Relation of Dissolved Nitrogen to Phototropy of Reduced Silicate Glasses

Soda‐silica glasses melted under strong reducing conditions (graphite crucible) contain the precursor of a 570‐nm color center. Glasses thus prepared under an N 2 atmosphere are readily colored by exposure to ultraviolet light; the uv‐generated color fades rapidly at room temperature, giving phototropic response. However, with glasses melted under N 2 ‐free atmospheres (for example, Ar), this phototropic response is greatly attenuated or absent. The precursor of the 570‐nm center evidently results from strong reduction of the glass independent of the covering atmosphere, since X irradiation generates the same characteristic optical absorption in glasses melted under both N 2 and Ar. For compositions melted under N 2 , the amount of N dissolved correlates positively with increasing SiO 2 content and phototropic response. The dissolution of N markedly affects the uv absorption of otherwise “clean” glass. An N‐free reduced soda‐silica glass exhibits essentially the same uv absorption as a similar oxidized glass. However, as N dissolves in the glass, the uv cutoff moves to longer wavelengths. The phototropic response of reduced glasses under long‐wavelength uv light is greatly enhanced by incorporation of europium. Again, an N 2 atmosphere during melting yields improved phototropic response. Dissolved N lowers the dissolved H 2 O content and increases the concentration of Eu in the lower valence state (Eu 2+ ).