TEMPERED GLASS HISTORY
The first patent on tempered glass was held by chemist Rudolph A. Seiden, born in 1900 in Austria. Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In about 1660, Prince Rupert of the Rhine brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of King Charles II. These are teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. They have the curious ability to withstand a blow from a hammer on the bulbous end without breaking, but the drops will disintegrate explosively if the tail end is even slightly damaged. The teardrops were often used by the King as a practical joke.
Toughened glass is processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including automotive windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass, for diving masks, and various types of plates and cookware. The federal safety laws require that window glass be tempered if each of the following criteria are met: sill height within 18 in (0.457 m) of the floor, top edge greater than 36 in (0.914 m) from the floor, area greater than 9 ft² (0.836 m²), and horizontal distance to nearest walking surface of less than 36 in (0.914 m).