I have a 1934 home with a real wood burning fireplace. I noticed a head and bed joint in the mortar of the back wall were loose. I began to remove the loose mortar and the entire back wall basically crumbled. The existing fire bricks are in good condiditon and need to be re-mortared. There in no air space between the fire brick and the back of the exterior brick. The exisitng bricks have carbon build up on them. Do I need to wire brush the bricks to be clean to remortar? Should I purchase new fire bricks to rebuild the fire box? Is there a material I can use to insulate the cold exterior brick from the hot fire bricks at the back wall? I basically have a 1/4" between inside and outside brick.