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Why slather mortar on the outside of the dome - Forno Bravo Forum: The Wood-Fired Oven Community

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Why slather mortar on the outside of the dome

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  • Why slather mortar on the outside of the dome

    The guy at the firebrick yard asks me today why put any stuff on the outside of the dome. He thought that maybe it had to do with filling any holes from the initial build. He says that I should build the dome with SAIRSET mortar (premixed) and then cover the outside of the dome with insulating cement.

    So, what do you guys think. mortar all the way, or a combination of mortar in between bricks and insulating cement over the dome?

    Thanks

  • #2
    Re: Why slather mortar on the outside of the dome

    surely, someone must have an opinion. First, I assume that the slathering of mortar, or cladding, it used to stop smoke perhaps from going thru the brick. It could also be to add extra mass, although, if that were true, then, why are we cutting our bricks in half for the courses. it could be that the cladding is extra assurance that the dome won't move or shift. I believe that the guy at the firebrick store was implying that a well built oven hardly needs mortar. As in "the mortarless oven build"

    So, does anyone have advice as to the type?

    thanks

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    • #3
      Re: Why slather mortar on the outside of the dome

      Primarily to fill the gaps and make it a solid unit.

      Comment


      • #4
        Re: Why slather mortar on the outside of the dome

        So, should I clad the outside in Sairset mortar, or the insulating cement?

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        • #5
          Re: Why slather mortar on the outside of the dome

          You should fill the gaps flush with refractory mortar, then use insulating material to coat it (unless you want to add more mass, mine is 3" of brick and 1" of stucco).

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          • #6
            Re: Why slather mortar on the outside of the dome

            I think the exterior cladding is an attempt to solidify the unit, but I haven't heard of any less cracking in clad compared to unclad ovens. Cover it or not, as you choose.

            The insulating concrete is different: if you use this instead of the blanket you want to layer this on at least four inches thick.
            My geodesic oven project: part 1, part 2

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            • #7
              Re: Why slather mortar on the outside of the dome

              It is a good way to use up excess mortar and does add a bit of thermal mass. Otherwise it is not necessary.

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              • #8
                Re: Why slather mortar on the outside of the dome

                Dino had a nice explanation:
                Just to be sure everyone is on the same page, if by "skim coat" you mean the high-heat mortar you smear on the outside of the dome bricks while you build it, then that "cladding" is part of the ovens "mass". You have to decide how much extra mass you want just like you decide on the oven width and height. I wanted a 1/4" of extra mass added around the dome to 1) add mass that heats up and releases the btu's just like the fire brick, 2) gives you the feeling of somehow adding some structural integrity to the dome although it really doesn't but I suspect it might make the joints stronger if you add it to the outside of that joint while it's still moist. And 3) unfortunately it shows all the hairline cracks you'll get when you 1st fire up the oven but that's OK.

                Some have added 1/2" of extra cladding, others barely a skim coat. Your oven will heat up faster without it, but with it, you retain more heat for longer cooking times and works better for bread baking.

                You can add the cladding while you work on the rings to use up the extra mortar or go back after you're done make sure you have an even amount.

                Happy WFO building, -Dino

                Comment


                • #9
                  Re: Why slather mortar on the outside of the dome

                  I added an extra inch of refractory mortar to mine so that I would have more mass. This allows me to bake a couple of extra batches of bread.

                  Also, I think that sari set premixed mortar is an "air set" mortar. Air set mortars are not appropriate for outside building as they remain water soluble even after is has dried...

                  Drake
                  My Oven Thread:
                  http://www.fornobravo.com/forum/f8/d...-oven-633.html

                  Comment


                  • #10
                    Re: Why slather mortar on the outside of the dome

                    This raises the issue of cladding vs. vermiculite for insulation. I understand from Allen Scott's book that instead of putting a cement cladding layer over the dome bricks, he recommends 6 inches of vermiculite piled around the oven. Will this really cut down on the thermal mass and length of time the oven stays hot? Is only a one inch layer of refractory mortar really enough "thermal mass" to make a noticeable difference?!

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                    • #11
                      Re: Why slather mortar on the outside of the dome

                      This raises the issue of cladding vs. vermiculite for insulation.
                      In what way? They are entirely different things. You MUST insulate your oven either with vermiculite concrete, or a refractory blanket, or both. If you don't your oven won't get up to heat, and won't stay up to heat. Cladding the dome with mortar, on the other hand, is simply icing on the cake. Do it, or not, it doesn't really matter in my book, but don't confuse it with the mandatory insulation.
                      My geodesic oven project: part 1, part 2

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                      • #12
                        Re: Why slather mortar on the outside of the dome

                        If you clad it with an insulating refractory that will not contribute greatly to thermal mass because it's comparatively a low density and correspondingly weaker as a bonding layer. Better to use a denser refractory material that will give you both strength and mass. You need to decide if it is insulation you want or more thermal mass.
                        Kindled with zeal and fired with passion.

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                        • #13
                          Re: Why slather mortar on the outside of the dome

                          Thanks for the quick response, which helps to clarify my confusion. My question came from the following link: Pizza Oven Construction, where the builder was criticized by Alan Scott for his cladding/insulation. I'm not sure what the problem was, as the oven bricks were coated with refractory, then an aluminum foil sheet and then Vemiculite, as most people have discussed. In my planned build, I will only have about 4 inches once the firebricks are constructed. Hence my problem;refractory or vermiculite cement, and I'm thinking it needs to be the latter, yes?

                          Comment


                          • #14
                            Re: Why slather mortar on the outside of the dome

                            Refractory concrete = MASS (no insulation, some structural rigidity). Vermiculite concrete = INSULATION (no mass, no structural rigidity). Extra thermal mass is not required. Insulation is essential.

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                            • #15
                              Re: Why slather mortar on the outside of the dome

                              I really do not like that base design in the link above.

                              Comment

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