Color change monitoring to evaluate pollution impact on architectural surfaces

Environmental impact on buildings and monuments may lead to color changes on architectural surfaces which may be aesthetically unacceptable. Gaseous and aerosol pollutants are known to be the main cause of surface blackening in urban sites. It is widely recognized that atmospheric air pollution is changing over time due to legislation policies, different combustion sources and fuels. Consequently, the soiling pattern and composition on built heritage reflects these changes. In particular, as the mobile combustion sources are growing in importance and in determining the overall pollution in urban centers, the "future" soiling on architectural surface is most likely going to contain primarily organic carbon (OC). This situation will presumably lead to a yellowing-browning of surfaces, besides the "traditional" blackening. In this framework, one of the main objectives of the European TeACH project (Technologies and tools to prioritize Assessment and diagnosis of air pollution impact on immovable and movable Cultural Heritage) was to develop a new instrument for monitoring the deterioration of stone surfaces in terms of blackening and colour change. It consists of a colorimeter and an electronic interface mounted on a mechanical arm to automate the functions normally performed manually by spectrophotometers or colorimeters. After laboratory calibration and testing, the instrument was integrated in a kit also comprising microclimatic sensors. The prototype was tested at 3 historic sites in urban areas in Europe: the Opera del Duomo Museum in Florence (Italy), The Arriaga Theatre in Bilbao (Spain) and the Cologne Cathedral (Germany). At each target site samples of surface deposits and damage layers have also been collected and analysed from a mineralogical and chemical point of view. The results of the continuous monitoring performed with the new instrument will be presented and the data of colorimetric parameters will be discussed also referring to color measurements performed on the damage layer present on the monuments of the studied sites. The research work performed within the TeACH projects highlights how the development and optimization of appropriate technologies for color change monitoring on building surfaces constitutes a potential tool in preventive conservation actions.