Semi-hollow or solid amber 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!