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mekdigital 07-14-2011 03:01 PM

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!

shuboyje 07-14-2011 03:15 PM

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?

mekdigital 07-14-2011 03:21 PM

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;

shuboyje 07-14-2011 03:37 PM

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.

Tscarborough 07-14-2011 05:39 PM

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.

tinkerric 07-14-2011 08:18 PM

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.


mekdigital 07-14-2011 08:27 PM

Re: Perlite/Mizzou castable
we have a winner![/B]

david s 07-15-2011 04:29 AM

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.

shuboyje 07-15-2011 06:32 AM

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

david s 07-15-2011 09:08 AM

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?

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