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 Oxalic acid poultice (on
Rust
granite), sodium citrate or
ammonium citrate (on carbonate
stones such as marble or
limestone), citric acid.
Ammonium carbonate, ammonia
Copper and alloys
solutions, sulfamic acid,
ammonium chloride.
Commercial herbicides,
Lichen, moss, ivy, plants
detergents, household bleach.
Soap, commercial powdered
Algae, mushrooms, mold
cleanser with bleach (e.g.,
Ajax, Comet), calcium
hypochlorite (chlorinated
lime), leaching powders,
peroxide.
Bleaching poultices made with
Tobacco smoke, old coating
hydrogen peroxide or
residues
hypochlorite bleach (e.g.,
clorox).
On marble: paste of calcium
hypochlorite mixed with
hydrated lime.
(Based on material in Respectful Rehabilitation: Masonrv -
How to Care for Old and Historic Brick and Stone, by Mark London.
Washington: The Preservation Press, 1986.)
Take the time to get a clear idea of what the problem is and
what can safely be done about it. Any cleaning methods being
considered should be tried first on a test patch (about two
square feet on brick or one entire unit on a stone wall) on an
inconspicuous part of the building. Allow plenty of time for
results to appear. Deterioration or discoloration may take
months to show up.
Removing Graffiti. Historical graffiti (for instance, the
names of early residents written on an attic wall) should be
preserved. Modern graffiti (lipstick, spray paint, and
magic-marker messages) should usually be removed. Solvents must
be chosen to fit the particular stain and applied as a poultice
(a paste made of a chemical mixed with an inert substance such as
talc, fullers earth, chalk powder, clay, sawdust, whiting, or
diatomaceous earth.) The poultice is spread 1/3 to 1/2 inch
thick on the pre-wetted stained surface, covered with a plastic
sheet, and allowed to dry slowly. When dry, it is carefully
removed, then rinsed with water.
(See Figure 4-13.) A poultice
4-21
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