Kettle bbq with refractory cement walls and dome

[sohara]

Hi All,

I am planning to attempt to build a quasi pizza oven by converting a kettle charcoal bbq (? la pizza hacker / pizza forge). I have a number of unresolved questions about how I could go about this. Here's a link to an image of what I am hoping to achieve.

My question is how I can go about making walls and dome. I suppose I would need make a mold and then fill it with refractory cement. I've seen several posts/faqs here about mortar for fire brick, but nothing detailed about refractory cement. Since I'll need more than just a couple of gallons I would prefer to be able to come up with something I can mix myself rather than laying out for a premixed bucket. Anyone with some ideas about an ideal composition for such a concrete? I live in Montreal, Canada, so I am not sure about sources for masonry material, etc.

My other question is about insulation. It looks like there is no insulation of the modified bbq used by the pizza hacker, yet he apparently is able to achieve temps of over 800 degrees. Do you think I need some kind of insulation?

Thanks for any advice/thoughts,
Sean

[Tman1]

Re: Kettle bbq with refractory cement walls and dome

I think you're going to want to check out pizzamaking.com for info on these types of modified BBQ's. You'll want to research MBE's and LBE's.

As for insulation, that's a question no one can answer until you have a method to the madness. However, with that said, if pizza hacker does it without insulation, why would you think you need it?

[sohara]

Re: Kettle bbq with refractory cement walls and dome

Hi Tman1,
Thanks for the advice. I had done some browsing at pizzamaking.com, and I made a post there about my project. It's true there is an extensive thread about the LBE, but for the most part those don't have any casted refractory parts. They attempt to get top heat by adding reflective materials to the original lid of the bbq. The other difference is that they are mostly using gas burners instead of charcoal and wood.

I think you're right about the insulation. I'll try it first without, and worry about that later.

Right now my main question is what kind of material to use as a castable refractory cement.

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

Hi,
I've been studying the pizzahacker's oven for a while.
From what I've read, he started out mixing refractory cement and perlite to cast the top dome, using the Weber kettle top as a mold. His first version has fire bricks raising the dome off the grill.

Your linked pictures show his more recent version with a cast base.
I think he must be casting these parts with an insulating castable refractory. The insulating castable must function to absorb enough heat to be able to radiate and insulated enough to keep the inside of the oven hot enough for pizza, though when the fire goes out, it must cool down fast. The oven must be pretty light as he moves it around.
The Pompeii oven and the cast Forno Bravo ovens all have more mass, either from firebrick or castable refractory. They need insulation.

After cooking in a 27" cob oven, I decided to forgo the modified kettle bbq route and cast a small 22" oven.

I want an oven that heats up fast for pizza and I don't need a lot of retained heat for cooking after pizza.
I made inner mold from the bottom half of a Weber Kettle, it is a really nice 22" half sphere.The outer mold is from 1/8" flexible plastic.

I had one bag, 55lb, of a castable refractory called KS-4 (122lbs/cu.ft)from Harbison Walker. The mold required 2 bags, but I was curious to see if I could mix perlite with the KS-4. Maybe it will heat up fast and not require insulation? I think there is a good chance that the mix will not be strong enough. I've got the mold made now, so if this doesn't hold together, I will cast it again without the perlite.

The mixing and casting was pretty easy. I spent a lot of time making the mold. It would be easier to make a sand mold.

There is a you tube video of a rip off of the frankenweber.
YouTube - Rip-off of Pizza Hacker's Pizza Forge (Franken-Weber)
It looks like they used portland based concrete to cast it. It must be heavy and probably toppled the bbq.

My mix is probably too heavy for a bbq.
Look into the insulating castables.
You might find them at pottery supply houses as they use them for kiln construction.
Sorry for the rambling reply. If you have any questions, just ask
Rich

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

Hi,
I think pizzahacker is casting with lightweight insulating refractory.
Here is a link to the many possibilities
Refractory Castables, Refractory Plastics, Refractory Mortars
Look for one that can be a hot face material but is also insulating.

From what I've read,at first he was mixing perlite with refractory cement to cast the dome. You can find pictures of this on the web, the more recent version is probably cast from a commercial mix.

I just cast a dome using the bottom of a Weber Kettle as the inner form.
I used a general purpose dense castable called KS-4 from ANH, I added about 2/3 by volume of perlite to the mix.
Its an experiment, and may not have the strength of to stay together. I'm going to let it dry for a week and then cure it for another week.
If it works, I'll post more.
Rich
Sorry for the redundant posts, I didn't think they were going thru so I replies 3 times over the course of 24 hrs. My third post which didn't make it up was the most concise.

[sohara]

Re: Kettle bbq with refractory cement walls and dome

Hi Rich,
Thanks for the detailed and helpful responses. What you are saying about using a lighter weight castable instead of regular refractory cement makes sense. I will have to try to get my hands on some of that. I live in Canada, so I'll have to do some research. Do you think I should try adding perlite if I go for one of the insulating castables that you linked to above (or something like it)? I do think there might be a great deal of difficulty in obtaining such a product in Canada, so might have to try to come up with my mix of some sort.

One other question. What are you doing to cure your dome? Just lighting small fires inside?

Thanks again.

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

Hi,
I just removed my cast from the mold. It came out pretty easily and seems pretty strong. I'm hopeful. I was going to mix 1:1 refractory to perlite but I was afraid it wouldn't hold together. I did 1:2/3 and I was a little short on material.

It is 22" inner diameter , a half sphere,about 1 1/2" thick an weighs in a 60 to 65lbs. This might be a little heavy to sit on a bbq. I'm going to make some sort of metal stand and try firebrick splits (1 1/4" thick) as the hearth. The opening is around 7" high by 15.75" wide.
If it holds together after firing I will probably insulate it more.

I'm sure you can find castable refractory in Canada. I bought my KS-4 from a pottery supplier. Montreal is a big city, they must have industrial furnace suppliers who also sell this stuff.

Look up all the documented cast builds in the "other oven types" section.
That's where I found most of my information.

Check out Kiko Denzer's book Build your own earth oven.

You could make a sand form and then cast your dome in one pour. It would be really easy. For some reason, I was attached to using found objects as my form.

I suggested a commercial lightweight insulating refractory because they are designed to perform a certain way, my mix is an experiment. I don't know if it will survive the rapid heating and cooling it will be subjected to.

I'm thrifty, and I wanted to see if I could cast a working oven with one bag of refractory ($38 and less than $10 of perlite.)

I will let it air dry for a week and then follow the general guidelines for curing the forno bravo cast ovens, starting with a small fire and build up slowly over a weeks time.

[sohara]

Re: Kettle bbq with refractory cement walls and dome

Sounds great, thanks for the tips. Are you building a cylindrical section for your dome to sit on, or just using the dome formed from the bottom of the charcoal grill?

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

The dome is the entire oven. I'll post a pic soon.
After using a friend's cob oven, I moved away from frankenweber bbq as base idea. It seems to work, but I want to build a fire right on the hearth, sweep the coals to the side, and cook pizza.
I don't want to start charcoal and then lay the stone in, etc.
My oven might end up being portable, but I don't need to disassemble and move it every time I cook. It will be in my yard.
I started out trying to reverse engineer the frankenweber, then moved towards a small thin domed oven like the Forno Bravo Giardino and then was inspired by poster david s's 21" cast oven in Australia to go small again.

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

here is a picture of the oven I just cast.
Next time I will vibrate the sides as I fill the form. The top of the dome is denser than the sides.

[david s]

Re: Kettle bbq with refractory cement walls and dome

Hi tinkerric,
This is an interestingbbuild. I am a bit mystified as to why you used an insulating refractory. Generally the inner dome needs to be a dense material. By adding perlite to the mix you are also weakening it. The idea is to have a dense material to give you sufficient thermal mass to store heat then insulate all round it. Not saying your idea won't work, I think you should proceed and see how it goes. I think small is good, less cost, less time, easier stand etc. And most folk only cook one at a time anyhow.
Dave

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

Hi David,
I agree with both of your points.
I will probably end up casting the next one with just dense refractory.
But, the pizzahacker's lightweight dome is what got me moving on this project.
His oven gets up to temp and his pizzas bake pretty fast.
Maybe it will work. I can't wait to try it out.
My dome is probably more dense than his so I may need insulation anyway.
Then I might as well omit the perlite. Its an experiment.
I was all set to build a 28" oven but after seeing your oven, I went small again.
Thanks for all the information you've shared.
Rich

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

A few more pictures of my molds.
The entrance was to be cast as a separate piece if I had had enough material.

[tailbiter]

Re: Kettle bbq with refractory cement walls and dome

Hi Rich,

I'm new to this forum, but what you did here is exactly what I've been investigating. Unfortunately I have no experience in working with cement and forms. Is there any way I could get more details on how you set this up? Was this last post your final product or did you try any additional attempts?

I have a ceramic grill that I was thinking about trying to build a "pizza dome" for that would look very similar to your creation. It would set on the bottom half of my Big Green Egg.

[tinkerric]

Re: Kettle bbq with refractory cement walls and dome

Hi Tailbiter,
Here is another thread where I detailed my build and posted some pictures.
http://www.fornobravo.com/forum/f2/p...ble-16351.html

I've been making pizza in my oven for over a year and there are no cracks in the dome.

It gets up to temp in less than an hour with very little wood. I can bake pizza all night as long as I keep a fire on the side. I cook in the oven as it is heating up, and bake bread after I'm done with pizza and the oven is cooling down.

I added 2/3 by volume of coarse perlite to ks-4 castable refractory. It has performed fine and is very strong. The only advantage is that the dome is lighter. It probably doesn't perform as well as a dome cast from straight castable refractory. my dome still needed insulation. This dome might be awkward to take on and off your green egg.
If you have any specific questions I will do my best to answer them
Rich