By Jacob Silverman

Blastproof, Sort Of
Like the terms "military-grade" or "developed for NASA," "blastproof" is the sort of label that's easily used and abused by product marketers. The term's presence may not accurately reflect a material's properties. (In researching this article, we found an allegedly blastproof glass bathtub, yet the supplier didn't offer any supporting data.) It's also only true up to a point — that is, large enough explosives will certainly demolish any glass, no matter how well reinforced it might be. For example, the atomic bomb dropped on Hiroshima shattered glass in homes 12 miles (19 kilometers) away and melted all glass within a couple miles of ground zero, according to the U.S. Department of Energy.

Despite these caveats, blastproof glass has improved in recent years, in part in reaction to concerns over terrorist attacks. One research team from the University of Exeter found that, during the 1995 bombing of Oklahoma City's Alfred P. Murrah Federal Building, about 67 percent of head and eye injuries could be traced to flying glass. There are several ways to deal with these sorts of problems — blastproof curtains, like the ones that will surround One World Trade Center, come to mind, as does turning to tougher, more resilient forms of glass.

Spiderwebs, Not Shards
Most U.S. government installations, from federal buildings to military bases, are equipped with some type of reinforced glass. Frequently it's shatterproof, so that rather than breaking into dangerous shards, it simply cracks into a spiderweb pattern while staying intact as one solid sheet. In rebuilding the World Trade Center, engineers are using somewhat flexible glass that's tempered (or treated with heat) and laminated for added strength, as well as being somewhat flexible. Most high-strength glass is composed of plastic between two sheets of laminate. This sort of glass, however, can be expensive, while cheaper options — like so-called toughened glass — may still break into pieces without the addition of an anti-shatter film. There's a market imperative, then, to find glass that's both blast-resistant and cheaper than existing products. It also wouldn't hurt for it to be thinner, ensuring that the glass weighs less.

Let's Throw a Grenade at It
Scientists at the University of Missouri think they have a good alternative. Their new, thin glass — it's only 0.5 inches (1.3 centimeters) thick — uses a glass-plastic composite material. In test videos posted online, researchers detonated hand grenades right next to a sheet of the superstrong glass. It cracked but didn't shatter, nor did it show any holes.

Besides tempering and various chemical processes, there are other methods of strengthening glass. Numerous companies market so-called ""fragment retention films"" that are designed to boost glass strength and prevent glass from exploding into dangerous shards. These films are either attached with adhesives or by attachment systems that may require silicone caulking or metal battens.

With all these innovations in glass, the only thing the material will have to revamp is its fragile reputation.


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