It is most often in the spring, usually concurrent with pilgrimages to historic homes, that the "cemetery bug" bites. Well-meaning individuals and groups go for a cemetery-cleaning day, armed with chlorine bleach, scrubbing powders, and sometimes even machine-driven pressure washers. Although it gives everyone a fine feeling of satisfaction to attack old, darkened markers with vigor and elbow grease, sometimes, unfortunately, more harm than good is done. It comes as a big surprise to people who think of masonry as the most durable of materials to learn that the marble, granite and limestone of old grave markers is far softer and more porous than one would think. For this reason, the general rule of thumb in cleaning them is: ‘Use the gentlest means possible."
When you undertake a cemetery cleaning project you must first determine what you want in the way of "clean." Remember that old, weathered stone does not have to look new. You probably want to remove harmful pollutants or lichen, but you certainly don’t want to scrub until the surface of the stone erodes. (Or, worse, you would not want to blast the surface of a gravestone until you can no longer read the inscription!)
Approach each project, no matter what material the grave marker is made of, by asking yourself, ‘what is the gentlest means possible? The harshest, quickest, or cheapest method is often not the gentlest. And, certainly, blasting with chemicals, sand, pecan shells, glass beads, or even water is not the gentlest means possible.
Make test patches of your proposed cleaning technique on an area of the structure that is least visible. Begin with plain water (at garden hose pressure) and a soft bristle brush. You will find that some foreign materials are removed quite satisfactorily with this simple approach-dark algae from a marble surface, for example.
Should further action be needed, the next step should be the slow and cautious addition of a mild detergent to the water. However, before any detergent is added, the stone surface should be thoroughly soaked with water, again at garden hose pressure. It takes time to thoroughly soak stone-thirty minutes minimum, and some sources recommend soaking a stone for 24 hours! The reason for the soaking is to make sure that the detergent is applied only to the surface of the gravestone; if not thoroughly saturated, the stone will absorb the detergent, making it impossible to rinse away.
It is very important to understand that acids are very damaging to marble and limestone, and chlorine bleach (such as Clorox and Purex) is very bad for almost all stone, including polished granite. Many people have totally lost the polished face of a fine granite marker by conscientiously scrubbing with household bleaches. If you feel you need something in addition to a mild detergent, use a diluted solution of ammonia (1 part ammonia to 3 parts water). You can also try a photographic solution named Triton X, which is recommended by conservators as a gentle cleaner. Water and gentle scrubbing should do the job.
Keep in mind that a soft-bristle brush is all that is needed. Metallic brushes are entirely too harsh, and they also leave particles on the surface of the stone that can rust. Always watch carefully to make sure that none of the stone’s surface is eroding as you scrub.
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