When we think of glass, we automatically imagine that it can be broken with a hammer. Tempered glass, on the other hand, has a special quality in that it fractures the entire piece even if just a little portion of the tip breaks, and it does not break sharply.
The invention of tempered glass started with understanding the mechanism of Prince Rupert’s drop. Prince Rupert of the Rhine discovered this glass in 1640 (Wikipedia Contributors, 2020).
Simply putting molten, hot glass into water creates this object. Thermal expansion causes the surface of the glass to compress as it cools, increasing internal pressure. Additionally, the inner component eventually cools and shrinks as well (Destin, 2022).
However, the whole volume of the glass becomes trapped since the outer portion has already cooled and hardened, creating a very high internal tension. As a result, the inside receives excessive tensile stress while the exterior faces extreme compressive stress. This is how Prince Rupert’s drop is created (Kooij et al., 2021).
Shaped like a water drop with a long tail, the base part of Prince Rupert’s drop is hard enough to be bulletproof. However, the thin and weak tip of the tail can easily be broken with a finger and fall off. The whole glass drop won’t shatter but explode (Kooij et al., 2021; Destin, 2022).
Using this principle, tempered glass is produced. The stronger the glass, the higher the compressive surface stress. Once the glass is cooled, it’s challenging to process it into another shape. If the initial temperature and cooling temperature are miscalculated, the glass may not have enough compressive surface stress, or it could shatter without external impact. Thus, when making tempered glass, precise calculations and preplanning of the mold and glass color are essential (Bendhelm, 2016).
Tempered glass, derived from the intriguing principles of Prince Rupert’s Drop, represents a remarkable achievement in glass manufacturing. Its ability to resist shattering and break into non-sharp pieces has made it highly sought-after for various applications, including safety glass in automobiles, buildings, and even mobile devices. Understanding the science and history behind tempered glass allows us to appreciate the intricate process that transforms regular glass into an unbreakable yet delicate material.
Reference list
bendhelm (2016). Tempered Glass Production. [online] www.youtube.com. Available at: https://youtu.be/dTKFPEzDk44 [Accessed 8 july 2023].
destin (2022). Prince Rupert’s Drop EXPLODING in Epoxy Resin at 456,522 fps – Smarter Every Day 273. [online] www.youtube.com. Available at: https://youtu.be/X3o71W4uNHc [Accessed 8 july 2023].
Ford, M. (2001). How is tempered glass made? [online] Scientific American. Available at: https://www.scientificamerican.com/article/how-is-tempered-glass-mad/ [Accessed 8 july 2023].
Kooij, S., van Dalen, G., Molinari, J.-F. and Bonn, D. (2021). Explosive fragmentation of Prince Rupert’s drops leads to well-defined fragment sizes. Nature Communications, [online] 12(1), p.2521. doi:https://doi.org/10.1038/s41467-021-22595-1.
Wikipedia Contributors (2020). Prince Rupert’s drop. [online] Wikipedia. Available at: https://en.wikipedia.org/wiki/Prince_Rupert%27s_drop [Accessed 8 july 2023].
