Page 6 - Curriculum Visions Dynamic Book

(Above and below) Recycling of glass is only possible because glass can be remelted and re-formed
time after time. The only thing that restricts its reuse is the amount of impurity in the glass being recycled. That is the reason you are asked
to keep different coloured glasses apart.
Glass is so unusual because, unlike a metal
in which the molten material turns immediately into a solid made of crystals on cooling, in a glass there is a steady and gradual change that stiffens the glass and traps the atoms before they can rearrange themselves into crystals. That is why,
on a microscopic scale, the atoms have the same pattern as they had when they were liquid.
The same happens as the glass is heated – the glass softens steadily until it is finally all
liquid, while metals suddenly change into a liquid.
This unorganized structure accounts for the fact that most glass is transparent. However, it is not only hard and dense, but also brittle. It is easily broken by a sudden blow.
All glasses break with a fracture, which often produces sharp knife or spearlike edges on the broken fragments.
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