High-sensitivity Raman imaging of the surface of casted glass plates

Glass manufacturing processes are prone to induce local fluctuations of the glass properties, to which Raman spectroscopy is highly sensitive. In this work, Raman imaging is used to investigate the homogeneity of the Raman response at the surface of casted aluminosilicate glass pieces. Samples were probed at constant focus depth across 7 x 7 cm(2) surfaces using 500 mu m spatial steps, resulting in unusually large and detailed Raman images. We show that the extent of modification of the Raman response is small across the scanned area and that the information is mostly carried by the spatial representation of properly selected Raman parameters. Specifically, detailed macroscopic patterns correlating to the glass casting process were obtained from Raman parameters of two distinct Raman modes: the Si-O stretching mode involving Q(2) tetrahedral units, and the Si-O-Si bending vibrations envelope in the low-wavenumber range. The contrasts on the Raman images are assigned to fine local variations of fictive temperature (and hence of cooling rate) and chemistry resulting from the manufacturing process. From the evolution of these parameters across the surface of the sample, we were able to identify areas consisting of material from different stages of the casting. Structural and chemical changes originating from the manufacturing process are therefore printed at the surface of the glass pieces, and their fingerprint revealed by Raman imaging.