Glass

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Any amorphous and transparent solid that is a product of the solidification of a liquid is called glass. However, glass is generally referred to as the transparent substance obtained when white sand is fused with oxides and carbonates of alkaline earth metals and the 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 US, the history of glass dates back to the 17^th century and before.

Manufacture of Glass

Raw Materials

• Silica (in the form of sand)

• Compounds of alkali metals, like Na₂CO₃, Na₂SO₄, NaNO₃, K₂CO₃ and KNO₃.

• Compounds of alkaline earth metals, like CaCO₃, CaO, BaCO₃. (for glass with high refractive index)

• Oxides of heavy metals, like PbO, Pb₃O₄

• Calcium phosphate, Ca₃(PO₄)₂ (for opalescent glass that also contains arsenic and antimony oxides)

• Colouring materials - Metallic oxides like 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) Glass (Na₂O.CaO.6SiO₂)

Soda glass, also called window glass, is got by fusing sodium carbonate (Na₂CO₃), calcium carbonate (CaCO₃) and sand or quartz (SiO₂) in proper proportions. 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.

Manufacture of Glass

Since CO₂ is liberated in the initial stages, frothing is a common observation. All the CO₂ is driven out on continued heating and a clear viscous fluid mass is got. It is then poured into moulds of various shapes or stamped with die (a device for stamping, cutting, or moulding material into a particular shape) to produce different kinds of glassware.

Properties of Glass

As we have seen, glass is a mixture of number of silicates. Therefore, when heated, it does not melt at a fixed temperature. But, it softens gradually and hence can be moulded into any desired shape. It is this property that makes glass one of the widely used materials.

Annealing

Glass if cooled rapidly becomes brittle and fragile and if cooled very slowly becomes opaque because of devitrification. For this purpose, 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 is progressively cooled. This process is known as annealing and takes several days to be completed.

Varieties of Glass

Soda Glass

Otherwise known as soft glass

Composition

Sand /Quartz : SiO₂ : 75% (will be nice if this can be a mouse over)

Sodium Oxide : Na₂O : 15%

Calcium Oxide : CaO : 8%

Aluminium Oxide : Al₂O₃ : 2% (impurity)

Uses

Since it softens at a comparatively lower temperature and can be shaped into different forms, it finds use in making windowpanes, bottles, etc.

Potash Glass (Hard Glass)

Composition

Sand /Quartz : SiO₂

Potassium oxide : K₂O

Calcium Oxide : CaO

Aluminium Oxide : Al₂O₃

Uses

Has a higher melting point and can withstand higher temperature. Hence it finds use in laboratory ware.

Flint Glass

Composition

Silica : 45%

Sodium oxide : 4%

Potassium oxide : 4%

Calcium oxide : 3%

Lead oxide : 44%

Lead carbonate or oxide replaces calcium carbonate. Potassium carbonate partly replaces sodium carbonate.

Uses

It has higher density, transparency and refracting power than ordinary glass. Hence used for making optical instruments. It is also used for ornament purposes.

Pyrex Glass

Composition

Silica : 80%

Sodium oxide : 4%

Calcium oxide : 0.5%

Potassium oxide : 0.5%

Boron trioxide :12%

Aluminium trioxide : 3%

Uses

It has very low co-efficient of expansion and can withstand sudden changes in temperature. Therefore highly suitable for laboratory ware like flasks, beakers and oven proof cook wares.

Jena Glass

Composition

It has lesser alkaline and higher alumina content than soda glass. It also contains barium oxide, zinc oxide and boron trioxide instead of silica.

Uses

More resistant to the action of acid or alkali. Hence can be used to make acid and alkali containers.

Crooke's Glass

Composition

Contains cerium oxide as one of the constituents as one of the constituents.

Uses

It is used for optical purposes as it has the capacity to cut off the ultra violetrays.

Safety Glass or Unbreakable Glass

• It is prepared by placing a layer of transparent plastic (sheet of vinyl acetate resin) between two layers of glass and sealing the layers with adhesive.

Uses

This variety of glass does not break easily under ordinary impact. Even though it breaks under heavy impact, glass pieces are not shattered because they are held by plastic. Therefore it is used in making windscreens of automobiles, aeroplanes and trains. It can also serve as bullet proof glass to some extent.