What Is Safety Glass?
Safety glass, when struck, bulges or breaks into tiny, relatively harmless fragments rather than shattering into large, jagged pieces. Safety glass may be constructed by laminating two sheets of ordinary glass together, with a thin interlayer of plastic, or it may be produced by strengthening glass sheets by heat treatment. In 1909 the first successful patent for safety glass was taken out in France by an artist and chemist, Édouard Bénédictus, who used a sheet of celluloid bonded between two pieces of glass.
Other plastics were also tried, but in 1936 polyvinyl butyral (PVB) was found to possess so many safety-desirable properties that its use became universal. Bulletproof glass is usually built up using several glass and plastic components.
Safety glass is a 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 Édouard Bénédictus, 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, Bénédictus 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 of the Rhine, nephew of King Charles II of England, discovered that molten glass was turned into immensely strong pear-shaped drops when tipped into cold water.
Prince Rupert’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, Francois Barthelemy Alfred Royer 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.
In 1877 the German Frederick Siemens developed a different process, sometimes called compressed glass or Siemens glass, producing a tempered glass stronger than the Bastie process by pressing the glass in cool molds. The first patent on a whole process to make tempered glass was held by chemist Rudolph A. Seiden who was born in 1900 in Austria and immigrated to the United States in 1935. 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.