Outgassing of Glass

Abstract

The gas evolved from glass at temperatures below the softening point, which is of interest in bake‐out problems, is primarily water. The water evolved from unit surface at constant temperature above 300°C is linear with respect to the square root of the time of bake‐out. The intercept of the linear plot, which can be altered by different surface treatments, is a measure of the easily removed water residing at the surface. The slope is a measure of the rate of evolution of water that has diffused to the surface from the interior. Values of the diffusion constant for water and concentration gradients after bake‐out have been calculated for a soda‐lime glass. The diffusion constant is an exponential function of the reciprocal of the absolute bake‐out temperature. Values of the activation energy for the diffusion process are given for eight glasses. A method of calculating the amount of water that will diffuse from glass for any time‐temperature conditions following any bake‐out is presented.