Glass: almost a solid

Glass is a wonderful material with a variety of uses. We use it in many everyday items, from bottles to cameras. It is transparent, long-lasting and recyclable. However, glass is also a strange material – it is neither a solid nor a liquid, but something in-between!


Glass is made of liquid sand. The main component of sand and glass is silica, a compound of silicon and oxygen. Pure silica glass is called fused quartz.
Silica is very stable and so has a very high melting point of 1700°C (3090°F). Heating to this temperature to make glass is very expensive. It is also bad for the environment as it requires burning fuels and polluting the atmosphere.
Most glasses have other chemicals mixed in to change their properties. The most common type of glass used today is soda-lime-silica glass. Sodium carbonate and calcium oxide are added to lower the melting point. It is inexpensive and used to make bottles and windows.


Usually simply changing the temperature is enough to make an object change from a solid to a liquid and back again. For example, water will freeze to ice and ice will melt to water. However, if you cool molten glass back to room temperature it does not turn back into solid sand!
This is because glass is an “amorphous solid”. One way to think about an amorphous solid is that it is like a liquid that moves so slowly that it looks like a solid. Just as honey moves more slowly than water, glass moves more slowly than any other liquid. It then acts like a solid in some ways and a liquid in others.
Glass is not the only thing that can form an “amorphous solid”. “Amorphous ice” is made by supercooling water so quickly that it does not have enough time to freeze. “Metallic glass” can also be made.


When light strikes a surface it will bounce off of it. The more rough and imperfect a surface is, the more randomly the light bounces off it. We call this scattering.
Most solids scatter light because they have rough surfaces.
However, glass is more similar to a liquid. Liquids have very smooth surfaces with almost no imperfections.
The smooth surface of glass means it does not scatter light. The light can instead travel through the glass in straight lines, which makes it transparent.


Glass is amazingly recyclable. Waste glass is simply melted and then cooled down back into a new shape! Theoretically, there is no limit to the number of times one glass bottle can be recycled.
This means that recycling glass is very worthwhile. Recycling glass produces 20% less air pollution and 50% less water pollution than making new glass. If all waste glass was recycled, we could live in a world where no new glass would ever need to be made at all!



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