Loss of Ag and Cl from Photochromic Glasses during Melting in Various Atmospheres

Abstract

Batches of photochromic glasses (about 100g) containing various amounts of Ag and Cl (the nominal oxide compositions, SiO2 52.63, B2O3 20.09, Al2O3 6.89, Li2O 2.90, Na2O 1.80, BaO 8.18, PbO 4.81, ZrO2 2.10, Cu2O 0.014, Ag 0.44-5.80, Cl 0.15-2.18wt%) were melted at 1400° or 1450°C in the air or in the air saturated with AgCl, NaCl or BaCl2 vapor and amounts of Ag and Cl retained in the glasses after melting for various times were measured by X-ray fluorescence analysis.1) The glass batches containing large amounts of AgCl (more than about 3wt%), when melted in air, first separated into two liquid phases; a phase rich in AgCl and that rich in silicate. The latter remained as glass after cooling to room temperature. A great part of the AgCl added to the batches was thus lost almost instantaneously from the glass. After the phase separation, losses of the Ag and Cl from the glass melts continued probably by evaporation of chlorides such as AgCl and NaCl from the melts and also by oxidation of the chlorides in the melts by air: 4AgCl+O2→2Ag2O+2Cl2↑ In the latter mechanism only the Cl was considered to be lost because the Ag2O formed has high solubility in the glass melts. Loss rates of the Ag and Cl were both low but that of the Cl was a little higher than that of the Ag.2) The glass batch containing small amounts of AgCl (less than about 0.5wt%) showed no liquid phase separation but losses of the Ag and Cl occurred at slow rates as observed for the batches containing large amounts of AgCl.3) When the glasses were melted in the air saturated with AgCl vapor, the Ag contents increased rapidly, whereas the Cl contents were held constant. When the glasses were melted in the air saturated with NaCl vapor, the Ag contents decreased at slow rates, whereas the Cl contents were held constant. The loss rate of the Ag in the air saturated with NaCl vapor was a little higher than that in air. Effects of the air atmosphere saturated with BaCl2 vapor were intermediate between those in the air and the air saturated with the NaCl vapor.The above results were explained by supposing suppressing effects of the chloride vapors in the atmosphere on evaporation of the chlorides from the molten glasses and oxidation of the chloride vapors by air.4) Mechanical stirring of the molten glass did not give remarkable effects in accelerating evaporation of the chlorides from the molten glasses.