- The Washington Times - Wednesday, April 11, 2007

One pane of glass can stop a bullet. Another can shatter over a stuntman’s head and not draw blood.

The science behind glass makes such variations a reality, even if most people will never need it for either purpose.

Glass, made of purified sand also known as silica or silicon dioxide, isn’t as easy to quantify as some substances, says Edward R. Van Keuren, an associate professor of physics at Georgetown University.

“It’s similar to a liquid, but the flowing is so much slower because of the way the molecules are held together,” Mr. Van Keuren says.

Glass generally strengthens the thicker it is, but the beefy glass one sees in a bank or in front of a convenience store clerk isn’t strong simply because of its dimensions.

Tempered glass is commonly used in patio and entrance doors to offer extra sturdiness and increased safety.

The process involves reheating glass to near its melting point, then cooling it quickly. The resulting glass is tougher than before, and when it shatters, the pieces break off much smaller than they would otherwise. Smaller shards are less likely to hurt someone.

Once tempered, glass cannot be recut, but the finished product is much stronger than standard glass, according to Alcoa, which uses such glass to make bathroom enclosures.

The technique isn’t the only way to create durable glass.

Eric Carlson, vice president of armory with Pennsylvania-based North American Specialty Glass, says bullet-resistant glass is created during a fairly simple process.

“It’s just multiple layers of glass with inner layers between them,” a process called lamination, Mr. Carlson says. This style of glass is also used in greenhouses, showers and office partitions.

The inner layers usually are made of polycarbonate.

The number of layers and the overall thickness of a particular piece of glass depend on just how much protection a client needs, Mr. Carlson says.

For a pane that can stop a .44-caliber Magnum, a 22-millimeter-thick glass should work, he says. A sheet of glass that’s 71 millimeters deep can stop three shots from an armor-piercing rifle. A bullet striking glass will expend some of its energy both on cracking the glass layer and on dispersing its might along the broken pane. The force “blows out to the sides,” he says. “The energy keeps dispersing through each layer.”

In theory, if the glass is thick enough, the speeding bullet won’t have enough power to pierce the innermost layer of glass.

Mr. Carlson says his company’s glass includes an interior layer of protective coating as a final defense against any projectile that slices through the glass and still has some energy left.

That process of glass shattering after being struck is called “spalling,” he says.

“The bullet might penetrate through the last piece of glass, but nothing flies out” thanks to the final coating, he says.

Car manufacturers began using laminated glass for their windshields in the late 1920s, although not all windshields are bulletproof.

The plastic film used not only strengthens the glass but also keeps glass pieces from flying when the windshield is hit, much like with tempered glass.

These windshields aren’t thick enough to block bullets, but the glass is stronger than a typical pane, and it also blocks the vast majority of ultraviolet radiation.

Another way windshields stop some rays from entering the car is to put impurities into the glass, Mr. Van Keuren says.

Such impurities absorb light that otherwise would pass through the glass.

On a smaller scale, sunglass manufacturers use science to keep people’s eyes protected from harmful rays.

David Hagan, museum scientist for physical sciences with the Science Museum of Virginia, says some companies embellish the impact of their product.

“Some of them are selling properties that all glass blocks,” Mr. Hagan says. A microscope cover slide, which is about one millimeter thick, blocks some ultraviolet, or UV rays, he adds.

Standard glass blocks UVB and UVC rays, which are shorter than UVA radiation.

“But regular glass doesn’t completely block UVA, and that’s what can give you a bit of a tan,” Mr. Hagan says. “Any material will selectively absorb or not absorb different wavelengths of light.”

Glass once served as the main ingredient in spectacles, but more companies are turning to polymers to create eyeglass lenses, says Timothy Wingert, a professor at the University of Missouri at St. Louis School of Optometry.

“The number of spectacles made of glass is an ever-dwindling one,” Mr. Wingert says.

Polymer glasses are lighter and thinner, and they offer the kind of impact resistance glass can’t match, he says.

Plastics offer another advantage over their glass counterparts in their lighter weight.

“Back then, it was not uncommon to need their glasses adjusted on a regular basis,” Mr. Wingert says, and each adjustment made the lenses heavier and thicker.

Spectacle manufacturers also deal with protecting users from UV radiation.

“They add material to the glass to reduce that transmission of UV, all types,” he says, particularly keeping those wavelengths from hitting the eye.

Just know that looks can be deceiving, Mr. Wingert says.

“You can have a tinted lens that doesn’t give you much protection against UV, or have a pair that’s clear with 100 percent protection. You can’t tell by the color of it,” he says.

Eyeglass makers use multiple coating layers to deflect the rays, sometimes as many as 23 in a single pair of glasses.

“You wouldn’t notice one was thicker than the other,” Mr. Wingert says of the layers, often consisting of metallic oxides.

No matter the number of layers or tint, he advises consumers to check with a third party, such as the American Optometrist Association (perhaps on the product itself), before buying a pair of glasses or sunglasses.

The cumulative effect of UV damage to one’s eyes, which can include cataracts and macular degeneration, shouldn’t be ignored, he says.

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