PYREX® Class A Volumetric Flask With Polyethylene Standard Taper Stopper manufactured by Corning features machine-blown bodies sealed to heavy-beaded, heavy-tubing necks for increased strength. The close-bottom stopper is made of high density polyethylene to conform to standard taper stopper dimensions. PYREX® Class A Volumetric Flask also features sharp and permanent graduation line and large white block letters for easy reading.