What is glass:
Glass is a solid material giving total through-vision. When other materials such as metal, solidify, they become crystalline, whereas glass (a super cool liquid) is non-crystalline.
A solid that is transparent?
Let’s stretch the imagination . . .
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, but the result is water glass which is soluble in water.
- Limestone, a stabiliser, is thus added to the silica and soda ash. Cullet (scrap glass) is also added to assist the melting process.
Making flat glass (the float process)
The float process of glass manufacture was developed by the Pilkington Glass Group (England) in 1959. There are currently over 60 float glass plants operating around the world, some under license. 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. 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. |
