Prince Rupert's Drops

Though the underlying mechanism was not known at the time, the effects of "toughening/tempering" glass have been known for centuries. In the 1640's Prince Rupert of Bavaria, who was grandson of James I of England and VI of Scotland, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable tear-drop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. The resulting tear drop shape is very strong, resisting hammer blows. But breaking the tail causes the whole to shatter.