One episode of Squid Game (Netflix, 2021) features a glass bridge consisting of pairs of glass panels where one of the pair is tempered glass and the other is “normal” glass. According to the show, one of the tempered glass sheets is strong enough to hold the weight of two people. But the normal glass would break if one person stepped on it.
But was this realistic? We asked an ARCCA Expert:
- Is there as much difference between tempered glass and “normal” glass as claimed in the show?
- Did the glass behave realistically?
- Could you tell the difference between the different kinds of glass just by looking at it?
- Would anybody actually make a walkway out of glass like this?
Different kinds of glass
It turns out that there are several different types of glass with different material properties and uses. Most glass you would be familiar with is made from silica, also known as silicon dioxide. What makes it a “glass” is that it is amorphous – it has no crystal structure or orderly repeating arrangement of its atoms. (As opposed to the mineral quartz, which is silica with a crystal structure.) Silica glass can be processed in to several forms. The most common are annealed glass, tempered glass, and laminated glass.
When Squid Game refers to “normal” glass in the glass bridge, they are referring to annealed glass. Annealed glass is simply amorphous silica – and it tends to be very brittle. When it fractures, it tends to break into large, sharp pieces.
Tempered glass, on the other hand, has been specially heat-treated by cooling it from a high temperature and cooling the surface faster than the interior. This creates “residual stress” in the material that makes it about three times stronger than annealed glass. When tempered glass fractures, the energy from the residual stress is released – causing it to fracture into tiny, blunt pieces.
So, Squid Game does show the behavior we would expect from annealed and tempered glass. If we assume that the annealed glass was strong enough to almost – but not quite – support the weight of one person, then the tempered glass would be about three times stronger and able to hold two people. In the show, the way the annealed glass panels fracture is also correct. When a panel breaks, it fractures quickly into large, sharp pieces.
Could you tell the difference?
In the show, one of the contestants claims that because – from his experience making glass – he could tell tempered glass from annealed glass by faint marks visible when the glass was lit from the side. This is not exactly how we might tell tempered an annealed glass part, but it is close to some realistic possibilities.
Annealed glass is normally not birefringent – it does not polarize light. But the residual stress in tempered glass can cause it to become birefringent, and this is especially noticeable when there are slight variations in the residual stress. Sometimes, if you wear polarized sunglasses, you can see those variation in tempered glass car windows. So perhaps someone could tell the difference between tempered and annealed glass by how it polarizes light. But that would require some kind of polarization filter. The clever glass-maker contestant in Squid Game did not have any such filter – so he probably could not have used light polarization to tell the difference.
Another possibility is related to one weakness of tempered glass. Even though the faces of a sheet of tempered glass are very strong and resistant to fracture, the edges are more vulnerable. To offset this, the edges of tempered glass are often polished, and sometimes coated, to reduce the risk that a crack might start at the edge and cause the whole sheet to fracture. It may be that the glass-maker contestant was able to see some difference in the edge treatments of the annealed and tempered glass sheets.
What would you use in a real walkway?
While tempered glass would definitely be a better choice than annealed glass for making a walkway, in reality you would not use either. Even though tempered glass is much stronger than annealed, if and when it fractured, it would lose all ability to prevent a person from falling.
In the real world, glass walkways are made from either a transparent polymer, such as polycarbonate, or laminated glass. Laminated glass has at least one layer of tough polymer so that even if the glass layers fracture, the polymer layers do not and give the glass some ability to act as a barrier or support. This is part of why laminated glass is used in car windshields, but federal standards (FMVSS 205) prohibit windshields made from annealed or tempered glass.
Even though Squid Game is fictional, it illustrates a real-world idea: A designer can select materials with particular properties to get a desired effect on their product. The glass-maker character is also a good example of how an investigator might use differences in material properties to distinguish one material from another in the field. ARCCA experts use similar ideas to analyze the design and manufacturing of different products, and to assist in field inspections and laboratory examinations of evidence.
If you have a question about how a material failed or how a material performed as part of a product, then ARCCA has an expert who can help. Call us to speak to an expert today.