Wood: Most types of wood used for buildings are soft, fibrous
and porous, and are particularly susceptible to damage by abrasive
cleaning. Because the summer wood between the lines of the grain
is softer than the grain itself, it will be worn away by abrasive
blasting or power tools, leaving an uneven surface with the grain
raised and often frayed or "fuzzy." Once this has occurred,
it is almost impossible to achieve a smooth surface again except
by extensive hand sanding, which is expensive and will quickly
negate any costs saved earlier by sandblasting. Such harsh cleaning
treatment also obliterates historic tool marks, fine carving and
detailing, which precludes its use on any interior or exterior
woodwork which has been hand planed, milled or carved.
Metals: Like stone, metals are another group of building materials
which vary considerably in hardness and durability. Softer metals
which are used architecturally, such as tin, zinc, lead, copper
or aluminum, generally should not be cleaned abrasively as the
process deforms and destroys the original surface texture and
appearance, as well as the acquired patina.
 Decorative pressed metal interior or exterior features should not be cleaned abrasively. Photo: NPS files. |
Much applied architectural
metal work used on historic buildings--tin, zinc, lead and copper--is
often quite thin and soft, and therefore susceptible to denting
and pitting. Galvanized sheet metal is especially vulnerable,
as abrasive treatment would wear away the protective galvanized
layer.
In the late 19th and early 20th centuries, these metals were often
cut, pressed or otherwise shaped from sheets of metal into a wide
variety of practical uses such as roofs, gutters and flashing,
and facade ornamentation such as cornices, friezes, dormers, panels,
cupolas, oriel windows, etc. The architecture of the 1920s and
1930s made use of metals such as chrome, nickel alloys, aluminum
and stainless steel in decorative exterior panels, window frames,
and doorways. Harsh abrasive blasting would destroy the original
surface finish of most of these metals, and would increase the
possibility of corrosion.
However, conservation specialists are now employing a sensitive
technique of glass bead peening to clean some of the harder metals,
in particular large bronze outdoor sculpture. Very fine (75125
micron) glass beads are used at a low pressure of 60 to 80 psi.
Because these glass beads are completely spherical, there are
no sharp edges to cut the surface of the metal. After cleaning,
these statues undergo a lengthy process of polishing. Coatings
are applied which protect the surface from corrosion, but they
must be renewed every 3 to 5 years. A similarly delicate cleaning
technique employing glass beads has been used in Europe to clean
historic masonry structures without causing damage. But at this
time the process has not been tested sufficiently in the United
States to recommend it as a building conservation measure.
Sometimes a very fine smooth sand is used at a low pressure to
clean or remove paint and corrosion from copper flashing and other
metal building components. Restoration architects recently found
that a mixture of crushed walnut shells and copper slag at a pressure
of approximately 200 psi was the only way to remove corrosion
successfully from a mid-19th century terne-coated iron roof. Metal
cleaned in this manner must be painted immediately to prevent
rapid recurrence of corrosion. It is thought that these methods
"work harden" the surface by compressing the outer layer,
and actually may be good for the surface of the metal. But the
extremely complex nature and the time required by such processes
make it very expensive and impractical for large-scale use at this
time.
 Cast iron may be abrasively cleaned, but must be painted immediately to prevent rust. Photo: NPS files. |
Cast and wrought iron architectural elements may be gently sandblasted
or abrasively cleaned using a wire brush to remove layers of paint,
rust and corrosion. Sandblasting was, in fact, developed originally
as an efficient maintenance procedure for engineering and industrial
structures and heavy machinery--iron and steel bridges, machine
tool frames, engine frames, and railroad rolling stock--in order
to clean and prepare them for repainting. Because iron is hard,
its surface, which is naturally somewhat uneven, will not be noticeably
damaged by controlled abrasion. Such treatment will, however,
result in a small amount of pitting. But this slight abrasion
creates a good surface for paint, since the iron must he repainted
immediately to prevent corrosion. Any abrasive cleaning of metal
building components will also remove the caulking from joints
and around other openings. Such areas must be recaulked quickly
to prevent moisture from entering and rusting the metal, or causing
deterioration of other building fabric inside the structure.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
