Cars blamed for black patch blight on historic monuments

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.