Silica is the main constituent of glass. But it is to be added with sodium potassium carbonate to bring down melting point. To make it durable lime or lead oxide is also added. Manganese oxide is added to nullify the adverse effects of unwanted iron present in the impure silica. The raw materials are ground and sieved. They are mixed in specific proportion and melted in furnace. Then glass items are manufactured by blowing, flat drawing, rolling and pressing.
PROPERTIES OF GLASS
 Glass absorbs, refracts or transmits light. It can be made transparent or translucent.
 It can take excellent polish.
 Glass is an excellent electrical insulator.
 It is strong and brittle.
 Glass can be blown, drawn or pressed.
 It is not affected by atmosphere.
 Glass has excellent resistance to chemicals.
 Glass is available in various beautiful colours.
 With the advancement in technology, it is possible to make glass lighter than cork or stronger than steel.
 Glass panes can be cleaned easily.
TYPES OF GLASS
The glass may be broadly classified as:
 SODA LIME GLASS
It is mainly a mixture of sodium silicate and calcium silicate. It is fusible at low temperature. In the fusion condition it can be blown or welded easily. It is colourless. It is used as window panes and for the laboratory tubes and apparatus.
 POTASH LIME GLASS
It is mainly a mixture of potassium silicate and calcium silicate. It is also known as hard glass. It fuses at high temperature. It is used in the manufacture of glass articles which have to withstand high temperatures.
 POTASH LEAD GLASS
It is mainly a mixture of potassium silicate and lead silicate. It possesses bright lustre and great refractive power. It is used in the manufacture of artificial gems, electric bulbs, lenses, prisms etc.
 COMMON GLASS
It is mainly a mixture of sodium silicate, calcium silicate and iron silicate. It is brown, green or yellow in colour. It is mainly used in the manufacture of medicine bottles.
 SPECIAL GLASSES
Properties of glasses can be suitably altered by changing basic ingredients and adding few more ingredients. It has now emerged as versatile material to meet many special requirement in engineering.
The following is the list of some of the special glasses:
- Fibre glass
- Foam glass
- Bullet proof glass
- Structural glass
- Glass black
- Wired glass
- Ultraviolet ray glass
- Perforated glass.
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