Glass that Explodes: Prince Rupert's Drops

You may be wondering what a Prince Rupert's drop is. A prince Rupert's drop is a piece of glass that has a broad end, and a long, thin tail. Prince Rupert's drops are formed by dropping a glob of molten glass into a bucket of water, causing the much of the glass to clump up at one end, leaving behind a long tail, hence the shape. The broad end is extremely strong, and can often be hit with a sledgehammer and still not break. The long, thin tail however, which becomes increasingly thin as you move away from the broad end, is very brittle where it comes to an end, where it breaks so easily that it can be cut by scissors. Here is where it gets interesting. If you cut or break the thin end, the entire piece of glass explodes, including the very strong broad end.

Why Does It Explode?
Because prince Rupert's drops are cooled suddenly by dropping them into a bucket of water, the outside cools faster than the inside. Because glass contracts when it cools, this means that once the glass on the inside contracts slower than the outside. By the time the core of the glass starts to contract, the outside is already cooled to the point where it is a solid. The only way the inside can contract is by pulling in on the solid outer layer.

This is the reason the broad end is so strong, because when the inside of the glass pulls in on the outside, it causes the outside to compress. Ironically, this is also the reason why the glass explodes if the tail is broken. The outer part of the entire piece of glass is under pressure, and when the tail is broken, some of that pressure is released. When this pressure is released, the releasing of pressure causes more glass to break, which then releases more pressure, which then causes more glass to break, in a wave that travels down the tail toward the thick end. This video explains Prince Rupert's drops pretty well, and also shows it breaking in slow motion.

Similar To Tempered Glass
A Prince Rupert's drop is structurally similar to Tempered glass, which is also made by cooling the outside of glass faster than the inside. However, tempered glass is a sheet rather than a head with a long tail. Like Prince Rupert's drops, tempered glass is difficult to break, and also like the Prince Rupert's drop, once it breaks, the entire thing "explodes" or shatters. Because of that, it is impossible to cut through, because once it is broken, the pressure begins to be released, destroying the entire piece of glass.
The guy in the video below tried etching a hole in tempered glass with hydrofluoric acid, also known as HF, which is an acid that eats through glass. While the HF did manage to etch 3/4 of the way through the tempered glass, the entire thing still shattered.

Some Final Thoughts
I will do a post on hydrofluoric acid at some time, since it is so different from other acids, such as hydrochloric and sulfuric acids. For one, it is much more dangerous. Second, the types of burns it causes are also very different from other acids, and the burns are systemic rather than localized to the area of contact. As you can imagine, that means HF burns are much more likely to be fatal than burns from other acids. It is also virtually the only simple acid that will eat through glass.

Since watching the video about trying to etch through tempered glass, it made me think of the Prince Rupert's drop. It made me wonder if it would be possible to chemically etch through a Prince Rupert's drop.