On a side note: I have been asked privately a few times, of how I bonded any or all of the material onto the brick bell, all the while handling expansion and contraction.
The answer, to me so far has been simple, in that, I simply used silicon calk. From the very best to the cheapest (based on cost) These ranged from $12.99 to $4.59 a tube. While the "the time it last test" simply takes years...and you would have to preserve a application sheet, of what you used and where, it could be kept track of.
But: What I have found---
Knowing that the external temp of the inner bell would never exceed 275 degrees (and this would really be hot) And rarely do I touch 200 degrees with my large mass built in- Thus with the silicon limit for heat far above this, I have felt comfortable using it. Note: other bonding materials often have far less temp range, sometimes as low as 150 degrees. (and are not as flexible at all)
I did "just for an experiment", use it very close to some metal door structure that has got near 400-500 degrees, and it discolored a bit, but nothing more.
As for expansion/contractions, the bonded small pieces can wiggle a bit, but you would have to exert great effort to pull off. I generally only bonded the edge touching the bell, not the edges of one piece to the next, thus everything can move a bit, without any domino effect. Note: I am not creating a SEAL, but rather adding a exterior facade, and mass. I suppose one could preach that the tiny gaps created between bell and facade has a insulating effect... And I won't argue, but I also won't deliberate it either. Mine has worked well, and that is what is important to me.
Lastly, I don't know of any material used for mass that silicone won't stick to. But very porous material will certainly
use more. But we want to add as heavy and nice looking material as we can.. So the porosity issue really doesn't come to play.