When we moved into our 1930's era home, the fireplace had some cracked/broken bricks in the back of the firebox. I used a high heat fireplace mortar and filled in the cracks, then gave it a skim coat to make the area flush with the undamaged portion. We've used it a few times after that, and have enjoyed it. Aside from the questionable "aestetics" of having a patched area, next to the bricks, is there anything wrong "safety-wise" with my approach?