Tempered glass is an important safety feature that is simultaneously much stronger than standard glass yet at the same time shatters in a way that is much safer than traditionally produced glass, which along with lamination makes it an optimal window glass replacement.
It was initially invented and patented by Francois Barthelemy Alfred Royer de la Bastie in 1874, through a process that involved quenching molten glass in a bath of oil or grease.
However, the process behind tempered glasses had been studied for centuries before this, perhaps going back as far as the Roman Empire, though a phenomenon known as Prince Rupert’s drops.
Prince Rupert’s drops, named for Prince Rupert of the Rhine, a person who did not invent or discover the phenomenon but brought it to King Charles II of England’s attention, are glass beads that resemble teardrops.
They are made by letting molten glass drip into a bucket of water, which rapidly cools it.
What made it so fascinating to the Royal Society, Margaret Cavendish, Duchess of Newcastle and other great 17th century thinkers was the diametrically opposed properties it had.
The bulb end was exceptionally strong, to the point of being bulletproof or resistant to hammer blows. However, if the tail end was slightly touched it would crumble immediately into powder.
The reason for this is that inside the drop are very high residual stresses which are accelerated to a speed of up to 4,000mph, causing a disintegration that can only be studied with the help of high-speed cameras.
In 17th century high society it was a common party piece, and people in high society were expected to be very familiar with them, even if the understanding of why they were so strong at one end and so weak at the other would not be known for centuries.
The drops are one of, if not the earliest example of toughened glass, and the interest in solving the drop’s mysteries inspired Mr Royer de la Bastie’s experiments and later patenting of toughened glass.