EPR of mosaic glasses from the Mediterranean area 543 Archaeometry 44, 4 (2002) 543–554. Printed in Great Britain * Received 23 May 2001; accepted 15 May 2002. † Present address: Centro di Studi sulla Corrosione ‘A. Daccò’, Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, I-44100 Ferrara, Italy. © University of Oxford, 2002 ELECTRON PARAMAGNETIC RESONANCE OF MOSAIC GLASSES FROM THE MEDITERRANEAN AREA* C. B. AZZONI and D. DI MARTINO INFM and Dipartimento di Fisica ‘A. Volta’, Università di Pavia, Via U. Bassi 6, 27100 Pavia, Italy C. CHIAVARI,† M. MARTINI and E. SIBILIA INFM and Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Via R. Cozzi 53, 20125 Milano, Italy and M. VANDINI Dipartimento di Storie e Metodi per la Conservazione dei Beni Culturali, Università di Bologna—sede di Ravenna, via degli Ariani 1, 48100 Ravenna, Italy Ancient mosaic glass tesserae sampled from archaeological sites located in the Mediter- ranean 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. KEYWORDS: MEDITERRANEAN AREA, MOSAIC, GLASSES, CHEMICAL CHARACTERIZATION, ELECTRON PARAMAGNETIC RESONANCE INTRODUCTION The coloration of glass is the result of complex interactions between structural and chemical effects (for a general review, see Weyl 1959). In particular, the presence and the relative concentration of 3d transition metals play a fundamental role, acting as chromophores in the glass base (Sanderson and Hutchings 1987). Whether they occur as impurities in the raw materials or as deliberate additions, the light absorption in the visible spectrum is due to the electronic transitions between the d-orbitals of the chromophores. In addition, their interaction with anions such as oxygen, known as ligands, modifies the d-energy levels and, therefore, the