The mortar in most brick joints consists of sand, lime, and portland cement. But in a fireplace, that mix just crumbles away when subjected to roaring wood fires. For this application, masons rely on a refractory mortar made of magnesium silicate, which can withstand heat up to 2,000 degrees F. Yet even refractory mortar can fail as the brickwork expands and contracts with repeated fires.
Fortunately, fixing those damaged joints is a snap, thanks to high-temp fireplace mortars packaged in caulk tubes, such as the one shown above. The old, messy way of repointing—mixing mortar in a bucket and troweling it on—is a thing of the past. Mark Schaub of Chimney Savers shows how easy the job is now. (Schaub appears regularly on TV episodes of This Old House.)
Wash the Brick
Schaub dons nitrile gloves, then thoroughly scrubs soot off the sides of the firebox with an abrasive pad on a wet sponge. Soot dissolves fairly easily when wet. The washing also dampens the brick and mortar, a necessary step before repointing.
Dig Out the Loose Mortar
Using a grout-removal tool fitted with a triangular carbide blade, like the one made by Goldblatt, Schaub scrapes out cracked, loose, and crumbling mortar. He stops when the sides of the blade touch the edges of the brick. The blade’s triangular shape prevents it from going too deep (more than half the width of the joint). He then cleans the joints with a narrow, stiff-bristle scrub brush and sweeps up the mortar crumbs with a wet/dry vac.
Wet the Joints
Schaub goes over all the scraped joints with a nylon paintbrush, which he dips repeatedly into a bucket of clean water. Refractory mortar bonds best to a damp surface, and the brush bristles ensure that all the old brick and mortar are moistened.
Caulk the Joints
After slipping a tube of refractory mortar into a caulk gun, Schaub drags the tube’s nozzle over all the scraped mortar joints, leaving behind a fat bead of caulk. He also makes sure to fill any gaps between the hearth and the firebox floor.
Pack the Mortar
Schaub uses a 3⁄8-inch-wide tuck-pointing trowel to smooth and press the soft mortar firmly into each of the joints, then scrapes off the excess with the trowel’s edge. In corners, a gloved finger performs the packing and smoothing functions.
Wipe Away the Residue
Multiple swipes with a damp sponge remove most of the mortar residue. Schaub will wipe the brick again in 24 hours, after the remaining residue dries to a haze. This mortar’s heat-resistant qualities kick in when it reaches 500 degrees. Either a heat gun or a small, brief fire will serve that purpose.