Toughened glass, also sometimes referred to as tempered glass, is four to five times stronger than ordinary annealed float glass and, if broken, disintegrates into small fragments with dulled edges that are unlikely to cause serious injury.
Prepared sheets of glass, which have been cut to size, processed and edgeworked as necessary, are heated to about 700°C in a furnace, which is just above the softening temperature of glass.
They are then chilled rapidly by cold air blown onto both surfaces.
This results in the two outer surfaces contracting and solidifying before the interior, which induces permanent compressive stresses into the surfaces of the glass, effectively increasing the strength of the glass. The interior of the glass naturally balances the compressive stress surface layers by being under tensile stress. It is these properties which result in the increased strength and safe breakage characteristics of thermally toughened glass.
When the central tensile section of the glass is ruptured, the stress is explosively released, producing the fracture characteristics of small, relatively harmless fragments sometimes referred to as dice.
In addition to being a Class A safety glass to BS 6206, the increased resistance of toughened glass to mechanical stress and to large temperature variations, render it ideally suited for use in structural glazing systems, where the glass is to be bolt or clamp fixed, and also in areas subject to high levels of thermal stress.