the exterior of your oven warmer than the heat from the oven itself.

              Luis, thanks for posting those pics too

              I might chat with the local supplier (Alphacon) and gets some more details on the insulating value/rating of the various products he has. I think I'll go the insulating blanket, then some sort of insulating concrete and then finish with a weatherproof stucco/render (N or S).

              Just with how my dome is at the moment........I've had about 6 cooking fires (after initial curing fires) with my dome now. There has been some cracking of the outer skim coat on top of the brick layer. Inside the oven though there is no cracking at all of the assembled brick dome, I'm guessing the oven has now "settled" down. Once I add the blanket/insulating concrete/stucco layers I'm guessing there should be minimal cracking (assuming I dry/cure it properly).

              Working on the plans for my door (when baking bread etc.) - I was intending to make it out of 2 pieces of sheetmetal with a void in the middle. I could then fill this void up with some insulating material - Perlite I guess.
              To get a good seal around the door I was think of using glass fiber rope.

              ...ahhhh....freshly made, home cooked pizzas are the best

              Build #1

              Build #2 (Current)