Bung

A bung is an apparatus used to seal a container, such as a bottle, tube or barrel. Unlike a lid which encloses a container from the outside without displacing the inner volume, a bung is partially inserted inside the container to act as a seal. The lids for safety overpacks for 55 gallon drums sometimes may have a bung built in  for access of the contents of the container. These may be referred to as fuel bungs.

The most common every-day example of a bung is the stopper of a wine bottle. Other bungs, particularly those used in chemical barrels, may be made of metal and are screwed into place via threading.

Rubber bungs
In chemistry, bungs are usually made of hardened rubber. Some chemistry bungs may also include one or more holes so a glass tube or laboratory funnel may be inserted through the bung and into the container or another piece of apparatus. The rubber bung may be used to seal a flask because the user may require the contents to be mixed via shaking the flask or may require that the contents be kept inside the flask and prevented from leaking out. In all cases, the bung keeps the experimentation environment completely sealed so that liquids or gases cannot escape.

Apart from self-made glassware, rubber bungs are rarely used now. They have been supplanted by laboratory glassware which come with standard ground glass joints which allow the pieces to be fitted together easily. Ground glass joints are also much more resistant to solvents and chemicals used.

Plastic bungholes are commonly used to stop the air leaking from the lower parts of an air-bed. This type of bung is commonly made for two parts; the plastic outer screw on cap and the inner rubber bung used to seal the hole.

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