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My perlite/Concrete build - Forno Bravo Forum: The Wood-Fired Oven Community

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  • My perlite/Concrete build

    I started this project last Thanksgiving weekend(October/Canada).
    I decided to go the oven with a perlite , Portland cement, sand, fireclay mix
    5-1-1/2-1 mix,with water added until the mix was workable but not overly wet.

    I used an old yoga ball, 75cm and overinflated it.
    I cut out a section of plywood to fit in the yoga ball, wrapped the yoga ball in plastic wrap, then a newspaper paper mache mix, let it dry and then started to mix and cast the shape.
    I used a very thin aspenite/fibreboard purchased from Lowes for the form on the lower sides of the dome as well as for the inner form for the opening hearth.
    Melamin was used for the form of the rest of the oven door entrance.

    to form the flue area, I used an HVAC register duct and formed my casting around this.

    during the casting, I used an old sander to vibrate against my form to ensure I was getting a little bit extra flow to have a tight cast with minimal voids

    I cast the oven in one pass, three full wheel barrows of mix, then cast etc until the oven was complete.

    I used more fire clay in my mix as I got past the halfway point in the dome, hoping to have better heat retention an durability in the top section.


    The oven oven sat in my garage from Oct till May with no additional work done.

    i then sourced a couple of old stainless steel carts from a salvager in kitchen and modified them to become my rolling chassis for the pizza oven.

    i then had an company in Elmira custom fabricate the rolled (3"x3") aluminum angle into a circular ring. More straight aluminum angle was purchased from Beach Rd metal in Hamilton to turn the circular shape into a keyhole shape to fit the oven.

    once the keyhole shape was formed, it was then welded, braced and then I dropped in 18g mild steel as a floor pan,

    this floor pan was drilled as well as the stainless frame and 5 bolts were installed to keep the oven base/floor pan attached to the stainless cart

    a mixture of 5-1-1-(1/4) of perlite, Portland cement, fire clay, sand was used for the base of the firebricks

    sand and fire clay dry mix was used to level the bricks in the form.

    next came the job of installing the dome on top of the new base.

    i lifted the dome onto some 2x4s so it was up off the ground, i then inserted some extra firebrick spacers in between the oven and the wood to get it up high enough so I could roll my 2-ton engine hoist underneath. I used two straps in a crisscross pattern front to back, picked up the oven, rolled the cart underneath and set the oven down on some kindling sticks

    i then bought a pail of Vesuvius refractory cement an proceeded to fill the voids with refractory cement and then lower the oven down on top. Remaining gaps along the outside of the oven base where it met the oven were filled with the refractory cement.


    cracks....
    during the curing of the oven, one cracked formed at the top of the chimney flue opening and that was all that was visible.

    yesterday I lit a small fire, then continued to build a larger more progressive fire. the original crack opened up and there was a subsequent crack on the opposite side, nothing leaking out in the way of steam or smoke just a crack.

    on the dome there were a couple of hairline cracks. I did notice as the fire was at its peak there was what appeared to be two hot spots on the hairline crack on the dome and what appeared to be smoke coming out. After ,
    more investigation, I believe it was excess moisture in those two places, as the oven cooled down, the hairline crack is barely visible now..

    i have 2 sections of doubled walled stainless (wood stove) chimney. I used an 18" long section with decent results yesterday but I may switch to the 4' section to get more draw


    my plans now are to do an insulator over the dome, securing it and the dome to the base with a chicken wire/safety wire and then put a perlite mix or stucco over top

    any suggestions from people who have done this part before would be appreciated

    i reached 965 degrees on the floor of the oven yesterday and several times the ceiling temp was over 1000 Deg, as the infrared devices went into error mode
    max external temp on the top of the dome was 180 degrees, I know that due to the amount of sand and fire lay used in the dome, that there was better heat retention,

  • #2
    I am looking to build a similar oven and live in Ontario as well..Would be interested where you sourced your material. Additionally how it has held up to multiple firings.

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    • #3
      Mate, think carefully, and read a lot, before you do this.
      Without seeing photos of the above oven, there are still a couple of things that bother me about it.

      1) There isn't much binder in this dome. Richfergie says he started out with 5-1-1/2-1. Once the Portland cement gets degraded on the inner surface of the dome where the flames hit it hard, the only thing preventing the sand and perlite falling in your pizza will be that lonely little one part fireclay.

      2) He boosted the clay hoping for more heat retention as he went up the dome.
      My take on that is - if you want heat retention you want thermal mass. So delete the low density perlite, and consider going with a commercial castable.
      If you can't get or afford commercial castable (it is expensive) consider using the home brew mortar. People have had some success with this.

      This thread is a good read. I think the welded steel reinforcing was over-kill and potentially counter productive (possible expansion cracking) but Michele says it has held up well.
      https://community.fornobravo.com/for...-oven#post9110

      3) This dome is self insulating. 1000 degrees on the inner surface, with only 180- degrees on the external surface says large internal temperature differential in the dome.
      The larger the temperature differential between the internal and external surfaces the more likelihood you'll crack it.
      So, again, delete the perlite in the hot layer.
      The idea is to have a dense non-insulating dome, that will get hotter than a summer's day in Marble Bar, and insulate over that.

      Never forget to insulate under the floor when you first build the oven. You can't go back and do it later.

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