International Year of Glass Physics World  June 2022

The many secrets of glass

Glasses are much more mysterious than their crystalline counterparts, yet have a wealth of practical uses, says Jon Cartwright

(Courtesy: Shutterstock/funkyfrogstock)

At the British Museum in London, there is a small turquoise-blue jug, originating from Egypt under the reign of the pharaoh Thutmose III. About the size of a salt shaker, the pretty opaque object was probably designed to hold perfumed oil, and is made almost entirely of glass. Yet despite being over 3400 years old, it is not considered one of the earliest examples of human glass making. Historians believe that Mesopotamians were amongst the leading glass-making cultures, fashioning beads and other simple decorative items from glass as long as 4500 years ago.

At first glance, glass does not seem very complicated. It merely refers to a material that has an amorphous rather than a crystalline structure – that is, one in which the atoms or molecules have no long-range order. Almost all common glasses, including those made by the ancient Egyptians and Mesopotamians, involve melting just three ingredients: silica (sand) for the basic structure; along with an alkali oxide (typically soda, or sodium carbonate) to lower the melting temperature; and lastly, calcium oxide (lime) to prevent the mixture from being soluble in water. In fact, the recipe can be simpler still, for we now know that almost any material can turn glassy if it is cooled from its liquid state so fast that its atoms or molecules are arrested before they have a chance to form a well-ordered solid state. But this simple description belies the depth of physics going on under the surface – physics that has been the subject of intense research for well over a century, with some aspects that still baffle us today.

The biggest question physicists want to answer is why a cooling liquid forms a hard glass at all, when no distinct change in structure occurs between the liquid and glass states. One might well expect glass to deform like a very viscous liquid. Indeed, there is a persistent myth that glass in old window panes is warped because it flows slowly over time (see box “The flowing myth”). In truth, glass is hard and brittle, and remains stable over surprisingly long periods. The stability of glass is one of its most attractive characteristics, for example in the storage of nuclear waste (see feature “A glassy solution to nuclear waste”).

An ideal glass is where molecules are packed together in the densest possible random arrangement

As seen through the conventional lens of “phase transitions”, put forth by Soviet physicist Lev Landau, there is no sudden shift in the underlying order (at least, no obvious one) when a substance turns into a glass – as would be seen for the emergence of any other genuine state of matter. The main difference between a liquid and a glass is that a liquid can continue to explore different disordered configurations, whereas a glass is, more or less, stuck with one. What makes a cooling liquid select a particular state on transition to glass is a question that goes back over 70 years (see box “In search of the ‘ideal’ glass”).

The fact that, as an amorphous solid, a material can potentially adopt so many different states makes glass incredibly versatile. With small changes in composition or processing, glass properties vary wildly (see “Two routes to better glass”). This accounts for the huge range in glass applications – from camera lenses to cookware, from windscreens to staircases, and from radiation protection to fibre-optic cables. Smartphones too, as we know them, would not have been possible without the development of thin-but-strong glass, such as “Gorilla Glass” glass, first made by the US manufacturer Corning (see feature “The unsung hero of the smartphone”). Even metals can turn into glass (see box “Mastering the metallic”). Often, the optical and electronic properties of a material do not differ greatly between its glassy and crystalline states. But sometimes they do, as is seen in phase-change materials, which, besides being of importance for data storage, are offering fundamentally new insights into chemical bonding (see box “The future of phase-change materials”). 

Perhaps the most surprising question to ask about glass is not what it is, but what it isn’t

However, perhaps the most surprising question to ask about glass is not what it is, but what it isn’t. While we are accustomed to thinking of glass as a hard, transparent substance, a vast swathe of other systems exhibit “glass physics”, from ant colonies to traffic jams (see box “Glass where you least expect it”). Glass physics helps scientists to understand these analogues, which in turn can shed light on glass physics itself.