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Dome Location on Hearth - Forno Bravo Forum: The Wood-Fired Oven Community

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Dome Location on Hearth

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  • Dome Location on Hearth

    I am starting to lay-out my 42" diameter dome. My slab is 76" deep x 69" wide and I'm not sure where the center of the dome should be in the 76" direction. The FB plans suggest a 12" oven landing and 4" vent landing and a 4" upper wall. Perhaps I'm missing the obvious but what is the difference between the oven landing and vent landing? I assume the upper wall is the enclosure around the insulation?
    Thanks in advance for any advice.

    ~Bob
    My Oven Progress: http://www.fornobravo.com/forum/f8/p...uild-7155.html

    If you fail to plan, you plan to fail!

  • #2
    Re: Dome Location on Hearth

    Hi Bob,

    I simply made a round template the size of my dome diameter, centered it, and then pushed it back so the space from the back hearth edge to the template was the same as the side hearth edge to the template. Mine is deep compared to some but I wanted it to sit back a bit more because we intended (and do) use it as a fireplace and didn't want the inferno opening too close to the edge so I extended my hearth.
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    Man I miss the build part!

    Good luck and be sure to post a lot of pictures.

    Bill
    Bill,

    Check out my build http://www.fornobravo.com/forum/f8/b...egin-5443.html

    Comment


    • #3
      Re: Dome Location on Hearth

      Bill,
      Thanks for the advice - that sounds like a good way to locate it.

      How does your 4 1/2" of vermicrete perform? Have you noticed if the temperature of the underside of the structural concrete layer gets warm? I was worried about the 6:1 ratio not being enoguh thermal mass and not being enough insulation. So I poured the 4" structural layer per plan but on top of that I will place 2" blocks of foamglas insulation. These blocks come in 24" x 18" 2" pieces and I am using six of them to cover the area under my 42" cooking floor. Then I'll pour a "frame" of vermicrete around these blocks and bury them as well with 2" of this vermicrete. I like how you framed your vermicrete section. It seems as if that would provide a better bae around the edges to build the enclosure.

      If you miss the building process that much then let me know when you'ree in the Connecticut area.

      Best Regards,
      Bob
      Attached Files
      My Oven Progress: http://www.fornobravo.com/forum/f8/p...uild-7155.html

      If you fail to plan, you plan to fail!

      Comment


      • #4
        Re: Dome Location on Hearth

        Hi Bob,

        The vermiculite mix works good but like I've stated before I know thats the weak link in my oven. I can hold temps for a very long time so for what I do it may not matter. I've had readings on the underneath hearth hit 130f I believe. That just seems like it's pulling way too much heat from the floor. I have had the dome at or near 1000f for a loooooong time and the exterior shell doesn't even get above ambient temp. If it really bothers me in the future I may pull the floor bricks up and remove just enough vermicucrete to place a layer of insulating board. That may not be as easy as it sounds as I have the dome walls resting on the floor bricks.

        I don't think I would bury the insulating blocks under the vermiculite mix; I'd go the other way. Not knowing what your boards are you may not be able to but if they're intended for this temp then by all means put your floor bricks on them. Vermiculite works well but the cement makes it a heat sink by default. Way better then concrete but no where near as good as the insulating boards. I've read the specs on most and they're very efficient. The only reason I didn't use them was lack of availability and lack of patience at the time.

        I do miss the build process but my list of projects continue to grow for some reason and they normally involve a lot of power tools. If I was remotely close I'd certainly lend a hand.

        Keep up the good work and post a lot of pictures.

        Bill
        Bill,

        Check out my build http://www.fornobravo.com/forum/f8/b...egin-5443.html

        Comment


        • #5
          Re: Dome Location on Hearth

          Hello Bill,

          I went to buy the vermiculite today and had no clue how many bags I need for a 4" slab! The volume of the slab is approximately 1/2 yard. The bags of vermiculite are 4 cubic feet each. So if I made this layer 100% vermiculite it would require 13.5 cubic feet which equals 1/2 yard. So at a 6:1 ratio it would be 2.25 cubic feet of Portland and 10.75 cubic feet of vermiculite. So three bags should do it?? Not sure what happens when water is included in this mix. How many bags did you require for your 4 1/2" layer?

          Thanks,
          Bob
          My Oven Progress: http://www.fornobravo.com/forum/f8/p...uild-7155.html

          If you fail to plan, you plan to fail!

          Comment


          • #6
            Re: Dome Location on Hearth

            Hi Bob,

            I bought 3 bags and it was more than enough. I mixed most of it on a tarp and some by hand. I made mine just moist enough to hold a formed ball when squeezed by hand. It took a few days to set but it set great. It was easy to make a hole into with a screwdriver but hard enough to hold any weight you're putting on it. Weird stuff.

            Don't sweat it, I'm sure it will turn out great.

            Bill
            Bill,

            Check out my build http://www.fornobravo.com/forum/f8/b...egin-5443.html

            Comment


            • #7
              Re: Dome Location on Hearth

              So I poured the 4" structural layer per plan but on top of that I will place 2" blocks of foamglas insulation. These blocks come in 24" x 18" 2" pieces and I am using six of them to cover the area under my 42" cooking floor. Then I'll pour a "frame" of vermicrete around these blocks and bury them as well with 2" of this vermicrete.
              Foamglas sounds really good: I think this is the first we've heard of it:

              Throughout the manufacturing process, system temperatures swing from -150°F (-101°C) to 600°F (315°C), a challenging process control range for most industrial insulation materials. The system must maintain the precise temperatures or the olestra will not move through the system properly, and the plant would shut down. A reliable insulation system is essential for the plant to run. Additionally, as in virtually any manufacturing operation, there is the potential for pipe leakage. At the P&G olestra plant, because of the flammability of the manufacturing process, the selection of the insulation system took on added significance. To meet all of the system challenges, P&G chose FOAMGLAS® Insulation for the plant because it is noncombustible, nonabsorbent, impermeable and nonwicking – it cannot absorb or transport combustible liquids. Because it is 100 percent glass, without binders or fillers, it cannot burn, even when in contact with the most intense flames. FOAMGLAS® Insulation was also specified because of its high load-bearing strength. Many of the pipes rest on half-pipe saddles that were designed to be used as support bases. The FOAMGLAS® Insulation was capable of supporting those heavy loads without deformity.
              I wonder how it compares to cal-sil and mineral wool products. I like the fact that it's impermeable, and will not absorb moisture. Keep us posted, availability, price, workability. As you've probably figured out, we're a little insulation obsessed around here.
              My geodesic oven project: part 1, part 2

              Comment


              • #8
                Re: Dome Location on Hearth

                I have been "scouring" the internet myself for comparison of Foamglas to vermiculite and finally found something. It appears the originator attempted to maintain a consistent R-value for various insulation materials by varying their thickness. The comparison chart does not include vermiculite/ Portland mix but it does have straight vermiculite. So I would believe that the addition of Portland cement (6 parts vermiculite to 1 part Portland) would require the thickness to be greater in order to maintain an R value equal to the 2.5 this chart utilizes.

                Check out the attached link and let me know your thoughts. Perhaps it might prove useful to you or others. It wasn't easy to find this and I don't know its accuracy but it must be true...it's on the internet!

                >http://www.ais-group.com.au/homeinsu...ive_survey.htm

                Pittsburg Corning is the Foamglas manufacturer but they only sell to distributors. if you follow this link and ennter your zip code it should display your local distributor:
                Pittsburgh Corning FOAMGLAS Insulation

                I purchased mine from Northeast Specialty Insulation. The Foamglas blocks come in 18" x 24" size and vary in thickness from 2" to 6" with increments of 1/2". My blocks were 2" thick and cost me approximately $9.50 each.
                I called and spoke to an engineer from Pittsburgh Corning and he actually never heard of his product being used for pizza ovens. I sent him some information I found and he was pleased to receive it. He went on to explain that the service temperature of the Foamglas is 900 degrees Farenheit. But that has a safety factor built in. I don't think it will ever see temperatures over 900 degrees with firebrick and vermiculite/Portland layer over it. Here is the data sheet link:
                Pittsburgh Corning FOAMGLAS Insulation

                Best Regards,
                Bob
                Last edited by pizza_bob; 06-11-2009, 05:29 PM.
                My Oven Progress: http://www.fornobravo.com/forum/f8/p...uild-7155.html

                If you fail to plan, you plan to fail!

                Comment


                • #9
                  Re: Dome Location on Hearth

                  Yes, the 900f is marginal, for direct brick contact, but the 900 degree temperatures tend toward to the top of the dome, and on the fire contact side. This is maybe where our thermocouple equipped members could help: How hot does the center of the oven floor get under full pizza heat? Anyone got any readings?

                  I'm going to read the data sheet tomorrow, when I'm not so bleary eyed.

                  Just for a comparison, I have my bricks laid directly on 2 1/2 inches of insblock19, by HW, which I think is not so nearly as good an insulator. 19, btw, means 1900 degrees farenheit: It's used for pottery kilns all the time, and is rated for higher temperatures than we get. In any event, the bottom of my support slab doesn't get hot, only warm to the touch the next day when things are cooling off.
                  My geodesic oven project: part 1, part 2

                  Comment


                  • #10
                    Re: Dome Location on Hearth

                    Because it is 100 percent glass, without binders or fillers, it cannot burn, even when in contact with the most intense flames.
                    So, why the nine hundred degree limit? Hmmm.
                    My geodesic oven project: part 1, part 2

                    Comment

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