Pink! Why not? On the unusual colour of Évora Cathedral

• Investigation of pink/orange stains in inner walls of the Évora Cathedral.
• Microbiological study of the inner walls of the Évora Cathedral colonisers.
• Mortars characterisation does not reveal inorganic chromophores presence.
• Raman and FTIR analysis detected carotenoids compounds in the pink stain walls.
• Carotenoids could be attributed to Rhodotorula sp. development and metabolic activity.

The inner walls of the Évora Cathedral have a pink/orange hue of unknown origin that covers the majority of the surface. In this work, an integrated approach was envisaged to explore whether the walls pink colour is due to anthropic intention or to natural sources, by combining material characterisation of the surface layers with its microbiological study.Mortars material characterisation showed no presence of inorganic chromophores and therefore the use of inorganic pigments. The microbiological characterisation revealed that Rhodotorula yeast is the major microorganism isolated from the stained sites. Carotenoids were identified in the mortars from pink walls and in the isolated yeast culture by Raman and FTIR analysis. In this context, carotenoids detection could be tied mainly to the development and metabolic activity of Rhodotorula sp. in the surface of the walls. The extension of Rhodotorula sp. colonisation and the results obtained by the spectroscopic analysis are a strong indication that this microorganism is one of the main agents responsible for the colour alteration on the inner walls of Évora Cathedral.Therefore, the perceived pink colour of the Cathedral is due to natural processes rather than to Human intention.