Raman analysis of silica glasses with composition typical of the Roman age

Raman spectroscopy is one of the most used techniquesto investigate glasses and amorphous materials and in the field of art and archaeology1-2. A method able to correlate the position and shape of Raman bends of silica glasses with their structure was proposed for the first time by Colomban et al. [3], based on the band area ratio (A500/A1000) of the Si–O bending at 500 cm-1and stretching at about 1000 cm-1. This method allows to calculate a polymerization index, useful to have a clue on the fictive temperature of the glass. Furthermore, by the analysis of the wavenumber and relative amplitude of the different Si–O stretching components, the amount of the Qn units constituting the glass can be estimated. In this work a series of glass samples resembling the composition of ancient Roman ones was synthesized in order to investigate the dependence of the Raman spectra on glass compositions and fabrication procedures (fluxing agents and quenching environment). The influence of the experimental setup (kind of spectrometer and, mostly, excitation laser line) is also investigated. The first results suggest that the polymerization index,determined as the ratio between the Si-O bending and stretching bands, does not increase, as expected, with increasing “glass formers/glass modifiers” ratio. This is true at least for the investigated range of compositions, resulting sometimes in lower bending bands for increasing amounts of SiO2. On the other hand, the spectral deconvolution of the Si-O stretching band has shown to be very effective in the identification of Qn units. The population of Q1 and Q2 modes is higher when a large amount of modifiers is present while Q3 and Q4 contributions dominate the stretching band for higher amounts of silica. This is a powerful tool for the study of the glass structure.The spectral resolution of the spectrometer, and in particular of the CCD detector, largely influences the relative intensity of the different parts of the glass spectra. In particular, blue excitation enhances the high wavenumber bands, as expected. A suitable calibration is necessary before the comparison of the spectra obtained with different laser lines. [1] Colomban, P., Tournie, A., Bellot-Gurlet, L., J. Raman Spectrosc., 2006, 37, 841–852 [2] Colomban, P., Journal of Cultural Heritage, 2008, 9, e55-e60 [3] Colomban, P., J. Non-Crystalline Solids, 2003, 322, 180-187