What Is Glass?

My son and I were in my car when this happened. I was on my way to get an estimate on some body work for my car, and this got my son interested in things about the car. That combined with his interest in ice and cold, and he starts telling me about ice. “And then they melt the ice, and you can see through it!” he announces, because they’d been looking at ice. Then he asks, “how do they make the ice into windows?”

“They don’t,” I tell him. “Windows are made of glass.”

“Glass is made of ice!” he tells me. I… guess it makes sense? Glass and ice can look very similar, after all.

“No,” I tell him. “Glass is made of sand.”

He laughs at me. “No it’s not!”

“Yes, it is.”

“Then,” he asks, with the air of a prosecutor delivering the final damning bit of evidence, “how do you see through it?”

It’s a good question, really. The idea that sand – something he’s seen a lot of and [i]knows[/i] is opaque – can be seen through is really absurd sounding. I don’t have a good answer for that, really.

What is glass?


Dictionary.com defines glass as:


  1. a hard, brittle, noncrystalline, more or less transparent substance produced by fusion, usually consisting of mutually dissolved silica and silicates that also contain soda and lime, as in the ordinary variety used for windows and bottles.
  2. any artificial or natural substance having similar properties and composition, as fused borax, obsidian, or the like.

The Corning Museum of Glass offers this definition instead:

Glass is a rigid material formed by heating a mixture of dry materials to a viscous state, then cooling the ingredients fast enough to prevent a regular crystalline structure. As the glass cools, the atoms become locked in a disordered state like a liquid before they can form into the perfect crystal arrangement of a solid. Being neither a liquid nor a solid, but sharing the qualities of both, glass is its own state of matter.

Intuitively, I think most of us know what glass is. We interact with it all the time, in the form of windows if nothing else. Our everyday experience of glass tells us that it is hard (but obviously not indestructible, particularly when thin) and transparent.

How is glass made?

British Glass has a page titled All About Glass, which provides a general description of how glass is made:

Glass is made by melting together several minerals at very high temperatures. Silica in the form of sand is the main ingredient and this is combined with soda ash and limestone and melted in a furnace at temperatures of 1700°C. Other materials can be added to produce different colours or properties. Glass can also be coated, heat-treated, engraved or decorated. Whilst still molten, glass can be manipulated to form packaging, car windscreens, glazing or numerous other products. Depending on the end use, the composition of the glass and the rate at which it is allowed to cool will vary, as these two factors are crucial in obtaining the properties the glassmaker is seeking to achieve.

So, in other words, you melt sand with other stuff. Going back to the Corning Museum of Glass, they explain that “typical glass contains formers, fluxes, and stabilizers.” A former is the thing the glass is made of (silicon dioxide, aka silica, in standard windows and bottles). A flux is something that lowers the melting temperature of the former – soda ash (sodium carbonate) is a flux, as is potash (potassium carbonate). A stabilizer is something that makes the glass stronger (and often water resistant) – limestone (a form of calcium carbonate) is a stabilizer.

Interestingly, the page also states that without the stabilizer water will dissolve glass. Put this firmly into the category of Things I REALLY Didn’t Know.

Window glass is generally 73.6% silica, 16% soda ash, 5.2% limestone, 0.6% potash, and 4.6% other materials. By contrast, your glass baking dish is 80% silica, 4% soda ash, 0.4% potash, 2% alumina, and 13% boric oxide. Your fine lead crystal is 35% silica, 7.2% potash, and 58% lead oxide.

Why can we see through it?

Because it’s transparent.

No, really.

No, seriously. Transparent Glass doesn’t absorb photons of light in the visible spectrum. A glass like volcanic obsidian, on the other hand, does. In fact, it absorbs nearly all the light in the visible spectrum, which is why it looks black.

Could you be more specific?

I’ll try. Let me warn you in advance that I’m leaning heavily on Transparency and translucency from Wikipedia for this.

Any given wavelength of electromagnetic energy will be either reflected, absorbed, or transmitted by a given material. The human eye interprets reflected (visible) wavelengths as color, radar dishes and detect reflected radio waves to calculate distance and direction, and so on. Absorbed wavelengths increase the temperature of the material, because they’re pumping energy into the material. Transmitted wavelengths pass through unhindered to a greater or lesser degree. A material that transmits no portion of the visible electromagnetic spectrum is optically opaque, and a material that transmits all of the visible spectrum is optically transparent. Here’s a good quote from the article:

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with negatively charged electrons in matter. When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur:

  • A molecule absorbs the photon, some of the energy may be lost via luminescence, fluorescence and phosphorescence.
  • A molecule absorbs the photon which results in reflection or scattering.
  • A molecule cannot absorb the energy of the photon and the photon continues on its path. This results in transmission (provided no other absorption mechanisms are active).

Most of the time, it is a combination of the above that happens to the light that hits an object. The states in different materials vary in the range of energy that they can absorb. Most glasses, for example, block ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum. But there are also existing special glass types, like special types of borosilicate glass or quartz that are UV-permeable and thus allow a high transmission of ultra violet light.

Interestingly enough, when I first wrote this out I wrote the following statement: “a material that transmits some portion is optically translucent to a greater or lesser degree”. This is actually incorrect. Optically translucent objects are considered optically transparent, but there is a quality to the material that prevents image formation. That is, visible light will pass through but it is scattered in such a way that you can’t make out what the source image on the other side is.

So. Glass is transparent because the molecules that make up the glass literally cannot absorb most of the electromagnetic energy in the visible spectrum (although it can reflect it, which is why you can sometimes see yourself in glass).

Is it a solid or a liquid?

You hear quite often that glass is a really slow-moving liquid. It isn’t. Glass is an amorphous solid, and to understand what that means we’ll need to explain what a crystalline solid is first.

Please do.

A crystalline solid is also called a crystal, and it is a solid where the atoms or molecules that make up the substance are arranged in a highly organized and periodic structure. Think of the atoms or molecules as the bricks in a brick wall or Lego structure. Individual crystalline solids can get really, really big.

A real picture, from the Naica Crystal Cave

Most solid objects are polycrystalline, meaning they are made up of multiple crystals. Each crystal can be arranged in a haphazard fashion relative to the other crystals – imagine taking that Lego structure above, breaking it into chunks, and then covering those chunks with glue and tossing them in a box. (Clearly, this analogy breaks down quickly. Just bear in mind that we’re imagining structures here, not chemical properties.)

Amorphous solids have no organization to the component atoms or molecules. Using the Lego analogy, they’re a sack full of individual Lego bricks (that were all coated with glue). They’re still solids – the individual atoms still have strong connectivity – but they have some liquid-like properties. They can flow, slightly, as the disorganized components try to arrange themselves into crystalline structures. As the Scientific American article I linked to notes, though, this is not why some antique glass looks thicker at the bottom. “[A]ncient Egyptian vessels have none of this sagging, says Robert Brill, an antique glass researcher at the Corning Museum of Glass in Corning, N.Y. Furthermore, cathedral glass should not flow because it is hundreds of degrees below its glass-transition temperature, Ediger adds. A mathematical model shows it would take longer than the universe has existed for room temperature cathedral glass to rearrange itself to appear melted.”