My Science School

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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