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Saving my exercise ball project

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  • Saving my exercise ball project

    For some reason my original post and the two replies I got have been deleted and my account gone. Had to re-register.

    Thanks 7yearswait and KJB for the replies, here is what I wrote in response:

    Thanks for the info! I found a lot of this out after the fact. I would have started with a homebrew layer first then the vermicrete over that with refractory cement instead of portland. But I'm this far so going to try to save the project!

    & days was yesterday so I removed the plastic film from the outside and took the ball and tunnel mould out. Inside is definitely damp so I'll let it sit for a while and the air can get in there and dry it out.

    I think I might get some castable refractory cement and use that to make an inner liner. Then figure out the render layer later once I get it on a base and stand.

    @7yearswait - Yeah I'm in Bristol, CT about an hour from New York City. I can probably get Vitcas shipped over but might be expensive. I'll see if I can find any in the US.

    @KJB - The flue in that picture is being formed inside that cardboard box, and there is a long wooden plug wrapped in plastic in the center. Slid out with some gentle tapping.

    Going to let the dome sit until next week, too much going on to work on it now. The additional dry time will be good for it. I have an air dryer that you place in boats during storage to keep the moisture away. I may put that in there closed off for some time. It creates gentle heat.

    Thanks again for all the help I really appreciate it! I'll do my best!!

  • #2
    Seems the forum was hacked and has been reset back a couple of weeks. That time relates to my joining so I have also had to to rejoin.

    I like your progress since your initial post. I am just making up my entrance and I will think about a rectangular rather than circular flue. This would allow me to maximise my flue size but keep my tunnel length to a minimum. I keep reading that for this size oven you need a 6 inch flue. My memory of your original post was that your dome was 30 inch. Mine is over a 750mm gym ball.

    Have you done any estimates as to the approximate weight of your build to date.

    Don't have a boat dehumidifier or the like but sore reference to someone using cheap candles. Wonder about a heating mat like used in homebrew beer or a aquarium heater that are thermostat controlled.

    Are you going for a skim coat only over the chicken wire or continuing on with blankets and more vermiculite.

    My plan is a cm or two of homebrew two inches of perlite and then a skim mortar coat. I wonder about weight and efficiency.
    I am not going for a baking oven but a relatively light and efficient pizza oven.
    It has to be better than the commercially made stainless steel and wood pellet oven that started this quest. Not to mention cheaper along with the satisfaction of building ones own.

    I will follow your posts with great interest.


    • #3
      Two inches of dry loose perlite would be minimal insulation for the dome. If you mix it with cement, even at 1:10 which is about as lean as you can go and still be workable to stick to the hemisphere, its insulation value is reduced considerably. (see attached table). The higher the temperature the greater the heat loss and as pizza temperature is pretty hot ~400C then its loss is also pretty high, especially with the oven door open. This is why a fire on the side must be maintained to hold the oven to pizza temperature. Experience has proved a minimum of 3" of vermicrete insulation or 2" of blanket is about as thin as you can go, most would prefer more.

      In the building of your oven there is lots of added water, depending on size you may have added around 100 litres of water. lighting some candles will do little to drive out the water. Try around 10 kg of bbq fuel briquettes and a little kindling to keep them alight for some serious gentle heat.

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      Last edited by david s; 05-22-2020, 12:30 AM.
      Kindled with zeal and fired with passion.


      • #4
        Okay I did think maybe the forums got set back a time period due to an issue. That explains things!

        I do think my flue is a bit small but there is enough material where I can open it up some more. I kind of like the square/rectangle look of it.

        The air dryer is one like this:

        Might leave that in there through the weekend. As far as weight I'm not sure. I used 5cu/ft of vermiculite (maybe 10lbs?) and appx 50lbs or portland cement. So 60lbs of material dry. Wet it has to be double that. I gently lifted one side up by the tunnel and while heavy I could lift it.

        The plan for now is to let it dry more and then line the inside with refractory. Still unsure if I want to do the ceramic blanket then render or just render straight over the dome. I have time to think about it. I may invest in the blanket if the liner comes out good.

        This was supposed to be a cheap build but as usual its snowballing haha.
        Last edited by BristolWFO; 05-27-2020, 06:55 AM. Reason: spelling edits


        • #5
          Can anyone provide input on the best way to line my vermicrete dome? What material and thickness should I use?

          I see refactory in Rutland castable, Mizzou castable, and a couple others. Or is there a better material that will stay in place?



          • #6
            One company that sells refactories sent me this link as a possibility for a dome liner:


            Not sure about it...


            • #7
              I read the MSDS and I am not sure I would want this material inside the dome where the food is cooking. I suggest you reach out to David S on parging the inside of a vcrete dome although I know from past post he believes that most applications will not work or fail.
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              • #8
                There are many different castable available from many different manufacturers. You need a dense castable not an insulating one. The product you linked to might work, but anything applied in a thin layer is likely to crack, so make it as thick as you can. Also the substrate it’s applied to needs to be somewhat damp. Too wet won’t work, too dry won’t work. Cover it for at least two days to retain moisture while the stuff hydrated. Calcium aluminate based products cure in 24 hrs, but best to give it 48. Good luck, I hope you can save your casting.
                Kindled with zeal and fired with passion.


                • #9
                  You're right UtahBeehiver, that looks like some nasty stuff. I will pass on that!

                  I was also looking at the "ITC-100HT Pint Ceramic Radiant Heat Reflection & Protective Refractory Coating". That goes on like a thick paint or a slip. Thinking maybe that would sort of seal the vermicrete and prevent spalling. It has good flexibility for expansion too. But it has a warning in its MSDS as well but not sure if that's for before curing only.

                  Then insulate and render.


                  • #10
                    Originally posted by UtahBeehiver View Post
                    I read the MSDS and I am not sure I would want this material inside the dome where the food is cooking. I suggest you reach out to David S on parging the inside of a vcrete dome although I know from past post he believes that most applications will not work or fail.
                    and BristolWFO,

                    I believe the MSDS apply the non-cured product. Any hydraulic setting agent will be hazardous due to the reactivity and high pH if in content with water.

                    Unterminated Si-O bonds in crystalline silica is what causes lung cancer if exposed over time. If in a mortar/casting, the silica is locked in and covered by the reaction products of the CAC and water.

                    Have I gotten this wrong or can someone please explain?
                    Last edited by Petter; 05-27-2020, 09:33 PM.


                    • #11
                      Yes the silica is locked in. Pretty much all refractory material, clay and concrete contain silica, but it’s only a problem if you inhale large amounts of it over a long period. Inhalation is the problem not ingestion, so it’s safe to use for an oven. Eg. Breathing clay dust indoors is bad, sanding fired clay indoors really bad especially for children. Dangerous activity is cutting or grinding, especially if done dry, as tiny shards of silica material damage the silica in the lungs.
                      Last edited by david s; 05-28-2020, 02:27 PM.
                      Kindled with zeal and fired with passion.


                      • #12
                        Thanks Petter and david s
                        The supplier contacted the manufacturer of Insul Stick to check on it and they said that the contents are really no different than firebrick, regular brick, concrete and mortar. All containing silica which as you guys pointed out has the danger of inhaled dust and handling of the material when applying.

                        Given that I think I will give it a try. Should I add melted extract needles as well for strength during heat & cool cycles?


                        • #13
                          No, I don't think you should on such a thin layer. You don't want any needles close to the surface and it will make application of the stuff that much more difficult.
                          Kindled with zeal and fired with passion.


                          • #14
                            Update - I ordered the Insul Stick, waiting for that to arrive. Been working on a stand for the oven. When I started it I needed it to be on wheels since I rent a house and cant make something permanent. But last week I have a down payment on a house so the wheels will help me move it to its new more permanent home!

                            If you look at the top deck supports there are 2 untreated 2x4s sitting in notches. Those are not secured and will be able to be slid out and replaced with longer 2x4s to act as handles for 4 guys to lift and move the whole top. I've built an upper wall and will be filling with vermicrete soon.

                            That's all for now!Click image for larger version

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                            • #15
                              Timber is generally not recommended as a material for an oven stand as it moves with moisture. Any movement will be transferred to the heavy brittle material it supports. If the stand is out in the weather this problem is far worse. I have no idea what size oven you plan to sit on top of the stand. Have you calculated or estimated its weight? They nearly always turn out to be heavier than you’d think. Movement of a timber stand, especially one that is mobile, can be reduced by the addition of diagonals. Small wheels also increase the danger of movement or at worst collapse because of the high centre of gravity if the wheel strikes a bump. Your horizontal beams appear to be only held in place by a couple of screws, or in the case of the ones checked in reduce the beam thickness by half.
                              On rereading your thread I see you’ve already cast a lightweight dome, so maybe you’ll get away with your stand, but at least change to large wheels for safety and add diagonals for strength.
                              Last edited by david s; 06-03-2020, 12:01 PM.
                              Kindled with zeal and fired with passion.