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 Always use the gentlest cleaning method possible. Avoid
abrasive cleaning. Abrasive cleaning methods, such as wet or dry
sand or grit blasting, even at very low pressure, should never be
used on historic masonry or wood buildings. These methods
destroy the hard outer skin of bricks and terra cotta and even
stone. Once this protection is gone, the surfaces become much
more likely to soak up water, leading to problems such as
spalling, exfoliation, efflorescence, and decay. Abrasive
cleaning can also damage nearby building surfaces, such as glass.
It is a hazard to workers, bystanders, and automobiles, as well
as to shrubbery and other landscape features. If it is necessary
to remove all the paint, the following methods may be considered:
o Wet cleaning. It is usually safe to begin cleaning
masonry with a low-pressure spray of plain water from a
hand-held garden hose and a gentle scrub using a soft,
natural bristle brush. Do not use wire brushes-- they are
hard on the masonry, and particles of the wire may be left
behind to rust and stain. A tablespoon or 2 of liquid
nonionic detergent, such as household dishwashing liquid,
can be added to each gallon of water. If that fails,
prolonged spraying or misting with plain water at low
pressure may soften surface dirt enough to allow it to be
rinsed off. The spraying should be done intermittently
for only 3 or 4 minutes at a time with a pause of several
seconds in between. This will keep the surface from
drying and also avoid penetrating masonry joints or
damaging the surface. (Attach a timer to the hose.) The
softening process may take up to a week. High-pressure
washing (600 - 1800 psi) should not be used on any except
the hardest surfaces, since softer stone (like limestone,
sandstone, and marble) can be eroded by high pressure
water. Steam cleaning is expensive, slow (average working
time: 1 minute/square foot), and somewhat hazardous to the
worker. It is useful for cleaning intricately carved
areas without heavy brushing, and it is less likely to
cause staining than prolonged washing.
There are some problems associated with wet cleaning.
Chemicals in the water may react adversely with those in
the masonry. For instance, water containing copper or
iron can cause stains on the surface of building stone.
(Adding chelating chemicals to trap the metal in the wash
water can help to prevent this effect.) Water may bring
mineral salts in brick or stone to the surface, causing a
hazy, white film called efflorescence. This discoloration
is harmless in itself and can usually be brushed away
without permanent damage to the masonry. Check the pH
level of the water before starting. Prolonged washing
with even slightly acidic water, such as that from most
public water supplies, can dissolve limestone or marble
surfaces. Masonry can be structurally weakened by
saturation. Water that penetrates through mortar joints
to interior surfaces can saturate insulation and damage
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