It’s fascinating how much history, science and the unknown is hidden under the cover of one (ordinary) drop. In truth, it is not a drop of water, but a drop of molten and then suddenly cooled glass. Well, let’s start with the story…

Some call it the Prussian tear, others the Dutch tear, however the most common name for this unusual creation is Prince Rupert’s tear, referring to Prince Rupert of the Rhine, Duke of Cumberland (17 December 1619 – 29 November 1682), a German-English military officer, admiral, scientist and colonial governor. It was he who brought the glass drop or tear, as it is often described, to Britain in 1660 to be studied in more detail.

It is unknown when it was first made. There are indications that the glassblowers of the Roman Empire had already known about it. The first records of the making of this fascinating drop come from 1625, and describe its production in Mecklenburg, a province in Northern Germany, from where it was sent to the rest of the world as an eccentric product of craftsmanship.

Since then, until today, it has been arousing interest because of its unusual properties. It is a piece of glass that is toughened by a process known as tempering, that is, by sudden cooling of a highly heated material. A similar process produces hardened iron, which is characterized by greater strength than ordinary iron.

The manufacturing process itself is very simple. A small piece of glass is heated in the hissing flame of a Bunsen burner (which reaches a temperature of about 1200 °C) above a beaker filled with water. At that temperature, the glass softens enough to start behaving like a liquid, forming a drop that falls into a glass of water. At the same time, sudden cooling of the material occurs, from the outside inwards.

Given that the density, as well as the volume of the glass, depends on the temperature to which it is heated, during cooling, the formed shape is suddenly compressed. Thus, the outer layers of the drop, which cool faster than the inner ones, form a very large pressure towards the inner layers, which leads to the creation of strong internal forces and stresses. Owing to them, the drops are featured by extraordinary mechanical properties – an ordinary piece of glass is now almost harder than steel, and if we try to crush it by hitting it with a hammer, we will almost certainly fail.

However, if we gently break the thin tail, the drop shatters into billions of tiny pieces, forming glass dust. Overstressed glasses on cars and bus stops behave similarly, because they break into pieces that are not sharp, which reduces the risk of injury when they break.

Prince Rupert’s tear is the first example where the man encountered artificially obtained overstressed material. Although at first glance they seem to be nothing special, if we look at them with a source of polarized light, we will discover various shapes in different colours. Each of those colours represents one domain, and precisely their order and number indicate how physically inhomogeneous the drop itself is.

Author of the text and photos: Alen Bjelopetrović, Dr.Sc.