Cracked or deteriorating brick or mortar, typically in the back wall, is one of the most common problems with the firebox. Cracks in the walls of your fireplace or the interior chimney lining are considered a serious fire hazard. This is because smoke carries waste from the fire, which eventually causes deposits on the walls, inside and behind these cracks. These deposits, called creosote, can reignite from sparks, potentially causing extensive damage to your home.
Cracked mortar may be repaired by chiseling away cracked areas and repointing with fireclay. If a brick is broken, the affected wall will probably need to be rebuilt with new firebrick and fireclay.