Category Chemistry

Glass objects can also be made by pressing or drawing. Pressing can be used to make items, like vases, with quite complicated patterns on them. The patterns come from a mould. These can be expensive to make but are stored in the factory to be reused again.

Drawing is the method used to make tubes like fluorescent lights or scientific glassware. Drawing uses a hollow, rotating tool called a mandrel. Molten glass is drawn over the mandrel while air is pumped in at the top. The thickness of the tube depends on the pressure of the air and the speed of the drawing.

In pressing, the gob of glass is dropped into an open mould and the top half of the mould, called the plunger, is pressed down into it. Only dishes and objects which are wide at the top can be made by pressing. It must be possible to get the upper part of the mould in and out. Pressing can be done automatically or by hand.

Glass tubing is made with a tool called mandrel. Glass tubing is made by drawing. The molten glass is shaped over the rotating mandrel with air blowing through it. This forms the tube which keeps its shape as the machine pulls it away from the furnace. The machine is very long — the tractor rollers may be as far as 120 m (400 ft) from the mandrel.

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Bottles and jars are made by blowing air into glass. Today, bottles and jars are mass produced using machinery. To make a jar, molten glass is dropped into a mould which has a plunger attached to it. The plunger is used to press the rough shape of the jar. The jar is then finished in a second mould by blowing. Bottles cannot be shaped using a plunger because they have a narrow neck. Instead the glass is blown at both stages.

A big modern bottle machine has a number of moulds operating at the same time. Some factories can make up to 12,000 bottles an hour. As with all glass, bottles must be annealed before they can be used.

Glass bottles are now mass produced in factories where they are quicker and cheaper to make.

Making a bottle

A lump of molten glass, known as a gob, drops into the first mould which is upside down

(1). The bottom of the mould is the exact size of the neck of the bottle with a plug in the centre. Air is blown in at the top so that the glass is pushed down into the mould to form the neck

(2). The plug at the bottom is removed. Next a plate is fixed over the top of the mould. More air is blown in from the bottom to form the rough shape, or parison

(3). The parison is removed, turned over and placed in the second mould

(4) Where the final shape is blown

(5). The bottle is now ready for annealing

(6). When the bottle has cooled, it is stacked and stored, ready to be filled.

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It can get very hot inside a room with big glass windows. This is an advantage for a farmer growing vegetables in a greenhouse, but it can be uncomfortable in an office. Double glazing or special solar control glass is used to help reduce the amount of heat that comes into a building.

If the lighting in a room is right, coated glass can be used as a one-way mirror for security purposes. Those on the inside can see outside perfectly, but those on the street will just see their own reflection. Patterned glass is also used for privacy, in a bathroom window for example.

Patterned glass is made by an old method of glass-making in which the glass from the furnace is poured between metal rollers to flatten it into a sheet. The pattern is imprinted into the hot glass from the top roller.

Many different designs are possible, some quite complicated. Notice how the pattern makes it more difficult to see through the glass and that it is smooth on one side. Patterned glass is often coloured for use as decorative screens.

Many offices use coted-glass windows for security purposes.

Double glazed windows provide better insulation than normal windows. They are made from two panes of glass with a layer of air trapped between them. In winter, double glazing reduces the amount of heat that escapes and keeps the inside warm, while in summer it limits the amount of sunlight coming in, preventing a room from becoming too warm. Double glazing also reduces noise pollution in a similar way.

In office blocks, where the whole wall may be made of glass, solar control glass is often used to help keep the heat out, while still letting the light in. The glass may be coloured or coated with a thin layer of metal.

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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.

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