These Pyrex® volumetric flasks are made from low actinic stained glass to provide protection for materials sensitive to light. The protective color is an integral part of the flask, which retains its mechanical strength, chemical stability and thermal resistance. The strength of these flasks has been increased appreciably through machine-blown bodies to which are sealed heavy-tubing necks tooled for ST glass stoppers. For replacement stoppers see Cat.No. 1-7650. The graduation line is sharp and permanent and large white block letters make the inscription easy to read. These flasks have been manufactured to Class A tolerances as established by ASTM E- 694 for volumetric ware, ASTM E-542 for calibration of volumetric ware and ASTM E-288 for volumetric flasks.