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Hearth question too - Forno Bravo Forum: The Wood-Fired Oven Community


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Hearth question too

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  • Hearth question too

    I like the idea of building my oven dome around the cooking floor, instead of on top of the floor, for all the reasons stated on this forum. But Im a little concerned about the pressure that the dome footprint will exert on my Calcium Silicate insulation boards. The load of the dome will bear directly on the insulation, rather than being spread over, well, more than just one brick anyway!

    So I was wondering if a thin piece (eg 1/16 inch, cut to the same external diameter as the dome) of aluminium or galvanised / stainless sheet steel might be a good idea between the insulation and the first ring of the dome and the cooking floor. This might also provide a suitable slip plane between the Calcium Silicate which I understand shrinks on heating and the firebricks of the dome and floor which expand on heating.

    However, I can think of potential trouble with this idea too Id be introducing a third material with different thermal expansion characteristics, sandwiched between two dissimilar materials. So alternatively, some 1/4 inch cement sheet might provide enough rigidity to minimise any local compression caused by the weight of the dome bricks, and be a better bet thermally.

    Ive already consulted one member privately on this, but would welcome other members views, especially regarding the metal sheet idea.

    Comments please!


  • #2
    Re: Hearth question too

    Originally posted by Hendo View Post
    I like the idea of building my oven dome around the cooking floor, instead of on top of the floor, for all the reasons stated on this forum. But Im a little concerned about the pressure that the dome footprint will exert on my Calcium Silicate insulation boards. The load of the dome will bear directly on the insulation, rather than being spread over, well, more than just one brick anyway! <snip>
    I am not sure of the weight difference between my Casa1010 FB oven and a brick oven, however, my FB shows no sign of any issue between either the floor or the dome and the SuperIsol board beneath them.

    J W


    • #3
      Re: Hearth question too

      I think JW is right. I don't have the compression strength numbers in front of me, but they are very high. There is no way your oven is going to make the proverbial "dent" in it.

      I think you can move ahead confidently with that.
      Pizza Ovens
      Outdoor Fireplaces


      • #4
        Re: Hearth question too

        Further to this topic: I read somewhere in the sales literature for CalSil (quoted on this forum?) that the product is intended for backup/backstop insulation which I take to mean in non-load bearing situations such as behind furnace walls etc.

        I referred elsewhere to the quoted a table from mattweb (?) which gives a compressive strength of just 2.6 MPa to calcium silicate insulation. This compares rather unfavourably with the 25 MPa for quality concrete. So, I am still apprehensive of putting my dome on the stuff, even though it would make more sense from the point of view of minimising heat loss at the foot of the oven wall...


        "I started out with nothing, and I've still got most of it"


        • #5
          Re: Hearth question too

          I can't comment on compressive strength numbers because I don't understand their practical application.

          I can tell you that the insulation boards I've seen are way more rigid than the kind of foam insulation boards that are used in ordinary construction, and those are rigid enough to put tons of concrete on top of.

          Here's the test I suggested to someone:

          Put a brick on the board.
          Stand on it.
          See if it dents. (I don't think it will)

          166 pounds on 1/3 square foot equals five hundred pounds per square foot.
          divided by 144 = about 3.5 pounds per square inch.
          Last edited by dmun; 04-15-2007, 04:23 AM. Reason: sq. in. calculation
          My geodesic oven project: part 1, part 2


          • #6
            Re: Hearth question too

            Here's my 2 cents...

            I'm nearing completion of my dome right now and used 2 layers of 1" thick insblok 19. Insblok 19 is not cal sil board, but in the same general area from an insulating value perspective (probably not quite as good an insulator, but pretty good).. The data sheet at hwr.com indicates a compressive strength of .3 MPa (whatever that means...) or 38 lb's / sq in... I also built my wall outside around the perimeter of my hearth brick floor.

            As I said, my dome is nearly complete, and I can see absolutely no deformation around the perimeter of my dome.. I have noticed that when this stuff gets wet, it does get a little "mushier", but when it dries out, it dries out firm again... 2 days ago we had a torrential downpour here.. we got about 2" of rain in a couple hours... the wind ripped away the tarp I had covering my construction area (luckily I was home at the time...) and the whole work area got pretty wet... Aside from a bunch of moisture getting sucked into my project, I have seen no ill affects on the part of my insblok underlay...

            Heres anther way of looking at it.. 250 bricks should weigh roughly 2000 lbs.. ballpark... let's just assume for a second that's what you're oven will weight... the area (in inches) of a 48" external diameter oven is a little under 1800 inches.. the area of a 42" internal diameter oven is around 1385 inches. That's a net area of around 425 inches that is "under brick"... That puts about 4.75 lbs of pressure per inch.. (somebody correct my math if it's wrong.. it's pretty late/early over here..).. That is, far, far less than the 38 lbs/inch that would lead to a 10% deformation that is listed in the product data sheet. The cal sil spec you quoted was 2.6 MPa.. I'm working with .3 MPa... and I'm doing fine.. I don't think you have anything to worry about.(BTW, the #'s above assume 3" walls, which is what I did.. if you went with 4" or 4.5" walls, your area would be even greater and thus less lbs/sq in.. Also, the 2000 lbs oven weight is high.. I just wanted to illustrate the math here...

            So, while its good to double check on things, better be safe than sorry, make sure, etc, etc... Based on my experience, you don't have anything to worry about..

            Get your build going and good luck.



            • #7
              Re: Hearth question too

              Thanks guys, that sort of settles the question in my mind!
              I believe the weight of the firebricks and mortar of my 39 in (1000 mm) internal diameter oven would be around 600 lbs (500 kg), so I should be right.

              BTW, I contacted to local distributors of calcium silicate boards. One said this:

              "Our 1,000C cal-sil has a density of 220kg/M3. Its compressive strength is noted as being 0.414 MPa or 4.22 kgf/cm2. We would suggest that if using our board that you have a layer of firebrick on top of it." (Don't know what he means by the 'f' in 4.22 kgf/cm2, or whether the 2 in (50 mm) hearth tiles would qualify here...

              The other hasn't replied yet.

              Tomorrow I'll take off the form work and proceed from there...


              "I started out with nothing, and I've still got most of it"


              • #8
                Re: Hearth question too

                I agree broadly with John's calcs and for what it's worth, under my dome (43" dia and 100 firebricks @ 9lb 9oz ea) the pressure on the Cal Sil sheets works out at 1 psi! That equates to 0.1 kgf/cm2 for metricated thinkers.

                So things should be well within failure limits.

                BTW, if my memory serves me correctly (dubious these days), the 'f' in kgf is 'force', and purists would probably correct me by saying 1 lb-f per sq in. I seem to recall from high school physics days that it has something to do with the difference (perhaps purely in nomenclature) between weight and mass ... or something to do with gravity???

                On with the build!