There’s a story longtime Pittsburgh residents like to tell about downtown workers bringing two white shirts each day because one would turn black from soot midday. Old images show a smokey, smoggy city surrounded by steel, coke and coal manufacturers. Pittsburgh’s history of poor air quality was top of mind for Good Question! asker Caily Grube.
“I noticed on the Sixteenth Street Bridge, the metal globe structures on either side, one of the sides the metal is oxidized and the other it’s not,” Grube said. “So I’ve wondered if there might be an air quality difference on either side?”
The Sixteenth Street Bridge spans from the Strip District to the North Shore. The bridge was built in 1923 and is the fourth span to stand in that location. It was dedicated to Pittsburgh native and Pulitzer-prize winning author David McCullough in 2013. The yellow steel truss bridge shares coloring with the sister bridges down the Allegheny River, but is more ornate thanks to a push in the early 20th century to make new structures both functional and visually stunning.
At each end are two towering pillars with bronze ornamental globe structures, formally called armillary spheres. The globes are wrapped by an iron band containing 12 animals to represent the zodiac.
Today, the North Shore-side globes are greenish-blue, while the Strip District side appears tarnished and darker. Structures like this made of copper or bronze naturally change color over time, developing a patina. It’s why the Statue of Liberty isn’t the color of a penny, and why the downtown Pittsburgh Koppers building appears to have a green roof.
Pollution, wind and rain on the Cathedral of Learning
Air pollution has long plagued Pittsburgh, and can have an impact on the city’s structures, too. Cliff Davidson, a civil and environmental engineering professor at Syracuse University, spent more than 30 years at Carnegie Mellon University studying air pollution. While he taught there, he embarked on a National Park Service-funded project involving the Cathedral of Learning in Oakland.
“[We were] trying to identify what were the most important sources of pollutants that were soiling the limestone of the Cathedral,” Davidson said.
The Cathedral was built in the 1930s, when Pittsburgh was still a booming manufacturing hub. When Davidson looked at the 42-story skyscraper, he noticed there was dark soiling on lower floors, but it was less pronounced as his eyes moved up. Each side of the building was different, too.
Davidson and researchers placed sampling monitors and air filters at various heights and measured the amount of raindrops striking the structure. Air pollutants on the building’s surface would sometimes change their chemical makeup in reaction to the rain, contributing to different soiling patterns.
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“Sulfur dioxide gas has a property that can react chemically with a surface like limestone, which is basically calcium carbonate,” Davidson said. “It actually forms a new chemical, which is calcium sulfate. This particularly happens when the surface of the stone is wet, for example, right after a rainstorm, or maybe in the morning when there’s dew present.”
These chemical changes could result in the limestone cracking and leaving holes. Diesel emitted by passing busses and trucks, Davidson said, deposits in these cracks and causes the building to look darker and dirtier.
Researchers also looked at old photographs of the Cathedral. Davidson said in the 1930s and 40s, dirty air particles kept sticking to the limestone.
“Starting in the 1950s and 1960s, when pollutants were being reduced, the rain wash-off was really dominating,” Davidson said. “Every year you could see the patterns changing.”
Wind patterns have a role, too. Davidson and the researchers discovered that the west and north sides of the Cathedral were much cleaner than the east and the south.
“My guess is that on the Sixteenth Street Bridge, the direction of the rain, the winds carrying the rain against the bridge may be very important here in terms of the areas that have been cleaned by the rain, and the areas that have not,” he said.
‘A little history behind those globes’
The span is maintained by Allegheny County and before prompted with the question, chief bridge engineer Richard Connors said, “there’s a little history behind those globes.”
In the 1970s, Connors said there was a movement to restore or update public art. Crews repainted structures like the panthers on Panther Hollow Bridge, the Robert Burns statue outside of Phipps Conservatory and the Greek-myth inspired figure in the Mary Schenley Fountain called A Song to Nature. “The city actually went in and painted [the artwork] brown, some sort of lacquer,” Connors said, to preserve the artwork from the elements.
At the Sixteenth Street Bridge, crews painted the globes on the Strip District side with the brown lacquer, and planned to do the same on the North Shore, until some local historians and sculptors intervened.
“[They] were saying, ‘You shouldn’t be doing that, bronze is meant to oxidize, it looks better, actually,’” Connors said. “So they stopped what they were doing midstream, and here we are today.”
In this case, the differences in coloration weren’t caused by air pollution. Just some misguided art restoration. Connors said it’s unlikely they’ll ever reverse the painting, mostly because the county hasn’t embarked on a project of that size, and likely won’t in the near future. Until then, the armillary spheres on each side will always be a little different.
This is part of our Good Question! series where we investigate what you've always wondered about Pittsburgh, its people and its culture.
