Reduce, Reuse, Rehab: Green Cleaning and Historic Exteriors

Gentle, green cleaning can remove mold growth from historic masonry. If mold growth is perpetual, there may be a drainage issue that requires remediation.

Gentle, green cleaning can remove mold growth from historic masonry. If mold growth is perpetual, there may be a drainage issue that requires remediation.

As spring blooms and temperatures rise, the effects of the harsh winter weather become apparent on historic exteriors. Excessive precipitation and wind can leave biological growth, dirt, grime, and rust clinging to walls, which, if not cleaned properly, can cause damage to the historic fabric.

In the September issue of Reduce, Reuse, Rehab, Laura L. Thornton highlighted green cleaning the interiors of historic homes, offering simple, natural methods, and describing the harmful effects modern household chemical cleaners can have on people, historic building materials, and the outdoor environment. This issue will explore green cleaning and historic exteriors by showing how gentle cleaning is green cleaning, explaining recommended cleaning treatments, and providing information on new green cleaning products.

Gentle Cleaning is Green Cleaning

With regard to the exteriors of historic buildings, the Secretary of the Interior’s Guidelines for Rehabilitating Historic Buildings recommend cleaning surfaces with the “gentlest means possible.” In simplified terms, don’t use a lot when a little will do.

Luckily for historic-home owners who want to practice environmentally friendly cleaning, the “gentlest means possible” frequently translates to the greenest means possible. A little water and a natural bristle brush – used with a gentle hand – are often all that is necessary to clean a soiled surface. This method avoids the use of potentially harmful pollutants that could be ingested by people or released into the air and soil.

Imagine the cleaning agents available at the time of the historic buildings’ initial construction. In most cases, the environmentally and personally harmful chemicals and high-powered mechanical equipment were not yet available, so the materials could usually be cleaned with natural means. For the sake of the building materials and the environment, basic and natural methods of cleaning should be tried before any potentially harmful chemicals.

Cleaning Treatments

When cleaning historic masonry (which includes brick, stone, terracotta, stucco, and concrete) in good condition, water treatments are often the gentlest – and greenest – means possible. The National Park Service’s Preservation Brief 1: Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings describes four water treatments: soaking, water-washing, water-washing with detergents, and steam/hot pressure washing.

  • Soaking involves the continuous, gentle spraying or misting of the exterior wall, followed by a rinse. Soaking works well with limestone and marble, particularly in the removal of soot and gypsum or sulfate crusts.
  • Water-washing uses very low pressure (under 400 psi) to remove dirt and other soiling agents. Often, this method is paired with the use of natural bristle brushes.
  • Water-washing with detergents again uses very low pressure, along with non-ionic detergent (not soap). Non-ionic detergents do not leave residues that could damage the materials and further contribute to run-off. This treatment is particularly effective with decorative details and is recommended for use with natural bristle brushes and a final rinse.
  • Steam/hot pressure washing involves heated, low-pressure water washing. This treatment is recommended for stones sensitive to acid (limestone, marble) and for the removal of plant materials.
If water and a natural bristle brush do not fully remove a rust stain, a poultice treament may be appropriate.

If water and a natural bristle brush do not fully remove a rust stain, a poultice treament may be appropriate.

Sometimes, chemical cleaners, poultices, and abrasive or mechanical methods of cleaning could be appropriate to use on historic masonry. These methods present greater challenges – both to historic materials and the environment – and so should only be explored if simple water treatments are not fully effective.

With wood, soiling is best removed with a small amount of water and a very gently used, natural bristle brush. Soaking, water-washing, and steam washing can be destructive to wood, as it soaks in water differently than masonry. To remove paint, scrape and/or sand by hand. As with masonry, chemicals – in this case, chemical strippers – are sometimes appropriate for use on wood, as when they supplement other paint removal techniques and are researched well.

Cleaning architectural metals requires the identification of the specific metal type, as various metals require different approaches. Soft metals like lead, tin, copper, and zinc should not be cleaned with abrasive blasting, as it damages the metals’ surfaces. With hard metals like steel and cast or wrought iron, scrape by hand or with a wire brush before attempting mechanical abrasive blasting.

Green Cleaners for Your Historic Exterior

When water and a natural bristle brush are not fully effective with a small-scale masonry cleaning job, the use of diluted chemical cleaners may be appropriate. As more people practice green cleaning, more environmentally friendly cleaning products become available. Instead of using a modern cleaner full of harsh chemicals, try one without dyes, perfumes, or synthetic preservatives. A good rule of thumb is to use a product that lists all of its ingredients on the label.

Manufacturers like Simple Green have full lines of environmentally friendly cleaners that can be diluted and used with water and natural bristle brushes to remove soiling from historic masonry. Further, many of these products are GreenSeal-certified.

For larger-scale masonry cleaning projects requiring the use of chemicals, some companies that cater to historic buildings now make “green” products. A member of the U.S. Green Building Council (USGBC), PROSOCO, Inc., one of the nation’s largest building cleaning, sealant, and repellent manufacturers, recently launched its Green Group and Environmentally Preferable Products lines. Environmentally Preferable Products feature some that are meant specifically for use on historic buildings.

Important Notes to Remember

Low-pressure washing with hot water and gentle scrubbing with a natural bristle brush could remove the plant materials and grime from this stone wall.

Low-pressure washing with hot water and gentle scrubbing with a natural bristle brush could remove the plant materials and grime from this stone wall.

Before undertaking any cleaning treatment on a historic building, there are important notes to remember:

  • Consult a historic-building contractor, particularly with large-scale projects and when using chemical or powered mechanical methods.
  • Always spot test any cleaning method before pursuing it on the full-scale project. It is better to see any harmful effects a cleaning treatment may pose in a small, contained area.
  • If there is recurring mold growth or soiling at the same spot on the exterior, there may be a drainage problem that needs to be addressed to prevent deterioration.
  • When using chemical cleaners, even if diluted, make sure to take the proper precautions to catch run-off, soiling materials, and paint chips (plastic troughs at the base, plastic sheeting, etc.) and dispose responsibly (See http://www.earth911.com for information on safe dumping).

Routine exterior cleaning is a part of any building owner’s agenda. Owners of historic buildings must use the gentlest means possible in order to thoughtfully maintain the historic exterior materials. Because gentle means are usually also green, historic-building owners can easily practice environmentally sustainable cleaning. When cleaning your historic building, keep it simple: don’t use a lot when a little will do.

For more information:

Inglesby, Tom. “Clean, Green, and Sealed,” Masonry Magazine, September 2008

National Geographic Green Guide Products

Pennybacker, Mindy. “What Cleaning Product Ingredients to Avoid,” National Geographic Green Guide, 1 March 2006.

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