Black layers of decay and color patterns on heritage limestone as markers of environmental change

Air pollution induces the development of black crusts on the surface of built heritage materials. Black layers on the limestone used on an emblematic Madrilenian building dating from the early twentieth century, mainly built up in the 20 years lapsing between two façade cleaning operations, was studied with POM and SEM-EDS. Particulate matter deposited on surfaces in the same period was analyzed with XRD and IC. Climate change in the environs was also studied and façade coloring patterns were compared. Since black crust and settled dust composition, as well as façade soling intensity, were found to be closely related to the surrounding environment, both are proposed as environment and climate change markers. These are considerations, moreover, that must be addressed when designing conservation strategies. Domestic heating systems and vehicle traffic were identified as the two main sources of pollution throughout the period studied in the target area, where the temperature steadily rose and relative humidity declined. The progressive replacement of coal with gas oil in domestic heating boilers and the proliferation of vehicles with diesel engines have mostly determined the evolution of the pollutants emitted. The color of façade soiling, in turn, has been primarily conditioned by the typology of the particles deposited on the limestone surface, declining humidity and the passage of time.