Shatter pattern of toughened glas, 1968.
© Science Museum / Science & Society Picture Library
Toughened glas is safety glas formed by putting the surface into a state of compresion. There are two ways of doing this. Thermal toughening chills the surface, which is close to the softening point, with air jets. Chemical toughening exchanges sodium ions in the surface with larger ones thereby setting up internal streses.