Supplied complete with a special open top cap which prevents the pressure in the vial from exceeding 150 PSIG. Made from 33 expansion borosilicate glass which conforms to ASTM e-438 Type 1-class a, USP Type 1 and ISO 3585. The high resistance to thermal shock along with its maximum chemical resistance makes this the glass of choice for reaction vials along with most all laboratory glassware.