Why do some headstones at Oakland look more weathered than others? Why are some crisp and as easy to read as the day they were carved while others appear to have melted? The primary reason for this has to do with the material the headstone is made out of and its exposure to moisture. In a previous post, we outlined the deleterious effects of the freeze/thaw cycle on stone. Today I’m going to talk about another danger associated with moisture: acid rain.
Acid rain is more common in cities than in rural areas because cities tend to be heavily populated with automobiles and industrial sites, resulting in high levels of sulphur dioxide and nitrogen oxide emissions. When combined with water, these chemicals form sulfuric and nitric acids which come in contact with stone in the form of snow, fog, and rain. The dry gases themselves can also be trapped in the water on a damp stone. This is called dry deposition.
Certain stone types contain compounds more susceptible to acid rain than others due to their minerology. Granite, for example, is made mostly of quartz, feldspar, and mica, which are all silicates. Marble is usually composed of calcite, which is a carbonate mineral. Silicates do not react with acids, and therefore, acid rain does not affect most granite. However, if the granite has some carbonate minerals, those minerals will be affected. Calcite, on the other hand, reacts strongly with acids, which dissolve the Calcium. Over time, this causes a major loss of material. This is the reason many of Oakland’s oldest marble headstones are no longer legible.
Minerology is the main reason that marble is susceptible to acid rain. But some marble markers look better than others, even if they are similar in age. There are a few reasons for this: The design and location of the monument and the type of marble. Monuments with lots of nooks and crannies for water to sit in tend to age poorly because acid is in contact with the stone longer. Similarly, a stone in the shade does not dry as quickly as one in full sunlight. Marble that is coarse-grained, such as that quarried in Tate, Georgia, weathers more quickly from acids than a fine-grained Marble from Carrara, Italy because it is much more porous.
Knowing all this, what can be done to protect Oakland’s marble monuments? Unfortunately, there is very little we can do to prevent acid rain from falling without eliminating industrial and auto emissions. However, there are a few steps that can slow the process: Re-setting headstones that have fallen so that they do not accumulate moisture on the ground, keeping shrubs trimmed away from markers to make sure they can shed water properly, and removing thick biological growth which can also trap moisture and acids against the stone. The most important step we can take, though, is to properly photograph and document all headstones at the cemetery. Sadly, many (many) years from now, all marble headstones at Oakland will be gone. At least we’ll have a lasting photograph of them.