ABOVE ALL, DO NO HARM! Rocks are hard. They are durable, right? Gravestones are made of rock, generally. So they are durable and last forever, right? They can be scoured clean regularly, to keep them looking respectable and honoring, right?
Not so fast. Most of the gravestones you see in historic cemeteries are extremely fragile, vulnerable to abrasion and chemical action. Most are sandstone, limestone, marble, slate or granite, depending on the preferred material of the era when they were erected. They may be smooth, even polished, yet each is porous, allowing liquids to percolate through their crystalline structure. Natural weathering processes, chemical action or abrasive action can weaken or break the adhesion between the crystals of the stone. The most well-intentioned cleaning can remove the surface layer of the stone, or worse, cause structural damage deeper than the surface, hastening the deterioration of the stone. Tap the stone. If it sounds hollow, stop. No scrubbing. No scraping. The gentlest cleaning touch may be damaging. Sound solid? A soft, clean brush, clean water and lots of elbow grease is the best method, if you must scrub. Use no metal implements at all...ever! Stone too fragile? Feeling a bit puny or lazy? Skip to D2, my darlin'!
Biological growth is the most obvious attack on gravestones. Moss and lichens have the ability to adhere to stone, even polished granite, by infiltrating the tiny crevices between the crystals of the stone. The best approach to removing this macro growth is to soak it well, and then use a plastic or wood scraper to gently remove it. Haste and gentle are not compatible. Work slowly, and keep the offending growth wet. In this way, you can remove the surface growth, but you will not remove the root of the problem. Note: Sandstone is extremely fragile, and moss and lichens will take part of the surface stone structure with them. Sandstone was easy to work, and is also easy to destroy. Know your stone!
D2 Biological Cleaning Solution is THE recommended cleaning product for gravestones, no matter the material. It has been tested extensively by the National Center for Preservation Technology and Training, and found safe for fragile stone structures. (In 2013 D2 was applied to the Washington Monument in the nation's capitol, and is the chosen solution for cleaning the headstones in Arlington National Cemetery). Other cleaning products have hit the market to compete with D2. You see them on the internet and in the national media, self-recommended by their manufacturers, but lacking extensive unbiased analysis. Regardless of their hype, do not use 30 Second Outdoor Cleaner, Wet And Forget, or any of the multitude of household cleaners or home cleaning recipes. Part of the physician's traditional oath is, “Above all, do no harm.” What applies in medicine applies here.
Now for the joy of the work-challenged descendants wanting to honor their forebears. With D2, you spray and walk away. Spray D2 full strength on the dry stone, thoroughly wetting all surfaces. Use a clean garden sprayer, or the spray bottle used to market smaller quantities of the cleaner. D2 seeps into the microscopic crevices in the stone where the biological offenders cling, and removes the deepest penetration. It takes time. White marble or gray mottled limestone, granite of any color, sandstone or slate, discolored by even a hundred year accumulation of algae and lichens, will over a period of months of natural cleansing, shed its cocoon, and emerge in its original beauty. D2 costs a bit, but the safety of the stone is worth the price. The frequency of application depends on the climate. Here on the coast, a treatment every year or two will keep the stone pristine. As an added benefit, testing has shown D2 safe for folks, as well as for the wee birdies and beasties.
DISCLAIMER: This is posted in the interest of preserving historic grave markers, with no intention of promoting or marketing D2.