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Cleaning Limestone Wall Caps and Post Caps

Cleaning Limestone Wall Caps and Post Caps

How to Clean Limestone Wall Caps and Post Caps

Limestone is a common material used in architecture, especially throughout North America and Europe. From the Great Pyramid in Giza, Egypt to Canada’s “Limestone City” of Kingston, limestone is utilized because it’s readily available, durable, weather resistant and relatively easy to cut into blocks or to be carved with elaborate imagery. The porous nature of limestone also means dirt and grit readily adheres to the surface and cannot tolerate many cleaning products. However, you can still maintain the aesthetic value of your limestone walls or wall caps and post caps if you maintain it properly with the appropriate methods and materials.

Three procedures are recognized for cleaning masonry and stone buildings:
The “Wet” methods include using a pierced hose which is placed to allow water to run gently over the surface for specific periods of time; using hand-held hoses at pressures ranging from normal city pressure up to 1,200psi, and hot water at relatively low pressure. “Wet” methods are often enhanced through the use of soap or detergent.

The “Dry” methods include grinding or sandblasting. These can produce excellent results by removing layers of stone along with grime. Conservationists have a tendency to consider “Dry” methods as excessive because of their potential for damage, but in some instances sandblasting or grinding may be necessary. Another method is using a hybrid of “Wet” and “Dry” methods, combining relatively low-pressure water with an abrasive (i.e. sand) discharged from a nozzle with multiple hoses and ports. Here, the water “cushions” the abrasive element.

The “Chemical” methods include the application of muriatic acid or another acid in various levels of dilution. Proprietary products tend to be acidic which may include detergents or other surfactants for the purpose of lessening the potential for damage attributed to the use of acids and caustics.

Experimentation may be required depending on the complexity of the stonework and the type or degree of grit to be removed.

The high-pressure water method is generally used. Water from a pressurized rig at a maximum pressure of 1,200psi, from a 45 fan-shaped nozzle, held no closer than 6 inches to the face of the limestone will normally produce acceptable results.

Keep in mind that the complete removal of grime almost always removes some of the original surface and increases surface area. The high-pressure method described above will perform satisfactorily in most cases, removing 80% or more of the visible dirt. However, even high-pressure water can cause damage especially if pressure, nozzle size and working distance are abused.

One type of grime that may accumulate on limestone is algae. This may, to the naked eye, appear to be a collection of dirt. Magnification may reveal algal flowers as small spots of red, green and orange. This can be removed using full-strength hydrogen peroxide scrubbed over the wetted stone. The algae, once agitated, will loosen and can be hosed away without further harm to the surface.

A good practice is to invite operators to clean a sample area using their preferred method to see if it is right for you.

For more information on cleaning your limestone wall caps and post caps, visit the Indiana Limestone Institute:

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