In addition cork is readily shapable, usable. Historically cork would have been the go to bung making material and flask makers would be equipped with machines to turn cork into bungs for lids. read more
I can only assume this will be about the sealing bung in the screw lid of the vacuum flask. The job the cork bung is supposed to be doing is sealing, with a modicum of insulation provided too. read more
To create vacuum. Heat doesn't travel from one side to the other (in either direction) if there is no matter. read more