Masonry Magazine August 2001 Page. 13
b. Acid rain
Disintegration and spalling are encouraged by the greater quantity of acid which is held on the surface for a longer time by the grime crust.
c. Biological action (stains and chemical disintegration)
Results when trapped water aids in the propagation of micro- vegetation.
TACTICAL
Cleaning a building is often useful for its visual enhancement or public-relations effect on preservationists (those who can better appreciate a building when they see it restored).
Varied professional chemical solutions, when used properly are safe and effective ways to restore an old building. Choosing the proper equipment and cleaning solution is imperative. The days of "one chemical cleans all" are over. The detergent processes are accepted for cleaning and restoring buildings on the National Register of Historic Places. Before we discuss the chemical method, we should mention some of the other methods.
The abrasive methods are the most damaging no matter how cleverly disguised. Whether it be blasting with sand, Black Beauty, walnut shells or corncob, they are all damaging to the surface to be cleaned, and more so on historic brick surfaces. The hardest part of a brick is its outside face. When this face is either removed or etched, the softer, inner portion is exposed to the elements. Moisture from rain, snow and even sprinkler systems can be readily absorbed into the masonry, along with atmospheric pollution. Moisture and dirt will result in deep-set stains that will prove difficult, if not impossible to remove later, and the moisture can lead to delamination and/or sugaring of the masonry surface. None of these methods are accepted for use on registered historic landmarks and will jeopardize the structure's historic grant standing and/or eliminate the structure from consideration as a nationally registered.
Sandblasting can be damaging to masonry surfaces.
landmark. Abrasive cleaning is prohibited by the sensitive and educated consultant or conservator.
There are several other abrasive methods for cleaning old buildings. First is the French Facade Gommage system that utilizes low pressure air to blow fine particulate at the surface to remove surface soiling, but will not remove deeper set dirt, stains or paint easily.
Second, is the German JOS technique, which, like the Gommage system, uses low-pressure air and a fine powder, along with low-pressure water. Acceptable applications on limestone have been accomplished, but on dirty, porous brick or on painted brick, both processes will etch. When removing multi-layer paint buildup, which is hard like an eggshell, all abrasives ricochet and cause hills and valleys like a miniature Grand Canyon on the masonry.
Thirdly, there is the soda blasting system that applies bicarbonate of soda and water under pressure to a substrate. This method has lately been promoted for the removal of graffiti from brick and stone. It is still an abrasive method and it has an ad- (more on next page)
Accumulated grime hides not only colors, but subtle molding profiles, delicate carvings and textural differences between materials.