Semi-hollow or solid DURAN® glass stoppers can be used to close glassware items such as flasks, separatory funnels or reagent bottles with ground joint necks. The Standard Taper ground glass joint forms a tight, leak-proof seal. Use grease for heating or vacuum applications. With octagonal shaped head. Note on using DURAN® glass stoppers with soda-lime glass bottles: If the bottle and the stopper have a temperature difference greater than 30 °C, the glass stoppers can become stuck!