by Don Friedman on December 15, 2016

That’s the view out my window in the new office. The facade of the building across the street is dirty.

No big surprise, right? New York has many fine qualities, but cleanliness is not among them. But that facade dirt is a little more complicated than that. First, different materials show dirt differently. In this case, matte-finished off-white brick and limestone tend to both hold the dirt and display it better than a lot of other materials. If the ornament were glazed terra cotta, for example, rather than limestone, there would be less dirt adhering; if were colored glazed terra cotta, the dirt might be barely visible.

I’ve increased the contrast in the copy of the photo below:

The staining at the limestone at the parapet (to the right of A) is obviously related to water running off the parapet cap. However, the effect of wind elsewhere shows that the dirt was airborne. There are areas (for example, to the right of B) that are almost clean. The very corner is particularly dirty and also shows a lot of striations in the dirt (for example, to the left of C). Wind forces increase at the edges of buildings, where vortices form when the wind blows. As an engineer, I usually think about this topic when I’m looking at wind loading on the bulking but it obviously affects how dirt is despoiled on masonry.

Finally, and most importantly, the nature of facade dirt has changed over time. We used to burn a lot of coal in New York, we used to have garbage incinerators in buildings rather than compactors, and we used to have cars and trucks without pollution-control devices built in. The stone of St. Patrick’s Cathedral used to be very dark with dirt. It was cleaned in 1978 and became noticeably lighter, and then cleaned again a couple of years ago and became lighter still. This process is certainly not unique to New York: I’ve visited Edinburgh several times, starting in 1990, and the cleaning of hundreds of old masonry buildings in the city center has had a dramatic effect. The ratchet effect of old masonry getting lighter and cleaner is caused by the fact that we have less dirt in the air – in formal terms, less particulate air pollution – than we used to, so buildings don’t get dirty as fast or as much.

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