What is glass?
Glass is a solid material giving total through-vision. When other materials (including metal) solidify, they become crystalline, whereas glass (a super cool liquid) is non-crystalline.
How is glass made?
By mixing raw materials such as sand, limestone, soda, dolomite, felspar and saltcake, then heating them together at a temperature of over 1 500°C. When the materials are melted, they form a viscous liquid called the quiescent melt. As this melt cools, the viscosity increases until the glass becomes hard. If the liquid cools too quickly, the glass does not have time to release stresses. This process is done by heat treatment and is called annealing.
- Silica sand is the main raw material in glass, but very high temperatures are needed to melt it.
- By adding soda ash, a fluxing agent, the glass can be melted at a lower temperature. The result is water glass which is soluble in water.
- Limestone, a stabiliser, is added to the silica and soda ash. Cullet (scrap glass) is also added to assist the melting process.
The float glass manufacturing process
The float process of glass manufacture was developed by the Pilkington Glass Group (England) in 1959. In the float process, a continuous ribbon of molten glass moves out of a melting furnace (1 500°C) and floats along the surface of a bath of molten tin. The ribbon is held in a chemically controlled atmosphere at a high enough temperature (1 000°C), long enough for irregularities to melt out and for the surface to become flat and parallel. Because the surface of the molten tin is flat, the glass also becomes flat. The top surface is subjected to nitrogen under pressure, for smooth, polished finish.
The ribbon is then cooled down while still advancing across the molten tin until the surfaces are hard enough for it to progress through the annealing lehr without the rollers marking its bottom surface. The glass produced has a uniform thickness and bright fire polished surfaces, without the need for further grinding or polishing.
Chemical Composition of Glass:
|Iron Oxide (Fe2O3)||0.075 %|
|Alumina (AL2O3)||0.75 %|
|Magnesium Oxide||2.5 %|
|Sodium Oxide||14.5 %|
|Potasium Oxide||0.5 %|
|Sulphur Trioxide||0.25 %|
|Calcium Oxide (CaO)||7.5 %|