Glass characterization through spectroscopic investigations of the Co(II) complexes

Since ancient times cobalt compounds have been extensively used to obtain dense blue colorations [1]. Even though the references about the first use of cobalt in glass are not in agreement, it could be attested between the XII and XV centuries [2]. Starting from the XVIII century, the developments in chemistry introduced the use of pure CoO and cobalt became the most diffused coloring agent used to produce blue glasses. Optical spectroscopies are a useful and non-invasive tool for the investigation of glass and the present study aims to focus on the application of Fiber Optic Reflectance Spectroscopy (FORS). As changings in glass manufacture along the centuries (mainly concerning the use of fluxes) are considered fundamental to evaluate the period of glass production [3], the effects of different fluxes on the absorbance spectra of some modern blue glasses have been studied. The absorbance spectra in the visible and near-IR regions of glasses containing Co(II) complexes are characterized by the peculiar absorption bands due to ligand field d-d electronic transitions [3] and the positive/negative band shifting due to the presence of different fluxes (potash or soda) have been discussed in literature [4,5]. The relationships between the silica network, the different fluxes and stabilizers and the cobalt complex has been studied and identified as the main reason for some discrepancies in literature [3, 4]. The results also suggested a relevant role of the flux and other modifying agents, such as zinc oxide (an important marker for modern glasses), on the equilibrium between tetra- and esa-coordinated Co(II), thus influencing the position and intensity of the sub-bands in the visible region. In conclusion, the present work aims to offer a more detailed investigation of the spectroscopic features of cobalt glasses, suggesting an improved tool to discriminate between ancient, early modern and modern glasses.