 On the left, grit blasting has obliterated the vertical tooling marks from granite, a very dense stone. Photo: NPS files. |
The crux of the problem is that abrasive cleaning is just that--abrasive.
An abrasively cleaned historic structure may be physically as
well as aesthetically damaged. Abrasive methods "clean"
by eroding dirt or paint, but at the same time they also tend
to erode the surface of the building material. In this way, abrasive
cleaning is destructive and causes irreversible harm to the historic
building fabric. If the fabric is brick, abrasive methods remove
the hard, outer protective surface, and therefore make the brick
more susceptible to rapid weathering and deterioration.
Grit blasting
may also increase the water permeability of a brick wall. The
impact of the grit particles tends to erode the bond between the
mortar and the brick, leaving cracks or enlarging existing cracks
where water can enter. Some types of stone develop a protective
patina or "quarry crust" parallel to the worked surface
(created by the movement of moisture towards the outer edge),
which also may be damaged by abrasive cleaning. The rate at which
the material subsequently weathers depends on the quality of the
inner surface that is exposed.
Abrasive cleaning can destroy, or substantially diminish, decorative
detailing on buildings such as a molded brickwork or architectural
terra-cotta, ornamental carving on wood or stone, and evidence
of historic craft techniques, such as tool marks and other surface
textures.
In addition, perfectly sound and/or "tooled"
mortar joints can be worn away by abrasive techniques. This not
only results in the loss of historic craft detailing but also
requires repointing, a step involving considerable time, skill
and expense, and which might not have been necessary had a gentler
method been chosen. Erosion and pitting of the building material
by abrasive cleaning creates a greater surface area on which dirt
and pollutants collect. In this sense, the building fabric "attracts"
more dirt, and will require more frequent cleaning in the future.
In addition to causing physical and aesthetic harm to the historic
fabric, there are several adverse environmental effects of dry
abrasive cleaning methods. Because of the friction caused by the
abrasive medium hitting the building fabric, these techniques
usually create a considerable amount of dust, which is unhealthy,
particularly to the operators of the abrasive equipment. It further
pollutes the environment around the job site, and deposits dust
on neighboring buildings, parked vehicles and nearby trees and
shrubbery. Some adjacent materials not intended for abrasive treatment
such as wood or glass, may also be damaged because the equipment
may be difficult to regulate.
Wet grit methods, while eliminating dust, deposit a messy slurry
on the ground or other objects surrounding the base of the building.
In colder climates where there is the threat of frost, any wet
cleaning process applied to historic masonry structures must be
done in warm weather, allowing ample time for the wall to dry
out thoroughly before cold weather sets in. Water which remains
and freezes in cracks and openings of the masonry surface eventually
may lead to spalling. High-pressure wet cleaning may force an inordinate
amount of water into the walls, affecting interior materials such
as plaster or joist ends, as well as metal building components
within the walls.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
