"Chemical or physical treatments, such as sandblasting,
that cause damage to historic materials shall not be used. The
surface cleaning of structures, if appropriate, shall be undertaken
using the gentlest means possible." The Secretary of
the Interior's Standards for Rehabilitation.
Abrasive cleaning methods are responsible for causing a great
deal of damage to historic building materials. To prevent indiscriminate
use of these potentially harmful techniques, this brief has been
prepared to explain abrasive cleaning methods, how they can be
physically and aesthetically destructive to historic building
materials, and why they generally are not acceptable preservation
treatments for historic structures. There are alternative, less
harsh means of cleaning and removing paint and stains from historic
buildings. However, careful testing should precede general cleaning
to assure that the method selected will not have an adverse effect
on the building materials. A historic building is irreplaceable,
and should be cleaned using only the "gentlest means possible"
to best preserve it.
 Abrasive cleaning can cause permanent damage to historic fabric, such as this brick wall. Photo: NPS files. |
Abrasive cleaning methods include all techniques that physically
abrade the building surface to remove soils, discolorations or
coatings. Such techniques involve the use of certain materials
which impact or abrade the surface under pressure, or abrasive
tools and equipment. Sand, because it is readily available, is
probably the most commonly used type of grit material. However,
any of the following materials may be substituted for sand, and
all can be classified as abrasive substances: ground slag or volcanic
ash, crushed (pulverized) walnut or almond shells, rice husks,
ground corncobs, ground coconut shells, crushed eggshells, silica
flour, synthetic particles, glass beads and micro-balloons. Even
water under pressure can be an abrasive substance. Tools and equipment
that are abrasive to historic building materials include wire
brushes, rotary wheels, power sanding disks and belt sanders.
The use of water in combination with grit may also be classified
as an abrasive cleaning method. Depending on the manner in which
it is applied, water may soften the impact of the grit, but water
that is too highly pressurized can be very abrasive. There are
basically two different methods which can be referred to as "wet
grit," and it is important to differentiate between the two.
One technique involves the addition of a stream of water to a
regular sandblasting nozzle. This is done primarily to cut down
dust, and has very little, if any, effect on reducing the aggressiveness,
or cutting action of the grit particles. With the second technique,
a very small amount of grit is added to a pressurized water stream.
This method may be controlled by regulating the amount of grit
fed into the water stream, as well as the pressure of the water.
"Dangers of Abrasive Cleaning" an Historic Preservation Brief
