A Prince Rupert's Drop is a bizarre and fascinating physical wonder. The structure itself is made from dropping molten glass into cold water, creating a bulb with a trailing "tail". Once the glass has hardened and is removed from the water, it somewhat resembles a tadpole. Although these drops have all the appearance of a delicate work of art that you might find in an exhibit or a museum, there is more to them than meets the eye, particularly the strength of the bulb compared to the fragility of the tail.
What makes the bulb so strong?
The simple answer: tension! When the molten glass is dropped into the cold water, the tail and outside of the bulb stiffen and solidify almost instantaneously. However, once the outer shell of the bulb has been "frozen," or set into place, the inside is still cooling down. As materials cool, they shrink, or compress. Therefore, the inside of the bulb is compressing while the outside has already been solidified. This creates a tension force inside the drop, as well as a compression force on the outside. Once the glass has completely solidified, those forces solidify with it, making the glass incredibly strong.
This type of method is not only used in artistry, but also in glass tempering. Tempered glass is roughly four times stronger than normal glass. Although different methods may be used than those which manufacture a Prince Rupert's Drop, the general physics is the same. Tension inside the glass and compression on the outside allow the glass to withstand significant external pressure.
Why is the tail so fragile?
Why was the tail so breakable, and why did the entire structure blow up when the tail was compromised? Since the tail is so thin, it was able to cool almost instantaneously when placed in the cold water. This means that it did not get the benefit of the inside cooling more slowly than the outside, which is what creates the tension and compression for the bulb. The tail is just a piece of normal, untempered glass. However, since it is attached to the bulb, if the tail shatters, there will be a chain reaction which spreads to the outside of the bulb, causing the outer layer to shatter as well. Once the outer layer is shattered, there is nothing left to create the tension that was keeping the inside together, and the whole thing blows apart.
The Prince Rupert's Drop truly is a marvel. Dainty and delicate, one would never guess that such a small, beautiful piece of glass could act much more like an orb of iron. To learn more about glass tempering and its uses in several industries, contact companies like TSS - Sales & Service.