Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption

Glasses of the systems Bi2O3–SiO2, Bi2O3–PbO–Ga2O3, Bi2O3–PbO–Ga2O3–GeO2 and Bi2O3–GeO2–Li2O have been prepared and the interaction of their melts with crucibles of different materials has been analytically determined. Silica and porcelain crucibles were very strongly corroded and the glass composition was noticeably altered. Instead platinum crucibles are not affected if the Bi2O3 content is not too high. The color of the glasses changes in all cases from pale yellow to deep brown when the melting temperature reaches approximately 1000 °C. The higher the temperature and the Bi2O3 content the darker the brown color, independently of the nature of the employed crucible. The addition of oxidizing ions (Sb5+, As5+ or Ce4+) to the glass batch prevents darkening. Nanoparticles of elementary bismuth Bi0 are observed by transmission electron microscopy in the glasses melted above 1000 °C. The partial thermoreduction of the Bi2O3 during the heating of the glass melt is proposed as the mechanism responsible for the observed darkening.