Hells Bells - questions regarding the bells of a RMH

So, I have two questions regarding the bells of a RMH:

Everybody I see uses 55 gal drums for the bell. Would fire brick be acceptable? I'm concerned with the possibility of the air inside not cooling fast enough to drop properly.

My second question is similar to my concerns on the first. I have an idea for incorperating an oven into the RMH. I was thinking of putting low density fire brick on top of the bell, (I'm invisioning the arched pizza ovens with removable plug). Again, my concern with that being the air may not cool fast enough to drop properly.

If anybody has tried something like this I'd love to hear how it turned out.

Check out Glenn Herberts thread "Rocket mass heater with 8" J-tube and bell." It should still be on the first page of this forum,  and it's pretty much exactly what you're looking for. Also, according to batchrocket.eu and donkeys forums, masonry bells are fine as long as the heat riser is properly insulated and the bell is sized for the system. I'm sure if you searched for "RMH pizza oven" in the forums something would pop up too. Hopefully someone with more technical knowledge will chime in to help you better than I!

Be certain to have a good chimney. Most recommendations on Donkeys site consider it a prerequisite.

Would fire brick be acceptable? I'm concerned with the possibility of the air inside not cooling fast enough to drop properly.

My second question is similar to my concerns on the first. I have an idea for incorperating an oven into the RMH. I was thinking of putting low density fire brick on top of the bell, (I'm invisioning the arched pizza ovens with removable plug). Again, my concern with that being the air may not cool fast enough to drop properly.  

 I'm sure that you could use fire brick, but I'm not even sure if you need fire brick for your bell.  Once you are past the Barrel/Manifold area the system is cool enough for regular bricks, I think.  I don't think that you have to be concerned about the cooler air not descending, so long as your bell's internal surface area is in the right proportion to your system.  The hottest gasses will go to the top of the bell, and this will force other gasses out of the system's exhaust, which will inevitably be cooler, and the system will form it's own thermo-siphon flow as new hot air is provided into the system and new cooler air is exhausted.  

The general recommendation is to make at least the top third of the bell of firebrick; the rest can be ordinary red brick. There is enough intense heat which builds up fast at the top of the bell to stress ordinary brick in that location. You can build the inner layer of bricks (firebrick and red brick) on edge, to make the inner wall thinner and have fewer joints. You would want a second outer wall, of cob or brick, in addition. The thin brick wall gets very hot quickly; I find that the face of my bell which will sit against the future stone faced block chimney gets too hot to touch in less than an hour, and even the bottom of the bell wall is uncomfortably hot after three or four hours of burning. The six inch thick cob facing on the other walls is barely warm after four hours, and still warm 12 hours later. You might want a slightly thinner outer wall than that if you use cob. You would definitely want a steel access panel or something set into the bell somewhere to get some instant radiant heat, as a solid bell as I built without such panels would not heat the space at all until a few hours after starting the fire.