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Smoke leakage vermiculite gym ball cast

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  • Smoke leakage vermiculite gym ball cast


    I decided on the gym ball vermiculite mix approach to the oven. The entire thing is cast and in situ on the firebricks. It was built as a single cast over an 80cm ball so is 80cm in diameter. The dome was cast with 1:5 cement fondu and vermiculite and set quite hard after about a week. We then moved the heavy lump onto its platform and vermicreted it in using the same mix. After another couple of days I added another less compact layer of vermiculite to portland as an insulating layer, so that the first layer is about 2-3 inches thick, followed by an insulation layer of around 2 inches again but less compact.

    I let that rest for a couple of days then did a very small fire just using scrunched up paper to help push the moisture outwards as suggested. This created quite a low heat and I was planning on doing the same once or twice a day for a while to gradually get rid of the moisture before rendering. The burn produced a lot of smoke so I could make sure the chimney worked ok (which it did). However I noticed some very fine whisps of smoke escaping from some places in the dome itself. I took a torch to the inside of the oven and there are some gaps about a cm or so in places that do seem to correlate to where the smoke leakage is occurring.

    My choices are I either adding a ceramic blanket then chicken wire, followed by a render to add insulation and keep the heat directed back into the oven. I have about a fifth of a bag of vermiculite left so I could add some more compressed vermiculite cement (but I've run out of ciment fondu so it would be an ordinary cement/vermiculite mix), use a small tub of fire cement and plug the holes inside or any or all of these maybe?

    What are the thoughts of anyone who has made a vermiculite/cement dome using the gym ball method. I appreciate that others have used firebricks etc and may not like the vermicrete approach, but the cost of the firebrick floor was more than enough so it wasn't an option to go for a full dome as well.

    Last edited by Vespasian; 07-22-2019, 09:34 AM.

  • #2
    Did you use the 1:5 cement fondu:vermiculite mix as the casting material? I am not an expert on casting but I do not think the vermiculite should be the layer that is in direct contact with the heat source. I think you needed a refractory cement or similar as your first layer, and then the vermiculite as a second insulating layer.

    david s could you chime in on what you use as a casting material?
    - seth s.

    my build (in progress)

    Google Photo Album:


    • #3
      Hi thanks for the reply. Its quite a common method apparently. We're not intending on staying in the current house for more than about four or so years as I we're going to move when I retire (can't wait). So i wanted something temporary and not too costly. So this method seemed the better option. I expected to be trouble shooting little snags as they cropped up so this is the first one thankfully.


      • #4
        Ok, sounds like a tried and true method, I just hadn't heard of it before.

        I wonder if you could mix up some of your mixture and patch the holes that you can see from the inside? Even though they don't line up with where you are seeing the smoke, there could be some tiny air channels/pores that lead out to where you are seeing the smoke exit the outside of the dome? Again, I am not an expert on casting, but that would be the next thing I would try.

        Hopefully someone with more knowledge than me will respond!
        - seth s.

        my build (in progress)

        Google Photo Album:


        • #5
          Iím not surprised that youíve had trouble. A 5:1 vermicrete is an insulation mix and not particularly strong. This leads to a material exposed to bumps and abrasions. Your problem is caused however, by steam expanding the mix leading to cracks. A 5:1 vermicrete takes on around a third of its volume in water and that is a huge amount to eliminate. As one litre of water creates over 1500 litres of steam and you started fires almost immediately, the resulting steam has split your casting. In addition as you used a5:1 ratio instead of the 4:1 recommended, then your casting is further weakened. You could try filling the cracks with the same mixture but it is unlikely to be successful. The now dry casting will quickly suck any moisture out of the repair mix preventing the required hydration period. We have not heard of any reports of the long term success of this style of build despite requests. Anyone reading this who has one of these ovens please chime in.
          Kindled with zeal and fired with passion.


          • #6
            Thanks for the early replies, much appreciated. I think its more holes that were there in the casting rather than heat related cracking. The fire was a bit rubbish really. I had literally started a very small paper burn to which I was going to add a few tiny fragment of wood, but as the paper was a bit damp and had ink on it the result was a brief flame followed by a long smoulder producing lots of smoke but naff all heat. I could put my hand into the oven to pull out a few of the sheet so I quickly dismissed the steam possibility. You couldn't have warmed a coffee if you'd had it in there for an hour. After watching the smoke I stuck a torch inside and unsurprisingly found some holes near to the smoking patches.

            I agree patching from the inside might be less succesful.

            I'm thinking that the best way forward might be to add some of the vermicrete to the outside where the smoke tends to escape to reduce the heat escape at this points, then cover with a 25mm insulation blanket followed by a couple of layers of what we call chicken wire here in the old world much as this guy did

            But I'll wait to get some more suggestions before ploughing ahead.


            • #7
              Hi Vespasian,

              There are several other gym ball builds in this same section (Other Oven Types). Most all were asking how to recover their builds from the infamous gym ball vermiculite/perlite/cement videos on the net . You may look through the first couple of pages and alert, private or visitor message them. I hope that you get some replies.

              But, imo the portland will burn out a little at a time with each firing. The vermiculite will turn loose and fall into your food. Probably not in noticable amounts and it is quite soft. No worries about braking a tooth and the asbestos content is mandated to be within the allowable limits

              Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build


              • #8
                Thanks for the links, very useful. Thankfully I used proper fire mortar called ciment fondu for the inner dome and 99.9% of it is smooth and solid, the only issues are the handful of 1cm gaps. Vermiculite looks like rice crispies when mixed. I should have added chocolate so if any fragments do fall in at least it would have tasted good.