Composition And Manufacture
Composition
Ordinary glass used in building is also known as soda-lime-silicate glass and has the following composition:
- silica, the raw material, in the form of sand (70 to 75 %), up to 28% of this constituent may be cullet, or waste glass.
- soda, the flux, as carbonate and sulphate (approximately 12 – 14 %)
- lime, a stabiliser, in the form of limestone (approximately 5 – 12 %)
- various other oxides such as alumina and magnesia, to improve the physical properties of the glass, including its resistance to atmospheric pollutants.
- for body-tinted glasses, metal oxides can also be incorporated.
Manufacture
Batch house (1)
Recycled glass (cullet) is added to the raw material mixture to lower the melting point.
The mixture is transported, weighed, mixed and charged into the furnace automatically and watered to prevent particles of the various constituents from separating and producing dust.
Furnace (2)
Float glass manufacture consists of three main stages:
- melting, when the raw materials become molten at a temperature of around 1500 – 1800 °C
- refinement, when the molten glass is homogenised and gas bubbles are removed
- temperature control, when the molten glass is cooled to a point where its viscosity is suitable for drawing into the tin float bath.
Molten tin float bath (3)
Liquid glass is floated onto the molten tin at approximately 1100°C forming a ribbon, which is naturally 6 to 7 mm thick. Since the glass is highly viscous, it does not mix with the very fluid molten tin, producing a perfectly flat contact surface.
The thickness of the glass is controlled by increasing or reducing the rate at which it spreads.
Annealing chamber (4)
On leaving the bath of molten tin, the now rigid glass ribbon passes through a cooling tunnel called an “annealing lehr”. The temperature of the glass is gradually lowered from 600 to 250 °C. The glass is further cooled in controlled conditions, removing any internal stresses, enabling it to be cut and worked.
Cutting (5)
The continuous cold glass ribbon, is now automatically cut into sheets measuring 6000 x 3210 mm.