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Planning my 32" cast oven

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  • Planning my 32" cast oven

    Finally getting started with my own build (or at least the detailed planning) of a cast pizza oven. I have been dreaming about this for years.
    Here is a summary of what I am planning to do and I would very much appreciate any feedback anybody has.
    I am thinking of a 32" / 800mm interior diameter, since I don’t have too much space (especially depth wise)

    • I already have an outdoor kitchen and am planning to include the pizza oven into the outdoor kitchen. I have attached a photo of the location. As you can see the kitchen is U-shaped. Ideally the pizza oven would go into the slope. I am thinking of placing it at an angle into the corner to give me more distance to the BBQ. Alternatively I could place it straight on. Two sketches are attached. Any thoughts?
    • Question: How much space do I need in front of the oven opening to easily maneuver with all the tools? Or in other words, how long a handle do you normally use on the tools. I am a bit concerned that I don’t have enough space to the BBQ
    Design of the oven stand:
    • I will have to dig out a bit from the slope (it will be above the current hip high retaining wall), place a foundation and pour the slab there. This means that I can’t have any weep holes as I have seen recommended, since the slab will be directly on the soil. We live in LA, so it doesn’t rain too much, I figure that should be OK, especially if I use FoamGlass for insulation.
    Oven floor:
    • Since the slab is right on the dirt I am considering using
      • 2” of FoamGlass (does not absorb water, and should help with keeping the floor dry)
        • Maybe make the Foamglass larger than the CalSil board so that the insulating blanket does not sit on the concrete slab
      • Question: I have seen people using mosaic tiles on top of the slab to insulate the Vermicrete from the possibly damp slab and help water / moisture finding its way to the weep holes. I am not planning on using mosaic tiles, since don’t have weep holes or Vermicrete. Any thoughts?
      • 2” of Cal Sil Board
      • Fire bricks
      • Question: I have seen different types of firebrick, light duty, heavy duty. Any preference, what should I look for when selecting firebrick?
    • I am planning leaving a 1/8” gap between the oven floor and the floor of the gallery and fill it most of the way with 5:1 Vermicrete (probably sift out the larger pieces), the rest will be filled with ash.
    Oven Dome:
    • I will cast the dome with Homebrew 3:1:1:1
      • Sand: max 3mm grain size, considering using pool filter sand
      • Hydrated Lime: planning on using Type S hydrated lime
      • Portland Cement: whatever is available at Lowes, Home Depot, etc.
      • Clay: planning on using any powdered clay I can find. Thompson Building Materials has some
    • 2” thick
    • Cast dome on top of the fire bricks. It will make it easier to cut the bricks (don’t have to be as exact). I know I won’t be able to remove the bricks around the sides, but from what I have read that is not really an issue.
    • Planning on casting the flue gallery separately, making sure that there is some extra space around the chimney pipe
    • Attach the flue gallery with 5:1 Vermicrete (probably sift out the larger pieces)
    • Question: would it be better to use high temp mortar made for fireplaces? I think the whole point is to allow for expansion of the dome separately from the gallery. If I decide to just use ceramic blanket for insulation (see below) I may not have Vermiculite. I was having some trouble finding it.
    Oven Dome insulation:
    • I will insulate with two layers of 1” 8# ceramic fiber blanket (2” total).
    • Don’t use chicken wire to tie down the insulation, should not be necessary, Vermicrete will keep insulation in place
    • Add 2” of Vermicrete 10:1 mix
    • Question: Would it make sense to just use 4” of fiber blanket and not use any Vermicrete? If my calculations are right, I should be able to get almost 4” out of 50 sqft of blanket that come in one roll. Dome and Gallery surface total area is 14 sqft total (without subtracting the opening of the dome where the gallery connects). I am having a hard time finding vermiculite in LA (all the horse supply stores and several nurseries I called were sold out and don’t know when they will get it back)
    • Use a breather vent (hydraulics breather cap) screwed into a ” brass pipe with holes at the top of the dome. I have seen other methods of using the chimney area to vent, but it may be a bit too complicated for me.
    Final render layer:
    • I am planning on using stucco to finish the oven. I won’t be able to build a roof over it. It will be too massive where the oven is placed. I am planning to simply cover it with a tarp (or maybe a homemade cover cut to shape) during the winter (when there is at least a chance of rain) or when it is not in use
    • Question: Any suggestion for what stucco to use? I am currently planning on using an acrylic stucco with some AR Fiberglass fibers (12 x 0.5mm), I am planning on a to in thick layer. I am not planning on painting it
    Again, any help, suggestions, criticism is VERY much appreciated.
    I will most likely not start this build for a month or two, will wait for colder weather to make sure the dome does not cure too fast.
    Last edited by AndreasP; 08-29-2021, 03:11 PM.
    My oven build thread

  • #2
    I like using vermicrete/perlcrete outside the ceramic fiber blanket, mostly for the ability to refine the shape of the dome. In my experience, it's challenging enough to just get the ceramic fiber blanket to conform to a generally spherical shape, let alone get it just the way you want it. With the vermicrete, you can add a little more here, a little less there, and wind up with the shape you want. If you skip the chicken wire, you may still need to use some type of wire or rope to hold the blanket in place, until you put the vermicrete on. With your extra blanket, you could do 3" all the way around, and/or add one more inch of insulation to the top 1/3 of the dome where it heats up the fastest.


    • #3
      Not only does the vermicrete layer over the blanket enable the restoration of the perfect hemispherical form, it also provides a firm substrate upon which you can render against. Attempting to apply a cement render against the blanket is difficult as it’s quite springy and therefore likely to result in cracks and a much thicker rendered layer is also required to fill any low spots. A 10:1 vermicrete has a similar insulation value as blanket and is firm enough to render on to.
      Most folk wrap the blanket in chicken wire. I don’t because it takes ages and although it restores the form somewhat, also compresses the blanket which reduces it’s insulating capacity. Also adding a highly conductive material like steel in an insulating layer also reduces its effectiveness. An inch and a half or so of vermicrete is plenty to restore the desired form.
      Last edited by david s; 08-29-2021, 06:05 PM.
      Kindled with zeal and fired with passion.


      • #4
        Thanks Ronstarch and david s , appreciate the feedback, I guess I will have to try and find Vermiculite. Will give it another try.

        What length tools do you use? My first decision is the location of the oven. I will build a little template and try to see how it feels, but any input on the tool handle length would be great.
        My oven build thread


        • #5
          AndreasP, you need long handled tools or fireproof arms
          Seriously, I think the shortest tool I use is an old fireplace poker that's about 3' long. When the oven is going and I am working a live fire I can singe my arm hair if I get anywhere in the vent area. I guess my point is whether you have a long or short vent area you will want some long handled tools.
          My build thread


          • #6
            I generally agree with everything david s said (as is usually the case). I use perlite rather than vermiculite, just because that's what is available near me, and it has worked well. I also use a richer mix, more like 5:1, because I figure I'm already getting great insulation from the ceramic fiber blanket, and I find it easier to work with the 5:1 mix, even though it's less insulative. I was buying perlite in 2 cu ft bags at Home Depot for about $20/bag, then found it at local farm/garden supply stores in 4 cu ft bags for $25.

            Regarding tool length, for my 29" diameter oven, I use wooden peels with a 24" handle, a stainless steel turning peel with about a 40" handle, and a 48" long blow pipe for clearing ashes. The turning peel and blow pipe could both be shorter, but I have the space and the extra length doesn't hurt. I also have a pair of welding gloves in case I need to reach into the oven.


            • #7
              Thanks Ronstarch and JRPizza. Looks like it might have enough space in front of the oven if I place it in the corner. I will have to mock it up and use a rake or something to try it out.

              On the Perlite / Vermiculite I calculated that I need approx. 3.2 cu ft if I use the 10:1 mix (which I will try) for a 2" thick layer.
              I just looked at Home Depot and they now have the Vigoro Vermiculite in 2 cu ft bags at $21, not too bad, since I only need two bags. Thanks for making me look again.
              They also have the Vigoro Perlite in 2 cu ft bags at $17. Any preference? Would either of them work? I remember reading something about the size of the perlite/vermiculite granules, any concerns?
              My oven build thread


              • #8
                The smaller the grains are the more workable the mix becomes, but the finer grade requires more water. I compromise and use a medium grade which requires 3 parts water for every 10 parts vermiculite or perlite. I’ve also found a 50/50 mix of vermiculite and perlite makes a more workable mix than either of the two alone. Also a little powdered clay added to the mix helps make a lean brew more workable. If you work out the volume required by using the volume of the perlite or vermiculite you’ll probably end up short because there is a reduction in volume when you mix with cement and water. Add 20% to account for this. Don’t use a mixer as it degrades the grains and sticks to the blades and sides of the mixer. Fold the dry materials together in a barrow then add 1/3 of the water at a time, mixing it in gently.
                Last edited by david s; 09-01-2021, 04:31 PM.
                Kindled with zeal and fired with passion.


                • #9
                  david s , thanks, appreciate the input.
                  Looks like the only Vermiculite (or Perlite) that I can find here is the Vigoro brand from Home Depot. I'll just have to go with that. Maybe if I have some ceramic blanket left I will just add more ceramic blanket (especially to the top). Then maybe the mix doesn't make that much of a difference, since it is already well insulated.
                  It will still be some time before I get to that stage anyway.

                  My oven build thread


                  • #10
                    Made some progress over the last few weekends, the base is cast and I am trying to figure out the exact dimensions of the oven that will fit into this space. Any input would be very much appreciated!

                    I placed a template on the slab that shows the location of the oven. It will be tight, but the template has 1" for the final render as well as approx. 1" extra round the outer render, so it looks like it will barely fit.
                    To make it fit I had to:
                    1. Reduce the opening width to 17" (I had planned 19")
                    2. make the area where the door is going to rest against 1" wide
                    3. reduce the wall thickness of the gallery to 1.5" Homebrew, 1.5" fiber blanket, 1.5" vermicrete, 1" render
                    I am wondering if a 17" door is reasonable for a 32" diameter oven.
                    I am also wondering if reducing the wall and insulation thicknesses to 1.5" is a good idea. In principle the main heat and therefore insulation requirement is in the main dome, correct?
                    Will a door close properly with only 1" overlap? I also want to use the oven for making bread the day after.

                    The other option of course is to reduce the size of the oven to maybe 30" diameter, which would give me 2" of extra room around the oven. Any thoughts?

                    A drawing as well as photo are attached.

                    My oven build thread


                    • #11
                      OK, made a bit more progress. The slab and wall are complete, and I have most of the materials.
                      I did change a few things in my plans:
                      • Door opening: went back to 19" and reduced the thickness of the insulation in the gallery from 2" to 1" of fiber blanket It seems 19" opening is somewhat of a standard for a 32" diameter oven
                      • Decided on a 1.5" "edge" against which the door will close, I felt 1" may be a bit small
                      • I think I will go with 3" of fiber blanket, will see how it feels once 2" are on, I have enough for 3".
                      • I will try to see if I can get the vcrete layer down to 1" or so, I don't have a lot of space around the oven, maybe add some clay or make it a bit richer (or stick with 2" blanket).

                      Floor insulation
                      • Bought 2" of Foamglass and 2" of CalSil board.
                        • I don't have any weep holes, since the slab is on the dirt (there is a bit of gravel between slab and dirt. Not sure if I should still drill weep holes or if it would cause more problems that it solves
                        • I'm hoping that the Foamglass will keep the CalSil and floor dry
                      • Any suggestions here?
                      Dome construction.
                      I think I have enough foamglass to extend it so that is is also below the Vermicrete. Is that a good idea? Any disadvantage? I am thinking
                      • 2" foamglass all the way to the outside of the Vcrete
                      • 2" CalSil to the outside of the fiber blanket
                      • 2.5" firebrick below the cast dome
                      Any concerns, suggestions? See the attached sketch. The goal is to keep all the insulation dry, and Foamglass does not absorb water from what I understand.

                      OK, one more questions regarding the materials. I just want to make sure I got the right stuff before I spend all the time and energy building the dome
                      • Sand: I got #30 Silica Sand. They also had one grade finer, not sure if that would have been a better choice
                      Thanks for any reassurances or corrections the community can provide.
                      My oven build thread


                      • #12
                        I'd be concerned about the supporting slab being on ground level as concrete is porous. I use additives in our precast supporting slabs that waterproof, or reduce permeability to prevent water wicking up from the stand.
                        The weep hole in your slab could be superfluous in your case and you wouldn't want them to act as an entry point for water. Using foam glass on the bottom should deal with the problem adequately.

                        Regarding the sand in the castable, any should work ok, but generally a mix of grain sizes is better (called graded sand) The clay does a good job of holding the materials together. You should make the mix pretty stiff to "ball up" constancy, ie. make the mix up, take a cricket (or baseball sized ball and toss it 2 ft into the air and catch it again. Too wet it falls apart and wants to slump when placed on the mould. Too dry it falls apart and when placed on the mould results in voids.
                        Last edited by david s; 10-02-2021, 09:08 PM.
                        Kindled with zeal and fired with passion.


                        • #13
                          david s , thanks for your comments. I live in Los Angeles, so doesn't rain too much, the soil should be pretty dry below the concrete most of the time. Let's hope the Foam Glass does the job. No weep holes for me then.

                          I'll try to make the Foam Glass as big as possible with the material i have, so it protects as much if the insulation as possible.

                          I'll see if I can find graded sand. With all the effort that goes into this Iight just as well try to use the best I can.

                          The clay I got simply says Mortar Clay on the bag, the invoice says fire clay. I think I have seen others use this product on this forum.
                          The lime is Type S, which if I recall is the correct one as well.

                          David, would be great if you could confirm my material choices
                          My oven build thread


                          • #14
                            As pretty much any powdered clay will suffice, the mortar clay is ideal. I'm not sure about the typeS lime as in our country we don't have that classification, but someone else will chime in. If you got it from a building supplier it's bound to be the correct item. The fine detail may also refer to it as either hydrated or hydraulic lime.
                            Kindled with zeal and fired with passion.


                            • #15
                              David, thanks, yes, got the lime from a building supplier and the invoice says hydrated lime, so should be OK.

                              really appreciate all your input!
                              My oven build thread