Etching quartz crystals in anhydrous HF gas

Exposing quartz crystals to anhydrous HF gas at 400°C results in a high density of helical etch channels, a white crusty surface, and other unusual features. The treatment results in a negligible thickness change. There is no similarity between the characteristics of these helical etch channels and the conventional etch channels produced by liquid etchants, nor is there a correlation between the incidence of helical etch channels and the type of quartz, or the surface finish. Exposure of quartz blanks to HF/H₂O vapors emanating from a 49% aqueous HF solution at ~20°C resulted in conventional etch channels, faceting, and measurable frequency changes, indicating that the presence or absence of water is responsible for the differences in etching behaviors