When heated, this glass reaches temperatures close to 1,300 degrees Fahrenheit (700 degrees Celsius), which is far higher than typical. The glass is then surface-hardened by an accelerated cooling process, which alters the glass’s physical characteristics. Tempering glass is a procedure that makes the glass more durable and secure than untreated glass. Large quantities of heat undergo a chemical reaction that strengthens the bonds between molecules. The molecules are then squeezed together because of the rapid cooling. It is this quality that allows the glass to endure damage from drops, knocks, and other misfortunes. http://www.glassinfo.net/
