Toughened glass (Fig. 7.10) is up to four or five times stronger than standard annealed glass of the same thickness. It is produced by subjecting preheated annealed glass at 650°C to rapid surface cooling by the application of jets of air. This causes the outer faces to be set in compression with balancing tension forces within the centre of the glass. As cracking within glass commences with tensile failure at the surface, much greater force can be withstood before this critical point is reached.
Toughened glass cannot be cut or worked, and therefore all necessary cutting, drilling of holes and grinding or polishing of edges must be completed in advance of the toughening process. In the roller hearth horizontal toughening process, some bow roller wave distortions and end edge sag may develop, but these will be within narrow tolerances; however,
Fig. 7.10 Process for manufacturing toughened glass
Fig. 7.10 Process for manufacturing toughened glass they may be observed in the more reflective glasses when viewed from the outside of a building. In the vertical toughening process the sheet is held by tongs which leave slight distortions where they have gripped the glass.
Toughened glass will withstand considerable extremes of temperature and sudden shock temperatures. If broken, it shatters into small granules which are not likely to cause the serious injuries associated with the accidental breakage of annealed glass. To be classified as a safety glass, toughened glass must be tested and marked according to required standard BS 6206: 1981. When toughened glass is specified for roof glazing, balustrades and spandrel panels, it is subjected to a heat soaking at 290°C, a process which is destructive to any sub-standard units. This removes the low risk of spontaneous breakage of toughened glass on site, caused by the presence of nickel sulfide inclusions within the material. All standard float, coated, roughcast and some patterned glasses can be toughened.
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