NCA maintains 3 million+ gravesites. A critical component of maintaining cemetery appearance is headstone cleaning. Individual national cemeteries tend to use a wide variety of cleaners, some of which can have harmful long term effects.
NCA contracted for the NPS study as a general guide for cleaning marble govt-issued headstones soiled by dirt/biological growth
Dirt (soil, mud) stains stone or gives it a dingy appearance. It leads to mold/mildew growth on stone, and minerals containing iron leave rust-colored stains. Air pollution (vehicle exhaust, industrial activities) generates pollutants that change the look of stone over time. For example, sulfur dioxide interacts with marble to cause gypsum crusts that capture pollution particles to make rough, gray surfaces.Biological organisms (bacteria, mold, mildew, algae, moss, lichen) establish a biofilm made of proteins and sugars that is hard to remove; they provide food for organism growth. Bacteria consume air pollutants and make acids that attack stone; fungi penetrate stone and carry bacteria deep into pores.Bird droppingsor other animal secretions can stain stone and be difficult to remove. Plant or tree sap is a sticky substance that drips from overhanging trees. It may contain resins not easily dissolved in water. Sugars in sap attract insects or provide food for molds, mildews.
Location Differences outweighed climatic differences in the observation of biological regrowth. One year after cleaning, Jefferson Barracks was the first to show visible biological regrowth. Based on biological studies Bath and Jefferson barracks showed colonies of bacteria and fungi. Santa Fe showed the least amount of regrowth.Results suggest that semi arid environments like Santa Fe will need to be cleaned less often than typical humid locations such as Jefferson Barracks, Bath, or Alexandria.The fogs of San Francisco can promote biological growth.
Less biological growth occurs in sunny locations, likely due to drier conditions and move intense UV radiation.Greater numbers of bacteria and fungi were found in shady areas, likely due to more moisture, and food sources from the surrounding trees and vegetation.Results suggest that an overall cleaning of the cemetery can then be followed up with spot cleaning in shady areas.
(Show Daybreak, D2, and a healthy stone)
Salt damage can disintegrate stone surface. Salts in the stone, surrounding ground, irrigation water and some herbicides and cleaners, migrate through the stone’s porous network damage. Salts are dissolved and transported by water. They can recrystallize and exert pressures in the pores that may exceed the strength of the stone. Thus, do not use cleaners that leave behind salts.Freeze-thaw cycles can increase stone weathering. Water gets into openings, cracks and pores of stone. If freezing temperatures exist, the water can freeze and expand. Since most cleaning efforts require saturating the stone with liquids, do not clean headstones during freezing temperatures or when a freeze is expected within 48 hours of the cleaning.Improper cleaning can stain the stone or accelerate deterioration. Poor cleaning methods can cause damage. This includes use of power-washing equipment too close to the surface, sanding equipment, not rinsing after application of cleaners, and using products in a greater strength than the manufacturer recommends.
Use the gentlest, least invasive method . Select methods and materials that do not affect the headstone. Chemicals and physical treatments should be undertaken using the gentlest means possible.Consider long-term effects. Cleaning impacts are not always obvious. Marble is made of porous carbonate mineral. As surfaces are cleaned, material is loosened and lost, and mineral binder is affected. Over time and cleanings, the surface is altered–called sugaring. Some marble is more prone to deterioration--Colorado Yule is more affected than Georgia’s Cherokee White.Minimize cleaning impacts. Limit the times a headstone is cleaned in its lifetime. While a cyclic maintenance plan is needed to maintain appearance, over-cleaning should be avoided. Historic headstones should not be cleaned more than once a year, if possible. Don’t remove original surface. Original surface may be polished, smooth; if it is altered, marble may weather poorly. A roughened surface will soil easier and inscriptions will erode. Never aggressively scrub the surface, use wire brushes or mechanical methods such as sanders/grinders on marble. Test cleaner first. Always test before wholesale cleaning, per recommended procedures, and let test area dry before inspection. For biocidal cleaners it may take days before the full effect is realized.
Bleach/bleach-like products. Household bleach or oxidizing cleaners will cause chemical reaction and leave soluble salts in stone pores that will lead to decay. Check the MSDS for these active ingredients: sodium hypochlorite, sodium perborate, sodium percarbonate, sodium persulfate, tetrasodium pyrophosphate, calcium hypochlorite or urea peroxide. Do not use products containing these. For example, Daybreak contains 14% sodium hypochlorite and is not recommended. Strong acids or bases. Strong acids (muriatic, hydrochloric) are harsh and will dissolve the stone surface. They are corrosive and can be hazardous to workers. Strong bases (concentrated ammonia, sodium hydroxide, calcium hydroxide, potassium hydroxide) may be too aggressive for the stone and hazardous to users. Power tools & pressure washing Harsh mechanical devices (sand blasters, sanders, drills w/wire brush) will remove original stone. Commercial pressure washers (750 psi to 30,000 psi) will damage marble headstones. The proper distance/pressure to clean a headstone is about 12”/500 psi or less. A test is recommended prior to cleaning.
A headstone-cleaning regimen should be based on environmental factors--humidity /precipitation, biological growth rates, tree cover/vegetation, and others activity that influence s the frequency of cleaning.Follow manufacturer’s recommendations. Two key factors are dilution ratio and dwell time. A small amount of cleaner should be added to water to create the required ratio; using a cleaner in a more concentrated form may damage the stone. Dwell time is the amount of time a cleaner is left on the stone before scrubbing/rinsing it. Biocidal cleaners are used against biological growth (algae, mildew, moss, lichen). Most biocidal additives also slow regrowth for a time. Recommended biocidal cleaners are D/2 Biological Solution, EnviroKlean® BioWash®, or others containing quaternary ammonium compounds. Follow directions and rinse thoroughly. Consult with the manufacturer to see if the biocidal cleaner contains buffers that may leave salts behind on the (stone and avoid these?.) Marble cleaned with biocides should continue to lighten over several days. The advantage of a biocidal cleaner is that it helps remove a wide range of soiling including biological growth. The disadvantage is that the cleaners are more expensive than other products on the market.Note: Sometimes vivid colors are seen upon application of the biocide. This is the result of organisms dying. The discoloration should rinse off immediately and have no permanet effect.