Tuesday, 18 February 2014

Annealed Glass As A Health Hazard

Annealed glass breaks in to shards.
Older Glass Windows Are Potentially Deadly: Lean against or touch certain glass windows and doors and you can actually be putting yourself in danger. CBS 2′s Dave Savini shows how the danger lies with how the glass is made. “This is a deadly weapon,” says glass safety expert Mark Meshulam as he shows a sharp, jagged broken piece of glass from a window. “This is just crazy.”

The type of glass he is talking about is called annealed glass. It is found in windows in older homes, schools and storefronts exempt from more current rules requiring safer, tempered glass windows. Tempered glass breaks into small pieces, while annealed breaks into large, jagged ones that can cause greater injury. There have even been deaths.

Fifty-two-year-old Michael Racky lost his life after falling through a glass window. His adult children, Meghan and Sean Racky, say their dad leaned against an annealed plate glass window in Oak Lawn, and it shattered, nearly amputating his leg. “It hurts a lot,” Meghan Racky says. “I have to wake up every day realizing, I’ll never see him again.” They say a BB-gun hole compromised the window three years earlier. “It’s actually a very big problem that many people aren’t even aware of,” Sean Racky says.

Tape was used to hold it together, says the Racky’s attorney Tom Paris. “Packing tape is not an answer for compromised glass,” he says. CBS 2 found that tape is being used on other annealed glass windows in Chicago, including at a Laundromat and a daycare. Meshulam says they are dangerous. The 2 Investigators found 6,069 city citations were issued for window code violations between 2010 and 2013. “Somebody, sometime, is going to go into that glass,” Meshulam warns outside one building.

Maria Elena Gonzalez was standing outside of a South Side apartment building in 2006 when she was struck by a falling window. “The pain was unbelievable,” says Gonzalez, who had never thought about the danger a window could pose. The 2 Investigators went back to the building where Gonzalez was hit by the window and found five more broken windows.

Annealed glass windows and doors are being blamed for deaths around the country, including a Pennsylvania art professor who fell through a glass door at a restaurant and bled to death. The solution, Meshulam says, is simple, and he demonstrated. He struck a tempered glass window with a special weight bag; initially, the glass failed to even break. It did break on the next test, but broke into tiny pieces instead of the jagged shards of annealed glass.

Building codes can vary by community, when it comes to the type of glass required in homes and businesses. Safety glass is pretty much mandatory in doors, but not all windows are required to have it. Houses and other buildings built before 1977 were not even required to have safety glass. If you are not sure what type of glass you have, look at the corners. Safer tempered glass typically has the manufacturer’s logo etched into it. Attempts to reach the business and building owners were unsuccessful.

Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass. The glass is heated until the temperature reaches a stress relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.

Tempered Glass links here.

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