“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. When it comes to ballistic glass, a great deal of a laminator’s art is invested in combining materials and membranes in order to get the full advantage of the strength and clarity of materials like acrylic while minimizing spall. But there’s one situation where laminators can turn spalling to their advantage, and that’s “one-way” ballistic glass.

Exotic Ballistic Glass

As the name implies, “one-way” (or “unidirectional”) ballistic glass stops bullets heading one way, but let’s them through the other. For example, a soldier at a checkpoint is protected from a spray of small-arms fire, but able to shoot back without leaving the safety of his post.

At first brush, this seems pretty counterintuitive. After all, 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. As Shawn Thomas explains it, spalling is what makes one-way glass’s counter-intuitive 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. According to Thomas, 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 that we came up with these make-ups [that] absorb energy in one direction, but don’t going the other way . . . it’s the actual combination and layering of the glass that makes this possible.” For example, Thomas might start with an exterior layer of hard, brittle acrylic, and laminate that to several thin sheets of durable, bullet-resistant polycarbonate using adhesive sheets of incredibly thin polyurethane. “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. “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.”

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 bellyflopper: 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 and 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. 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.