The diagnostic and monitoring approach for the preventive conservation of the Facade of the Milan cathedral

The importance of preventive conservation strategies for the built heritage has been debated in the last years, but there still is a limited number of applied research involving complex architectural sites. The identification and monitoring of the decay processes after the restoration activities can provide valuable information on the degradation rate and extent, thus supporting the future planned conservation. In the present work the methodology and some selected results of the post-treatment diagnostic and monitoring of the façade of the Milan Cathedral are presented. The main conservation issues have been identified and studied. A non-invasive colorimetric monitoring of selected areas of the façade has been carried out during a two-year period in order to evaluate the soiling effect. Fragments of stone and samples of the particulate matter deposits have been collected and characterized in laboratory according to a multi-analytical approach. As the early stages of deposition and erosion at the end of the intervention are particularly relevant for the evaluation of the degradation rate, several set of stone specimens have been also exposed on the façade in different conditions. The results of the in situ monitoring, supported by the study of the specimens, confirmed that soiling is the main and most rapidly-evolving deterioration effect and it is therefore expected to have a significant impact in the next future. Moreover, beside the carbonaceous fraction responsible for the surface blackening, the deposits composition showed high content of potentially harmful soluble compounds which can react with the stone matrix and therefore needs to be monitored over time.