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Spectral colors in the glass of a lighthouse Fresnel lens.


Now you see it, now you don't. Glass is a bit of a riddle. It's hard enough to protect us, but it shatters with incredible ease. It's made from opaque sand, yet it's completely transparent. And, perhaps most surprisingly of all, it behaves like a solid material... but it's also a sort of weird liquid in disguise! You can find glass wherever you look: most rooms in your home will have a glass window and, if not that, perhaps a glass mirror... or a glass lightbulb. Glass is one of the world's oldest and most versatile human-created materials. Let's find out some more about it.

Photo: Glass riddle: How does something transparent to light appear colored? The colors in this glass aren't really there! Glass lenses refract (bend) light rays of different wavelengths by different amounts, causing spectral colors to appear. This is a closeup of a Fresnel lens from a lighthouse.

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  1. What is glass?
  2. How is glass made?
  3. Is glass a solid... or a liquid?
  4. What do we use glass for?
  5. Find out more

What is glass?

Believe it or not, glass is made from liquid sand. You can make glass by heating ordinary sand (which is mostly made of silicon dioxide) until it melts and turns into a liquid. You won't find that happening on your local beach: sand melts at the incredibly high temperature of 1700°C (3090°F).

When molten sand cools, it doesn't turn back into the gritty yellow stuff you started out with: it undergoes a complete transformation and gains an entirely different inner structure. But it doesn't matter how much you cool the sand, it never quite sets into a solid. Instead, it becomes a kind of frozen liquid or what materials scientists refer to as an amorphous solid. It's like a cross between a solid and a liquid with some of the crystalline order of a solid and some of the molecular randomness of a liquid.

Glass is such a popular material in our homes because it has all kinds of really useful properties. Apart from being transparent, it's inexpensive to make, easy to shape when it's molten, reasonably resistant to heat when it's set, chemically inert (so a glass jar doesn't react with the things you put inside it), and it can be recycled any number of times.

Stained glass window by Edward Burne-Jones in St Philips' Cathedral, Birmingham

Photo: Stained glass is made by adding salts of metals such as iron, manganese, chromium, and tin to the ingredients of molten glass to give it a variety of attractive colors. This stained glass window, designed by artist Edward Burne-Jones, is in St Philip's Cathedral, Birmingham, England.

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How is glass made?

When US scientists tested a prototype of the atomic bomb in the New Mexico desert in 1945, the explosion turned the sand in the immediate area of the impact into glass. Fortunately, there are easier and less extreme ways of making glass—but all of them need immense amounts of heat.

In a commercial glass plant, sand is mixed with waste glass (from recycling collections), soda ash (sodium carbonate), and limestone (calcium carbonate) and heated in a furnace. The soda reduces the sand's melting point, which helps to save energy during manufacture, but it has an unfortunate drawback: it produces a kind of glass that would dissolve in water! The limestone is added to stop that happening. The end-product is called soda-lime-silica glass. It's the ordinary glass we can see all around us.

Glass is made by mixing and heating sand, recycled glass, calcium carbonate, and sodium carbonate.

Artwork: Glassmaking simplified: mix and heat sand and recycled glass with calcium carbonate and sodium carbonate.

Once the sand is melted, it is either poured into molds to make bottles, glasses, and other containers, or "floated" (poured on top of a big vat of molten tin metal) to make perfectly flat sheets of glass for windows.

A pane of float glass floating on a human hand

Photo: Perfectly flat panes of glass of uniform thickness are made by floating molten glass on giant tanks filled with molten tin. After cooling and solidifying, the glass is cut to whatever size is needed.

Unusual glass containers are still sometimes made by "blowing" them. A "gob" (lump) of molten glass is wrapped around an open pipe, which is slowly rotated. Air is blown through the pipe's open end, causing the glass to blow up like a balloon. With skillful blowing and turning, all kinds of amazing shapes can be made.

Blowing and spinning a piece of glass artwork

Photo: Making a piece of glass artwork by blowing and spinning. The glass (right) has been blown into a sphere on the long metal pipe you can see on the top right. It's now being turned on the same pipe to shape it further. Note how two hot blow torches are being applied from the left to keep the glass molten and flexible so it can be worked into shape. Photo by James Bunn courtesy of US Army and DVIDS.

Glass makers use a slightly different process depending on the type of glass they want to make. Usually, other chemicals are added to change the appearance or properties of the finished glass. For example, iron and chromium-based chemicals are added to the molten sand to make green-tinted glass. Oven-proof borosilicate glass (widely sold under the trademark PYREX®) is made by adding boron oxide to the molten mixture. Adding lead oxide makes a fine crystal glass that can be cut more easily; highly prized cut lead crystal sparkles with color as it refracts (bends) the light passing through it. Some special types of glass are made by a different manufacturing process. Bulletproof glass is made from a sandwich or laminate of multiple layers of glass and plastic bonded together. Toughened glass used in car windshields is made by cooling molten glass very quickly to make it much harder. Stained (colored) glass is made by adding metallic compounds to glass while it is molten; different metals give the separate segments of glass their different colors.

PYREX® cookery jug made from borosilicate glass

Photo: Borosilicate glass, such as this PYREX® jug (back), can withstand extreme changes of temperature, unlike normal glass (front), which shatters. The ordinary glass jar at the front is quite a bit thinner and considerably lighter. You can also see, very clearly that the borosilicate glass is a slightly blueish color (as is the boron oxide from which it's made).

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Is glass a solid... or a liquid?

Illustration showing a regular solid crystalline structure and an amorphous structure underneath it.

Artwork: Top: In a regular crystalline solid, the atoms are arranged in a neat and predictable way. Bottom: In an amorphous solid, such as glass, the arrangement is much more random.

It's a very interesting question.

The answer is both—and neither! There are widely differing opinions on how to refer to materials such as glass that seem to be a bit like liquids in some ways and a bit like solids in others.

In schools and in books, we tend to learn that solids all have a fixed structure of atoms.

In fact, there are different kinds of solids that have very different structures and not everything we describe as "solid" behaves in exactly the same way. Think of a lump of iron and a lump of rubber. Quite clearly they are both solids, and yet the rubber is very different from the iron. Inside, rubber and iron have their atoms (in the case of iron) and molecules (in the case of rubber) arranged in totally different ways. Iron has a regular or crystalline structure (like a climbing frame with atoms at the corners), while rubber is a polymer (made from long chains of molecules loosely connected together). Or think of water. As you may have discovered, water is an almost unique solid because it expands to begin with when it freezes. In short, not everything fits neatly into our ideas of solid, liquid, and gas and not all solids, liquids, and gases behave in a nice, neat, easy-to-explain way. The exceptions are the things that make science really interesting!

Amorphous solids

Let's return to glass. Peer through a microscope inside some glass and you'll find the molecules from which it's made are arranged in an irregular pattern. That's why glass is sometimes referred to as an amorphous solid (a solid without the regular crystalline structure that something like a metal would have). You may also see glass described as a "frozen supercooled liquid". This is another way of saying "glass is a liquid that has never set", which is the puzzling statement you'll sometimes find in science books. We could say glass is a bit like a liquid and a bit like a solid. It has an internal structure that is somewhere between the structure of a liquid and a solid, with some of the order of a solid and some of the randomness of a liquid.

Glass is by no means the only amorphous solid. It's possible to make a type of water called amorphous ice that could be described as in-between solid (water) and liquid (ice). You do this by cooling water very quickly. The ice forms so fast that it doesn't have time to build up its normal, crystalline structure. So what you get looks like ice but behaves in some ways like liquid water. Other substances can be made into amorphous solids too. Solar cells are often made from something called amorphous silicon.

Photo of an amorphous silicon solar cell panel on a rooftop.

Photo: A solar panel made from amorphous silicon. Photo by Dennis Schroeder courtesy of NREL (US Department of Energy/National Renewable Energy Laboratory) (photo id #22143).

What do we use glass for?

Items made from recycled glass

Photo: Glass can be used to recycle other materials. Uranium glass has an unusual yellow-green color and glows in ultraviolet light. These glass pieces were made using waste uranium from the cleanup of the Fernald uranium processing plant near Cincinnati, Ohio, USA. Vitrification (turning a material into glass) is one way to dispose of nuclear waste safely. Picture by courtesy of US Department of Energy.

Glass starts your day with a sparkle: a glance at your watch, a gaze through the glaze at the sun or the rain, a frown in the mirror, a song from the shower, as you wash with water trickling down warm from the solar panels on the roof. Glasses pack the breakfast table, which might, itself, be made from smoked glass, and there are bottles and jars of all shapes and colors. Making breakfast in your kitchen, you might be using a glass-ceramic cooktop or a microwave with a metal-lined window to keep the waves inside. Maybe you're watching croissants warm through the Pyrex oven door? (And is that a glass teapot?)

When you check your email over breakfast (bad habit), speed-of-light Internet data zips to your home through optical fibers, just as sunlight streams through the heat-reflective windows that keep you cool. You read the words through the glass LCD panel of your laptop or the toughened gorilla glass of your smartphone, both charged by solar energy from photovoltaic panels on the roof. Talking heads are muttering at you through the TV screen in the corner.

Then you set off for work or school, in a glass-wrapped car, bus, train (perhaps even helicopter), hunched under low-energy lamps covered by glass to make them last. If you're driving, the highway you're roaring down could be made from aggregates and asphalt including recycled glass; even the white stripes down the middle use tiny glass beads to make them shine in your headlights. Maybe you drop in the bank or the post office on your way, smiling at the cashier behind her bulletproof window, as you make a quick copy of your driving licence (which you carelessly leave behind on the glass plate of the photocopier).

Wayfarers Chapel in Rancho Palos Verdes, California by Carol Highsmith

Photo: Glass brings the outside in! This is the wonderful Wayfarers Chapel in Rancho Palos Verdes, California, designed by Lloyd Wright (son of Frank Lloyd Wright). Picture from The Jon B. Lovelace Collection of California Photographs in Carol M. Highsmith's America Project, Library of Congress, Prints and Photographs Division.

If it's a modern building, your office or school might be a mini glass cathedral; we think of glass as brittle and fragile, but toughen it the right way and you can make walls, floors, roofs, and staircases from it; shops show their wares through huge, laminated panels, polished to perfection.

And that's only a tiny selection of the things glass does for us. There are loads more places you'll find it hiding, from the bulbs in thermometers and the cermet fillings in teeth to the fiberglass hulls of boats, the "sandpaper" we use for decorating (which is often glasspaper), and even the strain gauges that warn us when buildings are cracking. Clear, clean, attractive, unreactive, cheap, strong, and effective. What more could you want? Glass is one of those magic materials we absolutely take for granted; everywhere and nowhere—"invisibly transparent," so we don't even notice that it's there!

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Text copyright © Chris Woodford 2007, 2022. All rights reserved. Full copyright notice and terms of use.

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@misc{woodford_glass, author = "Woodford, Chris", title = "Glass", publisher = "Explain that Stuff", year = "2007", url = "", urldate = "2022-02-12" }

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