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I always use a thermos jug when making tea. This time I filled the jug with tea leaves and hot water (around 100 degree celcius). I closed the jug and noticed that the tea was too weak therefore I opened the jug and added some more tea leaves and startet stirring gently with a stainless steel spoon. Almost immediately after the glass in the thermo jug exploded.

My husband is sure that it is common sense to not stir with anything in a thermo jug but I have never heard anything about it and couldn't find anything online.

I would really like to know why it exploded and if it applies to all kind of material or only stainless steel/metals? what about a wooden spoon?

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At the heart of a thermos jug lies a Scottish invention, the vacuum flask:

A vacuum flask (also known as a Dewar flask, Dewar bottle or thermos) is an insulating storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings. Invented by Sir James Dewar in 1892, 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 near-vacuum which significantly reduces heat transfer by conduction or convection.

Vacuum flask

Above, vacuum flask schematic (courtesy Wiki)

Vacuum flasks are great thermal insulators, capable of keeping your hot beverage hot for many hours but their structure makes them inherently fragile (which is why replacement vacuum flasks are for sale).

The near-vacuum that exists between the flask's walls makes the flask somewhat prone to implosion.

Stirring with an SS spoon is not recommended (although often will not actually cause a problem) because it can scratch the inside of the flask, making it even more vulnerable to implosion. Stir with a soft plastic or silicone rubber spoon.

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My thermos recently exploded at an elevation of 6,000ft and an ambient temperature of 50 degrees. However, when I filled the thermos with hot soup that morning, it was at an elevation of sea level, an ambient temperature of 68 degrees, and the temperature of the soup was about 180 degrees. When I closed the thermos in the morning, the air pressure acting on it from the outside was equal to the air pressure on the inside. However, when I reached an elevation of 6,000ft, the pressure acting on it from the inside was the same, but the pressure acting on it from the outside was significantly less. It's the same principle as airplane depressurization. Eventually, the difference in pressures overwhelmed the vacuum seal on the thermos and the conents exploded.

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  • $\begingroup$ The question was about explosions when stirring. $\endgroup$ – GiorgioP Jan 22 at 23:17

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