[Physics] Why did the thermos flask/jug explode after stirring with a stainless steel spoon

Question

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?

Answer 1

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.

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.