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It is transparent or translucent amorphous, super cooled solid solution of silicates and borates. Its general formula is :
R2O.MO.6SiO2 0r R2SiO3.MSiO3 .6SiO2
R is monovalent metal like Na, K etc.
M is divalent metal like Ba, Ca, Pb, Zn etc.
Manufacture: Raw material :
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1. Acidic oxide : some acidic oxide are required for the manufacture of glass. It depends upon its
quality . Some important acidic oxides are SiO2, B2O3, P2O5 etc.
2. Basic oxide : Na2O, K2O, Li2O, CaO, BaO, ZnO etc are important basic oxides. CaCO3 , BaCO3, Li2CO3 etc. can be used in the manufacture of glass.
3. Decolorizer: MnO2 or KMnO4 is used to decolorize the unwanted color in the glass.
4. Cullets: These are broken glasspieces. Cullets melts at low temperature and help in melting the other materials at low temperature.
5. Coloring materials:
It involves following steps:
a) Preparation of Batch:
The raw material is mixed in proper ratio and ground . The well ground mixture is called batch .
b) Fusion or melting of Batch:
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Cullets are added in the batch. The mixture is melted in the furnace heated by producer gas. The temperature of furnace is 1400o C. The mixture melts and formed transparent glass. To remove unwanted color, small amount of MnO2 or KMnO4 is added to moltenglass. If colored glass is required then required coloring matter is added in the moltenglass.
Following reactions take place in the furnace:
Na2CO3 +SiO2 ———–> Na2SiO3 (sodium silicate) +CO2
CaCO3 +SiO2 ————> CaSiO3 (calcium silicate) +CO2
Silica dissolved in molten Na2SiO3 and CaSiO3 at 1400o C and glass is formed.
Na2SiO3 + CaSi03 +4 SiO2 —–> Na2SiO3. CaSiO3.4SiO2 (soft glass or soda glass)
c) Annealing :
The process of slow cooling of glass is called annealing .The article are passed through a long tunnel like furnace . Its one end is hot and other end is at room temperature.Therefore the temperature of the articles decreases slowly. If articles are cooled rapidly then they became brittle and fragile.
Variety of Glass:
Soft or Soda glass :
Its formula is Na2SiO3.CaSiO3.4SiO2. On heating it became soft. Softglass is used in making tubes, dishes, bottles window pans & laboratory apparatus.
Hard or Potash glass :
Its formula is K2SiO3.CaSiO3.4SiO2. On heating it fuses or melts with difficulty. It is used for glass apparatus which can bear high temperature.
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Flint glass :
It is made from K2CO3, Na2CO3, PbCO3, SiO2 and boric acid. Reflective index of flintglass is very high. Therefore it is used for making optical instruments, prism and lens of camera, binocular etc.
Pyrex glass :
It is a mixture mainly of sodium and aluminium borosilicate. It has low coefficient of thermal expansion. It can bear sudden change of temperature. It is used for making high quality glassware like beaker, flask etc. It is also used in making cup, bowl, teapot etc.
It is a type of opticalglass it contain CeO2 (cerium oxide). It cuts harmful ultra violet rays. It is used in making lens.
It is a mixture of zinc and borosilicate. It is resistant to heat and acid. It is used in making better quality glassware.