Most historic buildings made of natural stone are suffering the deleterious effects of weathering, natural accidents and anthropogenic impacts. In this study we have evaluated the origin of black films observed on the Third Order of St. Francis Church, built on 1805 in Porto (Portugal) by using Double shot Analytical Pyrolysis (Py-GC/MS), X-Ray Diffraction (XRD), as well as optical and Scanning Electron Microscopy (SEM-EDS). These films were the most perceptible degradation pattern of the granite monument façade, coating over 70% of the surface, evidenced by the blackening of the stone surface. Analytical pyrolysis revealed a significant presence of biogenic derived material, signing that biological activity is playing a major role in the development of black films in this historical building. Biomass burning (wood, carbon or forest fires) was also a considerably source of particulate organic matter in the area, as is suggested by the noteworthy presence of levoglucosane. The use of a wax based cover and/or vegetal oil coating has been exposed by i) the presence of Vitamin E, ii) the distribution of fatty acids and iii) the detection of specific Cholestanes. Microscopic and mineralogical analysis confirmed those findings. They revealed that the activity of phototrophic microorganisms caused the detachment process. In fact, the growth of endolithic lichens, as Sarcogyne privigna, and cyanobacteria is compatible with that process because their growth can disolve minerals and generates a weak interface in the granite, prone to its detachment as plaquettes. In addition, stereomicroscope showed the presence of different coloured layers in almost all samples. The black films presented an average thickness of 1 μm and were tightly adhered to the rough etched surface.

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