Steel Dome Oven

[dmun]

Re: Steel Dome Oven

Several makers have reported that the cast refractory - SS needle combo is hard to work with. There have been more vent transitions made with the stuff than full domes. I have no personal experience with refractory concrete.

Our Australian builders have more experience with castable: for some reason firebricks are ruinously expensive "down under".

[Wiley]

Re: Steel Dome Oven

ClintonJ,
Thank You for the kind words. However regarding your questions I would suggest you search out Ed Schmidt or David S both of which built WFOs from castable refractory. Ed Schmidt's was constructed of pieces and assembled and David S (he uses lower case: david s) WFO is a single piece casting.

The material I used was "Fondu" (calcium aluminate cement) and crushed basalt and so far has worked well for the purpose I intended. I'm not as certain how well it would perform if it was in direct contact with flame. One of the benefits of using a steel dome in my construction is to protect my homemade cast refractory from direct contact with flames. With a wood fired oven it is possible to get localized temperatures in excess of the rated temperature of the cement and that cannot add to the durability of the WFO.

Regarding cracking, I suspect that larger WFOs might be more successfully constructed using cast sections rather than as a single piece. I think that perhaps the biggest concern with a single piece oven is how to accommodate the stresses created by uneven heating. Multiple sections would allow differential expansion due to uneven heat and thus not be as susceptible to cracking. Dmun's geodesic dome might be a good place to start if one was thinking along the lines of casting larger pieces that would fit together in a repeatable pattern thus minimizing the number of sectional molds

I'm not familiar with the "large fiberglass sand filter body" which you are suggesting to use for a plug mold. Do you have a link?

Sorry I couldn't be of more help. There are several people who have built cast ovens on the Forum who are far more qualified to address your questions than I am.

Bests,
Wiley

[ClintonJ]

Re: Steel Dome Oven

Hello Wiley- I'm new to the forum, but have been reading your thread and referring back to it often as I plan on building my own oven. I've been back and forth deciding on exactly how to build it. My first idea was just to build it exactly like the pompeii plans, but then I started reading about your project and was very impressed with the outcome. You seem to really know your stuff when it comes to insulation, refractory material, etc... My latest idea is to use a large fiberglass sand filter body (40") as a mold for cement refractory material. Basically cover the cut fiberglass dome with aluminum foil and pack the cement around it to a depth of 4", possibly including some wire mesh inside the cement for strength, allow the cement to cure and (with lots of help) remove the mold and set the cement dome in place. I would appreciate your opinion on this idea. My worries/questions are:
#1 - What type of cement/aggregate to use?
#2 - Should I use some type of wire mesh to strengthen the dome?
#3 - Will whichever type of cement I use eventually break or crack?

The rest of my plans are pretty basic. I'll Cover the dome with ceramic blanket and perlite or vermiculite, add a vent and hearth, all of this on a insulated base topped with firebrick (per pompeii plans). I've searched the forum and googled "castable pizza oven" and haven't gotten any reliable results. Any input would be greatly appreciated.

[Wiley]

Re: Steel Dome Oven

Randall,
I think the thickness I would choose would in part depend upon how good the join is between the mating pieces. I'm guessing you are speaking here of the piece I used between the dome assembly and the outer entrance ring. If it is a close fit the thicker stuff may not fit, if loose then the thicker would fill the extra space (sort of have to look and make your best guess). I don't think there is a magic number with this. The major idea is to stop the expansion and contraction of the inner dome from causing problems with the outer shell/exterior of the WFO thru the slip surface. The retardation of heat transfer from dome to rim is less controllable as the space for such insulation is small.

As for thermometers I have been using a Harbor Freight infrared thermometer, a cheapie (when on sale $29.00) and so far have been pleased. I did not opt for thermocouples in the floor or dome. In fact we are planning a trip to Italy this fall and I bought a second HF infrared thermometer to take along. Figuring there was a possibility of it being confiscated by our well meaning (but basically idiotic) homeland security people either exiting or entering our country. And if not then perhaps I can give it as a gift in Italy to a helpful pizzaiolo. The units work well enough and are cheap enough to be considered expendable. I'm most curious to get some real time data on temps in their ovens.

Hope this is helpful,
Wiley

[randall4483]

Re: Steel Dome Oven

Seattle Pottery does carry.
They have 1/32, 1/16", 1/8" 1/4" Would thicker be better or does that cause other issues? Thanks for the contact.
Also, did you install a thermometer in your oven or do you use a laser non contact thermometer?
Randall

[Wiley]

Re: Steel Dome Oven

Randall, If memory serves it came either from Seattle Pottery Supply or from one of the Seattle art glass suppliers. It is commonly used in slumping glass. I used to do alot of enamel work and SPS used to carry alot of stuff for enameling. If I were searching out a source today I would try them first and if no luck try a local glass blower or bead maker. I've seen bead makers use it to set their finished beads upon when annealing.

The thickness I used was about the same thickness as thick blotter paper. I'm sort of surprised they are only offering to sell it by the roll. When I bought mine it was so much per foot, they showed me what they had and I selected and bought. Then again, I was purchasing from a retail not wholesale outlets back then.

Hope this helps,
Wiley

[randall4483]

Re: Steel Dome Oven

Wiley, Where did you get the Frax paper and what thickness did you use? I can only find this paper in 100 sq.ft. rolls which is mega $. I will only need a small strip. Trying to gather all my supplies before I start.
Thanks,
Randall

[peculiarmike]

Re: Steel Dome Oven

WOW! What a thread! I'm worn out! NICE build!!!
Something I have seen no mention of -
The aluminum foil is serving two purposes, it's intended original purpose and as an infrared reflector. Infrared is heat. In the infrared world there are emittors and reflectors. Flat black is an excellent emittor, allows infrared to easily pass through. The foil, applied bright side in, would be an excellent reflector, driving heat back into the oven and improving efficency.
I build meat smokers and use a bright silver paint as a base coat when painting them for this reason. It makes a difference.
I am a FLIR certified thermographer.
Again - NICE BUILD!

[Wiley]

Re: Steel Dome Oven

Randall,
It's just regular aluminum foil, I used the widest my wife had to minimise joins.

I haven't had time to build what I consider an insulating door so how long the oven holds at what heat are figures I do not have. There is very little heat lost thru the insulation on my oven and I suspect that when I have a proper door it will retain heat as good as a brick oven. There may be a bit more loss as the flue is steel and connects directly to the inner dome, however, my "proper door" will seal the flue when in place. Any door that doesn't close off the flue allows the chimney to work and transfer heat away via convection from those non-insulated surfaces. As for the rest of the dome, the mantra of Forno Bravo Forum is "insulate, insulate, insulate". After several hours of running my WFO at pizza temps one could not detect heat thru its outer dome.

Even without a proper door I have not had problem cooking a single loading of bread (didn't bake enough to require a second loading) and it roasted a Thanksgiving turkey without dificulty as well.

Bests,
Wiley

[randall4483]

Re: Steel Dome Oven

Thanks, Is that just regular cooking aluminum foil or thicker stuff? If so, where do you purchase it. Approx. how long does your oven stay hot? say from 700 to 250? Once you remove the fire.
Randall

[Wiley]

Re: Steel Dome Oven

Randall, Although I do not have any figures to prove this it is my opinion that steel does not hold heat longer than firebrick, I believe that it conducts heat faster and that a solid steel oven would actually cool down sooner than a totally brick WFO. That's given that insulation would be the same etc.

The purpose of the aluminum foil was to cause a slip surface between the steel dome and the refractory molded over it. I don't have the calculations at hand but if I remember correctly if the circumfrence of the dome was a straight length of steel and if one heated it from a reasonable ambient tempreature to around 900 F degrees the length would grow something on the order of 1/2 inch. Pi x D gives the circumference so the diameter would grow something on the order of one third of one half inch or one sixth inch. Not much, however, that expansion is real and needs to be reckoned with. The refractory is going to expand as well but the steel will heat faster as it's closer to the heat source and I don't have any figures as to how much the refractory will expand. So by creating a slip surface between the steel dome and the refractory (and segmenting the refractory) I can control more or less the expansion. As the oven heats and everything expands the cracks between the segments grow and as if cools they contract. I think I included photos of the cracks somewhere in the thread. I heat cycled the oven several times to prove to myself that the oven was "working" as I wanted before continuing with the kaowool and etc. covering. I also placed strips of aluminum foil between the segments so the expansion cracks were more likely to form there than elsewhere.

I will admit I had some concern that the oven would expand and when it cooling the refractory would hangup and remain in the expanded shape leaving a gap betweeen the steel dome and the refractory. In my case the refractory cycled as I wanted and expected. I'm not sure if it had hung up there would be a problem with heat transfer to the refractory from the steel dome or not, but the worry turned out not to be warranted on my oven. I add that because I don't know of another WFO like mine and one is a pretty small sampling from which to make a definite statement.

Hope that answered your question, if not ask again and I'll try again :-)
Wiley

[randall4483]

Re: Steel Dome Oven

Wiley, does steel hold heat longer than the refractory? I was thinking of making the dome 2 layers thick. One dome on top of another then add the refractory. Of course I would need to slit the top dome 3 or 4 times to let it slide over. What does the aluminum foil do that you put on top of your dome before the refractory?
Thanks,
Randall

[randall4483]

Re: Steel Dome Oven

John, I have a 41" OD with the backer ring on it that I would sell for a $100 but I'm a long way from you so I'm not sure what shipping would be. My zip is 38017
Wiley, thanks for that very detailed explaination of the thermal break and rim.
After reading a few times I think I understand fully.
Also, still thinking about what you said on the steel floor welded to the dome. My wood stove is build like this and it gets much hotter without warping. Of course the floor is smaller. I make fire pits from these domes and never leave the backer ring and cut flames in the sides and I never had any of them become distorted so I don't think anyone needs to worry about it much.
Randall

[jcsforager]

Re: Steel Dome Oven

Hi Wiley,

Thanks for all the info! They are cutting the tank either way as they sell them to folks for BBQ's. I think I may hold off and go for a smaller one, 40" or so. All your points are convincing and I do want to consider firing time for a larger oven. Once I get the bug for something, such as this and start looking I usually find just what I need. Fun poking around all these yards as well. I wouldn't be suprised to find what I need tucked away in the woods around here. Plenty of people seem to have an obsession with collecting "stuff", piles of it. I was thinking of this as I lugged my new buoy, chain, a huge block, and other stuff associated with the buoy back to my home. All treasures I say! This is what I told my wife.

Keep your eye out for domes...

thanks,
john

[Wiley]

Re: Steel Dome Oven

Randall,
Sorry that the posts are unclear. In post #2 in this thread I do have the rim welded onto the dome. Subsequently I cut the rim such that the flanged end of the rim was separate from the portion welded onto the dome. That is sort of visible in the last photo in post #11 where there are three pieces spread out. Here's what happened between the photos:

The reason I cut the flanged portion from the rim welded to the dome was to create a thermal break. The dome gets hot, way hot and if the outer rim portion was part and parcel with the dome it was going to get very hot too. So my thought was to break that direct flow of heat with the strip that holds the transition to the flue. So heat has to flow out of the dome then over a strip of frax paper (Frax paper is a compressed ceramic material that looks like blotter paper...think kaowool compressed into a thin sheet) and into the strip or strap of steel that holds the transition then back thru the frax to finally heat the outer rim section.

Here's what I did (warning: probably confusing, although not meant to be)

Yes, there are three pieces, The dome with the welded on section of rim ; the transition section with the flue ; and the outer rim. The reason was stated above. Now I wanted to create the bent piece such that it matched the curve of the rim. So I tack welded a strip of metal to one edge of the rim (bottom edge) and bent it over the rim. Now anyone who has tried to bend a piece of metal over another can attest that when you let go the piece you bent didn't retain the bent shape but springs back in sort of the needed shape. That's not what we want so there is a way around this... heat the metal as you bend it and give it several smacks with a 48 oz hammer as it is being bent. For heat I used a carbon arc torch which is the easiest and cheapest way to get a real hot pressure-less flame (near 3000F). It runs off a welder and save for protection from the high intensity ultra violet light it generates and radiant heat it is a very neat tool. So bend the strip over until it reaches the opposite edge and tack weld it there. Now go over the whole bent strip get it good and hot with the torch and allow it to slowly cool, when it is cold it will retain the exact curve of the rim.

Now set the dome up so that it is level and determine the top of the strip of metal you bent. That is where the center of the flue will be. Lay out the flue and in the inside (nearer to the dome) drill 1/8 inch holes where the inside corners of the flue will be...only the two inside holes (there being four corners in a rectangle you are drilling the two nearest the dome). Next cut and weld the pieces of the transition together. Don't worry about cutting out the actual flue hole yet, having everything solid makes welding straight forward and fairly easy.

Ok still with me? Now drill four holes for flat headed bolts that will eventually hold the transition piece to the rim/dome section. I positioned them so they were about an inch up from the bottom on each side and then fairly close to the flue (two each side). The flat heads will be on the inside of the rim with the threads projecting thru the rim and then thru the welded transition piece.

Now grind out the tack welds that held the strip of metal to the rim. The strip will separate but retain its shape. Next mark and cut the rim into two from one edge to the other. This cut will be straight and end up being the front of the opening of the flue. Now mark and cut the rim and cut out what will be the flue passage thru the rim. That's why we drilled the corner holes that locate the exact flue location. However, (and here's where I expect to loose you) the flue hole thru the transition measured 3 x 9 and the section I cut from the rim/dome measured only 2 x9 the extra space is created by the outer flange rim being forward of the rim/dome piece...that's the thermal break area! Then cut out the section of the transition that will be the flue. Now lay the frax on the rim and push the flat headed bolts thru the frax. Cut away the part that would obstruct the flue and place the welded transition on the rim/dome and bolt down. The outer section of the rim that has the flange is supported by the base bricks and is held in position and captive by the transition piece. The thermal break allows for the dome to expand and contract as it needed as it heats in the completed WFO such that the dome can be very hot and not break the weathertight seal of the flange to the dome exterior.

Whew, if you followed that in one reading I'm amazed, Sorry to be so long winded and blow by blow.
Wiley

P.S. Last work before I retired was to gut a single story Victorian house (about 1/4 of the interior all the way to the dirt/basement) and rebuild. New joists, floors, plumbing, gas lines, wiring etc.etc. Finished with new oak floors and sheetrock, new thermal windows, clear fir panel doors the whole works. Did everything but the sheetrock myself. That was both my Victorian houses..my first and my last it took two plus years ugh!. Also delivered a 65 ft trawler thru the canal and up from Panama to Seattle as mate/engineer/navigator one summer and worked as an engineer on a fish buyer in Bristol Bay the next, so I had some fun too.