Experimental homebrew cast - thin walls and "zero-depth" flue gallery

[Ronstarch]

Hello fellow oven-casters! Ron Starch here with another homebrew cast oven with a twist. This time I'm testing how thin is too thin for a homebrew casting, and trying out an idea for making an extremely short flue gallery while still having a vent. I am shooting for a low-cost, lightweight, but fully-functional oven, primarily for pizzas, that will fit on a 30"x30" stainless steel table. I hope to be able to disassemble this oven and move it next year (assuming it works and doesn't crack apart), so any ideas in that regard are welcome. Here's what I've done so far:

Formwork:

I already had some existing formwork from previous builds that made this pretty easy. I've got a plywood sheet with a 29" diameter circle cut out, reinforced with 2x4s underneath, into which I can wedge an inflated yoga ball. This has been successful for me in the past, and I still have the same yoga ball (three castings and haven't popped it yet!). I just prop up the plywood on sawhorses, wedge the yoga ball in there, inflate, and start casting. I also had a couple other pieces of outside-curve formwork (4" garden wall plastic) that I used to cast a circular perlcrete base.


Dome Casting:

Used 3:1:1:1 homebrew for the casting, with AR glass fibers, stainless steel needles, and poly melt fibers. I aimed for only 1" thickness in my dome casting. Did this by eye, so some spots are probably thicker than others, but for the most part, I think I hit that thickness. My method was to basically dump some mix down onto the plywood, using it like a mortar board. I would then take a small trowel and flatten out a fistful of homebrew into a rectangle about 1" thick. Slide the trowel underneath, and place this "brick" against the side of the yoga ball, slightly overlapping the existing material to get a good bond. Once I hit a rhythm, this was fast; I think it took me about 2 hours to cast the entire half-sphere. After 1 day I deflated the yoga ball (a big benefit of using the yoga ball is how easy it is to de-mold), and examined the casting. Looked good for the most part; I filled the voids with a rich mix of the homebrew and then "sponged" the entire inside with a damp cloth. End result looks promising; nice and smooth. I estimate that the dome casting weighs about 110 lbs.

Oven Opening/Door:

Rather than build formwork for the oven door, I decided to just cast the entire half-dome, and then cut out the door after the fact with an angle grinder. This was a lot faster since it required no new formwork, and this also went along with my idea of trying to eliminate the depth of the flue gallery, which I'll describe later. It was pretty easy to cut the door out with a diamond blade on the 4 1/2" angle grinder. I did some shaving down/refinement of the opening after the initial rough cut, so the "core" doesn't quite fill the opening now. I plan to use this piece to make a functional door; still need to figure out those details. Door is not so important since this is a low thermal mass oven primarily for pizzas.

Next post will talk about the flue gallery/vent

[Ronstarch]

Flue Gallery (Smurf Hat?)

So the idea here is to make the flue gallery have basically no footprint in front of the oven opening. This should make it a lot easier to maneuver pizzas around in the oven, and also lets me put this oven onto my 30"x30" stainless steel table. I do this by venting smoke out the opening and back over the dome of the oven, and then out a chimney in the middle (naples style center vent). I cast the flue directly over the existing dome, which I covered in plastic sheeting so the casting wouldn't stick. I built some haphazard formwork using some thick vinyl, spray foam insulation, duct tape, and cardboard sonotube I had lying around. Not symmetrical, not perfect, very funny looking, but (maybe) functional? You can see in the third picture, the base of the flue gallery is basically even with the front of the oven, but the actual flue part overhangs quite a bit. This thing reminds me of the hats that the smurfs wear for some reason.

I'm a little concerned that the door on the flue is about 1.5" higher than the oven opening, which will probably let some smoke escape out the front, especially during light-up. Maybe I can add a decorative arch in front that drops down lower?

I will add a length of removable chimney pipe to the top of the little "starter" vent I made. Should I maybe add a venturi/taper on top of the vent to improve the draw, maybe going from 7" diameter to 5" or so?

[Petter]

I like your approach! Keep up the good work. Always inspiring with new solutions.

[david s]

Excellent work. Utilising the combination of casting and homebrew opens up great possibilities because it is both quick and cheap. If it fails you’ve only lost a little of your own free time, whilst it’s cost you little. But the experience is invaluable. I’m a great fan of pushing the envelope and I think you’re on the right track especially with the very shallow, lightweight flue gallery. Moving the thin dome casting into position might be challenging. One of the limiting factors might be the thickness (or should I say the thinness) of the dome. Strength in concrete is proportional to the square of the thickness, so halving the thickness results in 1/4 of the strength. A one piece cast, not being heated evenly is subjected to pretty extreme and uneven thermal expansion and from my experience with two of my one piece cast domes and that of another manufacturers one piece cast dome (in exactly the same place),results in a vertical crack rising from the base opposite the oven mouth. Not really a huge issue, my own oven at home is a single piece casting, around 12 years old with no crack (maybe it’s the careful way I fire it). This can be avoided by engineering the cast into sections to allow for thermal expansion, but that is quite a lot of involved mould work. You will just have to see how it goes. Keep posting

[Ronstarch]

Thanks Petter and David, I appreciate the encouragement. David, you mention possible cracking of the dome due to it being a single casting with uneven heating. What do you think about cutting the dome in half horizontally, and just resting the top part on the bottom part (no mortar)? With it being so thin, it would be pretty easy to slice it with the angle grinder. That would allow for the top of the dome to heat up/expand faster than the sides, maybe preventing the cracking (?) Or is that cure worse than the disease?

[david s]

Thanks Petter and David, I appreciate the encouragement. David, you mention possible cracking of the dome due to it being a single casting with uneven heating. What do you think about cutting the dome in half horizontally, and just resting the top part on the bottom part (no mortar)? With it being so thin, it would be pretty easy to slice it with the angle grinder. That would allow for the top of the dome to heat up/expand faster than the sides, maybe preventing the cracking (?) Or is that cure worse than the disease?

Because of heat rising by convection, especially because the chamber is fired empty, the top half of the dome will be considerably hotter, so your plan should be a good solution. I think the join should be filled from the outside, which is what I do with the top casting in my build. Fill it with sieved home-brew, so it's both finer and richer. But don't fill the join on the inside as you don't want any mortar to fall on your food. The mortar will seal the gap but there will still be an ability for the top section to expand at that join. It will also have the advantage of halving the weight which will make moving and reassembly of the castings much easier. You may also be able to skip the angle grinder and diamond blade step by casting up to halfway, allowing it to set, coating the join with some fine oil, then casting the top section. You should be able to pop the top section out easily enough using a car jack.

[Petter]

Just a thought: If the dome cracks naturally in the vertical direction, would that not be the way to with the engineered cut as well? Or have I missed something?

[david s]

Just a thought: If the dome cracks naturally in the vertical direction, would that not be the way to with the engineered cut as well? Or have I missed something?

Yes that would be preferable, but as I pointed out in a previous post, to engineer that in the mould is more difficult. The joints should really be half stepped if vertical. I have two in my build making the dome a three section one, front back and lid.

[Ronstarch]

Insulating slab

I again used existing formwork to cast a 4" thick circular perlcrete disc to use as underfloor insulation. I topped this with a thin layer of homebrew to strengthen it slightly and reduce crumbling, and give me a relatively flat surface for the cooking floor to sit on.

Cooking floor

I had a bunch of split firebricks from previous builds, so I'll use those. I decided to have the dome surround the floor, rather than sit on top. I used whole and half bricks arranged in a herringbone pattern to fill up the center of the circle. Then I flipped my oven mold upside down, rimmed it with a piece of flexible vinyl, and cast outside the bricks with homebrew (duct tape around the bricks so it wouldn't stick). This was instead of cutting all the outside firebricks to fit the circular profile. This seems like a decent idea, but in hindsight I should have cut the bricks in such a way that the homebrew part was a more uniform width. The way I did it, there are obvious weak parts where the homebrew is almost definitely going to crack.

[mrsrat]

Your idea for the form for the chimney is genious. I was thinking I was going to have to cut and weld some steel up to do essentially what you’ve done with a cement form I can pick up at Home Depot. Thank you for saving me hours and dollars!!!

[Ronstarch]

Update: Assembly

Stand Assembly and set-up
I bought a 30"x30" stainless steel table at the restaurant store for $150. It had a rated weight capacity of 450 lbs on the top surface, and 250 lbs on the lower shelf. This should be sufficient, although I recognize that the manufacturer of this steel table would almost certainly not sign off on this usage. I cast four 8" circular patio pavers out of quik-set cement, and set them level into my lawn on some gravel and then sand. I then set up the table onto the pavers, using the leveling feet to make sure it was solid.

Oven assembly
Once I had the table set up in the yard, it was pretty easy to assemble the oven components that I had cast. I happened to have a piece of 1/2" thickness cement board (hardiebacker), so I set that on top of the stainless steel table surface. Probably unnecessary, but I think it will protect the table top, and gives me a surface that I can tile or stucco later if I want. Onto that, I put the perlcrete floor, with the space already cast to receive the firebricks (splits). I set the firebricks in, using sand and clay to level them. Then I set the dome right on top, no mortar. The floor and the dome were light enough for me to place by myself.

Regarding the idea of an engineered cut to prevent cracking, I decided to chance it and not do it, even though it makes a lot of sense. The main reason: not having a cordless angle grinder. But also, I am curious to see if this dome cracks as others have; so I'm treating it a bit like an experiment.

Flue gallery
Here is where I ran into a bit of a problem. I had cast my base as a circle only just large enough to hold the dome. When I went to add my flue gallery, I realized that the legs of the gallery didn't reach down to the table. No big deal, I cut a couple of firebricks to the right height and mortared them into place. I debated whether to insulate the dome before adding the flue gallery. I decided not to, but as I'll show in my next post, I think this was a mistake...

[UtahBeehiver]

Even though a cast oven is lighter than all brick ovens I would consider some diagonal bracing on the stand.

[Ronstarch]

Update: Insulation, chimney, curing, and first use. How does it work, how does it vent?

Insulation
I got a 25' roll of 1" thick 8 lb ceramic fiber blanket for $90 off amazon. I tried to make sure I had at least 2 full layers (so 2" thick) everywhere on the dome. I added more to the top of the dome, so that part is 3" or even 4" thick in places. To hold it in place, I used a regular cheap bundle of 24" chicken wire, which I cut with tin snips and tied to itself with wire. I insulated over top of the flue gallery as well.

Chimney
I inherited a 4' length of triple-walled stainless steel chimney pipe (8" diameter) from a friend. This pretty much matches the diameter of the vent of the cast flue gallery. For now, the chimney just rests on top of the vent. It's quite vertical and well-balanced, so unless it's very windy, it should stay put. However, I'd like to come up with a better method of attaching the chimney, while still having it be removable.

Curing and vent performance
I tried to follow the recommended FB curing schedule. Temperatures always seemed to get a little higher than I was shooting for though (350 instead of 300, 450 instead of 400, etc), so by the 4th day of curing fires, I was already up to 900 F. Oops. I didn't get any dome cracking though, and everything seemed to be working well. Maybe because the dome is only 3/4" to 1" thick, rather that 2-3" of standard cast domes, the curing happened more quickly. I also had polypropylene burnout fibers in my homebrew, which should help water to escape the dome without cracking the casting.

The chimney vents absolutely perfectly. No smoke out the front of the oven, everything going up the chimney. Really happy with that aspect. I also really like to overall look/shape of the oven, with the chimney in the middle.

Issue with insulation of vent
It was when I finally got the dome to clear around 900-1000F that I noticed a strange pattern on the dome. The entire dome was clear/white, EXCEPT for the uninsulated part of the dome where the hot smoke vents back over the dome and out the middle, that part was staying black, and was cooler than the rest of the dome. I had intentionally not insulated this part of the dome, because I had read (on the FB website) that this center-vent design could be even MORE efficient than a standard chimney at the front. The idea being, the hot gases from the oven flow back over the top of the dome and therefore transfer more of their heat to the dome material. However, what I found is that, because a good amount of outside air is mixing with the exhaust gases from the oven, the temperature of the air within the flue was only about 500F, when the dome temp was close to 1000F. I fixed this by cutting a piece of ceramic fiber blanket to fit on top of the dome within the flue. I used a ceramic fiber blanket rigidizer (spray) on this piece of insulation, so that fibers from the blanket don't volatilize and come off in the exhaust.


First Pizzas
Not being able to wait to stucco/finish the oven, I had the first pizza party. What more can I say, other than, nothing beats a pizza from your own homemade pizza oven!

Not sure how I'm going to finish it yet. For now, just keeping it covered unless in use.

[david s]

While I’m a fan of a shallow entry to make the oven easier to work, I suspect you’ll have some issues with cross winds because you’ve eliminated the gallery side walls entirely. Looking forward to future reports in its performance.

[Ronstarch]

david s Thanks, I agree. Crosswinds haven't been a problem so far in the few cooks I've done, but I definitely recognize the possibility. I even brought up the issue of wind in the other thread about the offset entrance. I could add some little wind guards on the sides maybe, made of castable or cut firebrick. Will probably wait to see how it performs and then add something if needed prior to finishing the outside. This oven is kind of an experiment to see what I can get away with.

UtahBeehiver Yes, you're right. This oven is pretty lightweight, I think less than 220lbs total. Before I add more weight via stucco/tiling, i will see about some cross bracing.

mrsrat mine is far from the best example of what can be done with castable, but glad you found it helpful!