How is a vacuum flask made?

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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?

Blow two flasks, one bigger, one smaller

Crack off the bigger one at both ends, the smaller only at the neck

Now we have an inner flask, and one outer, open at both ends

Position the smaller inside the bigger, with asbestos studs

Close the outer bottom by melting, blowing from the neck end so the flasks stay separate

Melt and blow a hole at the bottom of the outer flask

Fasten a glass tube to the hole

Smelt the tops of both bottles to seal

Add silver nitrate solution, rotate and then heat

Evacuate the air and seal the tube

Done!!

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...)

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.

på svenska För andra språk erbjuder vi/ for other languages we offer Start page for the museum About us Machines and artifacts in our museum About the building Glass as a material, and the glass industry About hot glass techniques - Glass melting furnaces - Glass pots - Pressing, a general descripion - Swedish pressed glass - "Baking pancakes" - Centrifuging - Vacuum flasks About cold glass techniques Links Literature list Downloads, catalogues (opens in new window) museibloggen (written in Swedish) Facebook group for the Museum Contact us How to find us More about Bergdala	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...
	So: how are they made?
	Blow two flasks, one bigger, one smaller	Crack off the bigger one at both ends, the smaller only at the neck	Now we have an inner flask, and one outer, open at both ends
	Position the smaller inside the bigger, with asbestos studs	Close the outer bottom by melting, blowing from the neck end so the flasks stay separate	Melt and blow a hole at the bottom of the outer flask
	Fasten a glass tube to the hole	Smelt the tops of both bottles to seal	Add silver nitrate solution, rotate and then heat
	Evacuate the air and seal the tube	Done!!	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...)
	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.