Assessment of stone surfaces degradation induced by atmospheric pollutants : the case study of the Duomo di Milano

The issue of environment protection, including the conservation of the monumental heritage worldwide is related to atmospheric pollution. Anthropogenic multi-pollutants are the main factors responsible for the accelerating damage observed on cultural artifacts exposed to the atmosphere over recent decades [1]. Air pollution is one of the most important causes of surface decay in the urban environment. Currently, emissions from mobile combustion sources are the main agents responsible for pollution, although a significant decrease is expected in Europe within the next decade. Damage to stone surfaces is generated by the combined action of environmental and anthropogenic factors. Many studies in the past have assessed the erosion phenomena of stone surfaces by precipitation using both mechanical and chemical mechanisms and also the phenomena of blackening and sulfation due to the deposition of atmospheric particulate on the stone substrates [2-6]. Therefore, the issue of conservation of the monumental heritage worldwide is mainly related to atmospheric pollution that causes the degradation of stone surfaces. The deposition of particles on the surfaces of works of art of historic and artistic interest is not a simple phenomenon of adsorption to the surface since the powder is often cemented into a physicochemical process that includes the deposition of a thin layer of water, and chemical reactions between the material and the corrosive acids present in this solution can occur [5]. The powder deposits present on the stone monuments reflect the composition of the aerosol particulate matter (PM) to which the surfaces are exposed, so it’s the chemical characterization is necessary in order to adopt mitigation measurements which of conservative intervention and maintenance strategies, as well as for the development of emission reduction policies on a larger scale. In the present study Candoglia marble specimens and quartz fiber filters have been exposed at the façade of Milano Cathedral (Duomo di Milano). The field exposure activity started in 2012, few years after the completion of the restoration work of the façade. The objective was to characterize the soiling rate and composition during the very early stages of deposition on reference substrates exposed to the same condition of the actual stone of the façade. . In order to fully characterize those samples, a multi-analytical approach was used. Data acquired on samples have been compared with those collected by the regional environmental protection agency (ARPA Lombardia).