Category Glass and the Environment


Glass is strong enough for windows, jars and bottles in normal use, but it can be broken very easily. When safety is important, glass which has been specially strengthened is used. Cars used to be fitted with a toughened (heat-treated) glass windscreen. When toughened glass breaks it shatters into small pieces instead of leaving sharp splinters.

Windscreens are now made from laminated glass — a glass ‘sandwich’ with a layer of plastic in the middle. Laminated windscreens may crack but they don’t shatter, greatly reducing the risk of injury. Other sorts of strong glass include wired glass and bulletproof glass, which is made from several sheets of glass separated by plastic layers.

Laminated and wired glasses are both made by rolling. The plastic or wire is sandwiched between two sheets of glass. The strong glass is then annealed and cut to size. Laminated glass is perfectly clear and is ideal for car windscreens or shop windows, to prevent people from getting hurt if the glass breaks. Wired glass is used where falling glass would be dangerous. If the glass melts in a fire, the pieces are held in place by the wire. Invented in 1895, wired glass was the first safety glass ever made.

Today’s windscreens are made from laminated glass. On impact, the windscreen may crack, but because of its triple layer construction, it will not shatter. This reduces the chances of people being cut or injured by flying glass.

Fighter planes are fitted with a bulletproof windscreen – often up to 12 cm thick.

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Glass has been used for many centuries for windows, optical lenses and decorative purposes. But there are also many technological advances that have been made possible thanks to the unusual properties of glass. Here are just a few examples.

Fibre optics

Thin strands of optically pure glass, as thin as human hair, are used to carry digital information over long distances. Signals are sent along the core of each glass fibre as pulses of laser light and thousands of these fibres are bundled together to form a cable. Fibre optics is light, flexible and relatively inexpensive. They are ideal for investigative medical instruments used to see inside a patient, and for telephone, television and computer cables. Unlike metal cables that conduct heat or electricity, fibre optic signals are not affected by other fibres in the same cable. This means that you can get a clear telephone and television signal at the same time.


Shiny, smooth surfaces, such as metals, are the best reflectors of light. A mirror, made from a sheet of glass with a thin layer of silver on the back, reflects light almost perfectly. Glass for mirrors must be completely flat so that the image is not distorted. Float glass is ideal. The glass is first washed and then coated with a tin compound. This ensures that the silver deposit is embedded in the surface of the glass. The silver is deposited by the action of several chemicals. It is then covered with copper, red paint and varnish to protect the layers of metal.

Glass ceramics

Glass can be made stronger if its molecules are forced into a regular pattern. Chemical substances are added to glass and, through intense heat treatment, these particles act as ‘seeds’ around which crystals form. The crystallised glass is called glass ceramic. Glass ceramics can be heated or cooled without cracking, so they are ideal for ovens, freezers, stoves and fireplaces. Glass ceramics are also used for missile and rocket nose cones and as thermal insulation to protect space shuttles as they re-enter the Earth’s atmosphere.

Soluble glass

Glass made from silica and soda, which dissolves in water, has some unusual medical uses. Soluble glass pills can be made containing drugs or vitamins in the centre and are particularly useful in veterinary medicine. If the pill is fed to a sheep, for example, the glass slowly dissolves, releasing drugs or vitamins into the stomach. In this way, large doses of medication can enter an animal’s bloodstream.

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Glass is a part of our everyday life, but the processes used to produce this valuable material can also have a damaging effect on our environment. We have come a long way in reducing the environmental impact of glass, with an established recycling industry and more efficient methods of production, but we still need to do a lot more to reduce the impact that the glass industry has on our planet.

Pollution in production

The fuel and raw materials used in glass production release hazardous chemicals, such as sulphur and nitrogen oxides, into the atmosphere. Pollutants can also spread to nearby water sources. Factories now use filters to reduce air pollutants and closely monitor their drainage systems to minimise water pollution.


Extremely high temperatures are needed to melt the raw materials that make glass. Where possible, factories now use electricity as a heat source, instead of valuable natural resources such as gas and oil. Efficient furnace design also helps to prevent heat loss. Using waste glass alongside raw materials enables manufacturers to use lower temperatures, saving energy and reducing the level of oxides that are released into the atmosphere.

Tourism and pollution

In the developed world over 400 kg of domestic waste (per person) is generated each year. Beautiful locations are popular tourist attractions but they can easily be spoilt by careless waste disposal. More importantly, some litter can be very dangerous to humans, animals and other wildlife. Glass bottles left lying around can trap small animals, and broken glass is a serious hazard. Never leave your rubbish lying around in public places, and if glass gets broken, wrap it up before you dispose of it in a rubbish bin.

Raw materials

Hundreds of thousands of tonnes of raw materials are quarried each year for the purpose of making glass. Much of this glass is later thrown away as rubbish. Meanwhile, the quarrying continues, causing scarring of the landscape and the loss of many natural habitats.

What you can do

Did you know that recycling glass is one important way that you can help to improve the environment? Recycled glass can be used in glass furnaces to save on raw materials, cut energy costs and reduce pollutants. If you throw glass away it will be discarded in landfill sites, wasting precious natural resources. Instead, recycling saves hundreds of thousands of tonnes of raw materials from being quarried each year and conserves the countryside for everyone.

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When glass is heated, its surface and shape can be altered in many different ways. Techniques for shaping and colouring glass have been practised for hundreds of years. Stained glass was first used by wealthy Romans to decorate their villas and palaces. Over the years, clerics began to use stained and tinted glass windows in churches to keep these places of worship cool and dim.

At the end of the 17th century, the Bohemians discovered that adding chalk to glass created a much more brilliant version which, once cooled, was thick enough to engrave with elaborate patterns. Today, decorative glass is still popular and regarded as a highly-skilled craft.

Hand-made glass

In small glassworks and studios, glass is still made by hand. The raw materials are melted in a small furnace. The glassblower gathers a gob of glass on to the end of a long iron pipe known as a blowing iron . By blowing down the pipe and shaping the glass against a stone slab , the glass can be made into the shape required. A solid rod is attached to help hold the article and the top is cut off with shears. The glassblower will then finish shaping the work . If a handle is required the glassblower will get more glass from the furnace. Finally the rod is cut off .

Stained glass

Stained glass windows have been a feature of church architecture for centuries. The design is laid out on a table and small panes of coloured glass are mounted in lead frames to make an elaborate picture. Usually the colours are produced by adding metal oxides when the glass is made, although sometimes extra details may be painted on.

Decorating glass

Hand-made glass for wine glasses is often decorated. Lead oxide glass is especially suitable as it sparkles in the light. Deep patterns are made by cutting. More delicate designs are made by engraving using a copper wheel or a high-speed drill similar to the ones used by dentists. Glass can also be decorated by ‘sand blasting’, using a gun which fires sand particles.

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Special kinds of glass can be made if other chemicals, such as metal oxides are added. Borosilicate glass is made from the chemicals silica and boric oxide and is usually known by the trade name Pyrex. Pyrex is used to make casserole dishes, chemical glassware and industrial flasks and pipes because it does not crack when heated.

Metal oxides, such as zinc, lead and magnesium oxide, are added to make high quality optical glass for the lenses in cameras, microscopes, telescopes and some spectacles. Optical glass is hard to form and expensive to produce. It must be completely transparent so that light passes directly through it, without distortion.

            Binoculars and microscopes use optical glass to refract light and make objects look bigger.

The Hubble space telescope mirrors are extremely smooth, with precisely shaped reflecting surfaces.

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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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