An integrated approach to assess the origins of black films on a granite monument

In the present study, samples arising from the scaling of a Portuguese granite building were examined in an attempt to understand the mechanisms of surface blackening and detachment. The building, the Third Order of St. Francis Church, is located in the city centre of Porto which is an area characterised by moderate motor traffic. The Mediterranean climate and the façade orientation favoured the proliferation of microorganisms on the Third Order of St. Francis Church, in Porto. The scientific approach carried out in the South façade revealed that these coloured layers are essentially of biological origin. Subsequent chemical analysis confirmed microscope observations and pointed out to the presence of organic matter synthesised by cyanobacteria, algae and lichens. Numerous biological marker compounds indicated a significant presence of biogenically derived material, suggesting that biological activity was playing a major role in the development of coloured layers and in the detachment processes in this historic building.