Calibrated to Class A tolerances in accordance with ASTM E-542 and E-288. Each flask is individually serialized. The graduation line is sharp and permanent; large white block letters make the inscription easy to read. Supplied with a standard taper stopper. Low actinic stained glass provides protection for materials sensitive to light. The protective color is an integral part of the flask, which retains the mechanical strength, chemical stability, and the thermal resistance of PYREX brand labware. Tested and calibrated in an ISO/IEC 17025 certified laboratory.