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Any amorphous and transparent solid as a result of solidification of a liquid is called glass. However, glass is generally referred to as a transparent substance obtained when white sand is fused with oxides and carbonates of alkaline earth metals and then the resultant molten mixture is cooled. Glass is a super-cooled liquid, i.e. it is a liquid cooled much below its freezing point. The ordinary room temperature is much below the freezing point of glass.



From Egypt to Alexandria to other European countries and the United States, the history of glass dates back to the seventeenth century and early.
Glass is a homogeneous mixture of sodium silicate and calcium silicate.

Manufacture of Glass

Raw materials required:
  • Silica (in the form of sand)
  • Compounds of alkali metals such as Na2CO3, Na2SO4, NaNO3, K2CO3 and KNO3
  • Compounds of alkaline earth metals such as CaCO3, CaO and BaCO3 (for glass with high refractive index).
  • Oxides of heavy metals such as PbO and Pb3O4.
  • Calcium phosphate, Ca3(PO4)2 (for opalescent glass that also contains arsenic and antimony oxides).
  • Colouring materials: Metallic oxides such as ferric oxide (yellow), chromic oxide (green), manganese oxide (purple) and cobalt oxide (blue) are added to fused silicates to get coloured glass.
Manufacture of Soda (Ordinary) Glasses (Na2O ∙ CaO ∙ 6SiO2)
Soda glass, also called window glass, is obtained by fusing sodium carbonate (Na2CO3), calcium carbonate (CaCO3) and sand or quartz (SiO2) in proper proportions. A small amount of scrap glass (pieces and bits of waste glass from previous process runs) is added as a flux. The mixture is then fused in a tank furnace heated by producer gas (see Figure 10.3).



Na2CO3 + SiO2 Description: 36497.jpg Na2SiO3 + CO2

CaCO3 + SiO2 Description: 36497.jpg CaSiO3 + CO2

Properties of Glass

  • It is a mixture of number of silicates. Therefore, when heated, it does not melt at a fixed temperature.
  • It softens gradually on heating and hence can be moulded into any desired shape. It is this property that makes glass one of the widely used materials.
  • It is strong and transparent. It weakens only by surface imperfections.
  • It resists electric current.
  • It retains heat rather than conducting it.
  • It is unaffected by chemicals.


Glass if cooled rapidly becomes brittle and fragile and if cooled very slowly becomes opaque because of devitrification. Hence, before making articles, glass is passed through a long tunnel-like furnace that is very hot at one end and very cold at the other. When glass is passed through this furnace, it gets heated first and progressively gets cooled. This process is known as annealing and takes several days to complete.
Shows the characteristics of different types of glass.


Table Characteristics of Glass


Special Additives
Soda glass
Aluminium oxide (Al2O3)
Fuses easily at relatively low temperature
Window panels, bottles, tumblers
Borosilicate glass (also known as
pyrex glass)
Boron oxide (B2O3)
Has low coefficient of thermal expansion (therefore, resistant to thermal stress and thermal shock)
Laboratory equipments, 
Lead glass
Lead oxide (PbO)
Has high refractive index, absorbs radiation, highly
Prisms, window panels of nuclear installations, lenses
Potash glass
Potassium oxide (K2O)
Withstands higher temperature
Laboratory wares
Crooke’s glass
Cerium oxide
Cuts off the UV rays
Optical purposes
Jana glass
Zinc and barium borosilicates
Is resistant to heat and shock
Fibre glass also known as glass
reinforced plastic
Fibre reinforced polymer made of a plastic matrix reinforced by fire fibres made of glass
Is light, strong and is resistant to fire
Fire-proof curtains dresses, manufacture of automobile parts
Safety glass
Thin layer of flexible clear plastic film called polyvinyl butyral is sandwiched between two thin glass sheets
Withstands tension and compression
Bullet-proof glasses, windshields of automobiles
Coloured glass
CdS—Lemon yellow
Na3AlF6—Milky white
Window panels, decorative materials

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