Preservation Note 25

Repair of Scratched Limestone

Masonry

June-98

Masonry Restoration and Cleaning-04900

Scratched Limestone

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In the summer of 1992, damage to a section of exterior limestone wall
occurred along a ramp to a loading dock at the Department of Interior
building. The damage resulted in deep (up to 1/2\") scratches in the
limestone but fortunately; careful inspection by qualified persons
uncovered no cracking of the stones or mortar joints. In consultation
with the GSA Regional Historic Preservation Staff, the Department of
Interior honed the damaged area to eliminate the scratches. When
completed, the honed surface matched the adjacent aged limestone.
However in fall 1993, follow-up inspections by DOI and GSA Regional
Historic Preservation Staff revealed a marked difference in appearance
of the honed limestone. A year of weathering had highlighted differences
in texture between the honed surface and the aged limestone. The honed
surface texture had, over the year, become more reflective or smoother
as it weathered. This high contrast effect highlighted the precise area
where repair had been attempted.

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Currently, the repair attempt is significantly more noticeable than the
original damage. Future weathering will continue to modify the
appearance of the limestone.

CONCLUSIONS

1\. Because abrasive scratch repair methods such as honing,
waterblasting, or sandblasting change the surface texture of the stone,
areas repaired with these methods may become more noticeable over time
than the original scratch. Experiences at the Department of Interior
Building indicate that the guiding principle for repairing scratches in
exterior limestone may be to simply clean the stone non-abrasively to
diminish the visibility of the scratches, and then monitor the damage.

2\. Annual scheduled inspection accompanied by clear photographic
documentation every year will permit buildings management / architects /
facilities management to re-evaluate the condition of the damaged area.
This inspection and documentation over several years will indicate if
the damaged area is becoming less noticeable or more noticeable, via
accelerated deterioration or intensified staining.

3\. Extent of damage, depth and direction of scratches (horizontal,
diagonal, vertical); exposure to the sun, rain, wind; building
detailing; and time will all help to determine the visual effect of the
damage. A single scratch may be less noticeable than a repair.

4\. The decision to monitor damage may affect the Government\'s ability
to file claims for damages with the Contractor. Depending on the
potential cost of the claim, the Contracting Officer may want to
consider amending the contract to include the monitoring period.
Thorough documentation of the damage will assist the Contracting Officer
in making this determination. When only scratches are involved,
monitoring the damaged area is the preferred choice of action.