RMH Exhaust Materials

I was just sitting her thinking about the different materials involved in making an RMH. Then I was thinking about the discussion about using metal for the exhaust. Is this necessary? Does it need to be metal?

Why not just use cob?

Is there a structural reason? Safety? Is there a risk that the whole thing might collapse one day and spew fire?

How about using something like Sonotubes (cardboard tubes for doing fencing, deck foundations, etc) to make the form. Cover the whole thing with good cob or a suitable alternative. Light it and let her burn. The paper tube would of course burn, but would not the cob cure and harden creating a permanent exhaust?

Of course this makes me think of the saying "curiosity killed the cat". Am I the cat?

A metal liner will help prevent your cob from cracking and possibly collapsing.  Without such a liner, if it collapses, you have to start from square one and rebuild the system  With a liner, you just have to patch the cob.