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Perlite/Mizzou castable - Forno Bravo Forum: The Wood-Fired Oven Community



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Perlite/Mizzou castable

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  • Perlite/Mizzou castable

    Hi all,

    I am on my II attempt to build a (almost) portable WFO; my first attempt with clay did not work and now I am doing this a little bit seriously.

    I first tried to emulate @pizzahacker and put a dome on the weber grill, but I tried to be cheap and failed.

    I'm now working on a solid iron table 28" x 28", I secured a double layer of rebar grid and poured a three inch cement base. 18 fire bricks for the base and cooking floor and some more for the vertical walls.

    And I am at the DOME point again. I have the castable and I want to mix it with as much perlite as I can so I can reduce the weight. my question is: about proportions, how much perlite can I add without compromising the mix? And how much more water will I need for the mix? the Mizzou seems to require very accurate measurement for adding water... any hints?

    oh, by the way, I am going to lay 2" of the castable on top of the 22" weber, pop out the metal once the cement sets. I plan to sit that on the bricks and adding another layer to seal the bricks. Insulation on top and some more perlite/cement on top of the insulation layer.


    I am putting pictures on this public album!

    Last edited by mekdigital; 07-14-2011, 03:46 PM.
    Born in Italy, making pizza in San Diego

  • #2
    Re: Perlite/Mizzou castable

    This sounds like disappointment waiting to happen for you again IMHO. For an oven to work you need one of two things. Thermal mass surrounded by insulation OR a small amount of thermal mass with a hugely over sized heat source like people use in their little black egg pizza ovens made from weber grills.

    Lately SO MANY people just don't seem to get this. Between this forum and another I've seen numerous questions like yours were people want to add perlite to dense castable or just use perlcrete as an oven dome. Why would you add perlite to an expensive dense castable instead of just buying a ready mixed insulating castable?


    • #3
      Re: Perlite/Mizzou castable

      That's a valid question and I don't have a valid answer!

      I am aware of the performances of such a sub-engineered oven - I just hope the dome does not fall apart, that would be a great first step;
      Born in Italy, making pizza in San Diego


      • #4
        Re: Perlite/Mizzou castable

        I would personally cast the dome out of the Mizzou without the perlite and deal with the little extra weight. I would also look into adding some stainless needles for reinforcement, they are cheap PM me if you need a source. Lastly I would try and go bigger then the 22" dome of the grill. With a live fire in the oven that leaves very little space for a pizza. MY old 30" oven was too small IMHO and last week it got torn down because my 42" replacement is well underway.


        • #5
          Re: Perlite/Mizzou castable

          I see it often in my professional life. People wanting to save a lousy dollar end up spending more time, money, and ultimate satisfaction trying to reinvent the damned wheel than they would have had they not cheaped out on materials/taken the lowest bid/designed a project with no real knowledge.

          The plans are free, easy to understand and construct, and that is all there is to it.


          • #6
            Re: Perlite/Mizzou castable

            I have recently done this.
            I mixed KS 4 plus (a dense castable similar to Mizzou)with perlite.
            I cast 1 1/2" thick dome around a 22" weber kettle bottom.
            I posted some pictures in this thread

            I used 1 bag of ks4 and about 2/3 perlite by volume. I first mixed the ks4, using the recommended amount of water and then mixed in the perlite while adding water slowly. I had planned to go 1:1 with the perlite, but as I was adding the perlite, I stopped at 1:2/3. I was afraid the mix would be too weak.
            I used a spiral mortar mixer on a low rpm drill.

            The ks4 had a really nice consistency before I added the perlite. After, it didn't feel as plastic.
            I used a palm sander to vibrate the forms. Next time I will vibrate as I fill the form. The lower part of the dome ended up being loosely packed.It looks like perlcrete. The upper dome is denser.

            I made a wooden stand, placed lightweight cinderblock on it and then put down 1 1/4" thick firebrick splits as the hearth.
            I cured the oven over 5 days, making a larger fire each day.

            The dome is surprisingly strong. I think it weighs around 65lbs. I have had the top of the dome to 1100 degrees F. No cracking. I didn't use stainless needles. I have baked 4 times so far.

            After the first pizza bake, I realized the oven clearly needed insulation.I cast a 4" thick perlcrete slab, 6:1 perlite to portland. I put a little light wire fencing in it to help keep it together. I let this cure for a week. It came out nice, quite light and strong. I set my hearth bricks on it and fired it up.
            An improvement, but still too much heat leaving the dome. I could keep the dome at 750-800 with a fire on the side, but the hearth temperature dropped faster than I would like. Maybe I need full thickness firebricks, maybe the dome needs to be more massive, ie no perlite in the dense castable! Or maybe the dome needs insulation.

            This past Sunday, I covered the dome with 10:1 perlite to portland cement, 3 to 4".

            It is still curing, but soon I will find out if a relatively low mass dome will work.

            Without insulation, I could get the dome up to 900 degrees with a very small amount of wood in a little less than an hour.

            I imagine that it will perform much better now that the dome is insulated.

            I think that pizzahacker is using a very lightweight refractory.My dome would topple a Weber.

            My next oven will probably be cast without the perlite. Having more mass will probably not add much to heat up time and will give better performance.

            The 22" diameter is small,but workable. It takes less material to build and less wood to heat. I have been baking 10" pizzas, one at a time.
            Hope this answered some of your questions.



            • #7
              Re: Perlite/Mizzou castable

              we have a winner![/B]
              Born in Italy, making pizza in San Diego


              • #8
                Re: Perlite/Mizzou castable

                My oven is only 21" but can still feed a large crowd. Usually do 10" pizzas, one at a time, but can do two at a time if pressed. Most folk only cook one at a time, it takes way longer to prepare them. Did over 30 pizzas last Friday night, maintaining a fire on the side throughout, did not refire once and could have kept it going all night I think.
                Kindled with zeal and fired with passion.


                • #9
                  Re: Perlite/Mizzou castable

                  I'm happy you chimed in, I was actually gonna tell him if anyone would have any advice it would be you, king of the small oven, lol


                  • #10
                    Re: Perlite/Mizzou castable

                    Thanks, Next one I want to do is is a monster cob oven, but it'll probably never happen. You tend to fire small ovens more frequently.There are many other advantages. I guess it's like the difference between an electric recumbent bicycle and a V8. Economy or grunt?
                    Kindled with zeal and fired with passion.


                    • #11
                      Re: Perlite/Mizzou castable

                      Posting some pictures, so you don't have to click on the facebook page!

                      Tomorrow is DOME DAY !!!
                      Attached Files
                      Born in Italy, making pizza in San Diego


                      • #12
                        Re: Perlite/Mizzou castable

                        Hi again,
                        Are you still planning to mix perlite with the mizzou?
                        I covered my mold with shrinkable mylar( used for covering windows) to guarantee a good release. I still had to yank pretty hard to get it out.
                        Last edited by tinkerric; 07-18-2011, 04:01 AM.


                        • #13
                          Re: Perlite/Mizzou castable

                          I think I will go with a 1:1 mix. I really hope that won't make it too fragile, but 22" is not too large. And pizzahacker does that! LOL

                          how is your kettle-bbq-refractory-cement-walls-dome working ??
                          Born in Italy, making pizza in San Diego


                          • #14
                            Re: Perlite/Mizzou castable

                            I haven't fired my oven up since insulating the dome. It did pretty well with pizza before insulation, so I expect it to heat up faster and stay at temp more easily.

                            I have a few questions for you. Is your supporting slab insulated or just straight concrete? What is going to support the brick "arch" entrance?
                            Was your stand an existing table frame or did you make it? It all looks nice and think it will work fine.

                            As you said that you plan on insulating your oven, you could just cast your dome from straight mizzou and maybe a little thinner than 2". If you have a 55lb. bag,there should be enough material for 1 1/2". Though you should do the calculations so you don't run short.

                            From the pictures I've seen, it looks like Pizza Hacker may be using a commercial lightweight castable refractory, something much less dense than I made. The pictures of his early "oven" had what looked like firebrick walls and a lightweight dome sitting on top of them, very similar to what you have set up, but you've probably seen those pics as well.
                            I've seen recipes for a lightweight reftactory using premixed Rutland refractory cement mixed with perlite. I wonder if this is how pizzahacker did his first dome?

                            Silly as it may sound, one of the reasons I was cutting my dense castable was to add volume, so I could cast the dome with just one 55lb bag of ks4.
                            Having never worked with the material and never made a cast from a mold, I didn't want to possibly waste expensive material.

                            It has dawned on me that the perlite mixed in the dense castable, may actually be slowing down the dome's heat absorbtion, interfering with the heat transfer between all the particles of dense castable.
                            Well it seems to be working.

                            Enough ramblings for now. Hope your cast goes well.
                            Last edited by tinkerric; 07-17-2011, 08:55 PM. Reason: misspell


                            • #15
                              Re: Perlite/Mizzou castable


                              thanks for your comments!

                              the supporting slab is 1 50lb general cement and probably 1:2.5 perlite.. I added too much water to the cement and I had to add perlite and perlite and perlite until I got the desired consistency.. Is that considered some kind of insulation?

                              About the little arch, I am not sure. I was thinking of filling the joint with mortar and trying to put it in place once it settles and add more mortar on the wall bricks. Or I'll find a better solution

                              The frame is a table that lost its glass top and it's incredibly strong!

                              For me too, one of the three reasons for adding perlite to the mix was to grow the overall volume, in addition to weight reduction and adding some insulation.

                              I really hope things will work!

                              What about a little chimney?
                              Born in Italy, making pizza in San Diego