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A Prince Rupert's Drop is a very intriguing glass curiosity which exhibits a number of unusual properties. These glass oddities are also known as Dutch tears, Batavia's tears, Tzar's tears, or Prince Rupert's balls, and they are often on display in glass museums. The precise nature of the Prince Rupert's Drop was a mystery for some time; only after the development of high speed video did people really understand what was going on when a Prince Rupert's Drop was broken.
To make a Prince Rupert's Drop, a small piece of molten glass is dropped into a bucket of extremely cold water while the glass is still hot. The glass forms a characteristic tear-drop shape, with a fat head and a long, curving tail. The cold water causes the outside of the glass to cool extremely quickly, while the hot glass inside the head takes longer to cool. As it cools, it contracts, and creates a state of tension in the Prince Rupert's Drop.
No matter how hard you try, you cannot break the head of a Prince Rupert's Drop. The glass can be hammered, pinched with pliers, and run through a variety of mechanical trials, but the glass will still hold, because the state of tension is so high. If, however, the tail of the Prince Rupert's Drop is damaged in any way, the entire object will explode, because the tension has been broken. And “explode” is really the right word to use, as a Prince Rupert's Drop will shatter explosively when the tail is damaged.
The Prince Rupert's Drop is named for Prince Rupert of the Rhine, a German prince who has been credited with the development of this glass curiosity. According to legend, Prince Rupert enjoyed using the Prince Rupert's Drop as a practical joke, handing them to members of the court and then yanking on the tail so that the glass would explode in the hand of the unwary victim. One might assume that people grew wary of accepting presents from the fun-loving prince after that.
The mechanics of the Prince Rupert's Drop are quite interesting, and this piece of novel glassware is an early version of tempered glass, which is now used to make a wide variety of things, from laboratory glassware to car windows. You can see demonstrations of Prince Rupert's Drops at many scientific museums and glass museums, if you're interested; unless you are experienced in working with glass, this is a trick you should not try at home, as the shards of glass can be very dangerous, especially if inhaled.
My understanding is that Prince Rupert's Drops are actually UNtempered glass. Tempering removes the inherent stresses.
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