How they turn sand into glass

Five thousand years ago, on some beach in the Middle East, someone probably lit a fire and later found shiny, transparent globules like jewels among the sand. How were these new curiosities transformed into one of the major household and building materials of the 20th century - glass?

The raw material from which glass is made is silica, the most abundant of all the earth's minerals. Milky white in colour, it is found in many forms of rock, including granite. And as every beach in the world has been formed by water pounding rocks into tiny particles, sand is the major source of silica.

When you are next at the seaside, examine a handful of sand. Any grain which is semitransparent - rather than black, red, yellow or some other definite colour - is a grain of silica. Sand also contains other minerals, but silica is the main component because it is hard, insoluble and does not decompose, so it outlasts the others.

Pure silica has such a high melting point that no ordinary fire would convert it into glass. So the first Middle Eastern glass-makers must have lit their fire on sand which was impregnated with soda (compounds of sodium) left behind by evaporated water from a lake or sea. The soda reduces silica's melting point.

Today, lime and soda are combined with silica to produce soda-lime glass, used for making bottles, window panes and cheap drinking glasses. When glass cools, its structure does not return to the crystalline structure of silica, which is opaque. Instead, it forms a disordered structure rather like a frozen liquid, which is transparent.

Glass is made of many small molecules firmly bonded together, and the bonds between them will not stretch significantly. If submitted to sufficient force, they will break. These properties make glass hard, but brittle.

Transparent plastics, on the other hand, are polymers made by loosely bonding together very large molecules. The bonds are not very strong, so the molecules will slide over each other, making plastics flexible.
Ovenware and lead crystal

Other materials may be added to provide colour, or to improve the quality of the finished glass. Glass containing 10-15 per cent of boric oxide, for example, is resistant to sudden heating or cooling and is used for ovenware. Adding lead oxide, a technique discovered in the 17th century, produces a heavy glass with a brilliant glitter - lead crystal.

Modern sheet glass is made by heating the mixed ingredients in long tanks. The mixture always contains broken glass, known as cullet, which melts at a lower temperature than the other materials and helps them to combine thoroughly.

As newly made glass is taken out from one end of the tank, in a sheet up to 10ft (3m) wide, raw materials are poured in at the other, so that the level in the tank always remains constant.

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