Safety glass is glass that has been strengthened. There are two kinds of this protective glass-laminated and toughened-and both were discovered by accident.
In the early 1900s EdouardBenedictus, a French chemist knocked a glass flask on to the floor. Although the glass starred and cracked, it did not break. After examining the flask he realized that a coating of dried celluloid on the inside had held the fragments together.
Some years later, when injuries from broken car windscreens increased, Benedictus recalled this incident. Using glass sheets and celluloid bonded together in an old letter press, he produced the world’s first sheet of laminated, or layered, glass. Since then the clarity of the glass has been improved to equal that of ordinary glass. But it will withstand the impact of a half-pound steel ball dropped from a height of 16 feet. Toughened glass was developed later, although in the 17th century, Prince Rupert, nephew of king Charles I of England, discovered that molten glass was turned into immensely strong pear-shaped drops when tipped into cold water. Prince Ruport’s drops, as they are called, can be hammered on an anvil without breaking, but if the tail of the drop is broken they crumble into dust.
In 1874 a French scientist, de la Bastie, heated small sheets of glass and then quenched them in oil, increasing their strength dramatically. However these sheets of toughened glass were very small and it was not until the 1930s that sheets large enough for use in cars could be toughened.
Laminated or toughened safety glass is now used all over the world in cars, buses, trains, aircraft, ships and shops and has proved its safety value.
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