Steel Dome Oven

[Wiley]

This is what I am doing in the way of constructing a somewhat different WFO. It will still have a brick hearth and the actual design is not unusual, only the materials of it's construction are different. I'm going to try and stick to using commonly available recycled materials because (to me at least) going into detail about a project using truly unique one-of-a-kind materials, while perhaps interesting, is of limited use to others who might be so inclined to follow and build one of their own. That's providing, of course, that this works, which I believe it will or I would not be spending my time, money and energy on the project.

I'll try to add lots of photos. Suggestions as well as comments are welcomed.

First off this oven will be using one half of a condemed spherical propane tank. Tanks are condemed for various reasons, a common one being the serial number and spec plate has become separated from the tank. Once separated a tank is condemed and cannot be reused for propane. I got this one from our local scrap metal recycler. In point of fact, I got two, one a half tank with a diameter of 40" and a full sphere with a diameter of 48". The 40 inch one is what I'll be using in big part because it is aready cut in half and because the size is better for my use. The thickness of tank is approximately 5/16 inches.

This dome will serve as the interior surface of the oven. Refractory material will be exterior to this surface, with insulation outside of that, just like a typical WFO. These tanks were constructed of two hemispheres welded together. Where the weld is made there is a backing plate, in this case a backing ring. This backing ring aided in the welding because it allowed for higher amperages to be used in joining the halves resulting in a better weld (since it could only be welded from one side). The sphere was cut in two on one side of that backing ring and as a consequence when cut, the two halves were not equal. The half I have has the backing ring so that it is slightly larger than a true hemisphere. See photo of backing ring. The addition of the backing ring makes the interior height of the dome 20.75 inches.

This is all boring but the reason I included it will make more sense in a bit (hopefully).

So that will be the interior of the dome itself, a steel dome. But what happens where the interior makes the transition to the entrance/doorway and the entrance itself? My answer is one half of a steel "split rim" off a truck. This one came gratis from our local Les Schwab tire dealer. It was the biggest I could find with an interior diameter of 22.5 inches. That makes for a radius of 11.25 inches. This radius would be the height of the doorway/entrance to the oven. Split rims are thicker than conventional "clincher" rims, and this one is approximately 1/4 inch thick.

Now according to Alan Scott and others, in a correctly proportioned WFO the height of the doorway should be 63% of the height of the interior of the dome. So how far off are we with these two pieces? The height of dome (20.75 inches) minus the thickness of a standard firebrick (2.50 inches) creates an interior height of 18.25 inches. 63% of 18.25 inches is 11.497 (basically 11.5 inches). Height of split rim entrance 11.25. So matching the half split ring with the half dome is within 1/4 inches of being correct.

So it seems a straight forward matter of welding a piece of 1/4 inch steel plate to the bottom edge of the split rim (which will make the height of entrance to height of dome proportion correct); then welding the rim to the dome; cutting out the entrance/doorway and back welding the join. Then fitting, welding and cutting a suitable transition piece for the chimney. Followed by setting the whole assembly on a suitable raised base and covering with refractory and insulation etc. and finishing in a more conventional way with a layer of stucco.

For refractory I'm planning on using "traprock" or basalt (which is available locally) with LaFarge "Fondu" as cement. Per suggestions on this forum I am now expecting to use about 3 1/2" thickness of refractory. Insulation will be first a blanket of Frax (kaowool) and then a couple of inches of vermiculite and Portland cement.

Thoughts or suggestions anyone? I'm looking for ideas on the transition piece to the chimney, thanks.

Bests,
Wiley

[Wiley]

Re: Steel Dome Oven Update

I got a few days off and managed to get some work done on the WFO and figured I should post an update as to progress. Attached are some photos:

First shows the entrance (half split rim) welded onto the dome. View is from the top. Photo is out of order date wise as the original got lost somewhere's.

Second shows the dome inverted and ready to cut out the entrance itself. Entrance was welded to the dome before any cutting to minimize distortion of the dome from the welding.

Third shows the back weld of the inside of the entrance. Entrance had to be cut before back welding in order to weld to the edge of the dome.

Fourth shows the weld but ground and sanded to make for smooth flow of hot gasses in exiting the dome.

Fifth shows setting the dome back into the shop. It's easy with the right tools, in this case an excavator :-)

Next I'll grind the bottom of the entrance to the height of a standard firebrick and then build the transition/support piece to the chimney.

In order to slow the heat transfer from the steel dome to the front of the entrance I am planning on cutting the entrance so that it is essentially two arches. When finally assembled the outer arch will be spaced about one inch away from the arch that is welded to the dome. That space will be filled with insulating refractory. This is for safety, otherwise I can imagine the outer edge of the entrance getting quite hot.

If all goes well, I should be able to "test fire" the dome in a week or so to be sure it draws and (if necessary) make modifications before cladding the dome with refractory and setting on the platform... which I haven't built yet :-)

Wiley

[PeterW]

Re: Steel Dome Oven

I've been watching this with interest, and I'm really impressed with the re-use of old stuff...!

One think I did think, is expansion of the steel. This might sound an odd suggestion, but why don't you add a thin layer of a compressible fibre insulation, such as 10mm of rockwool before the outer refractory..? It will allow the steel to expand but not crack the casing..?

Just my 2p

Cheers

Peter

[Wiley]

Re: Steel Dome Oven

Peter,
Thanks for your thoughts :-)

My plan (subject to change, of course) is to cover the dome with a layer of aluminum foil. The refractory will be a concrete mixture using calcium aluminate cement and crushed basalt. The plan is for this to be mixed in small batches and hand packed upon the dome in a somewhat orderly fashion, but with definite expansion joints (probably with aluminum foil) between "blocks" of the refractory. My calculations put the expansion of the dome at less than 1/4 inch, that's radius of the sphere, for a 800 degree F differential. But the idea is the refractory can expand and contract and nothing will fall out and into the pizza. The loss from the lack of refractory where the cracks form will, I expect, be minimal, and the steel will "bridge" the cracks making for a uniform radiant surface. And when it cools it all shrinks back together.

The expansion of the whole will hopefully be allowed by a layer of Frax (insulating blanket) which covers the refractory. Over that will come the vermicrete and over that the stucco. If that fails and I get cracks on the outside of the dome, then I will turn to a rectangular "house" over the dome with the loose vermiculite etc. That is not my first choice I prefer the curves of the dome/igloo shape.

Wiley

And here's a photo of the entrance with the bottom ring ground to the height of a standard refractory brick.

[gjbingham]

Re: Steel Dome Oven

I'd start looking for stucco now. Port Townsend or points south are not quite the types of places I would expect to find it. Is the Keystone - PT ferry running yet? You may get lucky locally. Start calling masonry suppliers to see if they carry it.

[Frances]

Re: Steel Dome Oven

Fascinating! I'll be really interested to see how this one turns out... sounds as if you've got it well thought through Wiley.

[drogers]

Re: Steel Dome Oven

A very nice job of fabrication!

[jengineer]

Re: Steel Dome Oven

What a great bell. How did it sound before you finished fabing it. I cut a scuba cilnder and used the long part for a bell that my 2 yr old loves to swing. The 7 inch dome I kept and it has a real nice pitch to it. I bet the end of the this one was nice and deep.

[Wiley]

Re: Steel Dome Oven

Thank you Frances and drogers for your kind words and interest in my project.

gjbingham, Ferry service is back, but only one boat and only fifty cars per trip. They do have a reservation system and of course the summer rates have gone into effect :-( Fortunately we don't use it often as our island is served by a bridge:-)

I don't understand why should I be looking for stucco now. In the past I've had good success with using the formula and advice written in "Audels Masons and Builders Guide #3". It is basic and the mix is all available materials: cement, sand. and hydrated lime. Is there something I should be aware of? Another shortage?

jengineer, the sound was not as deep as I would have liked. In order to easily move the dome I welded a large staple to the top of the dome which made for ease in lifting and transport. The dome really isn't that heavy, a couple hundred pounds at most, but unwieldy. I move it with our tractor or the excavator. When suspended from a rope via the staple one could strike the dome to sound or ring the bell. Like I said it wasn't as deep as I would have expected. Perhaps had it been cut over center such that it recurved the sound would have been deeper and more full. I too have an old scuba cylinder that I have saved to make a bell/gong out of. The one I had seen cut into a bell/gong was quite a bit longer than it was wide and had a deeper richer tone. It was cut using the threaded hole as the top.

Previous I have used the dome as a cauldron for last year's Halloween Party complete with floating body parts and dry ice. See attached photo.

Today's rainy and so I have to work indoors. Most of the steel I'm using is recycled. I was given a steel box measuring several feet on each side a couple of years ago. It was built so that one could load wood outside a house and be able to access the wood from inside. Nice idea but they had a problem with yellow jacket hornets and his wife demanded it be gone. So I got it for taking it away. 10 guage or 1/8 inch steel, a little rusty on one side but fine under the paint on the other... and the price was right! I cut and use a carbon arc torch to remove the paint outdoors and so rain shuts me down.

I've got some questions regarding the chimney that I am going to post over at the chimney thread.
Thanks again,
Wiley

[gjbingham]

Re: Steel Dome Oven

No, you're probably right. The mix sound correct. There's a latex additive for the finish coat to make it waterproof though. Shouldn't be a problem for you.

[Wiley]

Re: Steel Dome Oven Test Fire

I've taken several photos of the intermediate steps but figure unless the oven works they are not really that valuable. So today I assembled the pieces I have built on the driveway in front of my home and had a test fire.

So lots of things were "mock-up", the bricks on the hearth are old red bricks and in order to insure some sort of air-tightness I packed sand around the outside of the dome itself. The chimney was something I found at the recycle center (see where I shop :-)

I am including a photo of the three pieces before assembly. Also I would like to know (even though the photo doesn't show it) I did get a small wisp of smoke out the front, is that common or do I need to consider rebuilding my transition piece... in this case the hoop band that connects the chimney to he FWO? Thoughts and opinions welcomed.

Wiley

[gjbingham]

Re: Steel Dome Oven

Wiley,
Very cool. A one in a million oven. I'd consider adding about 4 inches of cladding or some type of heat sink outside the dome to increase your thermal mass. That should help retain heat much better than just the metal dome alone. Insulation outside of that, a finish, and you're done!

[Wiley]

Re: Steel Dome Oven

Thank you for the kind words, George. Yes, the plan is to cover the dome with aluminum foil and then 3-4" of basalt concrete (basalt 1/4 minus mixed with calcium aluminate cement) scored deeply so the expansion cracks will be somewhat controlled. The a Frax blanket and vermicrete and stucco.
I'm a bit concerned about the small amount of smoke I'm getting out the front. Is a small amount normal? I mean I see photos of ovens with significant soot staining and don't know how my loss thru the front compares (due to small test fire) a real fire. Any thoughts? Maybe I'll try a much larger fire tomorrow.
Thanks,
Wiley
Oh, and here's a picture of me :-)

[Frances]

Re: Steel Dome Oven

I think a bit of smoke, specially while the fire is starting up, can be considered normal.... well, I know my oven spills quite a lot of smoke to begin with. Less would be better...

Your idea of building a larger fire sounds like a good way to go. Then you can decide whether you can live with the amount of smoke or not.

As long as the smoke stops pouring out of the front once the fire gets going properly you should be fine.

[dmun]

Re: Steel Dome Oven

I think another length of the chimney pipe would be more useful than a re-designed transition. Remember; this is all outdoors, where drafts push smoke around, and you don't have the temperature difference between outside and inside that drives fireplace draft. A whiff of smoke on startup is normal, but smoke pouring out once the fire is going strongly is not.