[dmun]

The vermiculite should expand at more or less the same rate as the exterior masonry. The interior brick dome will expand more than either of the exterior layers. The cured vermiculte concrete is fairly rigid: I used a one inch refractory blanket between the dome and vermiculite partly because it's more compressable, and might relieve expansion stresses between the two rigid layers. On the other hand, many makers have used the vermiculite concrete by it's self without difficulty.