Chemical analyses of Bronze Age glasses from Frattesina di Rovigo, Northern Italy

Within the frame of a project aimed to systematically characterize Bronze Age glasses in Northern Italy, a few glasses from Frattesina (Rovigo, Italy) were chemically analyzed by wavelength-dispersive electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS). Trace chromofores were also analyzed by atomic absorption spectroscopy (AAS) and X-ray photoelectron spectroscopy (XPS). Frattesina di Rovigo (Final Bronze Age (FBA), 1200–1000 BC) is at present the only recognized European locality showing evidence of glass working in the Bronze Age. The results of the investigation, which was initially carried out for standardization and comparison purposes of the chemical analyses of coeval glasses, yield interesting information if interpreted on the basis of the available chemical data from other Northern Italian and European glasses. The protohistoric glasses from Frattesina were likely produced with a rather advanced production technology. At least three different source materials were used, besides the pigments: a nearly pure source of silica, a source of stabilizers (mainly CaO, MgO), and a source of monovalent alkalis (Na2O, K2O) used to lower the softening point, possibly derived from wood ashes. The restricted range observed for the total alkali content and the presence at Frattesina of two classes of materials characterized by distinct Na2O/ K2O ratios indicate a rigorous control of the physical properties of the glass, and the availability of at least two different sources of alkaline elements. The color of the glass was determined by the use of chromophoric elements (especially Cu, Co, Fe, Mo) and by skillful control of oxidation/reduction conditions during the working process. A thin red coating on bulk blue glasses was obtained by exposing the material to reducing conditions and by inducing the exsolution of crystals and crystal aggregates of metallic copper in the surface layer. The metallic particles have been characterized by transmission electron microscopy (TEM) and have dimensions of the order of 0.2–0.4 mm. The technique is surprisingly similar to the one used to obtain metallic luster in renaissance glazes and optoelectronic effects in modern composite nanomaterials.