You live in a stately, prewar brick building with sandstone window trim, a copper cornice, terra-cotta moldings and years of city soot and grime. What cleaning methods are strong enough to remove dirt and stains while safe enough to protect the delicate details? Can you use the same treatments throughout, or is brick cleaned differently than stone and other materials? You're right to be concerned: The wrong treatments can scar the building and permanently harm its finer historical elements. Here's what you should know before hiring a cleaning company or cleaning the building yourself.
Different types of masonry call for different types of applications. Similarly, different types of grime, such as soot and smoke, require different cleaning agents than, say, oil or metallic stains.
As a rule of thumb, the gentlest treatment should be used when possible, and whenever stronger methods are called for they must be used carefully.
The three main methods for cleaning façades are water, chemical, and abrasive treatments. Abrasive treatments, such as grinding, sanding, and blasting, are not recommended, because they remove surface material along with dirt and paint. That leaves water and chemical treatments, both of which are effective and safe when properly used.
There are three kinds of water-based methods for cleaning façades:
• steam / hot-pressurized washing
Properly applied, water treatments are the least invasive types of façade cleaning and a conservative way to start the job, especially when dealing with typical dirt and grime.
The most common method is pressure-washing, in which you apply a low- to medium-pressure spray (100 to 400 pounds per square inch) to the surface of the building. (As a point of reference, the spray from a garden hose is about 60 psi.) The water pressure usually starts out low and is increased as needed, followed by scrubbing with a natural or synthetic (but not metal) bristle brush for stubborn areas and detailed elements.
Sometimes a non-ionic detergent (the type used in dish-washing liquids, which foams less than ionic detergents and does not react with ions in water) is added to the water to remove oil-based dirt from surfaces. Soap and other household detergents should not be used in water-based cleaning because they can leave a visible residue on the masonry.
Soaking involves spraying or misting the masonry surface for an extended period, usually up to several days at a time, to loosen heavy accumulations of soot and crusts, particularly in parts of the building not exposed to rain. Soaking, used in conjunction with pressure and followed by a final water rinse, requires repeated applications that can take up to several weeks. But because it's mild, it's ideal for historic masonry.
The third method, steam or hot-pressurized cleaning, is not commonly used, but it can be effective for removing built-up soil deposits and plants, such as ivy. It's also an option for cleaning stone that is sensitive to the acids used in some chemical cleaners.
Chemical cleaners are effective for removing dirt, and unlike water-based treatments, they can also be used to remove paint, coatings, metallic stains, and graffiti. Acid-based cleaners are effective on unglazed brick and terra-cotta, cast stone, concrete, granite and most sandstones. Alkaline cleaners are best used on acid-sensitive masonry, such as limestone, marble, polished granite and calcareous (chalky) sandstone.
Both types of chemical cleaners are sprayed or brushed on a wet surface and allowed to sit for a period of time. Depending on the substance being removed, several applications may be necessary. Both types are rinsed off with water; alkalines are given a slightly acidic wash first to neutralize them.
Before beginning a façade cleaning program, it is important to identify the types of masonry on the building and select the most appropriate cleaning method for each type. Distinguishing among different types of stone is especially crucial. Certain limestones, for example, can look like sandstone, and what looks like natural stone can be cast stone or concrete. Some bricks may contain impurities, such as iron particles that can react with certain cleaning agents, resulting in staining. Choosing the wrong type of cleaning method can cause irreversible damage to the building materials, so when in doubt, it's best to consult with a historic preservationist about the masonry's composition.
Also, be aware that some chemicals (and even water) that are safe for masonry can corrode or damage other building elements, such as decorative metal elements, glass, wooden window sashes, iron window bars and window air-conditioner sleeves. Any parts of the building not subject to cleaning but susceptible to damage should be covered or otherwise protected.
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