My Science School

The traditional tailor can take account of long arms or a spreading waistline and achieve a perfect fit. But made-to-measure clothes get more expensive every year, and the modern clothing industry has to make off-the-peg clothes that fit most people with no alteration.

One of the first proper surveys into people’s measurements was carried out by US Government, who measured 1000 recruits during the First World War to determine the best sizes for uniforms.

In Britain, 5000 women were measured in the early 1950s, with some unexpected results. Existing size charts were based on an average height for women of 5ft 6in (168cm) – but the survey found that the real average was 5ft 3in (160cm).

Today, in large companies, from a basic pattern produced by a designer, a computer produces a range of sizes to cover the normal variations of the population. Unusually small or large people complain that they can never find anything to fit them, and they are right; it does not make economic sense for manufacturers to produce the limited number of garments that would be sold.

The next step is to use the patterns to cut out the material for the garment. Rolls of material, which can be more than 100ft (30m) long, are laid out perfectly flat by machine. Hundreds of layers are spread on top of one another so that a large number of garments can be cut out at once. Computers are used to arrange the patterns on the material so that the minimum of cloth is wasted. A paper computer printout, called a marker, is laid on the layers of fabric ready for cutting.

The actual cutting of the material is done by knives guided from above, or in some modern factories, by laser beams controlled by computers. The laser, an intense beam of light, burns a clean cut through the material, far sharper than the cut of any knife.

Next, the pieces of material have to be sewn together. Many operations, such as buttonholing, can be done automatically. A hand-sewer averages 20 stitches a minute; modern machinery can sew up to 7000 stitches a minute. Some clothes are not stitched in the traditional way at all, but fused together.

Finally clothes are pressed, to mould them into the right shape and to make sharp creases or pleats. Special presses, called buck presses, are designed for each part of a garment.

 

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The Chinese have been exchanging gifts of richly patterned fabrics for thousands of years. At about the time of Christ’s birth the wife of a Chinese nobleman, Ho Kuang, gave another, Shunyu yen, ‘twenty-four rolls of a silk brocade with a grape design, and twenty-five rolls of thin silk design, and twenty-five rolls of thin silk woven with a pattern of scattered flowers.’

The Chinese mastered the art of weaving, using silk threads of many colours and complex weaves to produce brocades and tapestries. With primitive looms, weaving patterns into cloth was a job that needed a great deal of skill and patience.

Even with the inventions of the 18th century, a weaver had to know which of the warp threads (running down the length of the loom) to lift and which to leave to make a pattern. Only the threads that were lifted would be woven into the design when the shuttle carrying the weft (the threads running across the loom) was ‘thrown’ across the loom.

It was not until the beginning of the 19th century that a French silk weaver, Joseph Jacquard, found a way to make detailed patterns without skilled weavers. A chain of cards punched with holes was attached to a rotating block above the loom. Only where there were holes could threads be picked up by small hooks and become woven into the pattern. After each card had been used to make a small part of a pattern, the block was given a quarter-turn, bringing the next card into place.

It took 24,000 cards to weave a silk portrait of Jacquard, so accurate that it could hardly be distinguished from a portrait in oils. The cards were tied together in a long strip which slowly passed over the loom. Jacquard looms are still used to make luxury fabrics.

Many patterned fabrics can be woven on simpler machines. The timeless patterns of tweed are still woven on hand looms.

The direct printing of patterns onto woven fabrics originated in India, and the first printed calicos were brought to Europe in the 16th century. From the Hindi word ‘tchint’ comes ‘chintz’, which we still use to describe printed fabrics that are glazed to give them a slight sheen.

Modern textile printing uses metal rollers on which the design is engraved, with each colour applied by a different roller. The rollers pass through a colour trough as they rotate and then transfer the dye to the fabric. As many as 16 rollers may be used to produce a fabric.

Electronic control ensures that each successive roller matches its patterns perfectly with the one before. As the fabric comes off the final roller it passes through an oven where it is dried. Modern machines can print in 16 colours at speeds of 200yds (180m) of fabric a minute.

 

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For thousands of years silk has been traded from East to West, and it is still the most precious fabric by weight.

Silk is a fibre spun by the domestic silkworm, Bombyx mori, to create a cocoon in which it turns into a moth. Each cocoon consists of a single filament up to 1 mile (1.6km) long. It takes 110 cocoons to make a tie, 630 for a blouse and 3000 for a kimono.

Chinese legend dates the discovery of silk to the year 2640 BC, in the garden of Emperor Huang Ti. According to the story, Huang Ti asked his wife, Xi, Lingshi, to find out what was eating his mulberry trees. She discovered it was white worms that spun shiny cocoons. Dropping one by accident into warm water, she found that she could draw out a fine filament, and wind it onto reels. She had discovered how to make silk, and it remained a Chinese secret for the next 2000 years. Imperial law decreed that anybody revealing the secret would be tortured to death.

Manufacturing silk has four stages: the cultivation of mulberry trees, the raising of silkworms, the reeling of the silk fibre from the cocoons, and the weaving of fabric.

Silkworms will eat the leaves of a variety of trees – one type of silkworm feeds on oak leaves – but mulberry leaves produce the finest silk. In 1608 King James I ordered 10,000 black mulberry trees to be planted across England to create a domestic silk industry, but the project failed. He had unfortunately chosen the wrong variety – silkworms prefer the white mulberry.

In China the mulberries are cultivated as low bushes, so their leaves can be easily harvested and fed to the silkworms.

Silkworms are raised in the spring, in two months of intensive activity. The eggs, stored in a cool place from the previous season, are incubated as soon as the mulberry bushes come into leaf. They take about eight days to hatch, then the worms feed continuously on the mulberry leaves for almost a month. They increase their body weight 10,000 times in his four-week period. Even breathing does not interfere with their eating, because they breathe through holes in their bodies.

To be productive, silkworms must be cosseted. In China it was said that the worms liked warmth and hated cold, liked dryness and hated damp, liked cleanliness and hated dirt. But they were also said to dislike noise, the odour of frying fish, tears, shouting and women who were pregnant or had just give birth. Even today, in the Chinese province of Hangzhou, the women who look after the silkworms are forbidden to smoke, wear make-up, or eat garlic.

After their fourth moulting the silkworms set about making their cocoons. They begin to exude a semiliquid mixture from the two silk glands that run the length of their bodies. The single thread which emerges is made up of the two threads joined together.

First they anchor themselves by making a fine net. Then tossing their heads in a figure-of-eight motion, they slowly build up a waterproof cocoon that completely surrounds them. It takes a worm about three days to spin the entire cocoon, during which it will have shaken its head about 300,000 times.

Left to its own devices the worm will turn into a moth in about two weeks, exude an enzyme to weaken the cocoon and emerge to begin the life cycle once more. In practice only a few are allowed to do this, to provide for the following year. The rest are killed. By preventing the cocoon being damaged by the emerging moth, an unbroken thread can be recovered.

The process of obtaining the thread is called reeling. It is done by soaking the cocoons in warm water, finding the end of each silk thread and winding it onto a reel. Fibres from several cocoons, usually between five and eight, are wound on to the same reel to make a thread of sufficient thickness. Today automatic reeling machines do much of the work.

If two silkworms are placed together they create a twin cocoon. The silk that emerges is known as dupion. It has ‘slubs’ or lumpy places along the thread and is used to make fabrics with variations in texture.

World production of silk is small, around 50,000 tons a year, only a fifth of 1 per cent of total textile fibres. Its shimmering texture is created by fibres that are not round but triangular, and therefore reflect the light. It still makes the best ties and the most luxurious underwear.

 

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Primitive peoples wove fabric in just the same way as we do today. By the time of the death of the young Pharaoh Tutankhamun in the 14th century BC, immensely complex fabrics were being made, with delicate patterns in several colours. No items of ancient Greek fabrics survive, but the decoration on a vase of the 6th century BC shows both spinners and weavers. The loom, about 5ft (1.5m) high, is the same type as that used by Penelope as she waited for the return of her husband Odysseus in Homer’s epic poem The Odyssey.

Weaving uses two sets of yarn, the warp and the weft. The warp threads run parallel along the length of the cloth, and the weft is threaded through them, over and under successive warp threads.

The yarn is woven on a loom, a framework of wood or metal which makes the repetitive process of threading the weft is threaded through them, over and under successive warp threads.

In a simple mechanical loom, the warp threads run off a roller as wide as the finished bolt of cloth will be. The threads pass through a set of wires running vertically, which can be moved up and down. Each wire has a small eye, or ring, in the middle through which the warp yarn runs. By simple mechanical arrangements it is possible to raise every alternate ring, making a space through which the weft can pass. In traditional looms, the weft is carried in a boat-shaped device called a shuttle, but many modern looms are shuttleless and use a rapier-like rod, or jets of air or water to carry the weft.

When the weft has passed through the warp, it is pushed down tightly against the previous thread with a comb-like frame. The rings carrying the warp threads are now depressed, the shuttle is turned round, and a second pass between a different set of threads is made. The fastest industrial looms of today can make well over 200 passes a minute.

The result of this process is plain weaving, in which each weft yarn passes over and under each warp yarn. It makes a tough, hard-wearing material.

Many other possibilities exist. Satin weave, for example, results when the warp is interwoven with only every fourth or fifth weft thread. Because long lengths of the warp lie on the surface of the fabric, it has a lustrous appearance, but may not wear well as it is easy for these exposed lengths to become snagged.

A variation on stain weave is damask, used for tablecloths, furnishing and silk fabrics. Subtle colour variations are achieved by alternating areas in which the warp lies on the surface with areas where the weft does. Minute differences in the reflection of light create the pattern.

Other weaves include twill, with characteristic diagonal lines – used in gabardine, serge and whipcord – and pile weaves, used for producing corduroy, plush, velour and velvet. The thick ‘pile’ of velvet is created by cutting some of the surface threads after weaving.

 

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The principles of spinning are exactly the same now as when the task was performed by hand. The fibres are first ‘carded’ – arranged parallel to one another – by working them between two parallel moving surfaces faced with sharp points. Next they may be combed to remove short fibres, then fed into machines with rollers which draw out the fibres, making the yarn finer, and introducing a twist which holds the fibres together.

A very intense twist induces a kink in the yarn, as in crepe materials. Yarns may also be twisted together to produce a stronger, thicker thread – as in two-ply or three-ply knitting wool. Blended-fibre yarns may be made by spinning together fibres from different sources, mixing wool with polyester fibre, for example, to produce a better combination of warmth, strength and ease of washing.

Finally, the finished yarn is wound onto a bobbin, ready for dispatch.

 

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None of these fibres – animal or vegetable – is long enough to be woven into cloth without further treatment. In order to make a usable thread, the fibres have to be laid out parallel known as spinning.

Originally the tool for doing this was the spindle, a weighted stick which hung free and to which the fibres were attached. When spun between finger and thumb, the spindle imparted a twist to the fibres, which would then be drawn out from fibres stored on a second stick, the distaff.

Spinning machines achieve the same result mechanically. The first spinning wheel – which simply turned the spindle – was introduced to Europe, probably from India, in the early 14th century. But it was not until 1767 that a British weaver, James Hargreaves, built an eight-spindle spinning mass production to the industry. Throughout the Industrial Revolution spinning machines were improved and refined, and spinning machines – the ring-spinning frame – was devised in America. A modern ring-spinner may have as many as 500 spindles, each carrying up to 4 miles (6400m) of yarn.

 

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Wool was probably the first fibre to be made successfully into fabric, during the New Stone Age around 7000 years ago. It gave man his first alternative to wearing animal skins. Flax and cotton fibres were also well known in the ancient world.

In Egypt – where wool was thought to be ‘unclean’ – mummies from 3400 BC have been found wrapped in linen shrouds, made from flax, 1000yds (900m) long. Cotton was used in India in 3000 BC; and cotton fabrics from around 2000 BC have been found in Peru.

Two processes are needed to turn fibres like wool, flax and cotton into cloth. The first is spinning, in which the fibres are twisted together to form a yarn; the second is weaving, in which two sets of yarn are interwoven at right angles to form a fabric.

Spinning was traditionally a woman’s task, hence the term spinster for an unmarried woman. Weaving was done by men. Before the Industrial Revolution, when spinning was all done by hand, it took the combined output of five to eight spinsters to keep one weaver employed. Fabric was expensive, and clothes had to last a long time. In one day, a woman could spin about 550yds (500m) of wool.

The most important of the animal fibres is sheep’s wool. Most wool fibres are from 1in to 8in (25mm to 200mm) long.

Flax is a fibre found in the stem of the flax plant, from which it is extracted by splitting the stalk and soaking the fibres in water for several weeks to separate them from the resinous material that glues them together. The fibres are from 6in to 3ft 3in (150mm to 1m) long.

Cotton fibres grow in the seedpod of the cotton plant. They are much shorter than flax, forming flat, twisted ribbons from 1/8in to 2 ½in (3mm to 65mm) long. The fibres have to be teased out of the seedpod, and disentangled from the seeds, a process done by a cotton gin.

Other plant fibres include jute, used for making sacks, bags and carpet backings; and hemp, which is made from the cannabis plant and is used in sailcloth, canvas and tarpaulins. One of the most unusual plant fibres was made from stinging nettles. Mary, Queen of Scots slept in sheets made from the fine linen they produced.

 

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The discovery that paper could be made from wood was the key that made the mass market in books and newspapers possible. But unlike parchment, vellum or rag-based papers, paper made from wood pulp has a limited life. Librarians have begun to realize that modern books are deteriorating rapidly.

The problem is that they contain chemicals, including acids from the bleaching process, that eat them away. For most readers, it hardly matters, because they have read the books long before the decay becomes evident. But for archivists and librarians it is a disaster. It means that potentially all the books that have been published since 1850 could be slowly self-destructing.

‘The irony is that the paper of older books published since the beginning of printing in 1475 can be in much better condition than something printed only 40 years ago, which is collapsing’, says Mr Mike Weston of the British Library.

Librarians are now trying to find some inexpensive way of treating their vast stock of books. At present, the only way is to strip off the bindings and treat the pages one by one to remove acid. While this might be justified for some valuable first editions, it is impractical for the bulk of books. However, some manufacturers are now producing paper which has a neutral sizing, to prolong its life.

 

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The first watermark happened by accident at the Fabriano paper mill in Italy, where paper has been made since AD 1260. The mould that was being used to press the water from the wet paper had a small piece of wire projecting from it. The paper was thinner where the wire dug into it, causing a line that could be seen by holding the paper up to the light.

It was realised that if a complete design was made of wire, a decorative watermark would be created. In 1282 the first deliberate watermark was made – it was just a simple cross.

Much the same method is used today. The wet paper is squeezed by a roller known as the dandy roll. Soldered or sewn onto the dandy roll is the raised pattern that creates the watermark.

Watermarks have been used for centuries to identify the makers of fine stationery. More elaborate watermarks are used to make forgery of banknotes difficult, by impressing the portraits of heads of state or national heroes on the notes.

Foolscap got its name at the beginning of the 18th century from a watermark of a fool’s cap used on paper that was 13 ½in (340mm) wide, and 17in (430mm) long.

 

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It was a Chinese official attached to the Imperial court, Ts’ai Lun, who discovered how to make paper in about AD 105. Until then most documents had been written on parchment, made from the skin of sheep or goats, or vellum, which is made from the skin of a calf. The ancient Egyptians had used papyrus, made from reeds beaten flat, but this was not a true paper, which is made from fibres that have been pulped, then reconstituted.

Though serviceable and very long lasting, parchment and vellum could never have coped with the growing demand for a material on which to store man’s unending accumulation of information. It has been estimated that a single book 200 pages long would have needed the skins of 12 sheep.

Ts’ai Lun made his paper from mulberry fibres, fish nets, old rags and waste hemp. Almost any fibrous material can be used for making paper. It is mashed to a pulp with water, bleached, sealed with a sizing agent to prevent too much ink absorption, then pressed into sheets.

Until 1850 the basic raw material was linen and cotton rags, which made excellent paper. But by then demand was growing so rapidly that a new raw material was needed. Wood pulp – usually from softwood trees such as conifers – was the answer.

Wood – indeed all pants – consists of cellulose, an organic material which forms strong fibres about 1/10 in (2.5mm) long. After felling, trees are turned into wood chips and fed into huge digesters where they are mixed with chemicals (usually sodium sulphate) and pressures to separate out the fibres and produce pulp.

Impurities, such as resin and pitch, are removed, the pulp is bleached, and mixed with chemicals to give it the right colour, or to make it whiter. The mixture then flows from a large tank with a narrow slit onto a moving screen which allows the water to drain away but retains most of the fibres. The sheet is pressed to remove more water and dried by passing around a series of steam-heated cylinders.

The paper may finally be coated with pigments such as clay, chalk, or titanium dioxide to improve its surface.

 

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The structure of glass, though strong, contains a lot of empty randomly together like a pile of bricks, rather than lined up in neat columns as they would be if the bricks were made into a wall.

These cavities can be occupied by metal atoms which affect the way light is transmitted through the glass. Different metals absorb light of different frequencies, giving the glass that contains them a characteristic colour.

It was this principle that gave rise to one of the glories of the medieval cathedral, the stained-glass window.

When added to the motion glass, copper turned it ruby red, cobalt blue, iron green, antimony yellow, and manganese purple. Sheets about the size of this book were manufactured in different colours and then cut to the required shapes. They were then assembled into complete windows.

Variations in the thickness of the glass, inevitable with medieval technology, enhanced the beauty of the windows by providing a subtle variation of tone. When the techniques of glass-making improved, a lot of this subtlety was lost.

 

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The technique for making thin, flat window glass was perfected in Normandy, France, in the 14th century. Known as crown glass, each piece was blown by a craftsman. An accomplished glass-bowler could make only about a dozen windows in a day, making medieval window glass an expensive luxury.

For each pane, the molten glass is blown into a large bubble using a blowpipe. The bubble is then flattened and attached to the end of an iron rod, called a punty, which is rotated as possible by the craftsman.

The flattened bubble of glass fans out to form a circle 3ft to 6ft (1m to 2m) wide, depending on the size of the original bubble and the skill and strength of the craftsman.

The round, flat glass sheets were then cut for use as small window panes, particularly in churches. The ‘bullseye’ at the centre of the disc was the least transparent section, but because glass was so expensive, it would have been used anyway.

 

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Glass is made of many small molecules firmly bonded together, and the bonds between them will not stretch significantly. If submitted to sufficient force, they will break. These properties make glass hard, but brittle.

Transparent plastics, on the other hand, are polymers made by loosely bonding together very large molecules. The bonds are not very strong, so the molecules will slide over each other, making plastics flexible.

 

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Five thousand years ago, on some beach in the Middle East, someone probably lit a fire and later found shiny, transparent globules like jewels among the sand. How were these new curiosities transformed into one of the major household and building materials of the 20th century – glass?

The raw material from which glass is made is silica, the most abundant of all the earth’s minerals. Milky white in colour, it is found in many forms of rock, including granite. And as every beach in the world has been formed by water pounding rocks into tiny particles, sand is the major source of silica.

The next time you are at the beach, examine a handful of sand. Any grain which is semitransparent – rather than black, red, yellow or some other definite colour – is a grain of silica. Sand also contains other minerals, but silica is the main component because it is hard, insoluble and does not decompose, so it outlasts the others.

Pure silica has such a high melting point that no ordinary fire would convert it into glass. So the first Middle Eastern glass-makers must have lit their fire on sand which was impregnated with soda (compounds of sodium) left behind by evaporated water from a lake or ea. The soda reduces silica’s melting point.

Today, lime and soda are combined with silica to produce soda-lime glass, used for making bottles, window panes and cheap drinking glasses. When glass cools, its structure does not return to the crystalline structure of silica, which is opaque. Instead, it forms a disordered structure rather like a frozen liquid, which is transparent.

Ovenware and lead crystal

Other materials may be added to provide colour, or to improve the quality of the finished glass. Glass containing 10-15 per cent of boric oxide, for example, is resistant to sudden heating or cooling and is used for ovenware. Adding lead oxide, a technique discovered in the 17th century produces a heavy glass with a brilliant glitter – lead crystal.

Modern sheet glass is made by heating the mixed ingredients in long tanks. The mixture always contains broken glass, known as cullet, which melts at a lower temperature than the other materials and helps them to combine thoroughly.

As newly made glass is taken out from one end of the tank, in a sheet up to 10ft (3m) wide, raw materials are poured in at the other, so that the level in the tank always remains constant.

The tanks are lined with heat-proof bricks and remain in continuous production for as long as their linings last, which may be several years.

Stronger than steel

Glass is thought of as a fragile material, but actually it is very strong. If it is pulled lengthways, a flawless fibre of glass is five times stronger than the best steel. Glass fibres set in plastic produce a tough and resilient material suitable for boats or car bodies called glass-fibre reinforced plastic, or GRP.

Extra-strong glass is produced by heat toughening or by lamination. In toughening, the glass is heated to just below its melting point, then suddenly chilled with jets of air. This makes the surface of the glass cool and shrink before the inner part. As a result, the surface is compressed inwards. This built-in compression has to be overcome before the toughened glass will break. So toughened glass can be bent more, or stuck harder, before it breaks. When it does, it disintegrates into tiny fragments, rather than the dangerous shards of ordinary glass.

Laminated glass is a sandwich of two layers of glass and one of plastic. Although the plastic layer may be very thin, it is tough. Impacts may shatter the glass, but it will remain sticking to the plastic and does not form splinters, which makes it particularly for the windscreens of cars.

Aircraft windscreens must be able to withstand high pressure, extreme temperatures and impacts from flying birds. Three or four layers of glass are interleaved with layers of vinyl, and then bonded together. This produces a windscreen which is able to withstand the impact of a large bird while the aeroplane is flying at up to 400mph (650km/h). The same glass also gives the pilots of military aircraft protection against bullets.

 

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Very few metals emerge glittering and perfect from the earth. Nuggets of gold are sometimes found; in fact in 1869 a nugget of pure gold weighing 154lb (69.85kg) was found in Victoria, Australia. And in 1856 a 500 ton lump of pure copper was dug up from a mine in Michigan, USA.

Some other metals, however, appear in drab disguises, combined with oxygen, sulphur, carbon and other elements to form ores that look little different from rocks or earth.

The first step towards obtaining the pure metal is to separate the ore from the dirt and stones dug up with it. Different methods are needed for different metals.

One way is said to have been discovered by the wife of a lead miner who found that particles of lead stuck to the froth when she washed his dirty clothes.

Lead and copper-mining companies now add ore to aerated, frothing liquid containing a chemical called a collector that enables the mineral particles to cling to the surface of air bubbles, while the waste is wetted and sinks. The valuable material is carried away on the froth to be skimmed off and dried.

Heat is often used to extract the pure metal from the ore, in a process called smelting. Early man discovered that when ores were heated in a fire with charcoal, a spongy mass of metal was left which could be beaten into weapons, tools or ornaments.

Copper was smelted in this way in ancient Egypt, and later the same method was used to produce an even more useful metal, iron. In medieval England it was found that the use of furnaces, which bellows to produce a forced draught of air, would increase the temperature of the fire and produce not a lump of metal, but a stream of liquid iron that could be cast in moulds.

Iron ore is known chemically as iron oxide because the metal is combined with oxygen in its natural state. In the smelting process, the iron oxide reacts with charcoal, made by converting wood into carbon. The oxygen atoms are detached from the iron, and attach themselves to the carbon, forming carbon oxide gas. This escapes leaving behind the iron.

The modern version of the same process uses coke as a source of carbon rather than charcoal, and takes place in huge blast furnaces capable of producing thousands of tons of iron a day.

The iron produced is called pig iron. This contains too much carbon to be useful, so must be converted into steel, by removing the carbon, or into cast iron by blending. Steel is the most important form of iron.

Aluminium occurs in combination with oxygen in bauxite ore. Though it is the most plentiful metal of all, making up 8 per cent of the Earth’s crust, it was not produced in any quantity until the end of the 19th century, because it requires a large amount of energy to separate it from oxygen.

The method used in electrolysis. An electric current is passed through a molten bath of aluminium oxide, which removes the oxygen, leaving behind liquid aluminium. The major difficulty is the very high melting point of aluminium oxide – over 3600ºF (2000ºC), compared to about 2900ºF (1600ºC) for iron.

The problem is solved by mixing the aluminium oxide with a mineral called cryolite (sodium aluminium fluoride) which lowers the melting point to a more manageable and cheaper 1800ºF (1000ºC).

Gold is one of the metals produced by chemical means. It often occurs as fine grains in the beds of streams. The problem is to separate the very small amounts of gold from the mass of useless material.

In ancient times the fleece of a sheep, immersed in a stream, was used to collect grains of gold – perhaps the origin of the Golden Fleece sought by the Argonauts. And prospectors ‘panned’ for gold – swirling the dirt from a stream in a pan of water until the lighter gravel was washed away, leaving the denser gold in the pan.

Today a chemical is used. The crushed ore is mixed with a solution of potassium cyanide, which dissolves the gold. The solution containing the gold is then filtered, to remove undissolved impurities, and the gold is finally precipitated out. A ton of ore will produce just over one-third of an ounce (10 grams) of gold.

 

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