Atmospheric pollution
Atmospheric pollution & general grime is a particular problem in built up areas, such as city centres, where there is a high volume of traffic. This is because vehicle exhausts emit sulphur dioxide which reacts with stone, forming calcium sulphate crystals on the surface. These crystals trap pollution creating a black crust - the dark staining so often seen on buildings. Not only is this unsightly, it also clogs up the pores and affects the movement of water in and out of the stone which can in turn cause it to further deteriorate.