We all know that glass is really fragile.
And when it breaks, it usually explodes into dangerous shards that go everywhere.
Which is less than ideal when that glass is used for,
like, car windows or the outsides of buildings.
And how can glass be bulletproof if most glass isn’t even floor-proof?
Well, over five thousand years of us making the stuff, we’ve developed some tricks
for making glass stronger when it’s whole, and safer when it breaks.
And we’ve also decided that you can call something “glass” when it’s not really, well, glass.
Or, at least, not just glass.
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Now, glass is a solid, and no matter what keyboard warriors say, it is NOT a liquid.
That said, it’s what we call in the world of science, a weird solid.
Okay, technically the word is “amorphous,” but still.
See, most substances form crystals, where atoms and molecules
get locked into a pattern as the liquid freezes into a solid.
But not glass.
There’s no one most natural way for its molecules to lock together.
Some areas end up with molecules arranged one way
and some areas are completely different.
The transition from liquid to glass is so strange that no one
fully understands it: It’s an unsolved problem in physics.
And the end result is a chaotic mess of molecules that looks more like a liquid
than a typical solid crystal shape, hence the term amorphous solid.
Those messy molecules help make glass transparent;
the molecules are so disorganized that they don’t absorb any one visible color,
meaning that all the colors of visible light can pass through, more or less.
But glass’s disorganized molecules also make it brittle.
The molecules end up in tightly packed clumps in some places,
with lots of force binding the clumps together internally,
but a lot less force holding the separate clumps together.
And those areas with less force are all throughout the piece of glass,
meaning it’s chock full of weak points.
If the glass is hit or bends along one of those weak points,
the clusters might just separate permanently, forming a tiny crack in the glass.
And those cracks will grow and keep driving clusters apart,
until one last impact makes the whole thing go kaboom.
Those cracks spread across all dimensions of the piece of glass, too,
creating super-sharp edges that can be as thin as a single molecule.
Once a growing crack reaches the edge or circles back on itself,
energy stored between molecules is suddenly released and the glass shatters.
That makes glass sound like an awful choice for, say, a car window.
You know, something that’s constantly bonked
by little stones and sometimes hit by an entire car.
Fortunately, cars use a specially designed type of glass
that’s resistant to shattering - at least, shattering in a dangerous way.
Car windows usually have heat-treated or tempered glass.
Manufacturers melt sand and other stuff like they normally would to make glass,
but they don’t let it cool down like they normally would,
with basically all parts solidifying simultaneously.
Instead, they force the outermost layer to freeze first.
The liquid center would usually shrink as it freezes, kinda like an underbaked cake.
But it can’t do that, because the edge is already solid,
so there’s no slack for it to sink down with.
This creates a solid pane that’s constantly pulling against itself,
which also makes it resist anything external that might pull it apart.
Tempered glass can be up to five times stronger than untempered glass.
And when it does break, all that tension gets released at once,
immediately creating countless weak points in the glass.
Cracks immediately find other cracks, so they can’t create those
molecularly sharp edges that we usually worry about with broken glass.
Releasing all that tension at once means that tempered glass
doesn’t really shatter into big pieces.
It explodes.
But it’s an explosion of dull-edged clumps,
so the pieces are much safer to be around.
Windshields go one better than tempering and put a thin sheet of plastic
between two panes of tempered glass to make what’s called laminated glass.
The sticky plastic holds the glass in place even if it breaks a little bit, and anything
that might go all the way through the windshield first has to travel through a strong,
tempered pane; then sticky, bendy plastic; and then another tempered pane.
Because it holds together and has all those layers, laminated glass is even
stronger than the same thickness of tempered glass would be on its own.
And bulletproof glass takes this idea to the next level.
If laminated glass is a tempered glass sandwich, bulletproof glass is a
laminated glass sandwich: Glass, plastic, glass, plastic, glass, plastic, glass…
And so on until it’s strong enough to stop a bullet, while still being see-through.
That said, glass is heavy, especially once you start making sandwiches with it.
In most true bulletproof glass, some or even all of the glass
is replaced with a lighter plastic like polycarbonate or acrylic,
or designers might just go for one really thick sheet of one of those plastics.
So really, we might be better off calling bulletproof glass
“bulletproof-glass-and-plastic-and-other-plastic-and-glass sandwiches
that sometimes don’t have any glass at all”.
But that doesn’t quite roll off the tongue.
So there’s a lot more to making resilient, sturdy,
crack-proof glass than meets the eye!
Now, if only researchers would apply all this knowledge
to create a true shatterproof wine glass, then we’d be in business…
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