The identification of copper oxalates in a 16th century Cypriot exterior wall painting using micro FTIR, micro Raman spectroscopy and Gas Chromatography-Mass Spectrometry

Abstract

Analysis of paint samples from a 16th C. wall painting in the church of Agios Sozomenos in Galata, Cyprus resulted in the identification of a copper-based, pigment degradation product – a hydrated copper oxalate, analogous to the naturally occurring blue-green mineral Moolooite. The identification of copper oxalate, a deterioration (alteration) product more often associated with the deterioration of bronze was possible through the integrated use of both micro-FTIR in reflectance for spot analysis of areas on the surface of an embedded cross-section, and FTIR reflectance imaging for the localisation of the presence of copper and calcium oxalates within the stratigraphy of the painting. Further, micro-Raman spectroscopy was employed for the analysis of unembedded fragments of the painting, confirming the presence of both copper oxalate and calcium oxalate. Finally, novel methods for the removal of salt interferences were employed for analysis using Gas Chromatography-Mass Spectrometry (GC-MS), which revealed the presence of both oxalate and phosphate ions and, following multivariate analysis of the amino acid profile, identified casein as the binder of the paintings. The imaging of calcium oxalates within the stratigraphy of wall painting samples is important and significant not only for the study of copper-based pigments in general, but especially for the analysis of pigments used for painting on exterior surfaces.