I can only speak for myself and what was done in our family plot. This cemetery is outside the village of Canastota, Madison Co., NY. The plot for 10 was purchased in 1929, it accommodates 5 family members. There were only 2 headstones of the material that collects a black mold and etc. My husband and I purchased OUR headstone, and 3 others, and had the old stones (1915-16) cleaned. They turned out very well. We had gotten the caretaker, who also ran a nearby monument business, and he took care of all those phases, and it truly was not that expensive. Our stone of course was the more costly. If there was any permission gotten the caretaker did it. The old stones are of course starting to 'mold' again, something that cannot be prevented as the material is so porous. I would recommend contacting a local monument center to see what they might offer. As I said, I can only speak from my experience and in NY. Pat Jaquay