A Window into the World of Glass

2nd February 2025 | Caitlyn Koay

From windows to water vessels to spectacles, glass has been a staple in human life for the better part of our history. Versatile, transparent and chemically inert, it’s no wonder the material has found such widespread use in everyday objects, technology and decorative features. Yet for a substance so common in the mundane everyday, you’d find that glass is actually pretty interesting! Below are a few fun facts about glass that may make you appreciate it just a little more.

The science behind transparency

Though it seems non sequitur, leading into this topic it’s important to first talk about electrons. Note that electrons surround an atom’s nucleus, occupying different energy levels. To move up or down these energy levels, the electron has to either gain or lose energy.

When a photon (an elementary particle that can be seen as a discrete bundle of electromagnetic/light energy) approaches an atom, one of three things can happen:

The photon is absorbed. It gives up its energy to an electron, allowing the electron to move up an energy level.
The photon is reflected. It gives up its energy to the material, but another photon of the same energy is emitted.
The photon simply passes through the substance. This is what happens in transparent materials like glass! it doesn’t interact with any electrons and is completely transmitted.

Hanging on to scenario 3, glass’s transparency can be explained by photons not having sufficient energy to excite its electrons to a higher energy level. It is theorised that energy levels are discrete and exist in energy bands, while in between these bands energy levels for electrons don’t exist at all. Some materials just have larger gaps in these bands than others, meaning electrons require more energy to move up or down energy levels — something that photons of visible light don’t have. Keyword: visible light, as higher energy wavelengths such as ultraviolet are absorbed no problem, so glass actually appears opaque to UV light!

Sandy starts

The main constituent of flat glass is silicon dioxide (SiO2), the same stuff you’d find on the shores of beaches. It doesn’t just spontaneously undergo the transformation, however — there are 2 main steps to the process (quenching): heat to extreme temperatures, then cool rapidly. Sand has a regular, crystalline structure, composed of tiny quartz crystals of silicon dioxide in a tetrahedral arrangement. When heated to around 1700ºC, silicon dioxide melts, losing its original structure; molecules then arrange themselves randomly, thus becoming glass.

Additional fact: there are plenty of more unconventional methods to make glass. Fulgurite, also aptly called ‘fossilised lightning’, is a mass of vitrified sand, soil, rock or organic debris that forms due flash-vaporisation when lightning strikes the earth.

Volcanic glass (e.g. obsidian, perlite, pitchstone, apache tears etc.) forms when rhyolitic magma with a high silica, low gas or water content quickly cools to form a solid, skipping the development of the crystalline phase.

You can also produce glass using nuclear weaponry! After the Trinity Test (the nuclear test occurring July of 1945 in New Mexico), the sheer heat of the explosion caused surrounding soil to fuse into a glass-like consistency. This greyish-green residue was then found and named Trinitite.

Liquid but solid

There’s a popular myth that stained glass windows in churches appear to ‘flow’ over time, perpetuating a common misconception that despite appearances, glass is a liquid. It isn’t, of course, but this postulation does have a grain of truth behind it!

There are 2 main categories of solids: crystalline and amorphous. Crystalline solids are those in which the atoms, ions or molecules make up the solid in a regular arrangement (like in metallic or crystal lattices). Amorphous solids, on the other hand, are those in which particles are arranged irregularly, a little bit like a liquid. Given the liquid-like constitution, to answer the question, yes — glass does flow — just over a timescale of billions of years. For relative short-term purposes, say as long as a human lifespan, glass molecules barely move at all.

Lots of amorphous solids are formed through the aforementioned quenching process, which can be applied to other substances as well, such as metals! Metallic glass proves to be extremely strong and durable with tons of practical applications, but scientists haven’t been able to consistently quench metals yet — we don’t fully understand why molecules in glass rearrange randomly instead of reforming their crystal structure, and it’s still one of physics’ biggest unsolved problems!

To wrap it up, glass is a lot more fascinating than most usually give it credit for. From its light-transmitting and bending properties to how it never truly stills, it is as beautiful as it is compelling! Often, plenty of materials we utilise day-to-day and never spare a second thought about hold eccentricities like these; hopefully shedding a little more light on glass has made that notion abundantly clear.

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Calling All Chemists

A Window into the World of Glass

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