The strength of the types of walk-on glass that can be used to create incredible feats of architecture is the result of incredible cutting-edge materials science research, precision manufacturing processes, and a not-insignificant degree of serendipity.
The use of laminated materials to keep broken glass together was the result of a young chemist accidentally leaving cellulose nitrate in a glass flask and then accidentally knocking it over, in one of the few legitimate cases of two wrongs making a right.
The invention of tempered glass was developed through remarkably similar means. Whilst modern toughened glass is designed to be strong enough to walk on and safely break in situations where it would shatter, these unique properties were the result of trying to solve a centuries-long mystery.
At Once Bulletproof And Infinitely Fragile
The origins of tempered glass can be found in Prince Rupert’s drops, an extremely unusual byproduct of early glass production.
If molten glass droplets fell into cold water they cooled extremely rapidly, forming a tadpole-like shape which had two astonishingly different properties.
The head was strong enough to withstand the force of a bullet without the slightest hint of any damage, but if the tail was cut or even slightly nudged, it would explode into a burst of tiny glass fragments.
Prince Rupert of the Rhine originally brought them to England in 1660 largely as a party piece and a curiosity; it was often used to show off at dinner parties as guests would take turns hitting the droplet with a hammer only for it to cause no damage.
A very similar device, known as the Bologna bottle, had extremely similar properties and was often used for magic tricks. The bottle was used to perform feats of strength such as hammering nails before suddenly disintegrating, typically by rattling a coin or tiny object stuck in the bottom.
Given that the same phenomenon exists with lava (known as Pele’s tears), it is likely that Prince Rupert’s drops are as old as glassmaking itself, but whilst initially presented to the Royal Society more as a puzzle than a scientific experiment, the high interest generated by the strange droplets led to one of the most important discoveries in glasswork.
Prince Rupert And Toughened Glass
Tempered glass is manufactured using a fundamentally similar process of heat treatment followed by rapid cooling, which exponentially increases its strength, whilst also allowing it to break more safely.
What was discovered during the research into Prince Rupert’s drops is that the seemingly paradoxical properties are actually interconnected, although it took until the development of high-speed photography to categorically prove this.
The exterior surface of the glass is in a state of compression whilst the interior is in a state of tension, creating a state of unusual equilibrium.
Tempered glass was originally created in 1874 using the same process of thermal quenching that Prince Rupert’s drops were, although more even heat and chemical treatments are more commonly used today to make tempered glass more versatile.
This means that two of the most important developments in the history of glassmaking were the result of a fascinating glassmaking curiosity and an accident, both of which were refined by centuries of research.