Why 'Prince Rupert's Drop' Glass Is Strong Enough to Shatter a Bullet

Summary

For glassmakers, a “Prince Rupert’s drop” is simple to make but strong enough to shatter a bullet.The way a drop of hot glass cools in water is the secret to its strength. The physics that make the drop so tough have applications for thin, yet strong, protective surfaces.Formed of a glass teardrop that elongates into a skinny tail, a “Prince Rupert’s drop” looks like a delicate, clear tadpole. It’s easy enough to snap the tail, but the rounded drop at the top itself? It’s unbelievably strong. You can’t even break it with a hammer blow. When a glassmaker drips molten glass into cold water, it freezes into a bead with an extra long tail. Like Sauron’s One Ring, you can’t break it with a heavy blow from something as strong as an axe. You actually need the speed and force of a bullet to shatter it. The crazy part is that even a bullet can break apart when it hits the drop. (Check out the video at the top of the story from SmarterEveryDay.)The Science Behind the MagicWhen the bullet grazes the bulb of the drop itself, you can see a line of white take off from the tail and race down the drop. This is a traveling shockwave. Two factors—compressive strength built into the drop during its formation and the residual stress of the impact—cause the near-instantaneous destruction of the smooth glass. A fraction of a second later—a stunning sequence visible at 100,000 frames per second—the entire object vaporizes into sparkly smoke. Rather than exploding in all directions, the glass shatters in place because the compression and tension are released all at once.The bullet itself shatters due to the glass head releasing its compressive force. Bullet fragments fly off in a wide array, embedding themselves into the wall behind the setup.Glass melts between 2,552 degrees and 2,912 degrees Fahrenheit. When a glassmaker drops a molten glass bead into a tank of water, its surface cools rapidly, quickly forming a hardened shell around the liquid interior, similar to an egg white cooking f...