A while back I asked this question and got a lot of great responses.
I've been able to locate fireblock caulk, but one thing I didn't think of is the backer. I need some kind of fireblock/fireproof caulk backer for the gap in my fireplace's molding. The only caulk backer at Home Depot I've found is the polyethelyne backer. Something like this.
All of this stuff explicitly says to not expose it to heat, and that it's highly flamable, but I need something that will be fireproof.
What should I be using for caulk backer in an area highly sensitive to heat?