I have a cast iron open fireplace which is (loosely) attached to the chimney breast in my living room by the mantle piece. There is a small but significant gap behind it where the bricks have been hacked out so it will fit. What should I fill this gap with? Currently any soot which forms can fall down behind the fireplace.
a picture would help, but i sense that no matter what i will recommend you to use cob - that is, building with earth. i didn't understood exactly what gap you need to fill but cob can fill everything, it's cheap (cost nothing!) it's fixable, and it can stand the heat (with guaranty - in the past i have built some ovens with it).