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- Glass melting furnaces
- Glass pots
- Pressing, a general descripion
- Swedish pressed glass
- Centrifuging
- Vacuum flasks

Vacuum flask - thermos - Dewar flask

Practical to take on picnics, to have by the computer...
For warm beverages, for warm food, for cold beverages, cold food...

Often, we think about their (designed?) outside, but how often do we think about how it is constructed?

The base for the vacuum flask is a double-walled glass container. It is silvered between the walls, and then the air is evacuated.

The first isolating containers were contructed by the Scottish scientist John Dewar at the end of the 1800s.

They were soon a success: already in 1915 Termoverken in Jönköping started the manufacture for the Swedish market. There have been four vacuum flask factories in Sweden: Termoverken in Jönköping were the first, then Trelleborgs glasverk, AB Termoindustri in Kalmar and lastly Jönköpings vacuumindustri. The last factory for vacuum flasks in Sweden was closed in the 1960ies.

Curiosity: they are often called TV-pitchers here in Sweden. Most of us have probably thought that an example of the ubiquitous TV "everything" - TV tray, TV dinner...
But: we were all wrong. It started (albeit in the early TV era) as a trade mark for TermoVerken i Jönköping...

TeVe-kannan Kokhett from TermoVerken i Jönköping

So: how are they made?

picture 1
Blow two flasks, one bigger, one smaller
picture 2
Crack off the bigger one at both ends, the smaller only at the neck
picture 3
Now we have an inner flask, and one outer, open at both ends
picture 4
Position the smaller inside the bigger, with asbestos studs
picture 5
Close the outer bottom by melting, blowing from the neck end so the flasks stay separate
picture 6
Melt and blow a hole at the bottom of the outer flask
picture 7
Fasten a glass tube to the hole
picture 8
Smelt the tops of both bottles to seal
picture 9
Add silver nitrate solution, rotate and then heat
picture 10
Evacuate the air and seal the tube
picture 11
glass flask from Trelleborg
A vacuum flask "inner" from Trelleborgs glasverk
But: the thermos is not quite done: the glass flask has to be mounted in a suitable container to protect it, get some kind of stopper and some kind of top/cup.
From the beginning the container was often made of corrugated sheet metal. Sometimes the bottom was made to open (to make it possible to mount a new inner flask). Sometimes it had threads at the top, or the middle - on nearly all older thermoses it is possible to mount a new glass flask if the old was bust. Containers have been made in Bakelite, plastic, steel... Nowadays the outer shells are seen as design products, but at the heart of the best vacuum flasks still is a double-walled glass cylinder.
("Best"? Yes, all who have ever tried a vacuum flask all made of plastic, or all made of steel, have noticed that those do not keep the heat for very long...)
the bottom can be taken off

There have also been lots of variations to the stoppers: to begin with, they were made of natural cork.
AB Termoindustri in Kalmar put a twist to the natural cork: they put a spring at the top, to help keep the stopper tight with the top/cup.
Later, stoppers have been hinged, often with some plastic gasket.
Many manufacturers have used expansion stoppers - a good idea, in principle.

various kinds of stoppers