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Spectral colors in the glass of a lighthouse Fresnel lens.

Glass

by Chris Woodford. Last updated: August 17, 2014.

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.

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).

Items made from recycled glass

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.

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.

How is glass made?

A man fixing a large pane of glass into a window frame

Photo: Lifting a pane of glass into place. Picture by Kelly Barnes courtesy of US Navy.

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.

PYREX® cookery jug made from borosilicate glass

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. 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.

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.

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).

Is glass a solid... or a liquid?

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.

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

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.

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

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Text copyright © Chris Woodford 2007, 2009. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) Glass. Retrieved from http://www.explainthatstuff.com/glass.html. [Accessed (Insert date here)]

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