Category Glass and the Environment

Glass is most commonly used for making windows. Today, windows are made using the float glass process, a technique invented in the 1950s.

Earlier methods of glass-making produced sheets of rough glass that had to be ground and polished to make them smooth enough for windows. This was expensive because a lot of energy was needed. In the float process, the resulting glass is perfectly smooth. This is because the glass floats on a bath of molten metal which is smooth and shiny like a mirror. The bath is surrounded by an inactive gas so there is also nothing to spoil the top surface of the glass.

Float glass process

The raw materials are melted in a furnace. A ribbon of molten glass then goes into the float bath where it floats on the surface of molten tin. The thickness of the glass can be varied by controlling the rate at which it flows through the float bath.

If glass cools too quickly it becomes brittle and is no good for normal use. It must therefore be reheated (but not so much as to change its shape) and cooled slowly. This process is known as annealing. Annealing takes place in a long tunnel called a lehr.

The float glass process has revolutionized the manufacturing of glass windows.

From the annealing lehr, the glass is first washed and then cut up into huge sheets which are lifted off by machine. Any waste glass is collected to be used again. The cutting process and the moving and stacking of the glass are all controlled by computer.

Glass is mainly made from the chemical silica (silicon dioxide) which comes from sand. A very high temperature is needed to melt silica, so soda (sodium carbonate) is added to lower the melting point. Silica and soda produce a glass which dissolves in water. This is not suitable for making windows, so limestone (calcium carbonate) is added to make normal, strong glass.

The ingredients can be varied to make special kinds of glass. Adding lead oxide instead of most of the limestone gives a heavy glass which is used to make wine glasses.

Making glass

The raw materials are mixed together in the right quantities and melted in a huge furnace. The size of the furnace depends on how much glass is to be made. A typical furnace for flat glass may hold 2,000 tonnes of molten glass. Usually some waste glass (cullet) of the same colour and type is added.

Coloured glass is made by adding different metal compounds. For example, copper oxide produces blue glass while chromium compounds give green or yellow glass.

The silica found in sand is the main ingredient used to make glass.

Limestone is added to silica and soda to make glass stronger.

Glass is a strong transparent material which has been used for centuries to make many familiar objects. Windows, bottles and drinking glasses are just a few examples. Glass is cheap to make because it is mostly made from sand.

When hot glass cools, it hardens before the molecules can arrange themselves in a regular way (like they do in metal, for example). Although glass is rigid and solid at room temperature, scientists sometimes describe glass as a liquid because of the behaviour of its molecules. Glass even acts like a liquid. Light passes through it, and when glass is heated it flows easily and can be moulded into different shapes.

Glass is transparent because of the ‘loose’ arrangement of its molecules. Ultraviolet light rays are reflected, but white and infrared light rays pass through it.

Glass is perfect for making windows because it protects you from the wind, rain and cold, but still lets the sunlight in.

Picture Credit : Google