Blackening of copper pigments in wall paintings: impact of the fresco technique and the chemical composition of the pigments

This study addresses the blackening of copper-based pigments in fresco paintings. Historical evidence shows that copper carbonates, silicates and acetates were widely used despite often blackening, usually due to copper oxide formation in humid, highly alkaline conditions. To clarify the causes, eight fresco-applied copper pigments were studied after blackening. Physical, mineralogical and chemical analysis suggested that moisture, high alkalinity and carbonation promoted oxide formation. However, crystalline oxides alone did not explain the observed blackening. Instead, XPS analysis indicated poorly crystalline or amorphous reduced copper species as additional contributors. Pigment composition was found to be critical: carbonate- and acetate-based pigments showed instability, whereas silicate ones remained comparatively stable. Pigment origin and impurities further influenced alteration pathways, with copper-acetate verdigris exhibiting the most complex behaviour. These results highlight the multifactorial nature of copper pigment degradation and underline the importance of pigment identification for conservation strategies in fresco paintings.