Silica glass might be crystallized from the surface under existence of contaminants such as alkali metal compounds at high temperatures.1) The crystallization of glass from the surface upon heating is called devitrification. Devitrification of silica glass depends closely on the temperature, the holding time, impurities in the silica glass, and contaminants2) from the ambient materials. Wagstaff et al. reported dynamics of devitrification of silica glasses.3)6) They measured the crystallization rate of stoichiometric and oxygendeficient silica glasses. Under a humidified atmosphere, the activation energy of the crystallization becomes approximately half that in a dry atmosphere. In general, vitreous silica is crystallized at temperatures higher than about 1100°C without contaminants.7) Therefore, the studies reported in the literature are limited to temperatures higher than approximately 1200°C. Devitrification is promoted when in contact with alkali metal compounds; amorphous silica is devitrified at temperatures as low as 1000°C when in contact with alkali metal compounds.8)11) Bassett12) and Higuchi13) reported crystallization of silica glasses conducted with alkali metal compounds of various types. These studies specifically addressed the phase transitions of the silica crystal, and did not address the dynamics of the crystallization. In these studies, silica glass powder was used for quantitative analyses to assess the dynamics of the devitrification; plate-type samples must be used, as in the case of the study of Wagstaff and colleagues described above. Therefore, in this study, devitrification of the silica glass occurred when a grain of NaCl crystal contacted the silica glass plate. Four commercially available silica glasses provided by Tosoh Corp.®ES, ED-B, HR, and N®were used for samples. The ES and ED-B are synthetic fused silica containing ca. 1000 and <1 wt. ppm of OH, respectively. The N and HR are produced respectively by melting quartz powder containing ca. 200 and 1 wt. ppm of OH. These samples were cut to 20 © 20 © 1 mm and optically polished on two facing surfaces. The NaCl crystal grain was placed on the center of the silica glass substrate. Then the substrate was heated in a tubular electric furnace (type ARF-50K; Asahi Rika Co. Ltd.). The soaking area length («5°C) was 62 mm. The temperatures can thereby be controlled to within «2°C. The heating time was counted after the furnace temperature reached the target temperature. Heat treatments are repeated after measuring the morphology and depth. The heat-treated sample was cooled to room temperature without temperature control before measurements. The devitrification depth was determined from the distance from the backside of the substrate to the bottom of the devitrification region using a digital optical microscope (VH7000; Keyence Co.) with correction of the effect of the refractive index. Figure 1 portrays a photograph of the devitrification of samples that had been heat-treated at 1150°C for 10 min. The devitrified region consisted of double concentric circles around the NaCl grain (0.14 « 0.01 mg). Cross-section profiles of the devitrified region of ED-B heat-treated for 8 h at various temperatures are presented in Fig. 2. The mass of the NaCl in this measurement was 0.12 « 0.01 mg. At 800°C, the devitrification profile is flat throughout the devitrified region. At temperatures higher than 1000°C, the