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." 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 excessive 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.