After years of searing-hot blazes, fireplace mortar can crack, crumble, and fall out. Gaping mortar joints are not only unattractive, they leave the bricks more vulnerable to damage. So before wood-burning season starts, examine the condition of the mortar in the firebox and take an hour or two to replace any that has deteriorated. You'll need a carbide-tipped scoring tool meant for cutting tile backer board, a metal jointer, a couple of trowels—a brick trowel, and a tuck-pointing trowel narrow enough to fit in the brick joints—and a dry-mix refractory mortar such as Heat Stop II, which is specially formulated to withstand a wood fire's intense heat.
1. Scrape out the loose mortar. Rake out the joint with the scoring tool. Start gently, probing for areas that are loose. Dig out the deteriorated mortar until the joint is about ½ to 3/4 inch deep and the brick surfaces on both sides of the joint are mortar-free.
2. Brush the joint. Using an old paintbrush and a vacuum, sweep the joints clean. Then brush water liberally into the joints and onto the brick faces. Mortar doesn't cure or adhere well on bone-dry brick.
3. Pack the mortar. Mix up a small batch of mortar in a bucket of clean, room-temperature water. Use only as much water as directed. Stir until the mix turns pasty. Load mortar on the edge of the brick trowel, place it against the brick in line with the joint, and push the mortar into the joint with a tuck-pointing trowel. Pack the joint about halfway full, then press down firmly as you pull the pointing trowel, handle-first, along the joint a couple of times. Repeat until the joint is filled.
4. Clean up. Scrape the excess mortar off the brick faces
with the edge of a trowel, then wipe them down immediately with a damp sponge. Take care
not to touch the joint with the sponge. Now run the jointer over the soft mortar to make it harden properly. In 30 to 90 minutes, when the mortar is
firm to the touch, go over everything with a stiff-bristle brush. You can start enjoying a fire in your fireplace again as soon as the following day.