Watso I have an exciting update! I decided - after ‘fessing up to my fear this afternoon - to road test the grumpy old beast thermos after all.
And when I was pre-warming it, the metal neck exhibited a completely unexpected (by me) thermal principle on contact with the boiling water! …..It expanded! I was able to force in the cork more securely.
It was a Eureka moment although, as a security against any actual water displacement, I used the convenient hammer in the image below to thump the cork a a bit after I refilled it for my test, then screwed the steel beaker on tight in case the flask got moody in the night and tried to spit the dummy.
But it did not seem moody any more See how happy it looks! It was made for this
UPDATE - Just over three hours later, the beaker/lid and neck of grandfather vacuum flask are very warm :/ oh dear ….. Perhaps I should have used your ‘watso’s idea’ of extra insulation via a beer cosy, or perhaps woolly socks to assist with heat retention at this old Stanley’s potential ‘achilles heel’ ? Is this what Sir Edmund Hillary (with TWO LLs ) did in the Himalayas?
