Tempered glass is classified according to its manufacturing process.

(1) Physically tempered glass, also known as quenched tempered glass, is produced by heating ordinary flat glass in a furnace to near its softening temperature (600 ℃). This allows the glass to eliminate internal stress through deformation. The glass is then removed from the furnace, and high-pressure cold air is blown onto both sides of the glass through multi-nozzle nozzles, causing it to cool rapidly and evenly to room temperature. This type of glass is under tension internally and compression externally. If a local break occurs, the stress is released, and the glass shatters into numerous small pieces. These small fragments have no sharp edges and are unlikely to cause injury.

 

(2) Chemically tempered glass improves its strength by altering the chemical composition of the glass surface. It is generally tempered using an ion exchange method. The method involves immersing silicate glass containing alkali metal ions into molten lithium (Li+) salt, causing Na+ or K+ ions on the glass surface to exchange with Li+ ions, forming a Li+ ion exchange layer on the surface. Since the expansion coefficient of Li+ is smaller than that of Na+ and K+ ions, the outer layer shrinks less and the inner layer shrinks more during the cooling process. When cooled to room temperature, the glass is in a state where the inner layer is under tension and the outer layer is under compression, which is similar to the effect of physical tempered glass.