Roman glasses coloured by dissolved transition metal ions: Redox-reactions, optical spectroscopy and ligand field theory

A collection of Roman glass samples from Ancient Messene, Greece, was analysed non-destructively for colouring elements and non-colouring additives such as fining agents, opacifiers and decolourizers, by comparing the chemical composition with the observed optical spectra. The resulting information on ion speciation is important for the discussion of technological parameters such as the applied melting temperatures or the prevalent redox conditions. Speciation also helps to distinguish intentionally added dopants from impurities. This knowledge might be used in provenancing the raw materials. The Roman assemblage under study included purple coloured Mn3+-containing glasses, but also samples with high levels of colourless Mn2+. Manganese ions can either be considered a residue of recycling, or are indicative of an intentional addition, either as a fining or as a decolouring agent. Antimony oxide was either added as a fining agent, resulting in good quality transparent glasses, or as crystalline antimonates which act in glass mosaic tesserae as opacifiers. The much weaker molar extinction coefficient and the partial reduction of blue Cu2+ to colourless Cu+ explain why CuO is a weaker colouring agent than CoO. The colours of iron-bearing glasses range from light blue to green and from yellow to dark brown, reflecting a complex interplay between redox conditions and the presence of sulphur in the glass melt.Quantitative speciation of dopants will be reviewed on the basis of the ligand field theory. The different transition probabilities will be demonstrated on differently doped soda lime silicate glasses which were prepared for this purpose in the laboratory. © 2014 Elsevier Ltd

Modification of silicophosphate glass composition, structure, and properties via crucible material and melting conditions

Abstract Ceramic crucibles are known to corrode in contact with glass melts. Here, we investigate the effect of alumina and fused silica crucibles on the composition, structure, and properties of silicophosphate glasses. Glasses in the system 0.3 Na2O‐0.6 P2O5‐0.1 SiO2 were melted in platinum, alumina, or fused silica crucibles at 900°C or 1200°C for 0.5‐12 hours. Al2O3 and SiO2 were found to leach from the crucibles into the glass melt and alter the glass composition: Al2O3 content increased with melting temperature and time, resulting in up to 10 mol% Al2O3; SiO2 from fused silica crucibles was also introduced into the glass, resulting in a 25% higher SiO2 content compared to the nominal composition. Glass density, transition temperature, thermal expansion, and mechanical properties were strongly affected by these compositional changes. Based on vibrational spectroscopy, this is explained by increasing numbers of P–O–Al or P–O–Si bonds, resulting in a depolymerization of the phosphate network, and ionic cross‐linking by high field strength aluminum or silicon ions. With increasing alumina content, P–O–Si bonds were replaced by P–O–Al bonds. 31P and 27Al MAS NMR spectra revealed that aluminum is present in sixfold coordination exclusively and fully bonded to phosphate species, connecting phosphate groups by P–O–Al–O–P bonds

Preferential bonding in low alkali borosilicate glasses

International audienceIn an earlier review we discussed the connectivity in borosilicate glasses and compared our experimental findings by NMR, infrared and Raman spectroscopies with older structural models. We could show, contrary to the often cited reedmergnerite type model, that a significant preference exists in low alkali borosilicate glasses for trigonal rather than tetrahedral borate groups to link to silicate entities. Another often cited misconception is the application of the Loewenstein rule to borate tetrahedra. While linking of two "[AlØ4]– tetrahedra is disadvantageous compared to higher coordinated aluminate polyhedra, the borate tetrahedral units represents already the alternate higher coordination state and accordingly, many examples of linked [BØ4]– tetrahedra are known to exist in glasses as well as in crystalline compounds of boron at normal pressure conditions. We now present more NMR data on three different low alkali borosilicate glasses with Na2O:B2O3=0·2 to 0·35 and decreasing SiO2 fractions (74 to 43 mol% SiO2), for which we compare variations in the near and intermediate range structure of quenched and slowly annealed samples. None of the studied glasses showed a significantly higher fraction of trigonal BØ3 groups in the quenched than in the annealed samples, even though borate in the three-coordinated state is the preferred metaborate unit in the melt. However, for the two silicate rich glasses (including NBS 2), we observe at low temperatures a deviation of the viscosity–temperature plot from the ideal VFT-fit, that is from Tg at circa 440 to the expected 600°C. For samples prepared at any annealing temperature below 600°C, structural variations with thermal history are apparent, and are also reflected in many glass properties including density, fracture probability, or refractive index. Even though the glass NBS 2 shows no visible phase separation, DSC measurements indicate the presence of two different Tg values corresponding to the borate and the silica rich subnetworks. The values of the two Tg events shift with different cooling rates: for fast quenched glasses the two Tg values are closer together than for slowly annealed glasses. The study of these low alkali borosilicate glasses is ongoing, as we understand better how structural variations with changing thermal history impact the glasses' properties. Analogously, we can apply the same techniques to follow structural variations under external forces, such as irradiation, laser modification, pressure and mechanical impact