Category Science

If molten glass is forced through a small hole it can be drawn into very fine fibres. This is commonly known as ‘fibreglass’. Short strands are made into thick mats used for insulation. Many houses have a layer of fibreglass insulation in their roofs and outer walls to keep the heat in during the winter and out during the summer.

Long strands of glass fibre are also used to reinforce (strengthen) materials such as plastic. The glass fibres make glass-reinforced plastic (GRP) a strong and stiff material which is also light and waterproof. GRP can be brightly coloured and is easy to mould into shapes such as ‘hard-hat’ helmets and car bodies.

Glass fibre is ideal for making the body of vehicles because it is strong and light.

Making insulation

To make glass fibre for insulation, the fibres are chopped up and showered onto a moving web. A special binding spray (glue) is added and heated in an oven to stick the fibres together. The mat is then cut up into convenient sizes for roof, wall or floor insulation, for example.

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Today, bottles are designed with the aid of a computer. Glass manufacturers discuss the size and shape of a bottle with their customers before it is made. Manufacturers can also work out the minimum amount of glass that is needed to make a bottle.

Modern design means that bottles are lighter and cheaper than they used to be, but still strong enough that they do not break when they are opened or handled. Making sure that a bottle stands up and pours well is also important!

Making a light bulb

The glass parts of light bulbs, known as envelopes, are made on a fully automated machine running at high speed. A ribbon of molten glass from the furnace is blown into little blisters by a continuously moving chain of blow heads. Each blister drops into a mould as it passes under the glass ribbon. The glass bulbs are then cut from the ribbon by a rotating disc and collected on a conveyor belt, where they are taken to be annealed.

Glass bottles are made in a variety of shapes and sizes.

New bottle shapes are designed with the aid of computer images.

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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.