Mortar inside arch to dome or not

[Jim from Akron]

I've heard that some people mortar their dome to their inside arch, and some people leave a gap and fill it with high temp caulk. What are the advantages/disadvantages to each approach?

[Jim from Akron]

Here are a couple pics of how my dome and arch are lining up. Again, my question is should I mortar between the dome and arch, or should I leave the gap and fill it with high temp. caulk so that the two can expand/contract independently?

[UtahBeehiver]

Pics are too small to see detail. But I did stuff my arch and dome gap with a ceramic fiber rope then use a high temp caulk (2400 F) to keep the CF fibers encapsulated. I don't think this brand of caulk is available any more but there is one called Kaowool Moldable Caulk by Thermal Ceramics.