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Subj:.....Prince Rupert's Drop (S668) 
          From: sam.hutkins on 10/28/2009
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Drawing from Corning Museum of Glass...
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Source: http://www.cmog.org/dynamic.aspx?id=5660
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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.

These interesting demonstrations of the stress of glass were introduced
Drawing from PamelAbelle.com
to England in the 1640's by Prince Rupert of Bavaria (1619- 1682), grandson of James I of England, and nephew of Charles II.  Prince Rupert brought these to the attention of the King, and they were used as a joke.  The King would have a subject hold the bulb end in the palm of the hand, and then break off the tip, giving the startled person a small explosion right there in a closed hand.  It was harmless fun, though, as the
glass shatters into powder, not into jagged shards.
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