The back firewall on my masonry fireplace has a crack running top to bottom in the middle of it. I have heard that it can be repaired with masonry cement or caulk. Does anyone have any advise for me on how to do this and the best product to use? Can I find the materials I need at Home Depot or will I need to find a specialty store to find the correct fireproof cement/caulk? Any advise is appreciated.