Stone cleaning, like all conservation methods, requires a comprehensive understanding of the material, the construction, the type of dirt, grime, or damage causing discoloration, and the history of the stone’s condition. Contractors may be inclined to regard stone cleaning as a routine, basic process. However, unless careful attention is directed to the historic materials, there is risk of long-term damage resulting from improper cleaning methods. Not all stones receive the same cleaning treatment. Poor methods can result in permanent damage.
Marble eaten away by bad cleaning method at St. Matthew, Hastings-On-Hudson
The stability of the stone is not always determined by its cleanliness. In some cases, the layers of dirt and grime may have permeated the surface layers of the stone to act as a protective coating from the elements. Over-cleaning can be abrasive and risk stripping the stone of its top layers not only changing the aesthetic appearance entirely but also causing vulnerability to the long-term stability of the material.
Considerations
There are a number of things to consider when embarking on stone conservation:
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- How much of the surface area should be cleaned knowing that the cleaned areas will be in extreme contrast to the existing?
- If there are different types of stones, an understanding of how each ages and discolors will determine the end result of the cleaning. Some stones are more porous than others and will not come perfectly clean whereas others may look as new.
- It is important to remember that stone cleaning will significantly alter the aesthetic appearance and impact of the building, statue, etc.
Testing
Once the condition and material make-up have been analyzed, two questions must be determined: How much to clean? When to stop? By performing a series of tests on the stone, the proper cleaning method and the extent of the cleaning may be determined.
Once testing areas are selected, a number of trails are completed to determine the appropriate conservation approach. When cleaning stone, it is important to remember that the binding construction is also being cleaned. Thus, conservation methods must be compatible not only with the stone material but also the mortar. Consideration for the coloration and aging factors of the mortar are significant to aesthetic and structurally sound conservation results. During these tests, it may be found that repairs to the construction are also required to ensure the longevity of the construction. Once the tests are completed, it is possible that the conservator may suggest the stone remain uncleaned to minimize any potential risks of damage.
Washington Dc’s Union Station stone cleaning test showing the before and after cleaning.
Techniques
There are a number of standard cleaning techniques that have been used over the years that may be generally placed into two groups: physical methods and chemical methods. However, with the advancements of technology and materials, we also employ modern equipment and treatments. Not all methods are appropriate for all stones or conditions. During the analysis and conservation tests, the appropriate method is tailored specific to the project at hand.
The guidelines of the American Institute for Conservation (AIC) profess to “do no harm.” In the past, far too little consideration has been given to the process of stone cleaning and repair in the past causing grave consequences and permanent damage. Our goal is to preserve our historic monuments, buildings, statues, interiors, etc. for long term appreciation which requires a cautious and meticulous approach to structural and material stability of the stone before the aesthetic appearance. By adopting a limited and educated approach, the stone may not only be cleaned for greater appreciation but also preserved for the next generations to come.
