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Vermiculite Concrete Dome

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  • Vermiculite Concrete Dome

    Hi All, I'm new here today and could do with some advice. I've part built my oven as follows...

    Made a vermiculite/cement slab for the base, 4 part vermiculite 1 part cement left to set for 24 hours,
    On top of the slab I laid 1 inch fire bricks on 10mm of sand for the oven floor,

    Built a former from sand then covered in wet news paper. Then applied a 2 inch layer of vermiculite and cement same ratio as the slab to make the dome
    left over night then set a small fire.

    I noted some smoke escaping from the dome so applied another layer of vermiculite and cement. And that's where I'm up to, Can I ask how long should I leave the dome at this stage to set before starting to cure?

    After that I plan to cover in ceramic fibre insulation then apply a final layer of regular cement finish.

    Any suggestions or changes I should make would be very much appreciated


  • #2
    Hi Bill,

    Not being an expert myself i wouldn't like to say 100% for sure given your description, but it sounds as though you may have tried to make the cast part of your dome out of what should be used for the insulation.

    Vermiclulite, or Mica, is an inuslator. It is mixed with cement to hold it together where required i.e for the base. It shouldn't be used for the actual cast of the oven. That's where an 'off the shelf' castable refractory mix, or home brew refractory mix should be used.

    If you read some of the other recent threads you'll see some excellent builds and descriptions on how it should be dome for a refractory cast dome.

    Ideally, if you are making a home brew your oven should comprise:

    A sturdy base on which it sits;
    An insulation base layer - ideally a minimum of 100mm Vermicrete (5:1, vermiculite, cement), or, a Calcium Silicate boards, or a bit of both;
    Fire bricks for the cooking area
    The dome consisting of a homebrew cast of 3:1:1:1 (sharp) Sand, Cement, Fireclay, Hydrated lime with Stainless steel needles and polypropolyene fibres, or, an off the shelf refractory mix;
    Then an insulation layer of at least 100mm vermicrete (8-10:1) mix, or even better, 50mm of ceramic fire blanket and vermicrete.

    After all this has been done you can then decide how to put the final finishing layer/s on.

    All through the make, there are periods where you need water to escape and for the insulation layers to dry.

    Best having a look through all the threads.

    Hope all goes well
    Last edited by Robarb; 09-04-2020, 12:04 PM.


    • #3
      Welcome Bill!
      Robarb's comments are right on target as is his general overview of casting a WFO dome style oven. I would add a water channeling/exit layer between/through the hearth directly under the base insulation layer. I'm a little concerned with the description of your hearth construction. I am reading that your oven rests on a hearth constructed of 5:1 vermicrete rather than rebar reinforced concrete. Although, your vermicrete oven is far lighter than a brick oven...a supporting hearth of vermiculite and cement has almost no strength other than compression...and that's not much either for supporting an oven. I fear that you may have a collapse of the structure in the relatively near future if I'm reading your description correctly.

      As to your question, vermicrete holds tenaciously to its relatively large amount of water. It takes a week or two for it to cure and then longer to drive out the moisture. It sounds like you are rushing this essential process. Smoke escaping the dome definitely shows you have cracks that have already developed in the original casting. Covering it over has simply added another wet layer and lengthened the time it needs to dry out.

      You can hasten the drying process by using a fan or electric heater inside the cooking chamber...but you have a lot of water contained in those vermiculite/cement layers. I'd run the fan/heater for a couple weeks before starting any curing fires. I also hate to see you purchasing and using more insulation (the ceramic batting) over a material that is actually an insulation itself. The vermicrete has very little mass in which you can count on stored heat, so (IMHO) more insulation simply is a waste of your resources. You can cover the vermicrete directly with a cement finish...but that is not going to make it waterproof and you don't want to apply it until the vermicrete has completely dried out and the dome cured. (Use a piece of clear plastic over the oven on a sunny day or during curing fires...if you see condensation under the plastic, then there is still moisture being driven out.

      On the forum, we've had several projects started using vermiculite or perlite dome casting but have had very little feedback on their longevity. Both those materials will readily take on water again, so putting the oven under some cover is important or keeping a fire going and getting up to even moderate temperatures can be difficult every time the oven picks up moisture. Ovens made with these materials can be successful for limited use and we'd love to have you continue this thread with pictures and your successes (and failures) for everyone's benefit and adding to the knowledge base available in this forum.

      Good luck and keep posting!
      Mike Stansbury - The Traveling Loafer
      Roseburg, Oregon

      FB Forum: The Dragonfly Den build thread
      Available only if you're logged in = FB Photo Albums-Select media tab on profile


      • #4
        Guy's thanks for the advice, I wish I'd posted here 2 weeks ago!
        I should have said my base has rebar strengthening and sits on top of a regular 3 inch concrete base, block and steel supported. I'm not too worried about the structure of that part. You're other comments do give cause for concern, particularly the lack of a material to hold the heat. Maybe i could remove the dome flip it over and add an inner layer? If so how thick would it need to be to hold heat for a reasonable time? So whether to demolish the existing dome a build again or in the interest of science continue as planned ! I have plenty of the required materials to do both so maybe finish as is and post results
        (smug success or embarrassing failure) ?


        • #5
          Hi circo...

          I originally had the same plans as you, my goal was pizza and from what I saw, it appears your build will cook pizza... However, since congress wants to also cook via heat retention in the oven, breads, pies, roasts, etc, and wants longevity and has final approval $$$... I am convinced that the solid casted dome using the homebrew recipe (or refractory castable if your budget allows), followed by a layer of ceramic blanket and then the equivalent of the vermiculite dome you currently have over the top of it all for additional insulation and outer strength. That will all be built on top of a layered floor also designed to retain heat and prevent heat loss. It will definitely take longer and cost more but I am sure my grandkids maybe even great ones will one day enjoy the benefits of my efforts.

          Whichever direction you go, I would like to see how it goes for you. As for placing a layer on the hot face of your dome, there are some doing the same thing, however, my worry would be that after a few/several pizza nights (heat cycles), pieces of the layer might begin to separate from the inner vcrete dome, to become an additional ingredient for one of your pizzas...

          One bonus in keeping your current build is that you'll see if you really will use and enjoy your own wood fired oven... if it eventually fails you will have additional knowledge for your next build and motivation to build it in accordance with best recommendations.

          One bit of advice that has benefited me in my research is to use google to search this site... just enter your search term i.e, homebrew recipe followed by into the regular google search, I seem to get better results when I do that instead of using the search within this site.

          Take care


          • #6
            Bill, I suspect that turning the existing dome over to use it as a form for an inner casting of homebrew would be nearly impossible. If you did accomplish it flipping a two inch thick some back over would likely put your back out of commission at the very least. David S, our casting guru recommends casting domes in pieces for easier assembly of you don't want to create a form in place like you did for the vermicrete. Since you've got the dome built (and glad to hear on a solid hearth ), you would benefit from using it just to see if you would use an upgrade or just turn it into yard art. My main concern with using vermicrete domes is the higher possibility of "topping additions" from fallout. Definitely going to get more use and options with homebrew or refractory mix come and it will be less work now since you've gotten some experience with casting techniques. One good thing is you'll be able to tear the vermicrete apart quite easily. Lots of options, but as Rob noted, having something that will be used a lot and stand the test of time is a big consideration.

            Good luck and remember lots of people have gone through this process and it's as much a learning process and fun "I built it" hobby as anything else.
            Last edited by SableSprings; 09-07-2020, 07:48 AM. Reason: Autocorrect changed dome to democrats in first sentence, funny but not what I meant!
            Mike Stansbury - The Traveling Loafer
            Roseburg, Oregon

            FB Forum: The Dragonfly Den build thread
            Available only if you're logged in = FB Photo Albums-Select media tab on profile