Repointing an 1820s Wall: Charlestown Navy Yard's Granite Boundary

The granite wall along the northern edge of the Charlestown Navy Yard in Boston, behind the Commandant's House, was built between 1824 and 1826 to keep trespassers out and keep deserters from leaving. It has even been moved: in 1981–1982, the Alar Corporation cut it into sections and rolled them several feet on large rollers to make room for the Chelsea–Water Streets Connector. Its enemy now is the weather.

The granite wall between the Navy Yard and the Charlestown neighborhood, 1946. Boston National Historical Park, BOSTS-9187-1.

Why walls need repointing

In any brick or stone wall, the mortar between the blocks slowly crumbles and falls out. Left alone, the gaps let water work deeper into the wall and into the masonry itself, doing serious damage. The answer is repointing: clearing out mortar that has failed or soon will, and packing in fresh mortar.

The Student Conservation Association's Massachusetts Historic Preservation Corps began repointing the wall in 2023 and will keep going until the whole wall is done; there is no estimated finish date.

How it's done

1. Remove the old mortar with masonry chisels — or, where it is especially hard, an angle grinder.
2. Mix a fresh batch.
3. Pack it in: hold the mortar on a trowel or a masonry hawk — a flat metal board — and push it into the joint with a thin tuck-pointing tool.

Soft mortar for old stone

The key choice is how hard the new mortar should be. If it is harder than the stone or brick around it, water goes into the masonry instead of the mortar, causing damage that is far harder to fix. Historic bricks are usually much softer than modern ones, which makes this critical. Most repointing uses cement because it is easy to get, but carelessly mixed cement mortar can end up harder than the bricks. So the crew used lime putty: less common, but softer and more compatible with historic masonry.

Lime mortar fails if it dries too fast. Working through the summer of 2024, the crew ended each day by covering the fresh mortar with wet burlap and shading it with plywood, to keep the wall cool and damp.

Sources

Based on "Repairing the Granite Wall at the Charlestown Navy Yard," contributed by Greg Bsales, SCA Historic Preservation Corps crew member, National Park Service, Boston National Historical Park, citing Stephen P. Carlson, Charlestown Navy Yard Historic Resource Study (2010); a work of the United States government in the public domain; rewritten in hubnx's own words.