Multianalytical characterisation of a medieval red opaque glass tessera of Florentine manufacture

The cathedral in Florence was designed by Arnolfo di Cambio, famous architect and sculptor. The plans for the new cathedral, started in 1296, included the construction of the main façade which was begun but never completed. In 1587 the unfinished façade was completely dismantled with the purpose of building a new one. A red opaque glass tessera belonging to a medieval mosaics of the first façade, was investigated with the aim of providing more information on the glass manufacturing in Florence during the end of the Middle Age. The red/orange glass production has generated considerable interest due to the high complexity of the technical achievements and the multiple technological solutions used. The tessera was investigated by means of a multi-analytical approach, including micro-morphological and microchemical FEG-SEM investigations, and synchrotron radiation X-ray absorption and X-ray Fluorescence techniques. Optical and scanning electron microscopy show that this red medieval tessera has a inhomogeneous microscopical structure due to the presence of some optically transparent stripes within the main red stripes in which a bimodal distribution (~30 and ~160 nm) of copper nanoparticles is responsible for the red color. EDS raster microanalyses indicate that the tessera is a soda-lime-silica glass which has probably been produced with plant ash flux, with a particularly high MgO content (XX%). Moreover, an anomalously high Fe-content (6 wt%) was observed. X-ray Absorption Spectroscopy (XAS) and ?X-ray Fluorescence maps were performed to identify Fe and Cu speciation and distribution in the sample with the aim to verify if the formation of the copper nanoparticles is due to the reduction of copper oxides at the expenses of the oxidation of iron (II) species to iron (III) during the glass production process. The combined ?XRF and XAS information reveal that the transparent and red stripes regions in the tessera exhibit different redox balance of the main species: Cu(I)/Cu0 and Fe(II)/Fe(III). In the transparent stripes at least ½ of the total Cu is present as Cu(I), and ~20% of the total Fe as Fe(II). In the red stripes Cu is almost completely reduced, and the Fe(II) content decreases to ~10%. Conversely, ?XRF and XAS reveal a low concentration of MnO, homogeneously distributed throughout the whole sample, always present in its divalent state. A thermodynamical modelling of the chemical and redox evidences has been undertaken and a preliminary explanation will be provided.