Bleaches. Alkali-based bleaches such as calcium hypochlorite
can sometimes be used very successfully in a poultice to bleach or decolorize
certain dyes contained in some paints and inks that cannot readily be removed
by other means.
 Damaging graffiti removal methods have scarred the marble. Photo: NPS files. |
Mechanical or Abrasive Methods. Mechanical treatments include
dry or wet blasting, using abrasive grits, such as sand, dolomite powder,
aluminum oxide, ground-walnut shells, sodium bicarbonate (baking soda),
and others; high-pressure water washing; and mechanical sanding or grinding.
All of these abrasive methods will cause damage to masonry and, in most
instances, should never be considered as a method of removing graffiti
from historic masonry. Abrasive methods used mistakenly by untrained workers
to remove graffiti usually result in etching the outline of the graffiti
permanently into the masonry. Some historic masonry materials
can be easily damaged by pressure washing even at low or moderate pressures
(100-400 psi). Occasionally, however, under very controlled circumstances,
a micro-abrasive technique may be appropriate for removing graffiti
from delicate masonry surfaces, if used at low pressures of 35-40 psi with
fine abrasives.
This treatment, which must be done very slowly and carefully
to avoid damaging the masonry, should be tested first, and undertaken only
by a professional conservator. Another exception, even though it is not
strictly an abrasive treatment, is using a razor blade as a first step
to remove spray paint or felt-tip marker from polished granite. However,
this too, should be undertaken only by a professional conservator,
and only on polished granite, which is very hard and generally impervious
to scratches.
Laser Cleaning. Although not in general use as a cleaning technique,
laser technology offers great promise in the future as a non-damaging method
of graffiti removal.
Testing
Before selecting a removal method, all cleaning materials and techniques
for removing graffiti from a historic masonry building should be tested
on mock-ups or areas of the resource that are not highly visible, but which
are representative of typical conditions. Visual observation should be
supplemented by the use of a magnifying glass, and spot tests should be
carried out with various solvents to help identify the specific graffiti
medium, which will aid in its removal. More complex testing using laboratory
equipment and more scientific analytical processes may sometimes be necessary
in complex situations. Sample areas that represent the desired degree of
"cleanliness" should be approved in writing by client, architect,
conservator or other appropriate authority. The materials and all the other
data necessary to reproduce the desired cleaning results should be meticulously
recorded and the accepted sample area preserved for reference until the
end of the job. The existence of a "clean" sample for comparison
and a signed agreement can avoid unpleasant surprises, misunderstandings,
and perhaps legal actions.
When a type of graffiti appears for the first time that was executed
with a material not immediately recognizable and for which no countermeasures
have been developed, tests may need to be carried out by an architectural
conservator to identify the material and to determine effective removal
treatments. Agencies with large inventories of graffiti-prone buildings
and structures should watch for graffiti made with new materials and experiment
with different cleaning methods in order to be prepared when it appears.
Such early action can save large sums of money in the long term. (See "Development
of a Treatment Plan.")
"Removing Graffiti from Historic Masonry" an Historic Preservation Brief
