Even though fireplaces are almost always found either indoors or at least inside an outdoor shelter, their brickwork is still open to damage. Due to the high temperatures in and around the fireplace, the mortar in particular is prone to decay. Repointing a brick fireplace differs from repointing an ordinary brick wall in two respects. First, the fireplace's brickwork is usually a facade, and second fireplaces routinely use two different types of mortar.
Mix a batch of regular mortar for repairing cracks on the exterior of the fireplace. Do not apply this mortar to repairs inside the fireplace, as that requires specialised mortar.
Repair small cracks by smearing mortar directly on the crack and forcing it in with a trowel. Scrape extra mortar off the fireplace joint with the trowel, and drop it back into your supply of wet mortar.
Scrub out crumbling mortar with a wire brush. Remove solid mortar with big cracks by chipping it out with a cold chisel and a hammer. Proceed with care, as most fireplaces have only a brick facade and one layer of bricks, and whatever is behind the facade might be fragile.
Pour the mortar into a mortar bag, slide the snout of the bag into the empty joint and fill it with fresh mortar. Shape the wet mortar with the trowel to match the pattern of the old mortar in the adjacent joints.
Mix a separate batch of heat-resistant mortar and repeat Steps 2 to 4 for the inside of the fireplace.
If your house was built before the Second World War and the brick fireplace is an original fixture, it probably uses soft lime and sand mortar for the exterior brickwork. To repair cracks in the mortar, you must use matching mortar and not modern cement-based mortar. Modern mortar is much harder, and will damage the bricks in the fireplace.