Toughened glass is found everywhere, from cars and bus shelters to the windows, wallsand roofs of thousands of buildings around the world. lt's easy to see why. This glass hasfive times the strength of standard glass, and when it does break it shatters into tiny cubesrather than large, razor-sharp shards. Architects love it because large panels can be boltedtogether to make transparent walls, and turning it into ceilings and floors is almost as easy.It is made by heating a sheet of ordinary glass to about 620 to soften it slightly, allowingits structure to expand, and then cooling it rapidly with jets of cold air. This causes the outerlayer of the pane to contract and solidify before the interior. When the interior finally solidi-fies and shrinks, it exerts a pull on the outer layer that leaves it in permanent compressionand produces a tensile force inside the giass. As cracks propagate best in materials undertension, the compressive force on the surface must be overcome before the pane will break,making it more resistant to cracking. The problem starts when glass contains nickel sulphideimpurities. Trace amounts of nickel and sulphur are usually present in the raw materialsused to make glass, and nickel can also be introduced by fragments of nickel alloys fallinginto the molten glass.
As the glass is heated, these atoms react to form tiny crystals of nickel sulphide. Just a tenth of a gram of nickel in the furnace can create up to 50,000 crystals. These crystals canexist in two forms: a dense form called the alpha phase, which is stable at high tempera-tures, and a less dense form called the beta phase, which is stable at room temperatures. The high temperatures used in the toughening process convert all the crystals to the dense, compact alpha form. But the subsequent cooling is so rapid that the crystals don't have timeto change back to the beta phase. This leaves unstable alpha rystals in the glass, primed like a coiled spring, ready to revert to the beta phase without warning. When this happens, the crystals expand by up to 4%. And if they are within the central, tensile region of the pane, the stresses this unleashes can shatter the whole sheet. The time that elapses before failure occurs is unpredictable. It could happen just months after manufacture, or decades later, although if the glass is heated-by sunlight, for example - the process is speeded up. Ironically, says Graham Dodd, of consulting engineers Arup in London, the oldest pane of toughened glass known to have failed due to nickel sulphide inclusions was in Pilkington's glass research building in Lathom, Lancashire. The pane was 27 years old. Data showing the scale of the nickel sulphide problem is almost impossible to find. The picture is made more complicated by the fact that these crystals occur in batches.
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