Chromatic alterations in 15th and 16th century fresco paintings in northern Portugal

In the course of extensive cataloguing of wall painting ensembles in northern Portugal, a problem of biodeterioration featuring abundant production of a black colour film that stains both the granite wall and the painting is often encountered. Although the visual impact resulting from this alteration is a matter of importance in the aesthetic restitution of the painted surfaces, their stain removal is practically impossible. Whereas the microbial charge reduction is usually achieved through the use of biocide treatment, these products are unsuccessful in removing the dark pigmentation resulting of their growth. Despite frescoes’ substrate being mainly inorganic, deposition of atmospheric particles and the absorption of water by capillarity can promote the development of fungi, which secrete metabolites such as organic acids. Several fungi also produce pigments that vary depending on the species, the constituents of wall painting and environmental factors, among others. To ascertain how do the stains appear and what are the common characteristics between the monuments and their paintings that allow fungi growth, 24 Romanesque churches with 15th and 16th mural painting sets were selected. For this study, each mural painting was analysed and a series of variables collected for evaluation. While a great number of variables are amiss, the ones gathered underline that the majority of the churches presenting such stains are small, located on lowland, in the countryside and in isolated surroundings. Their orientation is mainly West-East, and date from 10th to 13th centuries while the wall paintings are mostly from the 15th-16th centuries. The chromatic alterations seem to appear when the paintings are uncovered from behind mortar and altars during conservation works.