Flawed Beauty Reading Passage
Flawed Beauty Reading Passage
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A large pane of toughened glass in the roof of a shopping centre at Bishop's Walk in the British town of Cirencester shattered and fell from its frame without warning on a particularly hot day on August 2, 1999.
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Experts from the global glass manufacturer Pilkington. which produced the shattered pane, analysed the pieces and concluded that tiny crystals of nickel sulphide trapped inside the glass resulted in failure.
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The glass industry is aware of the problem,' says Brian Waldron, chairman of the Glass and Glazing Federation's standards committee and standards development officer at Pilkington. However, he maintains that cases are few and far between. 'It's a very rare occurrence,' he says.
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While some may disagree, others may not. According to consultant engineer Barrie Josie, who is part of the Bishops Walk investigation, "on average, I see about one or two buildings a month suffering from nickel sulphide-related failures." Other authorities have made similar accounts. According to Tony Wilmott, one of the London-based consulting engineers Sandberg, and Simon Armstrong, one of the Hampshire-based CladTech Associates, each of them has heard of hundreds of such cases. What you're hearing is just the tip of the iceberg. Queensland. He thinks it's because 'no one wants bad press.'
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Toughened glass can be found in a wide variety of applications, from automobiles and bus stops to the windows, walls, and roofs of tens of thousands of buildings around the world. Why this would be the case is obvious. This glass is five times as strong as regular glass, and when it does break, the fragments are small and harmless rather than long and sharp. It is highly regarded by architects because large panels can be bolted together to create see-through walls, and it is almost as simple to transform into ceilings and floors.
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The process involves softening a sheet of regular glass to about 620°C, allowing its structure to expand, and then rapidly cooling the glass with jets of cold air to create the desired shape. This results in the outer layer of the window pane contracting and solidifying before the interior. A tensile force is generated within the glass when the interior solidifies and contracts, pulling on the exterior and permanently compressing it. While cracks spread more easily in materials that are under tension, the compressive force acting on the surface of the glass needs to be overcome before the pane will break.
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The issue arises when the glass has impurities of nickel sulphide. In addition to the usual presence of sulphur and nickel in the raw materials, nickel can also be introduced into the molten glass if fragments of nickel alloys happen to fall in. These atoms react with one another as the glass is heated, forming minuscule crystals of nickel sulphide. Up to 50,000 crystals can be produced from as little as a tenth of a gramme of nickel in a furnace.
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The alpha phase, in which the crystals are extremely dense, is stable at high temperatures, while the beta phase, in which the crystals are relatively loosely packed, is stable at room temperature. All the crystals are altered to the compact alpha form by the high temperatures used in the toughening process. However, the crystals are unable to revert to the beta phase because of the rapid cooling that follows. That leaves alpha crystals in the glass in an unstable state, ready to snap back to beta at any moment.
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This causes the crystals to expand by up to 4%. And if they are in the pane's central, tensile region, the stress that results can shatter the entire sheet. The time elapsed before failure is unpredictable. It could happen months after manufacture or decades later, though if the glass is heated - for example, by sunlight - the process is accelerated. The oldest pane of toughened glass known to have failed due to nickel sulphide inclusions, according to Graham Dodd of consulting engineers Arup in London, was in Pilkington's glass research building in Lathom, Lancashire. The pane had been in use for 27 years.
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The extent of the nickel sulphide issue data: Try as you might, but you won't be able to locate it. The fact that these crystals form in clusters further complicates the picture. If one pane of glass fails due to nickel sulphide, it's likely that more than one pane is affected, even if the average rate of inclusions is 1 per 7 metric tonnes of glass. According to Josie, he has worked on more than 15 failed buildings in the last decade.
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Waterfront Place, which was built in 1990, is a particularly bad example of this. The 40-story Brisbane building experienced a series of failures over the subsequent decade. Before professionals were called in, eighty panes of its toughened glass shattered due to inclusions. Each pane of glass in the structure was examined by John Barry, a specialist in nickel sulphide contamination from the University of Queensland. A photographer rode up in a cradle with a studio camera to capture images of each pane. Nickel sulphide crystals were searched for by running them through a customised microfiche reader. "We found at least another 120 panes with potentially dangerous inclusions," says Barry. It took about six months to complete and was very costly and time-consuming.
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The total cost of the project was A$1.6 million (nearly £700,000), but the cost of recladding the entire building would have been at least ten times that.
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