Portable Oven Designs

[cvdukes]

Threads I talked about various ferro-cement domes

http://www.fornobravo.com/forum/f3/f...oven-2938.html (Ferro Cement Pizza Oven)

http://www.fornobravo.com/forum/3212-michaelthebaker/

http://www.fornobravo.com/forum/f36/...ject-3464.html (SPRING!!! and a new project)

[mine]

I have a 44" dia. 1/4 thick sphere that I initialy was going to use as an oven until I happened upon this site.

The thought was to split it along it's equator, insulate the top and bottom with castable refractory, make a supported hearth with a gap around the circumference and heat from the bottom with wood/propane with the chimney protruding several inches below the center of the dome top.

I could not come up with a satisfactory way to line the inside, especially the top, and still allow the differential expansion of the refractory and the steel.

So I am making a "conventional" WFO.
Still open to the steel sphere if anyone has any good ideas.

[cvdukes]

Wiley,

As I gather, you got a metal dome already that you intend to cover with concrete. Problem you may run into is the metal expansion coeffiecent is higher than the concrete. The size of the metal dome is big enough that its going push the concrete a lot...best case is that the the materials will just separate but worst case could be the concrete will crack. May not be a major problem since its going to be encased in other materials... Also, that sounds like a lot of thermal mass to heat for up those kinds of thicknessess....start your fire midweek for the weekend parties?

[Xabia Jim]

Originally Posted by Wiley

I'm toying with the idea of a steel dome with 4-5 inches of refractory concrete with basalt as the aggregate over it. Topping that with 2 inches of vermiculite mix and then a 1" layer of frax. I have a 5/16ths inch thick 40" ID dome. Thoughts?

Wiley

Wiley: I had the same impression on thermal mass for your oven....lots.

If you read on thermal mass and insulation here (FB forum) you will find that many like a thinner dome with insulation...particularly for pizza

More thermal mass is okay, particularly for the bakers and commercial operations that will use the oven for longer periods.

Jim

[Xabia Jim]

Craig,

any reason not to consider some rebar in the ferro mix (is there a negative, beyond weight)?

...and how do you avoid voids when laying it up? You're basically packing mortar into a mash of wires right?

and again, how thick were your dome and liner layers?

Thanks

[Wiley]

cvdukes,
Actually I got the dome as part of a trade from our local scrap metal guy. He has a contract to cut up old propane spheres and tanks for recycle. He had the half dome and another full sphere one time when I was in dropping stuff off and they looked cool and we worked out a trade. The half dome is only 40" in diameter the other is much larger closer to 48".

As for thermal expansion I was thinking along the lines of a product our local quarry sells called "1/4 minus" for covering the dome itself. It's basically finely crushed basalt, they use it for top dressing on walkways and drives. It packs down quite well and I figured I could mix a bit of Fundu ciment as binder and basically "dry pack" the thermal mass layer. Perhaps creating deliberate score lines for "controlled cracking". I read where it is recommended to cover the bricks in conventional brick ovens with aluminum foil to allow movement. I was thinking doing this over the steel hemisphere. That it gets bigger and smaller shouldn't be a problem if that movement doesn't effect the weather resistant outer skin. I was even thinking at one point of simply cutting the larger sphere in two and using it over the other sphere with 3" of basalt pack and 1" of Frax. From what you'all are indicating that may have enough thermal mass for pizza and perhaps some bread too? Perhaps I could save on the cost of Frax and simply fill the extra space with dry vermiculite poured in thru a hole in the top?

I came across a company called "Pivot" located in Australia that makes ovens that are basically two hemispheres (one inside the other with a couple inches spacing) and which they fill that space with sand. Seems to me that without insulation it would cool down fast and when hot be pretty dangerous as in "Don't lean on that!!!"

Thanks one and all for your input. I'm not totally locked into using the hemisphere this way. I was even thinking one could use it as a "plug" in the manner some boats are built. Cast the refractory dome in sections over the top of the steel hemisphere and then remove and reassemble the cast pieces. Then there would be the chance of spalling, right? anybody cast their dome?

thanks,
Wiley

[Wiley]

cvdukes,
Thanks for the links on your postings on ferro cement construction. What threw me was the term ferro concrete. I'm familiar with ferro cement at least in regards to boat construction. It was a popular method in the 70s and 80s for DIY backyard boat construction. A good medium for freeform consruction.

I'm curous, you mention glass filaments in the mortar mix. Around here they have redimix with filaments in the concrete but I was under the impression that they were some sort of high tech plastic. Glass would make more sense considering the temperatures involved. Where do you get glass filaments for addition to concrete?
Thanks,
Wiley

[dmun]

Originally Posted by mine

I have a 44" dia. 1/4 thick sphere that I initialy was going to use as an oven until I happened upon this site.
Still open to the steel sphere if anyone has any good ideas.

I've long thought about building an oven upside down, so you could easily and properly finish the interior. The problem is that is the weight and logistics of turning the oven over when it's done. The thick steel hemisphere makes this easy, you just roll it over. Of course you'd want to set it in a sandbox while you were working so it wouldn't roll about while you were working. You could even tilt it to get a better angle at the place you were working on.

The simple answer to the expansion problem is that after you've coated the interior with castable refractory, you lay down squares or strips of insulating blanket, to give it an expansion layer, as you build your dome up from the peak.

[cvdukes]

Originally Posted by Xabia Jim

Craig,

any reason not to consider some rebar in the ferro mix (is there a negative, beyond weight)?

...and how do you avoid voids when laying it up? You're basically packing mortar into a mash of wires right?

and again, how thick were your dome and liner layers?

Thanks

I've never included rebar just because its harder to work with and hasn't been needed for anything I've built. Of course if I were building a really huge structure (say a 55 foot long ketch that I saw built down in Charleston harbor)... I would definitely want the rebar to give the shape to the structure because that large of frame built with just reinforcing wire (or my usual cheap hogwire) would warp before the cement mix was applied. If you have a 1/2 thick peice of rebar stuck in the middle of your chicken wire mesh layers, then you'll end up having to use several more layers. Even with just a relatively thin peice of reinforcing wire or hog wire in the middle of the mesh layers, you end up with a packet of wire mesh about 3/4 inch thick. By the
time the mortar is packed in, you're getting up around 1 1/4 inch thick. Putting rebar in would take you to 2 inches or thicker.

The hard part of ferro-concrete comes in that you basically end up packing the mortar in by hand. Filling voids just depends on backing up other side of the mesh so you can pack against. Easy enough if the structure is small enough to get your hands on both sides of it (or if you can get someone on the other side like stuffing my son up in the monster chimena). 3 layers of 1 inch chicken wire is about the minimum that you can easily pack... and its easier with more layers becasue there is more for the mortar to adhere to.

I'll address your last question on thicknesses, etc. back on my original thread on the pizza oven as I see you had a couple others there.

[cvdukes]

Wiley,
After seeing your and Mine's posts on the steel half-domes, I was wondering where ya'll are doing your shopping...thanks for clearing that up.

I don't know about this basalt you talk about (not a southern thing) but shucks, if I had one half dome 40 inches and one that was 48, I would sure try something with 'em, 'cause it don't sound like you got much to lose.

I would take the 48 incher and lay it with the open bottom side facing up (the sandbox idea seems good to me). Then take a piece of styrofoam about 4 inch thick and cut it out to shape of archway the width of a door into this thing. Glue the foam inside the bigger dome with the arch pointing down. Then slip the smaller one in, also with the open bottom side facing up. Figure out to make two bottoms even with each other...Maybe drill a few holes around the circumference of the smaller one so you can stick a few stiff wires to suspend in place. Pour a regular cheap 5000 psi concrete into the open space (use a finishing sander on the outside dome to vibrate it in place)...give it a fw days to set up good then cut out the metal of both domes where the foam arch is. Buy a case of beer and get your friends to help roll and lift it into place.
Insulate on the outside of the outside dome. If you wanted to cut down on the thermal mass a little bit, figure out how to insulate the inside of the bigger dome before you put them together... you want to keep the thermal mass next to the inside dome.

Just posted more info on the fibeglass concrete at http://www.fornobravo.com/forum/7/ex...html#post28234 (External crack)