Rock varnish growth also linked to sealants used in conservation
Traffic pollution could be the cause of mysterious black patches that have cropped up on historic buildings across the world. A team of researchers working on the Freiburg Minster in Germany found a variety of black patches on the building’s sandstone surfaces, many of which contained manganese. However, manganese patches were never found any higher than 7 metres off the ground. The team believe that the patches – also called rock varnish – are caused by car emissions, including dust from braking.