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The Secrets of Prince Rupert's Exploding Glass Drops

Prince Rupert's Drops are formed by dropping hot molten glass into cold water. Stresses created in the process make the glass resistant to smashing with a hammer. But they can also explode unexpectedly. Image (Image credit: Michael Grogan, University of Virginia)

Six years ago, I was looking out the window of a plane bound for England, admiring the sunlight illuminating the slightly pink clouds. My British companion remarked that the clouds were just like candy floss—Britspeak for cotton candy. Those sweet thoughts were rudely interrupted by a small popping noise from within my handbag, then another . . . and another. Unfortunately it wasn't candy. My heart sinking, I discovered that the bottom of my bag was littered with powdered glass. Three of the little Prince Rupert’s drops I had so carefully made for a lecture at Oxford University had exploded.

I first came across Prince Rupert's drops when I was reading an early "chymical" text at the British Library. In it the drops were called "Greatricks glasses." After a week of trying to find out who Greatricks was—mostly by reading up on a seventeenth-century faith healer named Valentine Greatrakes—I e-mailed a desperate SOS out to my colleagues. A kind soul informed me that "Greatricks" might have simply meant "great tricks": King Charles II of seventeenth-century England had used the drops as practical jokes after being introduced to them by his nephew, Prince Rupert of Bavaria.

Typical of "virtuosi" of this period, gentlemen who dabbled in everything and excelled at much, Prince Rupert had a varied career as an artist, military officer, and scientist. His scientific curiosity focused on the great-trick glasses: teardrop-shaped beads formed by dropping molten glass into cold water. The bulbous head can withstand hammering on an anvil, but breaking the curved, tapered tail shatters the entire drop into fine powder. The king would have subjects hold the bulb end in their palms, and then he'd break off the tip, startling the victims with a harmless little explosion.

Science gets involved

Charles II, the founding patron of the Royal Society, directed its members to explain why the "chymical glasses" exploded. So riveted was the Society by the glasses that in 1663, an enterprising satirist wrote a ballad about the group's obsession:

And that which makes their Fame ring louder With much adoe they shew'd the King To make glasse Buttons turn to powder, If off the[m] their tayles you doebut wring. How this was donne by soe small Force Did cost the Colledg a Month’s discourse.

Robert Hooke, the Society's Curator of Experiments, did the first detailed examinations of the drops: he coated them with a transparent glue, wrapped them in leather, broke off their tails, and then viewed the glass fissures under his microscope. But it was not until 1994 that their secret was fully revealed. Munawar M. Chaudhri of the University of Cambridge, England, and Srinivasan Chandrasekar of Purdue University in Indiana did high-speed photographic analyses of the drops, observing the cracks accelerating from the drop's tail towards the head at more than 4,000 miles per hour.

The two engineers realized that when a drop is formed, the cold water rapidly cools and solidifies the molten glass on the outside. The glass within, still hot, then gradually contracts, causing large compressive stresses on the surface as well as interior tension. That inner tension is enough to blow the drop apart at the tiniest crack, making the thin tail vulnerable. But that tension also accounts for the round head's toughness, by pulling the outer layers of the head tight.

Give it a try

Finding some instructions for making Prince Rupert's drops in The Art of Glass, written by a Florentine chemist, Antonio Neri, in 1662, I decided to try myself.

Neri advised, "The best way of making them, is to take up some of the Metall [molten glass] out of the pot upon the end of an Iron rod, and immediately let it drop into cold water, and there lye till it cool." Using a crème brûlée torch on steroids, I heated up a glass rod, placing it very close to the surface of a beaker of ice water to catch the drops. Some of the glass tears broke when I fished them out of the water, but soon I was armed with a dozen, carefully taped to an index card to take with me for my lecture.

Though a few drops did not endure the plane trip, because a change in air pressure can destabilize the glass, my audience was delighted with those that survived. Recently, a video of exploding drops has even appeared on YouTube. Prince Rupert had discovered a “great trick” indeed to entertain royals and Internet junkies alike.

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Anna Marie Roos is a research associate at the Wellcome Unit for the History of Medicine at the University of Oxford; she recently wrote The Salt of the Earth: Natural Philosophy, Medicine, and Chymistry in England, 1650-1750 (Brill, 2007).