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2 meter diameter New build oven with pumice reinforced with Basalt Rebar

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  • #61

    The expensive stuff in the bucket is a 100% acrylic product and they’re all expensive. Commercial cement renders, dry in a bag, are much cheaper and have a degree of waterproofing added to them. If you mix some water with it it resists mixing and you have to work a bit to get it integrated. Some of them are labelled PM which stands for polymer modified, but they’re still essentially a cement based render. There are others that recommend the addition of an acrylic bonding agent usually 50/50 water/ acrylic.
    You can make your own, but it won’t have any waterproofing qualities. The addition of clay in the mix increases the tendency for shrinkage cracks. A better recipe IMO is 4:1:1 sand,hydrated lime, Portland cement. The lime adds enough creaminess to make it nice and workable at the same time as providing some elasticity as crack healing properties. A non waterproof render can always be treated with a waterproof coating over it. The swimming pool tile adhesive could work, but will probably be expensive also. Is it suitable to apply as a render?
    Last edited by david s; 06-06-2020, 04:22 AM.
    Kindled with zeal and fired with passion.

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    • #62
      I moved this thread from Introductions to Other types of oven. I don't know why I did not do this earlier.
      Russell
      Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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      • #63
        Originally posted by david s View Post
        The swimming pool tile adhesive could work, but will probably be expensive also. Is it suitable to apply as a render?
        Thanks David for the great info.si is having a lime render on a metal mesh without acrylic would be waterproof?
        Or the acrylic is essential in making ir water proof?
        Any idea if the Vitcas fireplace render is suitable for exterior stucco?

        Cheers

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        • #64
          The render does not have to be waterproof. In Italy typically their ovens were outdoors and finished with a lime based render. It allows the transmission of water both in and out. If the oven gets wet a waterproof exterior makes drying it out more difficult. I think the best solution is to have an unsealed dome under a roof which keeps out the rain yet lets the oven breathe. As a roof is expensive the next best is to build a dog kennel style or enclosure around the oven which both shields the oven from rain and allows it to breathe. Waterproofing the exterior and having the oven in the weather is probably the cheapest solution but not the best IMO. If waterproofing the exterior a vent that allows the release of moisture from the insulation layer is pretty much essential for a good functioning oven. We live in the tropics and during our wet season in 90% humidity conditions, the oven interior and insulation layers get wet even if it hasn’t rained. A few long slow fires restore normal function though.
          I’d assume the Victas render is a calcium aluminate based product which means it will be expensive and also a waste of your $ because the outside never gets really hot.
          Last edited by david s; 06-07-2020, 04:51 AM.
          Kindled with zeal and fired with passion.

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          • #65
            Help!
            I am gutted!

            I opened up the entrance to the oven and have found that all the wooden frame shaping the domes were soaking wet.
            With the inside absolutely soaking wet from years of accumulating rain, and out of excitement I'd decided to start a small curing fire using a handful of twigs only to dry up the wood thinking which was soaking wet. This is since flexible plywood upon which the firebricks is sitting on was almost rotten from dampness as well as the structural ribs holding the sphere form were also very wet.
            With plenty of water on the side in case of uncontrollable fire occurs,I started the small fire and for about an hour it was going OK with plenty of steam coming our of the timber frame. Then my family asked me to join them for a late snack leaving the small fire unattended thinking the wood was far to wet to be inflamed by fire.
            After returning back to the curtilage, I have found that the structural wooden frame inside the oven was fully inflamed !
            The dome is riddled with cracks, the cracks seem to run in both directions. IIt is evident the crack run vertically where the wooed structural frame were. I have counted 6 vertical cracks running along vertically and starting from the first third of the dome upwards towards the apex.
            There are also horizontal cracks running in the last top quarter as a complete ring at the top of the dome!
            How bad is this? Do I need to demolish it?
            Attached Files

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            • #66
              I did warn you that if you see visible steam then you’re going at it too hard. What’s done is done. Your oven won’t fall over because the dome is a self supporting structure. Just continue with the drying fires gently until all the black soot has burned off right down to the base of the dome.
              Kindled with zeal and fired with passion.

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              • #67
                Originally posted by david s View Post
                I did warn you that if you see visible steam then you’re going at it too hard. What’s done is done. Your oven won’t fall over because the dome is a self supporting structure. Just continue with the drying fires gently until all the black soot has burned off right down to the base of the dome.
                David the steam I saw was coming from parts of the wooden frame shaping the dome and not from the bricks, My mistake was I lit a small fire amd left it for 2 hours unattended that must have started on a small part of the wooden frame and spreaded.

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                • #68
                  Ok. You can’t undo it short of a total rebuild and repairing internal dome cracks is rarely successful so just use the oven as planned. It should not alter its performance and it won’t collapse. You’ve just joined the crack club.
                  Kindled with zeal and fired with passion.

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                  • #69
                    Originally posted by david s View Post
                    So, the usual method is to give the blanket a layer or two of lean (10:1) vermicrete which evens out the lumps and bumps and once set and dry is firm enough to render against while also providing an extra insulating layer. If it is made any thicker than an inch or so, then drying becomes difficult. When I first began making ovens the older generation of blanket was not classified as safe and the safe stuff now used was prohibitively expensive. Consequently I insulated entirely with 10:1 vermicrete and quickly found that it was best done in layers of around one and a half inches thick with a week of drying before proceeding to the next layer. I now use a blanket layer over which I do a vermicrete layer, then after a week of drying the drying fires and finally the rendered outer shell. [ATTACH]n418076[/ATTACH]
                    Hi David,

                    Im about to set too with the v-crete layer. Am I mixing 10:1 vermiculite/portland for this purpose? And as far as hydration, what kind of consistency am I looking for in the mix?

                    From your post quoted above, am I right in thinking that 2 layers of 1.5" each are applied or is 1 layer of 1.5" sufficient (I have 3" of blanket insulation over the dome)?

                    Thanks,

                    Danny.
                    My Build:

                    https://community.fornobravo.com/for...and#post423032

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                    • #70
                      If you already have blanket one layer should be sufficient. To get the correct consistency for every 10 parts of vermiculite(or perlite) add 3 litres of water. If you have some powdered clay a handful of it for every litre or so of cement will give it mor workability.
                      Kindled with zeal and fired with passion.

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                      • #71

                        Given that the internal dome is 98cm in radius with a full brick in thickness (230mm) making the external shell of the sphere about 9.5 meters which is very expensive to insulate with ceramic blanket, would it be sufficient to have 1 inch of ceramic blanket and compensate that with 6 inches of pumice with vermiculite?
                        I have enough pumice to make it about 6' or even 8" inches thick. ALso, would fine wood ash be ok to be added to cement to make it workable ? I know ash is used in old cultures of the far east when mixed with clay as a waterproof.https://www.youtube.com/watch?v=mn1DEeyqaT4

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                        • #72
                          230mm thick dome walls is huge. Normally bricks are cut in half (4”) resulting in walls half that thickness. If you only have one inch of blanket and want to back it up with vermicrete then it’s important to keep it as lean as you can because the cement reduces insulating capacity. Not sure about the wood ash addition. I use a bit of powdered clay to assist workability. Wood ash is light but quite caustic when mixed with water and handled. We use it as an ingredient for glazes. It’s composition varies greatly depending on what wood is used.
                          Creating an insulation layer with wet material is a problem because you need to dry the stuff out. Apply it in layers around an inch and a half thick with at least a week drying between layers. Firing an uninsulated oven is a recipe for producing cracks and this probably contributed to your problem.
                          Last edited by david s; 06-21-2020, 01:21 PM.
                          Kindled with zeal and fired with passion.

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