Three Anisotropies in Vitreous Silica

Plasma-torch fused-silica boules of low OH content showed an optical anisotropy in the as-grown state which disappeared after annealing in O₂; this behavior was related to an EPR anisotropy which also depended on annealing, probably involving Fe²⁺ and Fe³⁺, respectively. High-temperature (1200° to 1500°C) deformation experiments performed in a modified DTA machine on slabs cut parallel and perpendicular to the preparation symmetry axis also showed an anisotropy, corresponding approximately to a factor of 2 in the viscosity. This anisotropy was also observed in commercial worked fused silica of high and low OH (water) content, made from purified Si compounds or natural quartz, and was not removed by annealing for 5 days at 1200°C in O₂. The deformation rate of fused silica decreases as water content is reduced from high to medium, but there is little further change when water content is decreased to a very low level.