I'm installing a Gallery Crown cast iron arch insert. The insert needs
to sit about 1" away from the wall to fill the rebate in the Mantel.
Its my understanding that the gap between the wall and the inside edge
of the insert will need to be sealed to stop any draft getting in and
stopping the proper draw of the fire, it's a real coal burning fire.

What should i use to seal the castiron insert to the wall.

Thanks in advance for any suggestions and help.

Chris