Brick and Architectural Terra-cotta: Abrasive blasting does not
affect all building materials to the same degree. Such techniques
quite logically cause greater damage to softer and more porous
materials, such as brick or architectural terra-cotta. When these
materials are cleaned abrasively, the hard, outer layer (closest
to the heat of the kiln) is eroded, leaving the soft, inner core
exposed and susceptible to accelerated weathering. Glazed architectural
terra-cotta and ceramic veneer have a baked on glaze which is also
easily damaged by abrasive cleaning. Glazed architectural terra-cotta
was designed for easy maintenance, and generally can be cleaned
using detergent and water; but chemicals or steam may be needed
to remove more persistent stains. Large areas of brick or architectural
terra-cotta which have been painted are best left painted, or repainted
if necessary.
Plaster and Stucco: Plaster and stucco are types of masonry finish
materials that are softer than brick or terra-cotta; if treated
abrasively these materials will simply disintegrate. Indeed, when
plaster or stucco is treated abrasively it is usually with the
intention of removing the plaster or stucco from whatever base
material or substrate it is covering. Obviously, such abrasive
techniques should not be applied to clean sound plaster or stuccoed
walls, or decorative plaster wall surfaces.
Building Stones: Building stones are cut from the three main categories
of natural rock: dense, igneous rock such as granite; sandy, sedimentary
rock such as limestone or sandstone; and crystalline, metamorphic
rock such as marble. As opposed to kiln-dried masonry materials
such as brick and architectural terra-cotta, building stones are
generally homogeneous in character at the time of a building's
construction. However, as the stone is exposed to weathering and
environmental pollutants, the surface may become friable, or may
develop a protective skin or patina. These outer surfaces are
very susceptible to damage by abrasive or improper chemical cleaning.
 Very high-pressure water has scarred this granite. Photo: NPS files. |
Building stones are frequently cut into ashlar blocks or "dressed"
with tool marks that give the building surface a specific texture
and contribute to its historic character as much as ornately carved
decorative stonework. Such detailing is easily damaged by abrasive
cleaning techniques; the pattern of tooling or cutting is erased,
and the crisp lines of moldings or carving are worn or pitted.
Occasionally, it may be possible to clean small areas of rough-cut
granite, limestone or sandstone having a heavy dirt encrustation
by using the "wet grit" method, whereby a small amount
of abrasive material is injected into a controlled, pressurized
water stream. However, this technique requires very careful supervision
in order to prevent damage to the stone. Polished or honed marble
or granite should never be treated abrasively, as the abrasion
would remove the finish in much the way glass would be etched
or "frosted" by such a process. It is generally preferable
to underclean, as too strong a cleaning procedure will erode the
stone, exposing a new and increased surface area to collect atmospheric
moisture and dirt. Removing paint, stains or graffiti from most
types of stone may be accomplished by a chemical treatment carefully
selected to best handle the removal of the particular type of
paint or stain without damaging the stone. (See section on the
"Gentlest Means Possible.")
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
