What I found out is that while human beings have been making glass for thousands of years, we have had a difficult time understanding it. The most common and oldest recipe for glass is mostly silicon and oxygen. We add impurities to change the properties of glass, like hardness, brilliance and melting temperature.
Glass can be difficult to classify because it does not quite act like a solid... nor does it act like a liquid. Most people think of it as an amorphous solid, which has different properties than a crystalline solid. Some people say, no it is not a solid, but super-cooled liquid. Other say, well, it's kind of like a polymer, but not really. Other people call it a rigid liquid.
So why all the disagreement? Glass does not fit neatly into our model for solids and liquids.
A good example that does fit the model is water. When water is cold, it freezes into a crystalline form that is hard and brittle (ice). Think of ice at a really cold temperature, for example -40 F. It is a hard, brittle crystalline solid. If you raise the temperature, the ice becomes warmer, but remains hard and brittle until you get to the point where some of the ice starts to melt into water. While the ice is changing form, the temperature stays constant at 32 F until all the ice has changed into liquid water. The difference between water and ice is very clear – water flows, ice does not.
Compare to glass. At room temperature glass is hard and brittle, like ice. But when you heat it, it gradually becomes softer and softer. The temperature at which it first starts to soften is called the glass transition temperature (abbreviated as “Tg”). If you keep heating it, eventually the glass begins to act like a liquid. The temperature at which this happens is the melting point.
Why does glass have such different behavior than water? Science does not have the complete answer to this question. I will describe what I think is a pretty good model that explains a lot of what we observe about glass. Bear in mind all models are wrong, but some are useful.
In ice, all the molecules are locked into place in a crystalline structure. As the temperature increases the molecules become more energetic, but they cannot break free of the structure (some days I feel the same way). When the molecules finally have enough energy to break free, melting occurs all at once. Glass is amorphous, meaning it does not have a set crystalline structure. Instead, the molecules have many different random arrangements. As glass is heated, some of the molecules are able to break free; the result is that the glass softens. As the glass heats, more molecules break free and it becomes softer. At a high enough temperature the molecules have enough energy to break free and the glass becomes completely liquid.
The softening temperature of glass depends on a lot of different things – what the glass is made out of, what kind of impurities are present, how fast the glass was cooled the last time it was heated, and the rate at which you heat it up. Because softening temperature can change so much, it can be tricky to measure accurately.
There is a lot more interesting information about glass - if you all are interested, I will add some more posts on the subject.