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Trinity Church Before — Trinity Church, Wall Street, NYC. The Church had worn a grim black mantle for so long that when the 1992 restoration cleaning began to reveal traces of the bright stone, beneath, many thought workers were slapping paint on the venerable building.
Trinity Church after — The church exhibits its original appearance. This is another case where restoration cleaning allowed conservators to identify and address structural deterioration.
Chemical vs Abrasive: Restoration cleaning for traditional masonry
Forget the hype. That's the first step in choosing between abrasive and chemical cleaning techniques for masonry. by David Boyer
All too often in North America, the default choice for masonry cleaning among restoration architects and conservators is whichever technique is currently favored in Europe. This approach fails to consider important differences in our buildings and our market expectations, and may produce results that are less than what the restoration professional would hope for.
There is now a broader range of sympathetic cleaning techniques available than ever before, both chemical and abrasive. Yet an alarming number of professionals continue to subscribe to the notion that only one or the other is worthy of serious consideration.
Many horror stories associated with abrasive cleaning hearken back to the days of dry blasting with coarse mason’s sand. Main streets and historic districts throughout the United States are riddled with masonry buildings irreparably damaged by sandblasting in the 1970s and 1980s. One such building in the Birmingham, Ala., historic district had the rich red glaze completely stripped off its terra cotta by an abrasive process meant only to remove atmospheric soiling.
Similar fears about chemical cleaning are inspired by damage from haphazard applications of concentrated hydrofluoric acid and sodium hydroxide solutions in the 1970s and 1980s. One popular example among conservators is Rutland Square, in Edinburgh, Scotland. Some of the square’s “blond” Scottish sandstone storefronts were improperly cleaned with sodium hydroxide, and the process mobilized metallic staining throughout the stone, discoloring it a rusty orange. Other shops received a hydrofluoric acid cleaning, which bleached the warm tones and color out of the stone, leaving it a sterile white.
Since then, significant advances have been made in both abrasive and chemical cleaning arenas. Still, each process has some potential for damaging building fabrics and the surrounding environment. The key to successful cleaning is recognizing the advantages and limitations of each technique.
