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Science Undusted: Prince Rupert Drops

Let’s talk about shattering glass. As one does.

Probably as far back as molten glass was first produced intentionally by man (i.e. not volcanic glass drops formed naturally, which are known as Pele’s tears) drips of molten glass fell into the glass blower’s water, kept nearby to cool the glass products in. Sometimes, these drops have particular strength, and these alone are true Prince Rupert Drops.

The English name is a classic example of who knew who, and who wrote the history books: The drops had been made Mecklenburg, northern Germany, since 1625, though some think they go back as far as the days of the Roman Empire; they were sold around Europe as toys or curiosities. In 1660, Prince Rupert brought some of them back to London as a gift for King Charles II, who then gave them to the Royal Society to investigate and attempt to reproduce. Thus, Rupert’s name has gone down in history – for bringing back a souvenir for the right person.

Their unusual strength comes from how they cool; when done right, they will come out shaped like a tadpole with a long, thin tail. The heads can be struck with a hammer or shot with a gun, and they will not shatter; but if the tail is knicked or disturbed, the entire drop shatters into glass dust instantly, from the tail down.

Not all drips of glass cooled in water produce Prince Rupert Drops, also known as Dutch tears, Prussian tears, or Batavian tears; the difference is their behaviour and is probably influenced by impurities or inclusions in the mineral composition of the material used. Higher quality glass produces pure Prince Rupert’s Drops; low quality may simply not have the tension created in the cooling process to be a shattering success. I have a drop of molten glass, found on a beach in the Scilly Isles (UK); it is the result of a shipwreck from around the 17th or 18th century, and the drop is full of the impurities of the orginal glassware plus black flecks of cinders from the burning ship. The tail was snapped off over time, yet the head survived – thus, it was not a Prince Rupert Drop.

For a cool video that explains the science behind the drops, including slow motion analysis of the shattering, just click on the image below; the video is from one of my favourite YouTube channels, Smarter Every Day. For further behind-the-scenes footage of another experiment that Destin did with these drops, click here.

Prince Rupert Drop

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