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40" Castable WFO Build

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  • 40" Castable WFO Build

    I'm part way through building a 40" castable WFO, and thought I'd share my progress & plans so far, in the hopes the community can help me refine things.

    I started out with a cinder block base, with a 4" concrete slab on top. After that, I have 4" of 5:1 vermicrete on top of the concrete. Unfortunately I did this prior to learning of the sloping technique. I will have to make sure it's well sealed as we get a lot of rain out here.

    My plans from here:
    - firebrick base and arched opening (I'm thinking 19" wide by 12.5" high)
    - 2" homebrew 5:1:1:1 refractory dome. I'm trying to find some polypropylene fibers to add as well, however having a little difficulty. I wrongly ordered some nylon (fiberglass) ones, then some polypropylene ones that were too long (1")... still trying to find the recommended half inch ones. I'm planning for this layer to sit fully on top of the firebrick. Insulation will then go down past the brick level to the vermicrete slab underneath.
    - I have about 24 sq ft of 1" ceramic blanket for insulating the dome. This is enough for one layer with a bit to spare... however I'm wondering if 1" is too little insulation?
    - After the blanket I will do another 5:1 vermicrete layer, 2" thick and probably mixing in some perlite this time as I found it much cheaper than vermiculite (too late).
    - After that, I will finish with probably an inch of stucco or something of that nature, to seal things off.

    Things I'm unsure about:
    - I'm not sure what to use for a chimney. I want to keep costs down. I'm planning on a 6" flue. This should be adequate for a 40" oven I believe?
    - Exterior finish.
    - Expansion gap between decorative archway and the castable dome. Does this only apply to the refractory layer? I assume then that the dome would attach to the arch in the vermicrete layer afterwards? That doesn't seem very strong to me.
    - My 4" vermicrete layer is the same shape as the lower concrete layer. I don't need this much, as it extends way beyond where the oven will sit. I'm thinking of simply cutting it back after I've done my final 2" vermicrete dome, and then applying stucco over all of that right down to the concrete slab. I tried cutting some, and it goes pretty easy with a knife.

    I posted some pics. Any advice/feedback/constructive questions is most welcome!


  • #2
    Hi Johnathon,

    a few tips: 5:1:1:1 is not really rich enough it should be 3:1:1:1 for the inner casting mix.

    The polypropylene fibres required at the really fine ones finer than human hair.

    One inch of blanket is not enough, but you can use a lean mix of 10:1 perlite cement which is a good insulator. 5:1 is far less insulative. To make the leaner mix more workable add a little powdered clay rather than more cement. Any blanket off-cuts can also go in this layer. Make this layer at least 3” thick so you’ll have 4” total insulation thickness.

    6” flue will work ok, but it is worth getting stainless. A single wall is sufficient.
    Kindled with zeal and fired with passion.


    • #3
      david s Thanks for the insight! Nice catch on the homebrew mixture - that was a typo on my part, I've been planning on the 3:1:1:1 ratio.

      Thanks for the tip on the 10:1 perlite, I think I will do that to keep the costs lower on the dome insulation. To be clear - I will have 2" of homebrew, plus 1" of ceramic blanket, plus 3" of 10:1 perlite, and then after we're just into sealing & finishing, is that correct?

      I will look for stainless, that's helpful. Thanks!


      • #4
        Yes, but do the drying fires before doing the outer shell so the insulation layer has purged most of its moisture.
        Kindled with zeal and fired with passion.


        • #5
          Will do.

          Regarding the polypropylene fibers, I believe half inch (15mm) is the recommended length I've seen on the forum here. I've managed to source 3/4" ones locally, and cheaply too. I imagine the difference in length shouldn't be too significant here?


          • #6
            Well, I managed to get my refractory dome built. A tip for anyone in the sand castle stage - I found using a trowel made it VASTLY easier to achieve the shape I was looking for. Also, I put about three arm fulls of firewood in first to save on sand. As it was, I used nearly a full Bobcat scoop, or half a yard (I have a 40" diameter oven).

            After about six days, I took the sand out, and on the 8th day I did my first very small (newspaper) fire. After that I've been slowly working up, adding about 100F each time, as measured by infrared on the top inside of the dome. I've got up to about 700F on the top inside now, however I notice the 40" oven doesn't heat up very much yet. So far the floor has been around 200F.

            Along the way, I noticed some hairline cracking, and yesterday I noticed some smoke coming through the cracks. More on this later.

            Today I added the ceramic blanket, reasoning that the oven wasn't getting hotter inside due to loss of heat externally on the dome. I'm also concerned about water leakage down into the lower vermicrete layer during the sand castle process. I don't think too much got down (I used plastic) but I know that some did. Could this be slowing things down? Will any moisture down there be eventually driven out by the fires?

            Now that the blanket is on there, after an hour of burning I noticed the blanket is wet to the touch, and steaming quite a bit. It seems there is either a lot of moisture still coming from the concrete.... OR... those cracks I mentioned earlier are allowing moisture through via the smoke.

            My big question is this - I know cracks are common - but if moisture is coming through the cracks, what happens once I've sealed this thing up (after the final vermicrete and stucco layers)? I don't want to be pumping moisture up in there, eventually it will run down and go into my lower vermicrete layer. Is this an issue others have faced? Any recommendations?


            • #7
              Well, I decided to use some of my fireplace cement repair mixture on this dome, to seal up some of the cracks. That was partially successful, but then I pushed the fire hotter - this time to about 550F on the floor, which was the hottest I'd had yet, and after that, I noticed much larger cracking on the dome. I think the dome expanded and simply re-cracked the patches. The largest of these cracks are now approximately 1/8".

              david s Could you please weigh in on the severity of these cracks? (see pics) I have more of this cement ( that I could use to fill in the crack, but is this going to keep happening? I still haven't reached target temp. Also, how much of a concern is it having moisture from the smoke escaping up into the insulation layers above? Any advice would be most appreciated.


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