First made over 5,000 years ago, glass is a thick liquid
that never completely sets (hardens). That is why old window panes are thicker
at the bottom than at the top. Glass is still in widespread use because it is
transparent (see-through), strong, and can be melted and recycled endlessly.
Molten glass can be shaped in many ways, including flat panels for windows and
threads for optic fibres. GLASS TECHNOLOGY is so advanced that
glass can be made fire-resistant and shatterproof.
Coloured glass is created by dissolving metal compounds into melted
sand. Different metal compounds create different colours. For example, selenium
sulphide makes glass red. Iron and chromium compounds produce a deep green
glass.
Sand, broken glass, soda, and limestone are heated in a furnace.
At around 1,500°C (2,732°F), the mixture melts to form molten
(liquid) glass, which is cut into individual globules of glass called gobs.
The gobs are dropped into bottle moulds. Compressed air blows the
glass against the mould walls. The bottles are removed from the moulds and
reheated slightly to remove imperfections.
The bottles are cooled slowly on a moving conveyor belt under
carefully controlled conditions. This ensures that no dust is trapped in them,
and that the glass does not shatter.
Material scientists have developed and improved the
properties of glass to suit a range of uses. Heat-proof oven doors are made by
adding chemicals to molten glass so that the glass lets light but not heat
through. Car windscreens are made shatterproof by cooling molten glass rapidly
with jets of air. Test tubes and other glass apparatus used in science labs
need to withstand the heat of a Bunsen flame. This kind of glass is made
heat-proof by adding boron oxide to the raw materials to make
borosilicate.
Molten glass can be pulled into extremely thin tubes called
optical fibres. A beam of light is reflected down the tube, even as it bends
around corners. Optical fibres are used in telephone cables. Pulses of light
pass down the tube, and transmit information. Optical fibres are also used in
endoscopes that allow doctors to see right inside our bodies.
