Toughened safety glass
Toughened safety glass is up to five times stronger than annealed (float glass) of the same thickness. Toughened glass qualifies as a Grade ‘A’ Safety Glass and complies with AS/NZS 2208 and AS1288. Toughened glass fractures into relatively small harmless particles when broken and has a greater resistance to thermal stress. Toughened glass can be subjected to temperatures ranging from 70°C to 290°C.
The Heat Strengthening Process
Heat-treated glass results from the controlled heating and cooling of annealed (float) glass. Firstly, annealed glass is heated inside a furnace to a temperature in excess of 600°C. The glass is then rapidly and uniformly cooled by blasting both sides of the glass with compressed air, a process known as quenching.
This ‘snap cooling’ or quenching action induces compressive stresses into the glass surface while the centre remains in tension. The cooling (or quenching) rate dictates how much stress is introduced into the sheet of glass. Toughened glass is cooled more quickly than heat-strengthened glass.
Most glass types can be heat strengthened although glass of thickness less than 4mm can not be heat strengthened. Once manufactured heat strengthened glass can not be cut or altered.
Heat Strengthened Glass
Is twice as strong as annealed glass of equal thickness. Heat strengthened glass does not qualify as a Grade ‘A’ Safety Glass. Has a greater resistance to thermal stress when compared to annealed glass. Typically breaks into large pieces, which tend to remain in the opening.