The origin of the blue colour of Bronze Age vitreous materials: a spectroscopic approach

An X-ray Absorption Spectroscopy (XAS) and Electron Paramagnetic Resonance (EPR) investigation of protohistoric blue vitreous materials was undertaken, aimed at ascertaining the valence state speciation of Cu and Co, suspected to play a role in the colour origin. Five different glass artefacts coming from Paduli (Colli sul Velino, Rieti, Italy) were investigated. A bichrome blue and white vessel fragment represents the only Natron glass. The other four beads, instead, are LMHK glass. A relevant question deals with concentration, distribution, and valence states of the transition elements Cu and Co. Two out of the five colored objects, in fact, contain only Cu, whilst the others exhibit both Cu and Co. A sample holder was specifically designed to ensure minimal invasiveness during XAS measurements. Multiple measurements (up to six) were performed for each sample at the Cu and/or Co edges to verify sample homogeneity. Fragments of the samples, when available, were investigated by EPR without manipulation to further characterize the Cu^II aliquot in the materials. The XAS spectra provided significant information confirming the presence of the Co^II chromophore in the samples where this species is chemically more abundant, and identifying and quantifying the presence of the Cu^II chromophore. Cu^II, as revealed by EPR, appears in a distorted (4 + 2)-fold coordination and partly clustered to form pairs. This spectroscopic approach, combining XAS and EPR techniques, proves to be successful in the characterization of Co- and Cu-based blue colors in the glasses of the Bronze Age, highlighting the high skill reached in the production.