Well I have heard of the product that Dick Eastman mentioned in his newsletter over the weekend, but haven't used it yet, but I will.
Without further ado, I will let Dick explain this in his own words from his column at (http://blog.eogn.com/2014/06/01/use-d2-biological-solution-to-clean-gravestones/)
"Genealogists and anyone else interested in preserving cemetery tombstones and other objects exposed to the weather should become familiar with D/2 Biological Solution. It is useful for cleaning tombstones without causing any damage to the stone.
"The solution is safe for use and does not harm the tombstone. Even the highly-respected Association for Gravestone Studies recommends the product in the organization’s FAQs (Frequently-Asked Questions) at https://www.gravestonestudies.org/knowledge-center/faq-s#faqnoanchor:
"D/2 Biological Solution is even used to clean the outside of the White House and also recently won a Veterans Administration contract to supply cleaner for over 3.5 million headstones and another contract to clean Civil War monuments at the Chickamauga battlefield. (Details may be found at http://d2bio.com/news.)
"D/2 Biological Solution is a biodegradable, easy-to-use liquid that removes stains due to mold, algae, mildew, lichens and air pollutants. It is effective not only on tombstones, but also on marble, granite, limestone, brownstone, travertine, masonry, terra cotta, concrete, stucco, wood, and other architectural surfaces, including monuments and sculptures.
"D/2 Biological Solution is easy to use. Apply it to the surface to be cleaned, preferably by using a soft-bristle brush. Wait 10 to 15 minutes, and then scrub the surface to be cleaned, again by using a soft nylon or natural bristle brush to loosen most biological and air pollutant staining. Never use a stiff brush or anything abrasive on a tombstone or other stone surface! Be sure to bring a watering can or other water source along so that you can rinse the solution off the cleaned surface when you’re done.
D/2 Biological Solution:
- is biodegradable
- will not harm plants, stone, animals or people
- contains no acids, salts, or chlorine
- is pH neutral
- will not etch metals or glass
- is not a hazardous material and requires no special handling or protection
- is used full strength with no in-field mixing required
- contains no carcinogenic compounds as defined by NTP, IARC, or OSHA
- is considered essentially non-toxic by swallowing
- requires no special ventilation during use
- has a shelf life of 5 years
"All in all, I’d suggest this is a good product used to clean many surfaces, including tombstones. You can learn more about D/2 Biological Solution at http://d2bio.com. It can be ordered from a number of distributors with a list available at http://d2bio.com/buy-d2. I also found it available in 1-gallon containers from Amazon at http://goo.gl/LfebAH."
Thanks Dick for sharing that with the rest of us. I have a tombstone in New Orleans that could use this stuff (see below).