Traditional Masonry Archives

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

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.

Top — This historical photo shows Philadelphia’s Pennsylvania Academy of the Fine Arts, built in 1876, as it appeared shortly after completion. Documentation like this aids U.S. conservators in their aim of restoring the original look of historical buildings. Center — After nearly 90 years, the Pennsylvania Academy of the Fine Arts needed cleaning. Along with revealing forgotten architectural treasures, restoration cleaning helps conservators identify areas of deterioration previously hidden by soiling and staining. Bottom — The U.S. approach to restoration cleaning is illustrated here. The Pennsylvania Academy of the Fine Arts is restored to a nearly new appearance by a well-conceived chemical cleaning, followed by pressure-water rinsing. All photos courtesy of PROSOCO.

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.

The European approach
The only references showing how a historic European building originally looked may be archival documents or drawings that are not part of public perception. Wall Street’s Trinity Church is a good example of this in our own country. The church was built in 1846 with a reddish-brown New Jersey sandstone. By the 1920s the church was completely blackened by atmospheric soiling. The growing public perception was that black was the church’s original color. When a 1990s restoration cleaning began to reveal the beautiful stone beneath the black mantle, New Yorkers were outraged. Many wrote to the New York Times demanding to know why workers were painting the church! The complaints subsided as more of Trinity Church’s original, almost golden appearance came to light.

Photos of such buildings in context with their surroundings often show elevations already blackened by the ages. For many, the appearance of age imparts a venerability that contemporary buildings can only begin to achieve after decades of exposure to the urban environment.

Many buildings that served as proving grounds for evolving European cleaning techniques are much older than their American counterparts. Europeans often perceive older buildings differently than North Americans, so the primary goal of their cleaning programs is conservation — that is, to extend a building’s useful service life by stabilizing ongoing deterioration. The notion of brightening or otherwise improving a building’s appearance is usually a secondary consideration.

On older architecture, films of atmospheric soot react with masonry and evolve into heavy soiling crusts. In addition to detracting from a building’s appearance, these crusts accelerate the deterioration of underlying masonry. Though removing them is required for conservation, conservators believe preserving the appearance of age is also an appropriate aim in a contemporary European context.

Carefully controlled abrasive cleaning is often the most efficient means of removing such crusts from large building facades. This method scours soiling crusts from the stone surface without removing staining or soiling within the stone pores. It results in a level of cleaning many European property owners consider desirable.

When conservation is emphasized and there is no driving desire to restore the building’s original appearance, the groundwork is laid for the development of increasingly sophisticated abrasive cleaning techniques.

The American approach
The trend toward restoring America’s historic buildings began in earnest in the 1970s. City centers reeled from the impact of urban renewal. They found themselves competing with sprawling suburbs for investment dollars.

Unlike their European counterparts, many American cities had grown rapidly over the span of still-living generations. Historical photographs and motion pictures available to the public showed many of our most prominent buildings in nearly new, pristine states.

Faced with the need to refurbish and modernize their aging real estate, property owners sought cleaning methods that could turn back the hands of time. They wanted to restore the color and accents that distinguished older facades from surrounding, nondescript storefronts and office towers. Because of an emphasis on aesthetics and less need for true conservation, chemical cleaning techniques developed rapidly in North America.

In recent years, an increasing emphasis on conservation and environmental impact has prompted restoration contractors to seek alternatives to aggressive chemical cleaners. Manufacturers have responded by introducing increasingly sophisticated, environmentally responsible cleaning formulations.

Gentle blasting
North American conservators who embrace the concept of chemical-free, high-performance abrasive cleaning systems do so out of respect for European conservation principles and to avoid the potential of environmental damage resulting from inappropriate or misapplied chemical cleaning processes.

The concept of soft abrasives such as baking soda, microsponges or crushed walnut shells gently scouring all types of soiling from the surface of vulnerable masonry is very appealing. It reduces the need to identify accurately the variety of substrates, soiling matter, and subsurface staining that frequently exists on older masonry buildings. However, the reality is that abrasives cannot discriminate between unwanted surface soiling and the underlying substrate. Abrasives impacting nonresilient surfaces with enough velocity to erode soiling matter will have some effect on the underlying surface.

If that effect is carefully controlled by the applicator to equal a gentle polishing action, the benefit is limited to the outermost surface being cleaned. This limitation is of little concern on nonporous surfaces, such as metal, but on porous masonry, much of the soiling and staining extends beneath the surface.

For even the most sympathetic abrasives to access and erode subsurface soiling from within the pores of a masonry surface, the outermost surface must be removed or substantially altered. If the soiling matter is resilient, such as a heavy accumulation of paint, the abrasive can do significant damage to the surface when it breaks through the soiling layer.

Nonabrasive abrasives?
One of the most encouraging developments in abrasive cleaning is the availability of a range of finer, more efficient cleaning abrasives. North American suppliers of finely ground limestone or dolomite “flours,” glass beads or ceramic microspheres, for example, can now match the abrasive to the surface and the soiling matter.

One misleading claim regarding abrasive aggregate selection attempts to compare the hardness of the aggregate, as measured on the Mohs scale of mineral hardness, with that of the stone. Since stone is a composite of various minerals and softer cementing materials, even aggregate with a lower mineral hardness can erode the matrix that cements individual stone grains together.

Efforts to tailor aggregate hardness to surface hardness presume that facades composed of varied stone elements suffering from varying levels of deterioration possess a uniform hardness. There are many instances where even soft, water-soluble aggregates have successfully polished ashlar surfaces free of dirt, but significantly damaged sculptural elements carved from the same stone type.

Cleaning contractors
It should come as no surprise to anyone involved in building restoration that the end result of any cleaning program continues to rely heavily on the touch and care applied by workers wielding the cleaning wands.

Europe will always have its “cowboy” contractors who don’t follow conservation guidelines. In America, professionals are often obliged to deal with underqualified low bidders. Regardless of the contractor’s qualifications or integrity, the specifier is responsible for designating cleaning procedures that produce results acceptable to building owners.

Professionals can exert some control by reducing the cutting power of the aggregate or the corrosiveness of the cleaning solution. They can oversee and approve test areas that meet the owners’ requirements, then base their specifications on the level of cleaning that passes the test. The spec can also identify techniques and products that should not be used. Even then, the cleaning operator wields the ultimate control, simply because the operator’s hand is on the wand.

The good news is that the attention focused on devising responsible cleaning programs has raised everyone’s expectations. The industry has responded by introducing cleaning processes and formulations that are significantly safer, more environmentally responsible, and more difficult to abuse, such as the previously mentioned “soft” abrasives. Today’s chemical cleaners contain a fraction of the aggressive active ingredients that were used in the 1970s and 1980s. Surfactants, wetting agents and other additives help make the smaller amounts of active ingredients much more effective.

Chemical cleaners that depart entirely from use of corrosive or caustic components are now in the marketplace. Chelating technology, for example, relies on chemical attractions between molecules to break stains loose from the substrate for rinsing away with clean water. A nonacidic, noncaustic chelating cleaner was used in 2005 to remove atmospheric and biological soiling from the limestone and marble exterior of the Civil War-era U.S. Customs House in Charleston, S.C.

Contractors who have built their reputations on successful restorations have learned from past mistakes and are taking more sophisticated approaches to projects under their control. Many are more knowledgeable in their areas of expertise than the professionals supervising their activities. It is unreasonable to limit such cleaning contractors to one cleaning method or another.

What is the task at hand?
Generally, the controlled removal of delicate surface crusts and loosely bound surface soiling is most effectively achieved with a controlled mechanical cleaning process. Dissolution of subsurface soiling, paint, or other synthetic coatings is most effectively achieved with a chemical cleaner in concert with appropriate rinsing techniques.

When cost and time are considerations on projects being cleaned for aesthetic purposes, a well-conceived chemical-cleaning process using controlled water-washing is most practical. It’s also best for prepping surfaces for subsequent treatments. Planning for this process starts with identification of the substrates and identification of the types of stains and soiling. The level of deterioration of the substrate must also be considered. This information permits conservators to go forward with testing of products and techniques. Laboratory testing by product manufacturers or consultants can eliminate many variables when it comes to field-testing. If laboratory testing isn’t possible, field-testing is even more essential.

When cost and time are secondary to the goal of removing soiling crusts, while retaining the appearance of age, the new breed of sympathetic abrasive cleaning systems is a wise choice. If additional cleaning and stain removal is called for, selective water or chemical cleaning remains an option.

Professionals and contractors involved in cleaning or conserving historic masonry should familiarize themselves with both abrasive and chemical cleaning technologies. Consider each a part of an ever-increasing pallet of options for designing a comprehensive cleaning program.

David W. Boyer is president of PROSOCO, an international manufacturer of products for cleaning, protecting, and maintaining concrete brick and stone. Trained as an architect, Mr. Boyer has aided in the preservation of historic properties in the U.S. and overseas.

TM