Prince Rupert's Drops are a glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 curiosity created by dripping hot molten glass into cold water. The glass cools into a tadpole

Tadpole

A tadpole or polliwog is the wholly aquatic larval stage in the life cycle of an amphibian, particularly that of a frog or toad.- Appellation :...

-shaped droplet

Drop (liquid)

A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop...

 with a long, thin tail

Tail

The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

. The water rapidly cools the molten glass on the outside of the drop, while the inner portion of the drop remains significantly hotter. When the glass on the inside eventually cools, it contracts inside the already-solid outer part. This contraction sets up very large compressive stresses on the exterior, while the core of the drop is in a state of tensile stress. It can be said to be a kind of tempered glass.

The very high residual stress

Residual stress

Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...

 within the drop gives rise to unusual qualities, such as the ability to withstand a blow from a hammer on the bulbous end without breaking, while the drop will disintegrate explosively

Explosion

An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

 if the tail end is even slightly damaged.

Damage

When the tail end is damaged, the large amount of potential energy stored in the drop's crystalline or amorphous atomic structure is released, causing fractures to propagate through the material at very high speeds.

Recently an examination of the shattering of Prince Rupert's Drops by the use of extremely high speed video has revealed that the "crack front" which is initiated at the tail end propagates in a disintegrating Drop within the tensile zone towards the drop's head at a very high speed (~ 1450–1900 m/s, or up to ~4,200 miles per hour, a number that in air would be Mach

Mach number

Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

 5.5).

Because of the transparency of glass, the internal stress within these objects can be demonstrated by viewing them through polarizing filter

Filter (optics)

Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....

s.

History

A scholarly account of the early history of Prince Rupert’s Drops is given in the Notes and Records of the Royal Society

Royal Society

The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...

 of London. Most of the early scientific study of the drops was performed at the Royal Society.

The drops are reliably reported to have been made in Mecklenburg (North Germany) at least as early as 1625. However, it has been claimed that they were invented in Holland, hence a common name for them in the seventeenth century was larmes bataviques or lacrymae Batavicae. The secret of how to make them remained in the Mecklenburg area for some time, although the drops were spread across Europe from there, for sale as toys or entertainments.

It seems clear that Prince Rupert

Prince Rupert of the Rhine

Rupert, Count Palatine of the Rhine, Duke of Bavaria, 1st Duke of Cumberland, 1st Earl of Holderness , commonly called Prince Rupert of the Rhine, KG, FRS was a noted soldier, admiral, scientist, sportsman, colonial governor and amateur artist during the 17th century...

 did not discover the drops, but played a role in their history by being the first to bring them to Britain, in 1660. He gave them to King Charles II

Charles II of England

Charles II was monarch of the three kingdoms of England, Scotland, and Ireland.Charles II's father, King Charles I, was executed at Whitehall on 30 January 1649, at the climax of the English Civil War...

, who in turn delivered them in 1661 to the Royal Society (which the King had created the previous year) for scientific study. Several early publications from the Royal Society give accounts of the drops and describe experiments performed. Among these publications was Micrographia of 1665 by Robert Hooke

Robert Hooke

Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

, who later would discover Hooke’s Law. His publication laid out correctly most of what can be said about Prince Rupert’s Drops without a fuller understanding than existed at the time, of elasticity

Elasticity (physics)

In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....

 (to which Hooke himself later contributed so greatly) and of the failure of brittle materials from the propagation of cracks. A fuller understanding of crack propagation had to wait until the work of A. A. Griffith in 1920.

Literary references

Because of their use as a party piece, Prince Rupert’s Drops became widely known in the late seventeenth century — far more than today. It can be seen that educated people (or those in “society”) were expected to be familiar with them, from their use in the literature of the day. Samuel Butler used them as a metaphor in his poem Hudibras in 1663, and Pepys refers to them in his diary.

The drops were immortalized in a verse of the Ballad of Gresham College (1663):

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 doe but wring.

How this was donne by soe small Force

Did cost the Colledg a Month's discourse.

Peter Carey devotes a chapter to the drops in his 1988 novel, Oscar and Lucinda

Oscar and Lucinda

Oscar and Lucinda is a novel by Peter Carey which won the 1988 Booker Prize, the 1989 Miles Franklin Award, and was shortlisted for The Best of the Booker.-Plot introduction:...

.

External links

• Prince Rupert's Drop Video demonstrating the creation, strength, and explosive fragility of a Prince Rupert Drop.
• Video showing the making and the breaking of Prince Rupert's Drops from the Museum of Glass
• http://www.popsci.com/diy/article/2008-06/shattering-strongest-glass Popular Science article with video detailing Prince Rupert’s Drops