Flat dome, and steel arch instead of brick?

[DragonStar]

After a few years of thinking about building an oven I am now doing it. I'm starting with a cob oven because my soil is expanding clay and there is lots of it.

I've found two things that I'm intrigued by. One is an aluminum reflector dome for flood lights. It is 11" tall by 30" wide, and has a really interesting pattern of flat spots that seems intended to focus light to the center. I'm thinking of using this dome as the basis of the sand form (ie, coating the inside with wet newspaper, filling it with damp sand, and removing the dome to leave behind the shape.) The dome is not round on top. There is a flat disc the size of a dinner plate. would this shape be better or worse than a dome (which I can do by simply shaping more sand on top manually).

The second thing is a heavy steel bracket that might be used to hold 55 gallon drums? It is really sexy looking and the perfect height. I can put my cast iron dutch oven in it with room to spare. The steel arch would help support the cob in the same way a brick arch would. However, I'm worried about the different expansion coefficients of steel and cob. Would such a steel lip at my oven entry be a problem?

Thanks, I'm really glad this forum is here. It is so nice to read about everyone's oven projects.

[v12spirit]

Re: Flat dome, and steel arch instead of brick?

Hi Dragonstar,
The dome rounded shape allows the fire to lick the dome and reflect over the inside of the dome and be scattered evenly in every direction, so the fire can eventually recharge the hearth evenly and keep it thermally consistent. IMHO eliminating that flat disc will allow for a "better" heat flow inside your oven so your hearth will be recharged more efficiently, while leaving it will make the disc act like a heat absorbent that impedes the heat circulation and transfer to the hearth.
Regarding the steel arch, It will definitely expand faster than any bricks or refractories being far more heat conductor. If it were my oven I would not go into that risk. I would better use it just for helping me shape the brick/refractory arch. If you intend to use it just as a decorative lip outside the vent landing, it may not get hot enough to make cracks but I'm afraid it may get hot enough to burn your hands.
All the best with your build.

[cobblerdave]

Re: Flat dome, and steel arch instead of brick?

G'day
I'm not adverse to using steel in an oven entrance my own has. I made the decision to use steel in the full knowledge that uneven expansion and rust could be a problem. It hasn't been to date. If it is I fix that.
I recon though sometimes when you have pieces like you do and use them they don't always suit the propose as well as something built to suit.
For a WFO to breath correctly thought its doorway that doorway should be 63 per cent of the internal height of the dome . That's a dome, tunnel or pear shape. And the door should be less than 1/2 the wide of the oven. And again that's a 1/2 round entrance part oval or square again it doesn't matter.
So if you keep this in mind and get as close as possible to these your going to be good.
Good luck ( post some pics were are simple folk and love pics)
Regards dave

[DragonStar]

Re: Flat dome, and steel arch instead of brick?

Thanks for the replies! I will definitely add a dome over the flat spot.

So steel might be okay, but I'm taking a risk of cracks or getting burned on really hot metal. Or it might not get that hot. I will have to think on that. Do either of you happen to know the average temps of your front lip?

My arch is 62.3% of the dome height, so I think I'm okay there. One figure that puzzles me is the door being less than 50% of the oven width. That would be a 14" door in my case, which seems unworkably tight given the tunnel with the chimney. What sacrifice, besides heat loss, am I making with a 25" wide door?

Thanks again for the replies. I'll post pics when there is something worth looking at.

[cobblerdave]

Re: Flat dome, and steel arch instead of brick?

G'day
A 25 in doorway the heat loss would be extreme large plus the entrance would intrude into the oven hearth floor space quite a lot. A standard pizza pie is her generally 12 in so you have an inch clearance either side. Squeeze out another inch either side which is not much in heat loss compared to 25 in and you'll have passage for all but the biggest baking dishes I'd recon.
As for the entrance/ chimney doesn't matter how large you make that the bigger the better I say. My doors roughly 12in by 18 in the entrance 21 in by 21 in x18 in deep. It doesn't matter much how big the entrance way its to the oven breathing but to the operation the bigger the better I say!
Regards dave

[DragonStar]

Re: Flat dome, and steel arch instead of brick?

You asked for pics so here is the build thus far, showing the good and the bad. I have a rock base built, with 6" of sawdust cob insulation under the base. Then sand, as level as i could get it, followed by the firebricks. A few of the firebricks picked up moss on the edges over the year I had them stored. I am happy to have finally begun this project and I apprfeciate your helpful comments!