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“Spalling” is the tendency for flecks or shards to come off of the surface of a material. It’s the bane of anyone working with protective surfaces, since many of these—from concrete to ballistic glass—tend to spall under impact, and those little fast-moving flecks can be dangerous. In the manufacturing of ballistic glass, a great deal of a laminator’s art comes down to their ability to combine materials, resin, and membranes in order to get the full advantage of the strength and clarity of materials like acrylic, glass, and polycarbonate while minimizing spall. But there’s one situation where laminators can turn spalling to their advantage: creating “one-way” ballistic glass.
Exotic Ballistic Glass
As the name implies, “one-way” (or “unidirectional”) ballistic glass stops bullets heading one way, but lets them through the other. For example, “one-way” glass can protect soldiers at a checkpoint from a spray of small-arms fire, but still allow them to return fire without leaving the safety of their post.
This seems pretty counterintuitive, at first. Most structural surfaces are basically symmetrical: a cinderblock wall stops a bullet regardless of which direction it’s headed, and flying through a windshield is just as uncomfortable if you’re coming or going. According to Shawn Thomas, spalling is what makes one-way glass’s counterintuitive behavior possible.
Shawn Thomas works for Protective Armored Services, a world-class laminator producing all manner of protective transparencies for government, transportation, commercial, and residential applications. Thomas explains that the secret to making one-way glass is using layering and lamination to control how the material spalls and absorbs shock. In this way, the glass forces bullets heading in one direction to waste significantly more energy than those headed the other way.
“It was through trial and error,” Thomas says, “that we came up with these make-ups [that] absorb energy in one direction, but don’t go the other way . . . it’s the actual combination and layering of the glass that makes this possible.”
For example, Thomas might start with a thin exterior layer of hard, brittle acrylic. He will then laminate that to several thin sheets of durable, bullet-resistant polycarbonate using adhesive sheets of incredibly thin polyurethane. “If you have a piece of 3/8 [acrylic] in the front, that will flatten the bullet more, and the bullet will be wider, making it easier to absorb all that energy. … What the urethan does is it allows that laminate to absorb the bullet’s energy” by holding the exterior sheet of acrylic together when it is shot, rather than allowing shards to fly out of the bullet’s path.
Bullets are meant to cut through a surface like a skilled high-diver, whose sharp, smooth posture allows her to cut through the surface of the water and slice to the bottom of the deep end. Contrast this to a bellyflopping goof, who makes a big slapping splash, but can’t go deep into the water. When a bullet pancakes against the hard acrylic exterior layer, it’s like that belly flopper: It makes a big messy splash on the surface but doesn’t have the energy to go anywhere. Having spent its energy on cracking through the acrylic, the bullet can easily be caught by the thin sheets of polycarbonate.
When the people behind the barrier return fire, their bullets do not have to contend with the hard acrylic: they pop through the softer polycarbonate first, retaining their sharp, cutting form. They can then burst out of the acrylic, throwing off shards and emerging largely undeterred.
One-Way Ballistic Glass in Action
The following video, prepared by Israeli ballistic glassmaker B.P. Developments is a great introduction to one-way ballistic glass:
As you can see at the 1:35 mark (when the announcer rubs his hand along the interior surface of the shot glass) there has been absolutely no spalling on the secure side of the barrier. There are tons of sharp shards on the exterior of the window, but the inside is perfectly smooth, with no debris to harm those within the protected area. When they flip the glass and simulate security personal returning fire, you can see how smoothly the bullet passes through the glass. Note that, for security personal, the chipping and flaking actually work to their advantage, flying like shrapnel toward their attackers. The shattered acrylic might make it a little harder to draw a bead for an accurate shot, but the shards certainly contribute to the physical and psycho-emotional impact of the return fire.