Electron paramagnetic resonance of mosaic glasses from the Mediterranean area*

Abstract

Ancient mosaic glass tesserae sampled from archaeological sites located in the Mediterranean area have been analysed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The aim of the research was to understand the role played by 3d transition metals in the optical properties of such samples. In order to better identify the specific effect of the main chromophorous element, modern samples have been prepared in the laboratory by adding to a colourless base frit controlled concentrations of colouring oxides. Samples with compositional and optical properties similar to the ancient ones have been obtained. For all these modern samples, as well as for all the ancient ones, electron paramagnetic resonance (EPR) measurements have been performed. The features of the EPR spectra could be successfully associated with the presence and abundance of the paramagnetic colouring ions (namely iron, manganese and copper) and to their oxidation states. This could be a basic step towards understanding the provenance and chronological attribution of mosaic glass tesserae.

The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of ‘gold’ tesserae

“…Little is known concerning the manufacturing techniques and production sites of ancient glass used in mosaics, so that these artefacts are an active subject of study and experimentation. Until quite recently, mosaics were generally studied in terms of their style or iconology, although, with reference to conservation issues, special attention is now being devoted to materials characterisation and glass working procedures (e.g., Freestone et al, 1990;Shugar, 2000;Azzoni et al, 2002;Fiori et al, 2004;James, 2006). In particular, due to their extraordinary variety of colours, the refined use of opacifying agents, the extensive use of metal coatings (gilding), and the complex technology of thermal redox treatments optimised in kilns, glass polychrome mosaics, such as those of the palaeo-Christian votive chapel of St. Prosdocimus in the Basilica of St. Justine (Padova) which are the subject of the present study, represent a stimulating yet challenging field of research in materials science.…”

Thermoluminescence and visible reflectance spectroscopy applied to the study of blue‐green mosaic silica‐glass tesserae

“…Both TSL and reflectance measurements, strongly depending on the presence in the matrix of very small concentrations of impurities, have shown their potentiality and can add useful information to the optical properties of these materials. Ancient mosaic glasses are under study for their promising characteristics for TSL dating [1] and have been deeply analysed before [2]. The ancient blue tesserae, whose coloration is due to cobalt and copper ions, usually have relatively high TSL sensitivity that was tentatively attributed to the colour centres induced by specific colouring ions.…”

“…The material of the entire ancient set, coming from the Mediterranean area and dating back to V-XIII century A.D., are defined as soda-lime silica glass. Their detailed description and characterisation is reported elsewhere [2]. The modern samples are instead lead oxide glass (the soda and lime fluxing agents are not used in modern industrial production), and cover a wide range of blue, bluish, green and greenish shades.…”

Thermally and optically stimulated luminescence of early medieval blue-green glass mosaics

“…EPR analyses appeared to be much useful than the elemental ones to understand the TSL processes in glasses (Azzoni et al, 2002), that are often associated to the oxidation states of the ions, and not to their total concentration in the glassy matrix.…”