The Prince Rupert’s drops
Also known as Dutch tears, these magnificent drops are incredibly strong, up to a certain point.
Prince Rupert’s drops are created by dripping molten glass into cold water, which causes it to solidify into a droplet with a long thin tail.
These drops are so strong that they can withstand the force of a hammer, or even a bullet fired from a gun, but they do have a significant weakness.
They explode when the weak part is damaged
Even though the drop is extremely strong, the tail of the drop is the biggest weakness. If the tail suffers any damage at all, it sets of an reaction which causes the entire drop to explode.
When the molten glass is dripped into cold water, the water rapidly cools and solidifies the glass on the outside of the drop, while the inside remains molten. When the molten glass on the inside eventually cools down, it contracts and creates a significant compressive stress on the surface of the drop.
When the tail of the drop is damaged in any way, it releases this stress and potential energy in such an explosive way that the entire drop is immediately transformed into flakes and powder.
The drops are named after Prince Rupert of the Rhine, who brought them to england in 1660, although they were reportedly being produced in the Netherlands as far back as 17th century, and had probably been known to glass makers far longer than this.
They were studied by the Royal Society and their findings probably led to the development of the process for production of toughened glass, patented in 1874.