You’ll know when it’s over.
Upon impact, a small amount of elastic energy is converted into acoustic energy — that resounding indication of breaking glass. But the majority of the elastic energy stored in the glass will be converted into two — or more — new surface energies. A crack. Or several cracks. With one eye closed, you’ll reach down gingerly, reluctant to turn it over. There won't be any shards to pick up off the pavement, only a phone with a freshly splintered screen, the jagged lines and spiderwebs forming some kind of painful abstract art.
You've just smashed your phone's screen.
The glass layered on the front of your phone is a lot of things: thin, flexible, transparent. But the only two things that matter when you drop it are its hardness and its strength. Hardness is resistance to abrasion. It’s your screen’s first line of defense against car keys, kitchen tables and concrete. Strength is the relationship between the surface compression and inner tension; it determines, among other things, how many blows a piece of glass can endure before it shatters completely. Glass only breaks in tension, when the force of impact finally overcomes the surface compression. A scratch doesn’t shatter glass, but small stresses will eventually lead to a colossal splintering.
Your phone is in fact covered with some very hard, very strong glass that's only 0.8 mm thick. Yet it takes 30 times more force to scratch the iPhone screen than it does a piece of plastic, according to Apple. And under perfect conditions, the latest chemically tempered Corning Gorilla Glass — which is more likely than not covering the front of the new phone in your pocket — can withstand around 100,000 pounds of pressure per square inch. Or in English, a 1.18-pound iron ball dropped from six feet.
Building glass that tough is an arduous process. A 400-degree Celsius bath of molten salt replaces the small sodium ions — the tiny particles that make up aluminosilicate sheet glass — with larger potassium ones that “stuff” themselves onto the surface, for increased compression. This makes the glass harder and more resistant to deeper scratches. Jets of cold air are then blown across the top of the glass, which allows the outside to cool and harden faster than the inside, giving it lots of compression on the surface, but tension in the middle. The push and pull between the two is what makes tempered glass so strong.
If your phone had belly flopped onto the ground, landing completely parallel to a flat surface, you'd have been a little luckier, because the stress of impact would've been spread across the entire surface. It would've emerged weaker, sicker, one blow closer to death, but unscathed to the naked eye. It would still feel smooth beneath your fingers.
But the uneven surface meant that the point of contact between the glass and ground was small and focused, directing the entire force of the impact onto one small point. And the screen had already been nicked by keys and pocketfuls of change, dinged by months of being tossed onto tables and countertops. Each tiny scratch moved the material apart at a microscopic level, almost like a piece of fabric slowly tearing, creating a shallow crevice at the surface. With every fresh trauma, those invisible crevices grew deeper. Sharper. The process accelerates over time. The flaw propagates. It grows and grows, until eventually it becomes visible and leaves you with two new adjacent surfaces: a crack.
The crack moves perpendicular to that initial surface scratch, racing to break the tension that holds your screen together from the center. Finally, it shatters. The phone simply slipped, so glass shards didn't eject themselves like thousands of fighter pilots upon the sudden release of pent up elastic energy. No, there's just a zig-zag spiderweb imprinted on the screen, one that you can clearly see but only sort of feel, like a quasi-3D etching. No other glass dies like this — it's a strange pattern that's unique to tempered glass, which is doing its job, reducing jagged edges that would render your texting days over. The glass, sandwiched between a thin coating, digitizer and display, is shattered, but it doesn't move.
It's impossible to determine the exact strength of a piece of glass, and there is no such thing as a flawless one. In the process of strengthening, imperfections are inevitable. For these reasons, we don’t know the exact force or number of incidental drops that our phones can survive; some say four spills is all it takes. Like people, each screen’s resilience is different. Some of us can withstand years of small scratches at the surface — for others, one big fall is enough to break us.
*Special thanks to Dr. Arun K. Varshneya of Saxon Glass for help with this story.