White gates turning black in Belgrade.
Once upon a time, long, long ago, a fortress of white limestone was built between the River Sava and the Danube in what is now Serbia. It later gave its name---Belgrade, or "white fortress"
---to the city that sprang up within and outside its walls, and in the twenty-first century, after more than a millennium of attacks by Huns, Bulgarians, Byzantines, more Bulgarians, Turks, and what-have-you, Belgrade fortress met its harshest enemy yet: fertilizer. Our story starts with scientists trying to figure out why the fortress's legendary white walls were turning black
. They took samples of the corrosion and examined it with a number of chemistry techniques to determine what it was made of, finding, as they had expected, that the black hue was partly due to sulfur dioxide released by the coal-burning fires heating the surrounding houses. Too much sulfur dioxide in damp air will trigger a chemical reaction in limestone, causing white calcium carbonate to convert to black calcium sulfate. But the researchers also found a substance called syngenite, which incorporates calcium, sulfur, and potassium. And that was strange, because syngenite, which often forms on medieval stained glass windows, hadn't been seen on limestone before. Noting that it appeared mostly at the base of the fortress walls, they traced the element back to its ultimate source: the potassium was coming from fertilizer used on the flowerbeds brightening up the old place. The fortress, in effect, was under attack by flowers. The moral of this story? If you want your limestone fortress to live happily ever after, check your chemistry textbook before landscaping. And remember, a flowerless fortress was enough to repel centuries of invaders. Why mess with a good thing?
Image courtesy of V. Matovic