Last updated: September 5, 2024
Article
Repairing the Granite Wall at the Charlestown Navy Yard
The granite wall of the Charlestown Navy Yard, located behind the Commandant's House, regularly experiences degradation from weather and the elements. Primarily, mortar between the granite blocks has been deteriorating over time. As such, the Student Conservation Association's Massachusetts Historic Preservation Corps (SCA HPC) provides regular repairs to the wall, chiseling out old, damaged mortar and replacing it with new mortar. The project began in 2023 and will end once the entire wall has been repointed. There is no current estimate for the project completion date.
Boston National Historical Park, BOSTS-9187-1.
Site History
Built between 1824 and 1826, the wall was intended to deter trespassers and prevent deserters from leaving the Navy Yard.[1] Between 1981 and 1982, Alar Corp moved the wall several feet to make room for the construction of the Chelsea-Water Streets Connector. This was done by cutting the wall into sections and moving them on large rollers.[2] The wall still stands today, marking the northern border of the Navy Yard.
Project Overview
Over time, in brick and stone walls, the mortar will start to deteriorate and fall out. If not addressed, water will seep further into the wall and into the bricks themselves, causing severe damage. Due to this potential damage, brick and mortar structures require regular repointing—clearing out any mortar that is or will soon degrade and applying newly-mixed mortar between the bricks. The Historic Preservation Corps has been addressing this need for repair and will continue to do so until the whole wall has been repointed.
Left image
One portion of the granite wall BEFORE repointing. July-August 2024.
Credit: Student Conservation Association photo, J. Richards.
Right image
One portion of the granite wall AFTER repointing. July-August 2024.
Credit: Student Conservation Association photo, J. Richards.
Preservation Efforts
To repoint the granite wall, the Historic Preservation Corps members first removed the old mortar, using masonry chisels or, for especially hard areas, an angle grinder. After this step, they mixed a batch of new mortar and inserted it between the granite blocks. Crew members completed this task by holding the mortar on a trowel or masonry hawk—a tool made of a horizontal sheet of metal—and then using a tuck point—a thin, trowel-like tool—to push the mortar into the empty space.
Student Conservation Association photo, A. Lueth.
Mortar Considerations
An important consideration for the repointing process is determining how hard the new mortar should be. If you use a mortar that is harder than the stone or brick it surrounds, then the water will penetrate into the stone/brick instead of the mortar, causing damage that is much harder to repair. When working with historic brick, identifying the best type of mortar is crucial, as these bricks are usually much softer than modern bricks.
Most repointing in historic structures is done using cement due to its wide availability. When not mixed carefully, however, cement mortar can end up harder than the brick it is being used on. To avoid this potential issue, the HPC used lime putty, a much softer material. While less common than cement, lime putty is more compatible with historic brick due to being softer.
When repointing, especially with lime, it is crucial to not let the mortar dry too fast or else it will fail. Since the 2024 crew did this project in the heat of the summer, at the end of each day they covered the mortar in wet burlap and shaded it with plywood to keep the surface of the wall cool and moist.
Contributed by: Greg Bsales, SCA Historic Preservation Corps Crew Member
Footnotes
[1] Stephen P. Carlson, Charlestown Navy Yard Historic Resource Study vol. 1. Boston, MA: Division of Cultural Resources, Boston National Historical Park, National Park Service, US Department of the Interior, 2010, 24. NPS History.
[2] Stephen P. Carlson, Charlestown Navy Yard Historic Resource Study vol. 1. Boston, MA: Division of Cultural Resources, Boston National Historical Park, National Park Service, US Department of the Interior, 2010, 220. NPS History.