… although it is strong enough to withstand the sledgehammer, the tail can be nipped with a pair of blunt-nosed pliers. It takes a little effort. And once it is done it is as if you have taken out the keystone, removed the linchpin, kicked out the foundations. The whole thing explodes. And where, a moment before, you had unbreakable glass, now you have grains of glass in every corner of the workshop — in your eyes if you are not careful — and what is left in your hand you can crumble — it feels like sugar — without danger.

This is protagonist Lucinda Leplastrier’s description of the Prince Rupert’s Drop (or the Dutch tear) from the Booker prize-winning novel Oscar and Lucinda by Peter Carey, which I have recently read.

 

Image from ‘Account of the Glass Drops’ by Robert Moray, 17 August 1661.
Royal Society Classified Papers Cl.P/3i/57

 

The Prince Rupert’s Drop is a tadpole-shaped droplet of glass with a bulbous end and long thin tail, created by dripping molten glass into cold water. The drops have the extraordinary property of being at once extremely strong (the bulbous end can resist a hammer blow) and very fragile (nip the tail with pliers and it explodes).

Lucinda’s witnessing of this spectacle sparks a lifelong fascination with glass which results in her buying a glass factory that she names ‘Prince Rupert Glass Works’. The Prince Rupert’s Drop is used throughout the novel as an analogy for strength and fragility; representing both human resilience and vulnerability.

The phenomenon of the drop really does capture the imagination, on one level it is fun and was often used as a party piece. This was just the kind of curiosity to intrigue the early Fellows of the Royal Society.

 

 

Prince Rupert (see his signature in the Royal Society’s Charter Book), who had a keen interest in scientific research, gave some of these glass drops to his cousin King Charles II, who in turn sent them to the Royal Society for their judgement. The Fellows jumped to it, keen to understand what made these ‘glasse drops’ behave in such a way, and conducted a series of experiments as described in a paper by Robert Moray (the Royal Society’s President at the time) from 17 August 1661.

As well as using cold water to create the drops, they tried dropping the molten glass into oil, milk, ‘spirit of wine’ and vinegar to compare the effects. They experimented with breaking the glass drops in the air ‘If it be broken so as the sparkes of it may have liberty to fly every way, they will disperse themselves in an orbe with violence like a little grenade’. They also conducted tests in other environments including exploding them under water; in a fire; ‘fastened’ in cement; and in a vacuum using Robert Boyle’s air pump: ‘One of them broken in Mr Boyles Engine when the receiver is well evacuated will fly in peeces as in the open air’.

Robert Hooke offered a largely accurate explanation of the phenomenon in his ground-breaking publication Micrographia (1665). Once dropped in the water, the molten glass rapidly cools on the outside while the interior remains much hotter. When the glass on the inside cools, it contracts within the now solid outer part. This causes large compressive stresses on the surface as well as interior tension. It is a kind of tempered or toughened glass; which has practical applications today in the manufacturing of car windscreens and glass doors.

You can read more on the Royal Society and Prince Rupert’s Drops in this article from Notes & Records. These little drops proved of interest to scientists from a range of backgrounds, including Italian astronomer Geminiano Montanari who sent a paper to the Royal Society on the subject in 1671, signing off with this rather philosophical quote:

‘So is a kingdom one and strong but when the top is broken shivers into men.’

 

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