Papier mache and pure clay?

I was thinking maybe one could build up a rocket mass heater and double bell system in papier mache (as the mold) and clay. Then burn away the Papier mache?

The challenge would be stability, as the expansion and contraction due to heat would be significant.  Would clay be stable enough if it wasn't kept absolutely pure and then fired in a kiln?  What you are suggesting is akin to a cob oven.  My concern is that if it cracks, you'd have a danger of a carbon monoxide leak.  Very dangerous.

But if it were professionally made and fired, like a kamado grill/Big Green Egg, it would be very stable and able to withstand the tremendous temperature extremes.  I'm waiting for the day when someone manufactures these professionally out of clay (again, glazed and fired like a Big Green Egg), with a tight gasket at the bottom that seals perfectly.

You will probably find that you need sand or other fillers/aggregate mixed with your clay to minimize cracking. It probably would be a good idea to pre-score lines for it to crack without being destructive to the piece, otherwise if it is monolithic it will crack probably where it will cause a structural or leaky problem.

Marco Banks wrote:
But if it were professionally made and fired, like a kamado grill/Big Green Egg, it would be very stable and able to withstand the tremendous temperature extremes.  I'm waiting for the day when someone manufactures these professionally out of clay (again, glazed and fired like a Big Green Egg), with a tight gasket at the bottom that seals perfectly.

But if you glaze it, won't the glaze just run on successive firings?