Today I was speaking with some fellow genealogists about gravestone cleanings when one of my colleagues stated she recently took a tombstone cleaning workshop and that you must not use chemicals on stones. This is something I say all the time, but she followed it by "and use distilled water". I responded with "I just use tap water". She stated her instructor was quite adamant that you mustn't use tap water because it has chemicals in it. While we all know this is true, chlorine and other chemicals are added to tap water, I was of the impression that it was not harmful to the stones.
I left the conversation doubting myself - have I been doing it wrong for years and years, unwittingly damaging stones?? What kind of monster am I! Then I remembered that I've done my research; I don't just go to the cemetery all willy-nilly. Also, I frequently revisit stones I've cleaned and check them for damage. I have not witnessed a stone I have cleaned having damage caused by my cleaning it. So...what gives? I know of the instructors that gave the class my colleague spoke of, and I know they are well educated. So, I dug through my old research and did some googling to see what else was out there, and this is what I found -
Per Jason Church of the National Center for Preservation Training and Technology, it is safe to use tap water on gravestones, as long as the chlorine level is less than 2 parts per million (PreserveAmSFASU, 2013).
In "A Guide to the Care & Cleaning of Natural Stone", the Marble Institute of America only refers to using distilled water when combined with a poultice material while describing how to remove a stain. In all other instances where they reference water, they make no mention of needing to use anything specific. They just say water.
As a matter of fact, all of the resources I found that had a reputable source only used the term water - sometimes they would say clean water.
I also learned that sometimes you in fact should not use soft water. "Soft water, for example, should not be used on carbonate stone because of the possibility of dissolution of the stone" (A Glossary of Historic Masonry Deterioration Problems and Preservation Treatments, 1984). Limestone is a carbonate stone, and American cemeteries are full of these markers.
While doing my research I did attempt to find sources that advised using distilled water for stone cleaning. The only sources I found were those that I could not determine to be reputable.
So, my findings on this subject are that there does not appear to be any reason to use distilled water when cleaning gravestones. If you know of additional resources that can shed light on this topic, I would be most appreciative!
Marble Institute of America. (2004). A Guide to the Care & Cleaning of Natural Stone [Pamphlet]. Cleveland, OH: Marble Institute of America.
[PreserveAmSFASU]. (2013, April 23). Preserve America- Gravestone Cleaning Workshop With Jason Church [Video file]. Retrieved from https://www.youtube.com/watch?v=CIQS-ZryE20
U.S. Department of the Interior National Park Service Technical Preservation Services. (1995). The secretary of the interior’s standards for the treatment of historic properties with guidelines for preserving, rehabilitating, restoring & reconstructing historic buildings. Retrieved from: https://www.nps.gov/tps/standards/treatment-guidelines-2017.pdf