Flat flange vessels are usually large volume reaction flasks which can be used as an alternative to standard round flasks. The wide neck opening of flat flange vessels allows easy access to the flask contents. They consist of a reaction vessel and separate lid. The two parts are held together by means of a clamping ring. The flange on these reaction vessels has a groove in which a silicone (VMQ) or FEP O ring can be fitted to improve the flange seal. Flat flange vessels are well suited for preparative work such as general reactions, distillations, evaporation and drying of solid and viscous substances. Their heavy wall construction and geometry make these vessels safe for working under positive pressure and vacuum.