The Rathaus was one of the first things I photographed in Hamburg. The picture is still in my camera roll, taken on 6 July 2017, five days after I arrived for an internship. Like most visitors, I was impressed by its size and its tower, but what I kept coming back to was the roof. Why was it turquoise, and why were so many other roofs in the city the same colour?

My first Rathaus picture

The answer turned out to be chemistry. The roof is copper, and copper slowly reacts with oxygen, water and carbon dioxide in the air. Over the decades this forms a green layer called patina, so the roof I was looking at wasn't the colour the builders had seen.