The Formation of Metal Oxalates in the Painted Layers of a Medieval Polychrome on Stone, as Revealed by Micro-Raman Spectroscopy

Abstract

This paper reports a detailed study of the degradation phenomena exhibited by the painted bas-relief sculpture on the portal of the Basilica of San Zeno Maggiore in Verona. This study extends the wide diagnostic project started previously by the Istituto Centrale per il Restauro (ICR, Rome). Micro-Raman measurements on polished cross-sections of samples from the painted lunette allowed detailed identification of the species present in the different layers, thanks to the high spatial resolution and chemical selectivity of this technique. One of the most interesting findings was the occurrence of large amounts of metal oxalates. In particular, in addition to calcium oxalates, copper oxalate was observed in layers containing copper-based pigments, and data on the distribution of both these oxalates within the painted layers were obtained. The results give evidence of the nature of the degradation processes taking place involving the painting materials, provide information about the degree of degradation, and make it possible to advance reliable hypotheses on the causes and mechanisms of ageing. Along with calcium oxalates, copper oxalate proves to be a reliable marker for degradation, which is of general interest for the study of outdoor paintings containing copper-based pigments.