Thanks for the help from this forum my fireplace is in an running well. I wanted to burn for about a month before I sealed and finished everything. I'm now at that stage- I lucked out with 60 degrees in WI on Dec. 1st to open up the outside wall and finish all that up on the outside. Inside I have a gap between the original stone work and the new ZC.(picture shows wire mesh filling gap). I have stone to fill gap and overlap the trim (smooth metal showing on the ZC) My question is: I will mortar the stone in place bonding it to the metal mesh and existing stone work, but wondered about what to seal it too the fireplace with? High Temp Silicone behind the stone so you cant see it or mortar it right to the metal? I was afraid with expansion and contraction of the metal the mortar would crack so thought the silicone would be a good idea, just not sure it is designed for this application. I don't need any structural integrity the mortar will take care of that, just want a good seal so air can't draft in behind the stone.