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Different Kinds Of Glasses, And How They Are Made
Glass has been used since very early times; the Egyptians used glass nearly four thousand years ago. Chinese learned to manufacture glass comparably later than the Mesopotamians and Egyptians. The first Chinese glasses are dragon-eyed beads produced during the Warring State period (5th century BC), . American Indian didn’t know glasses at all before Columbus. When Columbus and his sailors came ashore, carrying swords, speaking oddly, the Arawaks ran to greet them, brought them food, water, gifts. Columbus exchanged glass beads for food, water and gold. He later wrote of this in his log:
They brought us parrots and balls of cotton and spears and many other things, which they exchanged for the glass beads and hawks' bells.
The basic materials used for making glass are the same as those used centuries ago - sand, soda and lime. The simplest type of glass is made by heating calcium carbonate, sodium carbonate and sand (silica) until the mixture melts to give a transparent liquid. On cooling, this solidifies to give glass.
Glass is not a single substance but an amorphous, homogeneous mixture. It does not melt at a definite temperature and is not really a solid. It is said to be a supercooled liquid.
Kinds of glass
The major disadvantage of ordinary glasses is that they are brittle and crack when subjected to sudden changes of temperature. During manufacture, the ingredients used for making glass are changed suitably to obtain specific properties. Some common types of glass are described below.
Soda glass: This is a mixture of sodium and calcium silicates used for making bottles, beads, marbles, etc.
Potash glass: This is a mixture of potassium and calcium silicates, also known as hard glass. Potassium carbonate is used instead of sodium carbonate for making this glass. It can resist high temperature without melting.
Pyrex glass or borosilicate glass: This contains a mixture of borosilicate of calcium and aluminium. It is made from sand, sodium carbonate, alumina and borax. This glass can withstand sudden changes of temperature without cracking. Therefore it is used for special laboratory glassware and for ovenware.
Cut glass or optical glass: This contains some lead oxide along with potassium and silica. This kind of glass, also called lead crystal glass has a high refractive index. It is used for making lenses, prisms, high quality art objects and expensive glassware. Cut glass shows a high degree of brilliance and sparkle.
Crookes’ glass contains some rare earth elements and is used for special optical lenses.
Shatterproof glass or laminated glass: This is made of several sheets of glass cemented together with some transparent adhesive. Bulletproof glass consists of several alternate layers of plastic and glass. Laminated glass is used in aeroplanes, windshields of cars and bulletproof screens.
Fibre glass: if a glass rod is heated in the middle and pulled apart as it softens, it can be drawn into thin fibres. Glass wool is a bundle of loose glass fibres. It is an excellent heat insulator and is therefore used as insulating material in refrigerators, ovens, cookers and hot water bottles.
Glass fibres are light yet strong, weather proof, fire proof, corrosion free and poor conductors of electricity. They are used for sound proofing and insulation of buildings. Layers of glass fibres when bonded with adhesives produce fibre glass. This can be moulded into any shape and is used in motor car bodies, boats and aeroplane wings.
Coloured glass: Glass can be coloured by the addition of certain metals or metal salts to the silicate mixture during fusion.
The necessary raw materials (sodium carbonate, calcium carbonate and silica for ordinary glass) are taken and ground to a powder. Broken bits of glass are also added. This is called “cullet”. The addition of cullet lowers the melting point. This mixture called “batch” is heated to about 1400oC when it melts.
When all gases like carbon dioxide and air escape, the necessary colouring materials are added. Heating is continued until it melts. This worked to get the required articles. It can be blown into the desired shape using a long pipe or moulded into articles and cooled.
If the finished articles is suddenly cooled, it would become very brittle. Instead it is gradually cooled, and the process is called “annealing”. the articles to be annealed are kept on a slowly-moving belt passing through a narrow chamber called “lehr”. One end of the chamber is hot and the temperature gradually decreases until it is room temperature at the other end. It takes several days for the articles to reach form one end to the other end and so they are cooled very slowly.