Thanks Cristobal, that info matches my sense about cob oven design and performance. In this case, I'm making the oven more as an experiment, for occasional use based on its performance. I think it will be useful for some form of cooking/baking, but I'm not too worried about getting especially high performance for any one specific use. Ideally, when firing, it would swirl the hot flue gasses and perhaps allow some secondary combustion rising out of the fire box below, as well as some conducted heat through the oven floor / top of fire box, to pick up as much heat as desired. When its up to good temperature, the fire can reduce to coals and sustain heat for a while during cooking use (either with or without continued flue draft, since it can exhaust to the side as well).
So, my main question here is how to build up the walls and roof of the firebox, on top of an existing stone seat or 'frame', so that they will hold up to repeated firing, without installing a metal box or lots of brick. I've done a good bit of burning inside another stone-clay fire box underneath my hot tub, and that's survived okay with some repairs as needed. I guess I'm curious what else could be done to improve the durability of earth and stone, if that's my default solution, incorporate cement/lime/other additives for integrity, or another option I haven't considered.
And I should have mentioned, I do intend to use a metal front face and door with draft control to enclose and regulate the fire that way.
