What is glass?


When most people think of glass, their mind probably jumps straight to windows. And perhaps they’ve heard that old myth – that glass is actually a liquid, not a solid.

So what is glass?

Well, you’ve probably seen something like this before:


The three common phases of matter – gas, liquid, and solid. But you’ll notice that the solid picture is labeled crystalline state. Most people consider glass to be a solid, but it doesn’t quite look like that.

Crystals have a well defined structure, exhibiting long-range order. Glass is what’s called an amorphous material, exhibiting only short-range order. 

Basically, glass is a different kind of solid:


The quartz shown above is an example of a crystalline material. The molecules of glass on the other hand are disordered – yet still solid. 

To create glass, the liquid melt has to be cooled fast enough to prevent the substance from crystallizing. This fast cooling locks the atoms or molecules in the disordered state that looks like the liquid phase. 

Characterizing a substance as a glass also means that this glass transition is reversible. 

While most glass is optically transparent, the properties depend on the composition of the glass. Most of what you see every day is soda-lime-silicate glass, but there are many different kinds of glasses, including sodium borosilicate glass (Pyrex), lead-oxide glass, and aluminosilicate glass.

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(via materialsscienceandengineering)

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