5. Environment and Architectural Stone

It has long been recognized that the environment exerts a wide range of pressures on building stone. These can be loosely thought of in terms of the physical pressures of climate, particularly water and temperature issues and the chemical attack by pollutants. Twentieth century concern was especially focused on attack by acid rain, but, on longer time scales, weathering is likely to be the most important mechanism of degradation. For normal meteorological variables of temperature and precipitation, while valuable in parameterizing climate impacts on stone, it is advantageous to combine or accumulate simple meteorological data when assessing the effect of weather on building stone. Classic maps of climate offer some guidance about the geographic variability of pressures on stone, but these neglect air pollution, wind, and relative humidity. Climate maps need to be tuned to the impacts on heritage issues. Air pollution has been present in cities for thousands of years, but the extensive use of coal and the sulfur dioxide produced when it is burnt has been particularly aggressive towards stone. This sulfur is oxidized to sulphate and responsible for disfiguring gypsum crusts that characterized stone buildings in the 20th century. Contemporary urban atmospheres have lower concentrations of sulfur dioxide, but nitrogen oxides and ozone are increasingly found. Along with these, there is a range of organic acids and polycyclicaromatic compounds that potentially damage and discolor urban facades of the future. Climate change may also impose new threats to architectural stone.