The Chimney Clinic
In many cases the firebox in a fireplace can start to show signs of wear with either missing or loose mortar joints or by completely missing brick. This type of damage is usually caused by water leaching into the chimney from the exterior.
The joints can be either missing or sometimes dry rotted in which they simply turn to powder. Either way it makes the fireplace unsafe to use and the firebox simply needs to be repointed by removing the old or loose joints and replacing them with a high temperature refractory mortar. Regular mortar can not be used because it can not withstand the high temperatures that the firebox is exposed too.
A commonly missed but extremely dangerous repair that is common in 90% of all fireplaces is the side wall and lintel repairs. Unfortunately most masons that build fireplaces and chimneys do not know what fire code and building codes are. They simply learn from who taught them and they learned from who taught them. This cycle can go on for a hundred years. When a fireplace is finished the joint between the fireplace breast and the firebox needs to be sealed with high temperature mortar as well. MOST masons simply pack standard house insulation into this gap (which is considered flammable) The repair requires the insulation to be removed and a high temperature fireproof insulation be installed with a layer of high temperature air set mortar over top sealing it off.