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Glass - The Basics
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Glass can be created by heating up a mixture of approximately 75% sand and 25% soda ash to around 1700°C in a furnace. At this high temperature the raw materials melt and fuse together, forming a hot molten fluid that can be shaped before it cools. Over many years of product development and research, by adding other raw materials and chemicals to this base mixture glass manufacturers have changed the type of the glass they can create. These developments, together with automation processes; such as creation of the 'float process' (developed by Pilkington Glass) to form flat glass, and the use of compressed air for blowing glass have enabled glass production processes to expand and new glass products to be developed.
There are four main forms of glass used widely today, these are:
- Commercial Glass
- Lead Glass
- Borosilicate Glass
- Glass Fibre.
Each of these is made by making adjustments to the raw materials before they are added to the furnace.
- Commercial Glass - from the bottles and glasses we drink from, the jars that contain our food to the flat glass in our windows, commercial glass is used daily by everyone. Commercial glass is also known as soda-lime glass, as soda ash (sodium oxide) is added to the mixture in the furnace. However, the main raw material used in production of all commercial glass is still sand (silica - SiO2) at 70% - 74% of the mixture. The other raw materials used to create commercial glass are:
- 12% - 16% Na2O (sodium oxide)
- 1% - 3% MgO (magnesium oxide)
- 5% - 11% CAO (calcium oxide)
- 1% - 3% Al2O3 (aluminium oxide)
- Lead Glass - lead glass is more commonly known as lead crystal. It is used to make a wide variety of decorative glass objects such as drinking glasses, decanters, vases and goblets. The traditional English Full Lead Crystal contains at least 30% lead oxide (PbO). Glass containing less than 24% PbO is known simply as Crystal Glass.
Lead is poisonous, but the lead used in lead crystal is locked into the chemical structure of the glass, so there is no risk to human health. Adding lead to glass makes it sparkle brightly, it also makes it softer than commercial glass, so it can easily be decorated by grinding, cutting and engraving when it is cold. These processes are used to catch light, highlighting the crystal's reflective properties.
Cullet – (broken or waste glass for recycling) is added to the mix that goes into the furnace. It is important that the type of cullet is matched to the required new product. Using cullet lowers the melting point of the raw materials which reduces harmful effects on our environment and production costs.
- Borosilicate glass - made mainly of silica (70-80%) and boric oxide (7-13%), this type of glass doesn't break when changing temperature quickly. As a result, it is widely used in bakeware products (e.g. baking dishes, glass lids and fluted pie plates). The chemical industry also uses borosilicate glass for laboratory apparatus, for ampoules and other pharmaceutical containers. Other uses include high power lighting applications and as glass fibres for use in textile and plastic reinforcement.
- Glass fibre - glass fibre comes in two forms; glass wool that is used with textiles and for insulation in buildings, and continuous fibre, which has over 40,000 different uses (e.g. reinforcing plastics, protective helmets, boats, piping, car chassis, ropes, car exhausts etc.)
- vitreous silica
- glass ceramics
- aluminosilicate glass
- optical glass
- alkali-barium silicate glass
- sealing glass
- technical glass
In recent years continuous fibre glass has also been used to create "optical fibres" which can guide light and transmit images around corners. These fibres are found in apparatus such as endoscopes, used by medical staff for examination of internal human organs. They are also used inside the new traffic message signs seen above motorways where changeable warnings, such as speed restrictions, can be displayed. The other main use for optical fibres is in network cables (communications technology), without which communication via telephone and the fast broadband internet connections that are now widely in use would not be possible. A single glass fibre, the same width as a human hair, could transmit all the 40,000 movies made so far in around 20 minutes. But demand for faster systems, to support the growth of the Internet, is still high.
Glass can be formulated to meet almost any requirement. Recently, a new set of materials known as glass ceramics have been developed. Other types of special glass include:
More information about these types of glass can be found on the British Glass website www.britglass.org.uk/AboutGlass/TypesofGlass.html
