The transmittances and reflectances of cultured crystalline quartz, Suprasil, Suprasil W, and Infrasil were compared over the wavelength region from 2 to 1000 µm. The high-water content of Suprasil and the low-water content of cultured crystalline quartz, Suprasil W, and Infrasil were determined by their transmittances measured at 2.73 µm where water content causes high absorption in optical materials. The fact that the fused silicas, both with high- and low-water content, had identical far-IR transmittances and that their transmittances were greatly inferior to that of crystalline quartz led to the conclusion that their inferior transmittance is due to their amorphous structure and not to their water content.
James B. Heaney, K. P. Stewart, and Georg Hass, "Transmittance and reflectance of crystalline quartz and high-and low-water content fused silica from 2 µm to 1 mm," Appl. Opt. 22, 4069-4072 (1983)