Weber Grill Conversion ("Little Black Egg")

[mako]

With a lot of inspiration (EADavis's Forno Clamshell (Forno Clamshell - Another Charcoal Grill Conversion...), the Little Black Egg mentioned both here (Anyone ever made Weber Grill Pizza on a Stone?) and at pizzamaking.com, and the pizzahacker Franken-Weber that made the rounds a few months ago) I've decided to give a grill conversion a go.

There seem to be a broad range of opinions on how these things should work: propane vs wood-fired (with pizzahacker being the only one I've found opting for wood). Insulated (Forno Clamshell and FrankenWeber) vs letting the propane do the work (most of the little black eggs). Radiant vs convective heating to cook the tops of the pizzas (everyone probably falls somewhere a little different on a sliding scale). Side vs top venting. Etc.

My initial desires/design considerations are:

Big: I'd love to be able to cook an 18"-20" pizza.
Lots of BTUs: For minimal time to get from startup to operational temp
Heavily insulated: Both to be stingy with propane and to keep it at operational temp without too much work.
Thermal mass in the top: Because it just feels inauthentic to me if there isn't some heat radiating down from the dome onto the pizza. So what if this project is already super-inauthentic. :-)

Picked up a 22.5" Weber, cheap, from Craigslist. Shouldn't have too much trouble lobotomizing it and affixing a propane burner. Found a cheap high pressure cast iron burner at Academy -- still need a good regulator to pump out the maximal heat.

Also found a 21" cordierite kiln shelf at a reasonable price; laid it in the grill and it looks like it was made for it -- though it is a bit of a tight fit in terms of getting good air circulation. Might have to get a tileworking friend to open up some space on one side. I'll be using a round cast iron griddle

Here's where things get a little more hazy for me. I am a complete newbie to all of this, but I've read quite a bit in the last few days, and here's my thinking:

Insulation: I would like to line the entire assembly with ceramic blanket. Seems it can be affixed to the walls of the grill with Resbond 907GF. Or... could I just feather some sort of mortar in to affix it to? Is there a thinset that can handle the temperatures?

Then I'd need to cover it to keep bits from flaking off and giving my guests silicosis. People generally seem to suggest ITC HT 100 Ceramic coating, but others have mentioned covering with refractory mortar. I know this is completely backwards from the point of view of most oven-builders here, but do you think I could put together a homebrew* mortar to seal in the blanket without compressing it and killing its insulative properties? As I've not worked with any mortar/cement before (seriously, total noob) I don't have a good sense for whether these two materials would play nice together.

The other option is just to cover the blanket with heavy duty aluminum foil. Somehow seems like a bit of a cop-out, and less durable. But I don't think any part that's covered in foil will be nearing the melting point of aluminum, so that should hold up.

Top: Long term: I'd love to build a top like pizzahacker's. I haven't contacted him (though I probably should), but I assume the method to do that would be (if someone corrects any glaring problems, I will be very grateful):

Line the weber top with newspaper (?) or saran wrap (?). Make a fairly strong vermicrete mix (3 or 4:1 ratio?) to minimize crumbling. Fill the weber top, using a large bowl to form a dome shape on what will be the underside. Let cure for a few days.

Coat the inside of the vermicrete layer with a layer of refractory mortar. (Am I way off base here? Will it hold together? Will it handle the hopefully 900-1000 degF temps without losing integrity?)

Coat the outside of the vermicrete layer with something (more refractory mortar?) to smooth and parge (?) it. (Is that even a verb?)

Cast a C-shaped stand on which the dome will sit, with an opening at the front for pizza loading. (Like a first course of bricks, but thinner and made of something castable -- wish I could homebrew that.)

With everything miniaturized, there's going to be a lot more heat escaping through the entirety of the dome -- will the portland cement break down? Clearly it can be done, since I've seen video of it being done -- I'm just not quite sure of how, or whether my ideas are flawed.

Top: Short term: line the top with foil (with ceramic blanket underneath, when I manage to get some), somehow bolt a big kiln shelf to it.

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Wow. I sure can ramble on. My thanks and apologies if you read all of that.

I don't have any pictures yet, but I've got many of the pieces together, so hopefully there'll be something to look at within the week. If I can get the burner attached and the propane tank filled, there might even be shots of it in use.

Any thoughts? Ideas? Accolades? Dire warnings? I am listening. Cheers.

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* Can't seem to find the premixed in anything less than expensively large quantities.

PS. I tried to post this last week, but it got lost somewhere in the tubes. If a 2nd copy of this shows up, I apologize.

[EADavis]

Wow! Someone read my posts Actually, thank you - the whole reason I posted my experience was to get people thinking about other ways to cook pizza.

I like your ideas. At one point, I was thinking of building a bigger portable oven, but have decided to put those efforts into my Pompeii 80 kit. Here are some random thoughts on your effort - I hope some of it may help (you are welcome to ignore, or use, any or all of them):

• Covering insulation - if you want to keep this thing around for a while, you probably don't want the insulation exposed. I have heard waterglass/sodium silicate may work, but it seems like a large area to cover. In order to keep the weight down, and the integrity up, you may want to consider buying a larger 26" grill for the outside shell, and put the insulation between the two. I saw a cheap charbroil 26" charcoal grill on ebay for $60.
• With an oven the size you are proposing, having a cutout in the front is probably better than hinging the whole top, like I did. In fact, you can hinge a cover over the opening, to retain heat while heating up, but still be able to slide the pizza in and out.
• I tried the vermiculite mix on my first attempt on a smaller grill, and it really did not work well (and it is heavier than you think, at least in a portable application). I ended up throwing that prototype away.
• I loved PizzaHacker's approach, and may of actually gone that way, if I knew about it at the time. The only thing I would do different from him (and it looks like you are thinking along those lines) is to insulate it on the top. I really don't like the idea of a very hot surface externally, where someone can accidentally burn themselves.
• If you are going with the refractory on top, I would probably go with the real stuff (castable refractory cement). Form the cement about 1" thick in the inside of your 22" lid (after you cut out the entrance), using some bolts and washers to hold it to to the top. Then insulate on the outside of the 22" lid, using a 26" lid as the outside shell. At least that is what I was thinking of doing for my Forno Clamshell 2.0.
• I have found that convection plays a bigger role in cooking a pizza on my grill than I had thought. So if your heat source is from down below (i.e. propane burner) then make sure you have good clearance around the cooking surface to allow the heat flow to rise. Also, you may need to increase the mass of your cooking surface (I use two pizza stones on the bottom, a third up top, and have added a fourth to help diffuse the heat off the burner). Otherwise, the cooking surface gets too hot, relative the the rest of the grill, and you have burnt dough!

I think you are off to a great start, and have some great ideas. As with any engineering prototype project (face it, that is exactly what you are doing!), you may need to retry things several times before it works. But that is the fun of doing it. And the result is great pizza!
Good luck, and let's see some pictures!
Ed

[EADavis]

Since I had not revisited this till I saw your post, I went back to see what PizzaHacker was up to. There is another YouTube video that shows what must be a newer model of his FrankenWeber (it does use the lid to the grill) YouTube - Pizza Hacker - San Francisco, CA

Also, I found another YouTube video, to show what can happen if you make a FrankenWeber too top heavy YouTube - Rip-off of Pizza Hacker's Franken-Weber
Ed

[mako]

Ha! Guess I'll have to do some more testing before I try to cast anything. Don't want to end up with a pile of flaming rubble.

the pizzahacker fully cast model looks quite pretty. About how I'd like to end up, but with some more propane fire coming from the bottom. In that video you can see the coals/wood chunks burning right under the stone (as well as above) -- surprising that the heat distribution is workable like that.

Just to address some of your other notes:

No way I'll be putting a hinge on mine. Front door or pull the top off entirely. For now I'm trying to avoid making cuts that I can't later un-make, so I'll probably just pull the top off to load pizzas. Or maybe try building up a spacer out of aluminum foil.

Insulation I was thinking would go inside rather than outside. I really don't want to go any larger than the 22" grill -- it'll just get too much like having a whole brick oven, but on wheels. (Speaking of which... this all started from me wanting a brick oven on a trailer -- and I don't want it any less now)

That said, the idea of laying the castable refractory inside the 22" top, and the insulation outside it, is a very elegant solution.

And as far as convection, the 21" kiln shelf is a tighter fit than I expected, but I think pushing it to the front will leave enough space at the back to create a nice wind tunnel of hot air. At least, I'm hoping.

Close to being able to fire it up. Just need to figure out how to mount the burner. Haven't had any good ideas there yet. Oh; and I've gotta convince my friend to bring his propane tank over.

[mako]

An update on this project: Haven't gotten any insulation, but I put together something for a test run, and it worked pretty well (held 800+ deg F without complaint, and could've gone higher).

Not quite as complex as I had planned on, yet. 2 kiln shelves and a a few firebricks for thermal mass and support, big propane burner on the bottom, aluminum foil lining for a little bit of insulation/reflection.

It does pretty well, but the heat on the bottom and from the sides outstrips that coming from the top (even with the burner cranked up to the max to pour out plenty of convective heat, the top still lags behind).

But it'll crank out pizzas in 55-75 seconds until the propane runs out. The propane might run out pretty quickly with the heat cranked up, though.

Next test will be to either drop the top stone (flip the firebricks onto their sides), or throw a little wood into there to get some heat coming from above the stone, instead of just from below.

[mako]

Oh, I guess I should post a picture of the results, as well.

Not bad, but clearly there are hotspots on the bottom to deal with, and the top could use a little more heat.