<P> The vacuum flask consists of two flasks , placed one within the other and joined at the neck . The gap between the two flasks is partially evacuated of air , creating a partial - vacuum which reduces heat conduction or convection . Heat transfer by thermal radiation may be minimized by silvering flask surfaces facing the gap but can become problematic if the flask 's contents or surroundings are very hot ; hence vacuum flasks usually hold contents below the boiling point of water . Most heat transfer occurs through the neck and opening of the flask , where there is no vacuum . Vacuum flasks are usually made of metal , borosilicate glass , foam or plastic and have their opening stoppered with cork or polyethylene plastic . Vacuum flasks are often used as insulated shipping containers . </P>
what is the use of silvering the walls of thermos flask
Heat transfer by thermal radiation may be minimized