Pressure Equalization Set, for DURAN® HPLC screw cap, incl. 0 Also called marking field, white marking spot or marking square. Material: PP, BORO 3.3, Silicone: Boro 3.3: Borosilicate glass 3.3 has a very good chemical resistance, virtual inert behavior, a high maximum usage temperature, high light transparency and minimal thermal expansion.