DK Science: Glass
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.
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.
