The fire box is the space inside your fireplace where wood combusts to release smoke up the chimney and spread warmth into your home. Cracks in the masonry of the firebox can be dangerous since smoke and hot gasses can seep through the openings and possibly ignite wood beams inside the walls. Water damage is also a possibility. You can make repairs to minor cracks and chinks in the masonry using supplies from your local hardware or home improvement store.
Lay sheets of old newspaper across your hearth to minimize mess.
Empty the fireplace of all ash, burnt wood, and other debris. Then remove the log grate and set it aside.
Clean the inside of the firebox with damp rags, scrubbing the interior walls and flooring where the log grate normally sets. Then allow it to dry completely.
Inspect the firebox with a flashlight to note all cracks, loose masonry and chinks in the mortar.
Use the putty knife to spread fireclay across damaged areas, and work the material deep into the cracks using the flat blade of the putty knife to apply pressure.
Smooth out the fireclay for an even coat and uniform appearance.
Allow the fresh mortar to dry for 48 hours before using the fireplace.