Cleaning Old Gravestones
By Trina Purcell
At our last meeting, there was a lot of discussion both during the meeting and during the lunch break about cleaning and restoring early gravestones. This prompted me to review some of the literature that exists on this topic. The conservation of old stones can be tricky and there is a very real danger of inappropriate procedures causing irreversible damage to the stones.
With the help of the Association for Gravestone Studies, Rebecca Reynolds of the Museum of Fine Arts in Boston, and publications by conservators Lynette Strangstad and Tracy C. Walther, I’ve tried to put together a simple procedure for cleaning old stones. I would urge anyone contemplating a cleaning or restoration project of any kind to read Strangstad’s book before embarking on the project. You may also want to consult with the Association for Gravestone Studies. They can be contacted at (412) 773-0836 or at www.gravestonestudies.org and can provide expert advice on the cleaning and restoration of old stones. Tracy Walther’s invaluable article entitled “Review and Evaluation of Selected Brand Name Materials for Cleaning Gravestones” is available on the AGS web site.
There are three basic rules to keep in mind when cleaning or restoring old stones. The first is to be gentle. As many of us have discovered, these early gravemarkers are very fragile and it’s difficult to effectively repair damage once it’s been done. The second rule is to keep long-term preservation in mind. Many “cleaning procedures” that make the stone more pleasing aesthetically do irreparable damage. Think in terms of what you want the stone to look like in fifty years, as well as what you want it to look like when you are done cleaning it. Finally, never attempt to clean a stone that appears to be distressed. This is indicated by cracks, flaking, scaling, or a “sugary” (granular) surface. Tap the stone gently to make sure there are no internal hollow areas. If you have any doubt about the stone’s ability to withstand the cleaning procedure, don’t attempt it.
Let’s start by talking about bleach. There’s a real temptation to use bleach to clean discolored marble stones. They come out pristine and white, right? Unfortunately, while the stone looks better to the naked eye, bleach causes invisible irreparable damage by eating away at the surface of the stone and exposing a “softer” at-risk under layer. The exposure of this under layer to the elements causes the monument to decay much more rapidly than it would have normally. Secondly, bleach leaves a residue behind that no amount of rinsing will remove. Other commercial cleaners like Fantastic and Ivory soap pose the same danger. This is because calcium ions in the stone make bleach and other commercial detergents insoluble in water. Chlorides are inevitably left behind and these cause migrating salts to form beneath the surface of the stone. As the crystals grow and exert pressure from below, they will cause the face of the stone to “pop off.[i]” Other harmful cleaners include muriatic acid, phosphoric acid (e.g. Naval Jelly, Lime Away), alkaline, corrosive and biocidal cleaning materials.[ii]
So how do you clean a gravestone? Start with the least damaging product. Photographic supply houses sell non-ionic detergents that are safe to use on marble, limestone, soapstone, sandstone and slate. Brand names include Photo-Flo, Triton-X 100 and Igepal. Use one ounce to five gallons of water. Conservator supply houses can provide a similar product called Vulpex. Use one part Vulpux to 3 parts water.[iii]
Start by thoroughly wetting the stone with water. Gently scrub it from bottom to top with a soft brush. Pre-wetting the stone prevents the cleaning solution from penetrating too far into the porous surface of the stone. Scrubbing from the bottom up prevents streaks. Remember that whenever two objects (such as the brush and the stone) are rubbing against each other, some abrasion occurs. To minimize this, scrub as gently as possible. Once you have pre-cleaned the stone, follow the same procedure with the recommended cleaner mixed with the appropriate amount of water. Rinse often with plenty of clean water. Never let any detergent dry on the surface of the stone.
A wooden stick such as an ice cream stick or tongue depressor will help clean crevices on slate or granite stones. Don’t use anything other than a toothbrush on softer stones such as marble or sandstone. Never use metal implements.[iv]
Household ammonia can also be used to clean stones, but should be used as a second choice when non-ionic detergents don’t provide effective results. If you use ammonia, do not exceed one cup ammonia to four cups water. Ammonia can be quite effective on biological growth such as lichen.[v]
Other types of biological growth, such as stubborn black algae, can be removed by using calcium hypochlorite. This is available as swimming pool disinfectant. This is a granular product not to be confused with other swimming pool disinfectants such as “liquid chlorine” or sodium hypochlorite. Dissolve one pound of calcium hypochlorite in four gallons of warm water. Use only to remove biological growth. Follow the procedure described above.[vi]
In our next newsletter: Repairing Early Gravestones: Some Basic Do’s and Don’ts.