Remove loose mortar or cracked brick residue from the cracks in the walls of your fireplace using a small metal rod or mason's tuckpointing tool. Ensure that no chunks of residue remain in the cracks so the caulk used to repair adheres to the bricks when filling the cracks.
Go over the cleaned-out cracks with a dry paintbrush to remove any remaining small particles of loose mortar or crumbling brick.
Apply special fireplace mortar, which is actually a silicate-based refractory cement caulk, packaged in the same type of container as regular caulk. Insert the tube of fireplace mortar into a caulk gun to apply it to the cracks. This special mortar withstands temperatures up to 2,000 degrees F once it is properly cured. If the cracks are large, apply multiple layers of this caulk, with each layer approximately 1/8 inch deep and 1/8 inch wide. Overlap the layers of caulk slightly to ensure that you cover the entire crack. Allow the fireplace mortar/caulk to dry to the touch before continuing.
Start a small, low fire in the fireplace after the caulk is dry to the touch. The ideal temperature of this fire should be below 212 degrees F. Burn the fire at this temperature for one to four hours to completely dry the special fireplace mortar/caulk. After four hours, increase the intensity of the fire to about 500 degrees and keep it at that temperature for another one to four hours. After this curing by fire, the fireplace is ready for normal use.