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AS Oven Question - Forno Bravo Forum: The Wood-Fired Oven Community



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AS Oven Question

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  • AS Oven Question

    I am currently in the process of building an AS barrel type oven. I have modified the plans slightly and would like some advice. The inside dimensions are approximately 32 inches wide by 36 inches deep and 16 inhes high at the peak of the arch. I constructed the hearth slab with 4 inches of concrete, followed by 2 inches of 1900 degree insulation, covered with approximately 1 inch of 1200 degree refractory material, followed by fire brick layed 2-1/2 inches thick. My walls are firebrick layed 9 inches high by 4-1/2 inches thick. The dome is also layed 4-1/2 inches thick. My concern is as follows. I only have approximately 4 inches of space from the outside of my barrel walls to the inside of the final cladding (regular brick). I have read many posts that require the walls to be buttressed to support the dome when heat is applied. By following the AS plans this would require the dome to be covered with 3 more inches of concrete, thus adding to the thermal mass. I don't think I need this thermal mass and if I do so only have 1 inch of insulation space on the two side walls. Would a 4 inch layer of 5:1 vermiculite/portland be sufficient to support the side walls? Any advice would be appreciated.

  • #2
    Re: AS Oven Question

    "Would a 4 inch layer of 5:1 vermiculite/portland be sufficient to support the side walls? "

    As I understand, this would then be supported by your outside brick walls ? If so then you should end up with sufficient buttressing. 5:1 vermiculete has a surprising compressive strength. I would be tempted to back this off to about 8:1 for a better insulation value. For that size of arch, you would probably only need the vermiculite for about 6-8 inches up and go with either loose vermiculite or a 12:1 mix above that


    • #3
      Re: AS Oven Question

      Yes, the outside walls would add to the buttressing. However, since they will also support the brick arch cladding I assume that I may need to make this outer arch more of a semi-circle to avoid the same circumstance? My intention was to shape the vermiculite mix with a form to act as the "back up" for the arched brick cladding. Would you suggest following your original idea only eliminating the loose vermiculite? Another possibility that I thought of was to install a 3 inch reinforced concrete wall around the sides of the dome (dowel into the slab) and add 1 inch of rigid 1200 degree insulation (6 to 8 inches up), with the top of the dome following your 8:1 mix and keeping the outer arch around the same as the barrel arch. Again this adds a little thermal to the sides, but unsure of how much of an effect. Maybe I am overthinking things. I have very experienced commercial masons doing the install, but they don't have much experice with heat being applied to a barrel shape (only fireplaces). They don't seem concerned at all. When they built the barrel they used no side reinforcement (just let the walls sit for a couple of hours) and the arch forms were removed immediatley (looks great). Thank you very much for your advice as well as any others out there who may have an opinion. I will post pictures of the project when time allows.


      • #4
        Re: AS Oven Question

        I concur with Neill in that it would be sufficiently buttressed...but you may consider some type of layer that is a bit more compressable than the vermicrete...at the very outside of the insulated cladding...otherwise there might be possibility of cracking your outer brick walls when the oven fully expands...
        "Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity. " Charles Mingus
        "Build at least two brick ovens...one to make all the mistakes on and the other to be just like you dreamed of!" Dutch


        • #5
          Re: AS Oven Question


          Reviving an old thread, because I am in a very similar situation. The deal:
          Modified AS design from Rado in Australia. I've modified a bit, and now have:
          Concrete slab base. 4" of V-crete on top of base. 2" of refractory cement for mass above the v-crete (bread is the primary use here). On top of the 2" refractory cement is my firebrick, laid flat (2.5") Basically 4.5" of mass on the bottom of the oven. Oven floor size is 30" (length) by 34" width. Dome will be approx 16-17" high. Bricks for the dome will be laid thick (4.5") for more thermal mass. Side walls will go up 2 bricks (flat) and the third row will be beveled to meet the dome. So here comes the question...

          As in the above note, I have roughly 4" of room on the sides of the oven to provide both support to the dome AND insulation, before I hit the outer concrete blocks (halves). The plan calls for reinforced concrete around the outside walls of the dome for both support AND mass. I believe my options are/could be:

          2" of reinforced concrete around the sides, and 2" ceramic fiber board (which I have a lot of! ...don't ask) up against the concrete block. Or, 3" of re-inforced concrete and 1" of ceramic fiber board against the concrete block. The concrete block will have re-bar support. I've also been contemplating using 4" of ceramic fiber board as both the side wall support and insulation. This set-up would fully rely on the re-bar/concrete block to provide support.

          If YOU had 4" to support a barrel dome and insulate, how would you approach it? Plenty of materials...so don't be shy on spending money!


          Pic or two to help visualize attached.


          • #6
            Re: AS Oven Question

            First, let me say, once again, that the AS oven and Rado's design are not very good for home use, and only minimally efficient for a commercial oven.

            To heat saturate 4.5" of masonry floor is going to be exceedingly difficult, as well as not needed for home use.

            Nor is more mass than the brick required in the dome, nor is concrete reinforcement required for the support of the dome.

            Parge the dome with 1/2" to 3/4" of refractory cement and use the rest of the space for insulation.


            • #7
              Re: AS Oven Question

              I built a barrel oven and found myself in a similar situation as you are in. I went with the barrel (instead of the dome) because I had a lot of tapered bricks and wanted to make as few cuts as possible. It turns out that the barrel was a good choice since I have started a bread bakery using the oven (it holds 12, 1.5lb loaves at once).

              I looked at Rado's site and read a lot on this forum and went with a bit of a hybrid. I have 4" of insulation directly under my hearth floor (2.5" fire brick) and I have 4" to 6" of foamglass board/vermicrete on the three sides (2 sides, back) of the barrel and 4" of ceramic blanket on top of the barrel. I poured lose vermiculite on top of that.

              I have rebar reinforced half cinder blocks, just like you, for the exterior of my enclosure. Between the cinder blocks and the fire brick walls of the oven I have the foamglass/vermicrete. I didn't want any more thermal mass and Because of the compression strength of the vermicrete and the foamglass along with the strength of the cinder blocks I didn't think the walls were going to be able to shift.

              I'm very happy with my oven... I use 15 to 17 pieces of wood (20" long by 5" triangular split, some smaller) to heat it up and I can bake 60 to 80 loaves of bread 18 hours after I have put the fire out.



              • #8
                Re: AS Oven Question


                Thanks for your thoughts and advice on this. Plugging along...