Perhaps you are a politician who visits every nook and cranny of your country to campaign and canvas for votes; or a popular pastor who always stand in front of massive crowd of people to preach the gospel; or you are a cashier, banker, store keeper, etc, who always has to receive or hand out bundles of money to customers; whichever one you are, the chances are you had used bulletproof glass before or may even be using one even as I write.

_{[Source: Flickr commons. Author: John Murphy. CC BY 2.0 licensed]}

You feel completely safe addressing a crowd of people or attending to customers from inside a bulletproof glass cubicle on the premise that its barrier is impenetrable? How true is this? How does such a structure offer protection against bullet attacks?
What's the science behind it? What is the popular misconception people hold about bulletproof glass? Let's take a closer look in this article.

The misconception and the truth about bulletproof glass

You know when we hear folks talk about bulletproof glass, we intend to think it as something impenetrable by bullets, and perfectly safe for us for as long as we remain separated from our attackers by such structures. Well, if you are among those having this kind of conviction about bulletproof glass, think again as that's a total misconception.

In essence, the truth is no glass (bulletproof or no bulletproof) is completely impenetrable by bullets as much as there is nothing like bulletproof glass. The right name for such structure is bullet-resistant glass. This means it only constitutes a barrier that delays bullet penetration^[ref]. In other words, it would take more than just one bullet to shatter the glass either partially or completely, and get at you. However, this will depend on the thickness of the bullet-resistant glass and the type of bullet in use.

The keyword here is "the bullets getting at you" 😂😂😂. I know you wouldn't want that and will do everything within your reach to avoid being hit. So the best thing once you come under attack while being shielded by bullet-resistant glass is to reach for the phone and put a call across for a rescue operation. If you are in a car, just zoom off and drive in zigzag pattern in order not to allow your attackers the time to concentrate their bullets on one spot.

_{[In the above image, three bullets were able to slip away and pass through the interceptor with repeated shots focused at a spot. Hence, bullet-resistant glass isn't foolproof. Source: Flickr commons. Author: John Murphy. CC BY 2.0 licensed]}

Little wonder this source defined bullet-resistant glass as...:

...a type of strong but optically transparent material that is particularly resistant to being penetrated when struck. Like any material it is not completely impenetrable!.

The history of bullet-resistant glass and its construction

The history of bullet-resistant glass can be traced to Édouard Bénédictus (1878–1930), a French chemist who pioneered the making of the first ever known bullet-resistant glass which was made by sandwiching a celluloid plastic between two sheets of glass, and Bénédictus would later patent the idea in 1909^[ref]. This was long before 1936 when Earl Fix of the Pittsburgh Plate Glass Company suggested the idea of using polyvinyl plastics in laminated glass^[ref]. Hence, the modern bullet-resistant glass is simply a derivative of Bénédictus' concept with some variations.

With that in mind, bullet-resistant glass is produced through making alternating layers of glass and plastics built into varied thickness via a process known as lamination. The thickness of a bullet-resistant glass is dependent on the caliber of bullet it is intended to offer protection against. Generally, the thicker the glass the more protection it offers. The thickness of bullet-resistant glass ranges between 19 and 89mm^[ref][ref]. In the past, polyvinyl butyral plastics (PVB) were used to laminate layers of glass sheets to make bullet-resistant glass. This has since been replaced by stronger kinds of plastics including those of acrylic glass, ionoplast polymers, ethylene vinyl acetate or polycarbonate. Thinner films of plastics like PVB or polyurethane was used to separate thick layers of plastics and glass.

_{[A rough representation of bullet-resistant glass made of layers of plastic sheeting (grey) and layers of glass (blue)^[ref]. Source: Wikimedia commons. Author: McGeddon. CC BY-SA 4.0 licensed]}

Making PVB-based bullet-resistant glass

First and foremost, a laminate of PVB is made by sandwiching thin films of PVB with thicker glass. The whole unit is then heated under appropriate pressure condition to melt the plastic and enable it to bond to the glass. This is carried out in a vacuum to avoid trapping of air between the layers of plastic and glass, which would negatively affect the optical properties of the bullet-resistant glass. The cooking is done in an industrial scale autoclave under operating temperature of about 150°C or 300°F and 13 — 14 times normal atmospheric pressure condition.

Ideally, two or more types of glass (one hard and one soft) are combined in building the bulletproof layers. Like the plastics used, the softer one functions to cushion the effects of bullets, and prevent the glass from shattering into several pieces on impact. The two or more sheets of glass used must have the same or close refractive index in order not to interfere with optical properties of the final product when completed^[ref].

How bullet-resistant glass works

As has been said under construction of bullet-resistant glass, Bullet-resistant glass is made up of multiple layers of very thick glass alternated with interlayers of plastics. In some bullet-resistant glass, there a final layer of very thick polycarbonate pane that functions to prevent spalling so that dangerous fragments of glass and plastics do not splitter off and hurt someone when strike by bullets. Actually, what bullet-resistant glass does following impact with bullets is to absorb the bullet energy by distributing it to its component multiple layers of plastics and glass.

On hitting the bullet-resistant, the hard glass deforms it while the plastic component stops it; the bullet energy quickly spreads through layers of various plastic panes over a large area. In the process, the bullet lose its momentum and is therefore brought to a stop. Or even if it succeeds in piercing through the pane, it would not have enough energy to hurt its target. *Of course, the glass panes do break into pieces in all cases of bullet attack but the dangerous glass fragments are prevented from splashing off into different directions by the plastic components. Hence, the fragments stay there!

_{[Notice as the hard glass deform the bullet in the above image. Source: Flickr commons. Author: John Murphy. CC BY 2.0 licensed]}

Conclusion

We have been able to demonstrate how a simple bullet-resistant glass works. Essentially, it only delays bullet penetration, and doest not permanently stop it. It takes more than one bullet and the security of the subject may be seriously compromised. People should know this truth and make "hay while sun still shines" in order not to endanger their lives when they come under bullet attack on the premise that once someone is inside bullet-resistant glass cubicle, the person is perpetually safe. Far from that. However, the chance of bullets eventually penetrating through the barrier of bullet-resistant glass when repeatedly aimed at a spot, is dependent upon some conditions, among which are the type of bullet and the thickness of the bullet-resistant glass. At any rate, it is my pleasure that you read this to this point. Thanks for reading.

References for further reading

• _{Laminated glass}
• _{How does bullet glass work}
• _{Bulletproof glass}
• _{Bullet-resistant glass: how does it work?}
• _{Bulletproof glass}

Yours truly,
@eurogee

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