Prince Rupert's Drop and Glass Stress
Theoretically, because of the nature of its atomic bonds, glass should be about five times as strong as steel. However, glass tends to have less strength than theory would suggest. One of the main reasons for its loss of strength is surface and internal stress.
If glass is cooled too rapidly, high stresses are "frozen" inside, and may cause the piece to simply shatter, the warmer glass from the inside bursting its way out of the "cold skin" of the outside surface of the glass. This shattering may occur spontaneously, or with a tiny scratch produced on the surface.
Stresses are dramatically illustrated in an object called "Prince Rupert's Drop." A 17th-century drawing of the drop can be seen above. It is a curious tadpole-shaped solid glass object, having a bulbous end that tapers down into a thin, curved tail. They are formed by dropping a small gob of hot, molten glass into cold water, and leaving it to cool. This creates tremendous stress between the outside layer, cooled by the water, and the inside, which is warm. Because of the tremendous surface tension, the thick, bulbous end can endure a blow with a hammer. However, if you scratch, or break the surface of the glass in the thin, fragile tail, the glass releases the internal stress with such force that the entire piece shatters into fine powder.
These interesting demonstrations of the stress of glass were introduced 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. To avoid these internal stresses, glass articles must be annealed.
In the annealing process, the temperature of the object is raised to that which will allow the internal stress to be relaxed, but not so high that the glass will slump. At that temperature, the entire object is at the same uniform temperature, and there is no internal temperature difference. The temperature, which is held high for some time, is then slowly reduced to room temperature.
Samuel Pepys described them in his diary on January 13, 1662.
From a book written by Antonio Neri, and published in London in 1662 comes an account of "Glaƒs Drops" that refers to the illustration at the top of this page:
This account was given to the Society of Sir Robert Moray, MDCLXI
A B the thread, B C the body, B the neck, A the point of end of the thread. They are made of Green-glaƒs well refined; they do not at all ƒucceed, but crack and break, foon after they are drop't into the water.
The beƒt way of making them, is to take up fome of the Metall out of the pot upon the end of an Iron rod, and immediately let it drop into cold water, and there lye till it cool.
If the Metall be too hot when it drops into the water, the Glaƒs drop certainly froƒts and cracks all over, and falls to pieces in the water.
Every one that Cracks not in the water, and lies in it, till it be quite cold, is ƒure to be good.
The moƒt expert Workmen, know not the juƒt temper of heat, that is requiƒite, and therefore cannot promiƒe before hand to make one that ƒhall prove good, and many of them miƒcarry in the making, ƒometimes two or three or more for one that hits.
Some of them froƒt, but the body falls not into pieces; others break into pieces before the red heat be quite over, and with a ƒmall noiƒe; others ƒoon after the red heat is over, and with a great noiƒe; ƒome neither break nor crack, till they ƒeem to be quite cold; others keep whole whileƒt they are in the water, and fly to pieces of themƒelves with a ƒmart noiƒe...
(Note that glass is called Metall and ƒ substitutes for s.)