Tempered Glass

The tempered glass door on my balcony recently shattered. We still don't know what exactly caused it, but that's not important. What is important is how it broke into thousands of tiny shards. These shards may cause some minor cuts, but they are way better than traditional glass which would have broken into large dagger like pieces. Another lifesaving application of this kind of glass is in car windshields where you don't want a hundred knives flying through the air in the event of a car crash. To explain why it shatters into those tiny pieces, let me first introduce you to the Prince Rupert's drop.

A Prince Rupert's drop is formed when molten glass is dripped into water. It consist of a small glass 'head' and a long 'tail'. The head is immensely strong and can withstand hammer blows easily. But as soon as the fragile tail is broken, the whole thing explodes into what is essential fine glass sand. The reason behind this is pent up stress in the drop. When the molten glass is dripped into water, the out layer cools and solidifies immediately. But the inside of the drop is still liquid. As the inside cools, it wants to contract. But since the outside glass is already a solid, it cannot contract and so enormous amounts to tension is stored inside of the glass. This property makes the head strong, since all of the molecules are tightly held a fracture cannot start here. But if the tail is broken, a small shockwave starts that releases the stored energy. This shockwave breaks the next section of glass and this section breaks another section and so on until in a fraction of a second the entire drop is broken.

Tempered glass is like a less extreme version of the Prince Rupert's drop. The newly formed glass is cooled faster than usual using cold air jets. This creates the same tension inside of the sheet of glass such that as soon as a small section is broken the entire glass plane breaks.





Image Citations: 
N.d. Prince Rupert's Drop. Web. 5 May 2016. <http://www.oberlin.edu/physics/catalog/demonstrations/mech/princerupertdrops.html>.
N.d. The American Ceramics Association. Web. 4 May 2016. <http://ceramics.org/ceramic-tech-today/video-glass-science-of-prince-ruperts-drop-captured-with-high-speed-cameras>.