Prince Rupert Drops

Glass can be a beguiling and sometimes counter-intuitive material, and this unassuming drop of glass is no different. To the casual observer, it might appear to be at rest. But inside, its molecules are under immense stress, being squashed together and pulled apart with bone-crushing forces, poised at any moment to shatter into a thousand pieces. Yet for all this microscopic drama, this material is actually a form of safety glass. When it comes to glass, looks can often be deceiving.

Its secrets date back to the court of King Charles II, around 1660. Prince Rupert of the Rhine was a German-English army officer and keen scientist. During his scientific tinkering, Rupert observed that a blob of molten glass dropped into water gains some odd behaviours. Once fished out of the bucket, the bulbous end of the teardrop-shaped solidified glass could be walloped with a hammer without breaking. But the drop had an Achilles heel; even the smallest knock to the thin tail end caused the whole thing to catastrophically shatter into powder. Rupert showed this curiosity to King Charles II, and the Prince Rupert’s Drop takes his name to this day (although it may well have been known to glassblowers before this official “discovery”).

At the molecular level, the rapid solidification of the outer surfaces of the molten glass droplet put the molecules there in compression; they are squeezed together far more than they would prefer to be. This makes the glass at the surface extremely strong and tough; any cracks which try to form on the surface are immediately sealed shut by the compressive forces. To counterbalance this, the molecules towards the centre of the droplet solidify in tension; being pulled apart further than usual. At the bulbous end, this delicate glass in tension is buried deep inside the droplet, but it comes dangerously close to the surface at the narrow tail end. Snapping the tail suddenly releases the tensions, which shatters the droplet into bits.

To make toughened sheets of glass (which is basically a flat Prince Rupert’s Drop), an ordinary glass pane is heated to over 600°C, and the surfaces rapidly cooled with compressed air. Toughened glass can most commonly be found in settings where impacts are more likely, such as car windows and windscreens, phone boxes, shop fronts and home conservatories.