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Folding glass: how, why, and the truth of Samsung’s Z Flip

It’s glass, but not what we expected

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Photo by Nick Statt / The Verge

On February 11th, 2020, Samsung introduced the world’s first folding glass phone: the Galaxy Z Flip. Samsung declared that it had broken the laws of physics by bending glass, specifically by making “a leap from polymer screens to ultra-thin glass technology.”

Samsung made it sound like it had invented a new proprietary form of glass, and the company even dubbed it Samsung Ultra Thin Glass (UTG). “We’ve done the impossible and created ultra thin glass that folds,” the company’s presentation claimed, adding that it “protects your screen from scratches.”

But as the world would see days later, physics isn’t so easy to ignore.

GIF: Samsung

Glass experts could have warned you about what happened next, when it turned out that Samsung’s supposed “leap” does use polymer and is easy to scratch after all. Samsung’s dust-resistant “fiber shield” immediately went on to fail an aggressive dust test, and the company’s claims around glass have now retreated to the ridiculous tagline “tough, yet tender.”

But Samsung didn’t lie about the primary innovation here: the Galaxy Z Flip is truly a folding glass phone. It’s just that glass is actually made by German manufacturer Schott, it’s got a soft, scratchable plastic layer up top, and — hopefully — future folding glass phones won’t require that extra protection.

To understand why, you should probably first understand how glass can fold at all. 

Image: Schott

Flexible glass is real

If I’m being honest, I’m still wrapping my head around it myself. Glass is fragile; even if you’ve somehow never broken glass yourself, you definitely know how it sounds when it shatters. It’s tough to even imagine how such a brittle material could be turned into a bendable flip phone, which is why I’m not surprised when Samsung found itself facing accusations that the Galaxy Z Flip’s ultra-thin glass might not be glass at all.

And yet the physics of folding glass is remarkably simple, according to two material scientists, as well as representatives from Gorilla Glass maker Corning and Schott itself. The short version: practically anything can be bent if you make it thin enough. 

“One of the most common misconceptions about glass is that it breaks because it’s inherently weak” Corning

“All the materials we know of that are very rigid can be bent to some extent,” says Schott’s Dr. Mathias Mydlak, a chemist who now leads business development for the company’s ultra-thin glass. “If you think of wood, a 2x4 cannot be bent, but if you chisel a very fine piece of it […] the same is true for glass,” he told me late last year. While a normal windowpane might not seem to bend before breaking, a thin enough ribbon of glass actually can.

When you bend a material, glass or otherwise, what you’re doing is naturally stretching the material on the outside of your bend — and even glass has some flex. “You could imagine a metallic spring between every two atoms and the spring elongating when you stretch the two atoms apart,” says Erkka Frankberg, a research fellow at Tampere University who studies unconventional forms of glass.

A demo machine bends Schott’s earlier, slightly less flexible glass.
A demo machine bends Schott’s earlier, slightly less flexible glass.
Image: Schott

The main gotcha: you can only stretch a material’s chemical bonds so far before it breaks, a concept known as tensile strength. But if you use a thinner sheet of glass, you’re stretching less of the material across the same space, bending fewer layers and stretching fewer chemical bonds, Mydlak says. That puts less tensile strain on the glass, letting you bend it tighter before it snaps. Thinning out the glass is one of the “two main tricks” to making it bend, says MIT associate professor of materials science Juejun Hu, because physics students learn that tensile strain scales linearly with thickness.

When you get glass down below a hundred microns thick — around the thickness of a human hair — that’s when it can bend far enough for basic folding gadgets, experts say. If you want a phone that folds inward without a huge hinge gap, we’re talking tens of microns. Think aluminum foil.

Schott’s latest ultra-thin glass.
Schott’s latest ultra-thin glass.
Image: Schott

That thinness is already possible with glass that exists today. I touched a 70-micron sheet of Schott glass four years ago, and bent it with my own hands. Samsung says its Z Flip uses a 30-micron-thick glass, and Schott says it’s gone down to 25 microns in the lab as well. Both Corning and Schott use somewhat similar techniques, each creating a floating glass sheet in midair by drawing thin ribbons of molten glass out of specially shaped vats.

But thin isn’t enough by itself. The second “main trick” for bendable glass is to strengthen it against imperfections, particularly at the surface where they can cause catastrophic failures when the glass is stressed. A tiny air bubble, speck of dirt, or minuscule scratch during the manufacturing or handling process might be enough to destroy a piece of bendable glass, experts say. “All the stresses you produce in the glass tend to concentrate on those small defects, and that’s where it will produce a fracture,” says Frankberg. 

That’s where chemical baths and heat treatments can come in to strengthen and temper the glass, as we’ve written about before, but the trade-off is that they can make glass easier to scratch. And as your glass gets thinner — thin enough to bend — a scratch becomes even more worrying.

Scratches are deadlier

While Mydlak says that a piece of Schott’s glass “can go on forever” in controlled conditions if it survives the first bend test, he admits that one scratch could change that. “If you scratch it with something … this is going to be an initial defect that can cause problems later on,” he says. If a piece of glass with a large enough scratch is bent with enough stress concentrated on a “pre-crack,” says Frankberg, “it will basically propagate with the speed of sound and catastrophically go through the whole material.” 

Corning uses foam to easily illustrate why a crack might be a problem when bending.
Corning uses foam to easily illustrate why a crack might be a problem when bending.
Image: Corning

The thing to remember is that while glass is more scratch resistant than plastic, it’s not scratchproof — it may scratch at a Mohs hardness of 5 or 6, instead of 2 or 3 for plastic, but it scratches anyhow. I bet you have some tiny scratches on your phone’s traditional glass screen right now. But I’m also guessing you never need to bend your traditional screen like you would with a folding phone. Now it’s a way bigger deal.

Frankberg thinks that’s probably why Samsung’s Galaxy Z Flip still has a plastic screen protector up top. Not to prevent scratches or help it bend more easily, but as a “sacrificial layer” that gets scratched instead of the glass, so there are fewer potential catastrophic cracks down the road. “Probably they receive fewer product returns due to the extra layer,” he speculates. 

Schott’s Mydlak actually hinted as much to me late last year, before the Z Flip was announced: “Most probably you would not be touching the bare glass at all, but something on top of it,” Mydlak said when I asked about whether scratches would be something to worry about with the first batch of folding glass phones.

Zack Nelson, aka JerryRigEverything, removes the Z Fold’s “protective layer.”
Zack Nelson, aka JerryRigEverything, removes the Z Fold’s “protective layer.”
Image: JerryRigEverything

I should add that we don’t fully know what Samsung and its partners are doing with Schott’s glass screen before adding it to the phone. Schott could only confirm that it’s delivering its ultra-thin glass to Samsung, saying that it “cannot comment on any processing details of the raw glass material.” And while Samsung has admitted that it comes with a pre-installed screen protector, the company announced on Wednesday that it’s also “injecting” the glass “with a special material up to an undisclosed depth to achieve a consistent hardness,” whatever that means. Samsung hasn’t been willing to say more, and it sounds fairly hush-hush.

But Samsung does have another partner involved in that process. It’s been working with Korean screen manufacturer Dowoo Insys on this glass since 2013, Yonhap News reports, and recently locked it down by becoming the largest shareholder in the company. Allegedly, Samsung has an exclusive on Dowoo’s process but not Schott’s ultra-thin glass, and Samsung reportedly decided to use both for the Galaxy Z Flip’s screen. 

What good is glass?

All of this still doesn’t answer the question, though: what good is foldable glass if it scratches like plastic anyhow? One answer is that it may last longer; Schott says glass won’t break down like plastics naturally do over time. There’s also optical clarity to consider. Plus, as Dieter Bohn noted, the Galaxy Z Flip’s glass — polymer cover or no — just plain feels nicer than the pure-plastic-covered original Galaxy Fold. The crease at the fold is less prominent and though it can be indented with a fingernail, it feels firmer than what’s on the Galaxy Fold.

But Gorilla Glass maker Corning wants you to know the Galaxy Z Flip’s screen protector may not be necessary in a year or two. The company tells me samples of its new bendable glass are already in the hands of device manufacturers. It expects devices in the market in the next 12 to 18 months, and it’s shooting to deliver “durability, scratch resistance, and optical clarity” all in a single package for foldable devices.

While a rep couldn’t yet say how Corning’s doing that or explicitly confirm there’d be no plastic involved, plastic doesn’t appear to be the goal: “Today, when you buy a phone with Gorilla Glass, you’re touching glass … that’s what we’re working towards.”

Mass-producing glass thin enough to reliably bend is a tough challenge, Corning and Schott agree. Mydlak told me last year that Schott was still figuring out a few production issues, like how to properly cut, pack, and ship reels of the very thinnest glasses it had developed without damaging them along the way. It sounds like the existence of the Galaxy Z Flip means it’s worked some of that out — but low yields also may be one of the reasons folding phones are so incredibly expensive, and why Corning has yet to fully figure it out.

Corning’s ultra-thin glass.
Corning’s ultra-thin glass.
Image: Corning

It’s hard to blame Samsung for pushing so hard, even if both its foldables have stumbled out of the gate — for one thing, it wants to sell folding screens to other companies, too, as it all-but-revealed in its latest press release. But that doesn’t make it any less frustrating to see that a phone that was promised as scratch-resistant can easily be scratched, and it doesn’t mean you need to buy in. Though if you do, Samsung tells The Verge that you can get a free screen protector and / or a one-time $119 screen replacement if your device does succumb to scratches.

Image: Corning

Frankberg isn’t yet convinced. “My honest opinion as a materials scientist: I wouldn’t buy these kinds of phones at this stage because the size of the scratch you need to break it is very very small,” he says, pointing out how a single grain of sand could destroy a folding glass screen. “Buying a very expensive phone, I would step back and see how the technology improves.”

Mind you, Frankberg has an idea of how such screens might eventually evolve: he’s in the very early stages of developing a form of glass that uses aluminum oxide instead of the traditional silica, one that can bend instead of breaking, and is exploring other alternative materials, too. (It wouldn’t be the first time the world’s looked into “alumina” glass, and you may also be familiar with aluminum oxide’s condensed crystalline form, sapphire.)

I just wonder if we’d actually call such a material “glass” if or when it ever arrives. Like JerryRigEverything’s Zack Nelson is fond of saying when he scratch-tests a new glass screen, it’s hard to shake the stereotype. “Glass breaks. If anyone ever comes up with something stronger than glass, they’ll definitely be calling it something different.”