Zirconium fluoride is a volatile component of ZrF 4-based glasses above 450 ° C. A high temperature mass spectrometer measured the relative rates of evolution of the gasses that vaporized from ZBL and ZBLAN glasses. ZrF 4, ZrClF 2, and ZrClF 3 vaporized under different conditions; their presence was confirmed from the isotopic ratios expected from natural zirconium and chlorine. The chlorides result from chlorine gas used during glass melting to provide a reactive atmosphere. Atomic fluorine, rather than molecular fluorine, also vaporized as expected from thermodynamic calculations for the low pressures in the mass spectrometer. Crystals of LaF 3 and AlF 3 grew around evaporation sites. The weight loss from evaporation was proportional to time for ZBLAN glass and perhaps proportional to square root time for ZBL glass.