Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World

by Mark Miodownik

The melting point of a metal is an indicator of how tightly the metal atoms are stuck together and so also affects how easily the dislocations move.

Heating metals allows dislocations to move about and reorganize themselves, with one of the outcomes being that it makes metals softer.

Gold is another relatively soft metal, so much so that rings are very rarely made from pure gold metal because they quickly scratch. But if you alloy gold, by adding a small percentage of other metals such as silver or copper, you not only change the color of the gold—silver making the gold whiter, and copper making the gold redder—you make the gold harder, much harder. This changing of the properties of metals by very small additions of other ingredients is what makes the study of metals so fascinating. In the case of gold alloys, you might wonder where the silver atoms go. The answer is that they sit inside the gold crystal structure, taking the place of a gold atom, and it is this atom substitution inside the crystal lattice of the gold that makes it stronger. Alloys tend to be stronger than pure metals for one very simple reason: the alloy atoms have a different size and chemistry from the host metal’s atoms, so when they sit inside the host crystal they cause all sorts of mechanical and electrical disturbances that add up to one crucial thing: they make it more difficult for dislocations to move. And if dislocations find it difficult to move, then the metal is stronger, since it’s harder for the metal crystals to change shape. Alloy design is thus the art of preventing the movement of dislocations. These atom substitutions happen naturally inside other crystals too. A crystal of aluminum oxide is colorless if pure but becomes blue when it contains impurities of iron a...

The ages of civilization, from the Copper Age to the Bronze Age to the Iron Age, represent a succession of stronger and stronger alloys. Copper is a weak metal, but naturally occurring and easy to smelt. Bronze is an alloy of copper, containing small amounts of tin or sometimes arsenic, and is much stronger than copper. So, if you had copper and you knew what you were doing, for very little extra effort you could create weapons and razors ten times stronger and harder than copper. The only problem is that tin and arsenic are extremely rare. Elaborate trade routes evolved in the Bronze Age to bring tin from places such as Cornwall and Afghanistan to the centers of civilization in the Middle East for precisely this reason.

Steel, the alloy of iron and carbon, is even stronger than bronze, with ingredients that are much more plentiful: pretty much every bit of rock has some iron in it, and carbon is present in the fuel of any fire.

(the carbon in steel doesn’t take the place of an iron atom in the crystal, but is able to squeeze in between the iron atoms, creating a stretched crystal).

There is another problem, too. If iron becomes alloyed with too much carbon—if, for instance, it contains 4 percent carbon instead of 1 percent carbon—then it becomes extremely brittle and essentially useless for tools and weapons. This is a major obstacle because inside a fire there is rather a lot of carbon around. Leave the iron in too long, or allow it to become liquid in the fire, and a huge amount of carbon enters the metal crystals, making the alloy very brittle. Swords made from this high-carbon steel snap in battle.

The addition of chromium had not made the steel harder, hence he had rejected the sample, but it had done something much more interesting. Normally when steel is exposed to air and water, the iron on the surface reacts to form iron(III) oxide, a red mineral commonly known as rust. When this rust flakes off, it exposes another layer of the steel to further corrosion, which is what makes rusting such a chronic problem for steel structures, hence the need to paint steel bridges and cars. But with chromium present something different happens. Like some hugely polite guest, it reacts with the oxygen before the host iron atoms can do so, creating chromium oxide. Chromium oxide is a transparent, hard mineral that sticks extremely well to steel. In other words, it doesn’t flake off and you don’t know it is there. Instead it creates an invisible, chemically protective layer over the whole surface of the steel. What’s more, we now know that the protective layer is self-healing; when you scratch stainless steel, even though you break the protective barrier, it re-forms.

suffering probably from arsenicosis, an infliction common to smiths of the time, who were exposed to high levels of arsenic poisoning during the smelting of bronze, which resulted in lameness and skin cancers.

A tree’s core strength derives from a microscopically small fiber called cellulose, which is bound together by an organic glue called lignin.

Extracting the fibers of cellulose from the lignin is not easy. It is like trying to remove chewing gum from hair. Delignification of wood, as the process is called, involves crunching up the wood into tiny pieces and boiling them at high temperatures and pressures with a chemical cocktail that breaks down the bonds within the lignin and frees up the cellulose fibers. Once achieved, what is left is a tangle of fibers called wood pulp: in effect, liquid wood—at a microscopic scale it resembles spaghetti in a rather watery sauce. Laying this on to a flat surface and allowing it to dry yields paper.

This basic type of paper is raw and brown. Making it white, sleek, and shiny requires a chemical bleach and the addition of a fine white powder such as calcium carbonate in the form of chalk dust. Other coatings are then added to stop any ink that is laid on top of the paper from bein...

Paper yellows with age for two reasons. If it is made from cheap, low-grade mechanical pulp, it will still contain some lignin. Lignin reacts with oxygen in the presence of light to create chromophores (meaning, literally, “color-carriers”), which turn the paper yellow as they increase in concentration. This type of paper is used for cheap and disposable paper products, and is why newspapers yellow quickly in light.

It used to be fairly common to increase the textural quality of paper by coating it with aluminum sulfate, a chemical compound that is used primarily to purify water, but what wasn’t appreciated at the time was that this treatment creates acidic conditions. This causes the cellulose fibers to react with hydrogen ions, which results in another form of yellowing. It also decreases the strength of the paper. Large numbers of books from the nineteenth and twentieth centuries were printed on this so-called acid paper and can now be easily identified in book shops and libraries by their bright yellow appearance. Even non-acid paper is susceptible to this aging, just at a slower rate.

The aging process also results in the formation of a wide range of volatile (meaning that they evaporate easily) organic molecules, which are responsible for the smell of old paper and books. Libraries are now actively researching the chemistry of book smell to see if they...

books en masse are more than a library, they are a statement of identity.

This stiffness is controlled by the addition of “sizings,”—fine powder additives, such as kaolin and calcium carbonate, that among other things reduce the paper’s ability to absorb moisture, causing inks to dry on its surface rather than infuse its fibers, while also allowing the whiteness of the paper to be controlled. These powders, and the binders that bond them to the cellulose fibers of the paper, create what’s known as a “composite matrix.”

Control of this matrix determines the weight, strength, and stiffness of the paper.

To prevent forgeries the paper has a number of tricks up its sleeve. First of all, it is not made of wood cellulose, like other paper, but from cotton. This not only gives it greater strength and stops it disintegrating in the rain and washing machines, but it changes the sound of the paper: the crisp sound of paper money is one of its most notable characteristics. It is also one of its best anti-counterfeit measures because it is hard to fake with wood-based paper. The particular texture of cotton paper is something that bank machines monitor. Humans are very sensitive to it, too. If there is any doubt about a banknote, there is an easy chemical test that can confirm whether it’s cotton or not. This is done in many shops using an iodine pen. When used on cellulose-based paper, the iodine reacts with the starch in the cellulose to create a pigment and so appears black. When the same pen is used on cotton paper there is no starch for the iodine to react with, and no mark appears. This basic measure allows shops to protect themselves from counterfeits produced using color photocopiers. But the paper has got another trick up its sleeve: watermarking. This is a pattern or picture that is embedded in the paper but can only be seen when light is transmitted through the paper—in other words, when you hold the banknote up to the light. Despite the name, this is not a water stain or an ink of any sort. It is engineered by creating small changes in the density of the cotton, so that...

The technology relies on the ink being made into a form of the so-called Janus particle. Each particle of ink is dyed so that it is dark on one side and white on the other. The two sides are given opposite electric charges, and so each pixel on the electronic paper can be made dark or white by applying the appropriate electric charge. They are named Janus particles after the Roman god of transitions, who is depicted as having two faces and is often associated with doors and gates. Because the Janus particles are physical ink and need to physically rotate when the text is changed, they cannot be switched as rapidly as the liquid crystal display of an iPad or smartphone, and so at the moment they are unable to show movies and other snazzy stuff. They have a pleasing retro quality, which perhaps suits the written word.

However, it is unlikely that electronic paper will completely supplant books while it lacks paper’s distinctive smell, feel, and sound, since it is this multisensual physicality of reading that is one of its great attractions. People love books, more perhaps than they love the written word. They use them as a way to define who they are and to provide physical evidence of their values. Books on shelves and on tables are a kind of internal marketing exercise, reminding us who we are and who we want to be.

It was a habitual lie, born of living in London and finding ways of politely avoiding talking to strangers.

The setting of concrete is, at its heart, an ingenious piece of chemistry, which has powdered rock as its active ingredient. Not every type of rock will work. If you want to make your own concrete you need some calcium carbonate, which is the main constituent of limestone, a rock formed from the compressed layers of living organisms over millions of years and then fused together by the heat and pressure of the movement of the Earth’s crust. You also need some rock containing silicate—silicate being a compound containing silicon and oxygen, and constituting roughly 90 percent of the Earth’s crust—for which some form of clay will do. Grinding these ingredients up and mixing them together with water won’t get you anywhere, unless you want to create a sludgy mud. In order to create within them the essential ingredient that will react with the water, you need to free them from their current chemical bonds. This is not easy. These bonds are extremely stable, which is why rocks do not easily dissolve or react with many things—on the contrary, they last for millions of years. The trick is to heat them to a temperature of about 1450°C. This is a temperature far exceeding that of an average wood or coal fire, which is between 600 and 800°C if it is glowing red or yellow hot. At 1450°C a fire will glow white hot, with no tinge of red or even yellow in the flames but instead a hint of blue. It is so bright it is unnerving and almost painful to look at. At these temperatures, rock star...

As with any chemical reaction, if you get the ratio of the ingredients wrong, then you get a mess. In the case of concrete, if you add too much water there won’t be enough calcium silicate from the cement powder to react with, and so water will be left over within the structure, which makes it weak. Similarly, if you add too little water there will be unreacted cement left over, which again weakens the structure. It is usually human error of this sort that proves the undoing of concrete. Such poor concrete can go undiscovered but then lead to catastrophe many years after the builders have departed. The extent of the devastation due to the 2010 earthquake in Haiti was blamed on shoddy construction and poor-quality concrete: an estimated 250,000 buildings collapsed, killing more than 300,000 people, and making a million more homeless. What is worse is that Haiti is by no means unusual. Such concrete time bombs are scattered throughout the world.

And there is also now a textile version of concrete called concrete cloth. This material comes in a roll and needs only water to be added for it to harden into any shape you like. Although this material has great sculptural potential, perhaps its biggest application may be in disaster zones, where tents made in situ from rolls of concrete dropped from the air can create a temporary city in a matter of days, one that will keep out the rain, wind, and sun for years while rebuilding efforts continue.

Dark chocolate usually contains 50 percent cocoa fat and 20 percent cocoa nut powder (referred to as “70 percent cocoa solids” on the packaging). Almost all the rest is sugar.

The reason why any sugar molecule—whether in a cocoa bean or a pan or anywhere else—turns brown when heated is to do with the presence of carbon. Sugars are carbohydrates, which is to say that they are made of carbon (“carbo-”), hydrogen (“hydr-”), and oxygen (“-ate”) atoms. When heated, these long molecules disintegrate into smaller units, some of which are so small that they evaporate (which accounts for the lovely smell). On the whole, it is the carbon-rich molecules that are larger, so these get left behind, and within these there is a structure called a carbon–carbon double bond. This chemical structure absorbs light. In small amounts it gives the caramelizing sugar a yellow-brown color. Further roasting will turn some of the sugar into pure carbon

(double bonds all round), which creates a burnt flavor and a dark-brown color. Complete roasting results in charcoal: all of the sugar has become carbon, which is black.

Another type of reaction, which occurs at a higher temperature, also contributes to the color and flavor of the cocoa: the Maillard reaction. This is when a sugar reacts with a protein. If carbohydrates are the fuel of the cellular world, proteins are the workhorses: the structural molecules that build cells and all their internal workings. Seeds (in the form of nuts or beans) must contain all of the proteins needed to get the cellular machinery of a plant up and running, so there is plenty of protein in the cocoa beans. When subjected to temperatures of 160°C and above, these proteins and carbohydrates start to undergo Maillard reactions, reacting with the acids and esters (produced by the earlier fermentation process) and resulting in a huge range of smaller flavor molecules. It is no exaggeration to say that without the Maillard reaction the world would ...

for the nutty, meaty flavors of chocolate, while also reducing some of the ast...

In the USA the milk used has had some of its fat removed by enzymes, giving the chocolate a cheesy, almost rancid flavor. In the UK sugar is added to liquid milk, and it is this solution, reduced to a concentrate, that is added to the chocolate, creating a milder caramel flavor. In Europe powdered milk is still used, giving the chocolate a fresh dairy flavor with a powdery texture. These different tastes do not travel well. Despite globalization, the preferred taste of milk chocolate, once acquired, remains surprisingly regional.

taste. It also contains psychoactive ingredients. The most familiar one is caffeine, which is present in small proportions in the cocoa bean, and so ends up in the chocolate via the cocoa powder. The other psychoactive ingredient is theobromine, which is a stimulant and antioxidant, like caffeine, but is also highly toxic to dogs. Many dogs die every year from eating chocolate, mainly around Easter and Christmas. Theobromine’s effect on humans appears to be much milder, and the stimulant levels in chocolate are small when compared to coffee and tea, so even if you eat a dozen chocolate bars every day, it is only equivalent to drinking one or two cups of strong coffee. Chocolate also contains cannabinoids, which are the chemicals responsible for the high experienced from smoking dope. But again the percentages are tiny, and when blind taste studies were carried out to analyze chocolate cravings, researchers found little evidence that any of these chemicals were linked to feelings of craving.

This leaves another possibility to explain chocolate addiction. Rather than its being a chemical effect, it may be that the sensory experience of eating chocolate is

itself add...

In a list of the countries with the highest

consumption of chocolate, Switzerland comes top, followed by Austria, Ireland, Germany, and Norway.

The game of pool evolved from billiards, a fifteenth-century Northern European game that started in royal palaces and was essentially an indoor version of croquet. This is why the table surface was colored green, to simulate grass.

Sand is a mixture of tiny bits of stone that have fallen off larger bits of rock as a result of the wind and the waves and other wear and tear that stones have to put up with. If you take a close look at a handful of sand you will find that a lot of these bits of stone are made of quartz, a crystal form of silicon dioxide. There is a lot of quartz in the world because the two most abundant chemical elements in the Earth’s crust are oxygen and silicon, which react together to form silicon dioxide molecules (SiO2

The gaps between rows correspond to specific quantities of energy, or quanta. The way these quanta are arranged in glass is such that moving to a free row requires much more energy than is available in visible light. Consequently, visible light does not have enough energy to allow the electrons to upgrade their seats and has no choice but to pass straight through the atoms. This is why glass is transparent. Higher-energy light, on the other hand, such as UV light, can upgrade the electrons in glass to the better seats, and so glass is opaque to UV light. This is why you can’t get a suntan through glass, since the UV light never reaches you.

Even if light is not absorbed by glass, moving through the interior of an atom still affects it, slowing it down until it emerges from the other side of the glass, when it speeds up again. If the light strikes the glass at an angle, different parts of the light will enter it and emerge from it at different instants, forcing them to travel momentarily at slightly different speeds. This momentary difference is what causes the light to bend, or refract, and this is what makes an optical lens possible, with the curvature of the glass resulting in different angles of refraction at different points along its surface. Controlling the curvature of the glass means we can magnify images, which allows us to make telescopes and microscopes, and, for those of us who wear glasses, to see.

Pyrex is a glass with boron oxide added to the mix. This is another molecule that, like silicon dioxide, finds it hard to form crystals. More importantly, as an additive it counteracts the tendency of glass to expand when heated or contract when cooled. When different parts of a piece of glass are at different temperatures, expanding and contracting at different rates, stresses build up within the material as the different parts of the glass strain against one another. These stresses cause cracks to grow and ultimately shatter the glass. If this happens in a vessel containing boiling sulfuric acid, such a failure can maim or kill. The discovery of borosilicate glass (Pyrex is a trade name) put a stop to thermal expansion and the stresses associated with it. It released chemists to heat and cool their experiments as they wished, to concentrate on chemistry and not the potential dangers of thermal shock.

The biggest diamond yet discovered is located in the Milky Way in the constellation of Serpens Cauda, where it is orbiting a pulsar star called PSR J1719–1438. It is an entire planet five times the size of Earth. Diamonds on Earth are minuscule by comparison. The biggest yet found is the size of a football. Extracted from the Cullinan mine in South Africa, it was eventually presented to King Edward VII in 1907 on his birthday and is now part of the crown jewels of the British monarchy. This diamond was formed far below the surface of the Earth at a depth of approximately three hundred kilometers, where, over the course of billions of years, the high temperatures and pressures converted a largish-sized carbon rock into the huge diamond. The diamond was then most likely carried to the surface of our planet during a volcanic eruption, where it lay inert and undisturbed for millions of years until it was discovered a mile underground.

Each diamond is, in fact, a single crystal. In a typical diamond there are about a million billion billion atoms (1,000,000,000,000,000,000,000,000), perfectly arranged and assembled into this pyramidal structure. And it is this structure that accounts for its remarkable properties. In this formation, the electrons are locked into an extremely stable state, and this is what gives it its legendary strength. It is also transparent, but with an unusually high optical dispersion, which means that it splits light that enters it into its constituent colors, giving it its bright rainbow sparkle.

The combination of extreme hardness and optical luster makes diamonds almost flawless as gemstones. Because of their hardness, virtually nothing can scratch them, and so they keep their perfectly faceted shape and pristine sparkle not just throughout the lifetime of the wearer but throughout the lifetime of a civilization—through rain or shine, whether worn in a sandstorm, hacking through a jungle, or just doing the washing up. Even in antiquity diamond was known to be the hardest material in the world. The word diamond is derived from the Greek adamas, meaning “unalterable” or “unbreakable.”

But diamonds are not forever, at least on the surface of this planet. It is, in fact, diamond’s sibling structure, graphite, that is the more stable form, and so all diamonds, including the Great Star of Africa in the Tower of London, are actually turning slowly into graphite. This is distressing news for anyone who owns a diamond, although they can be reassured that it will take billions of years before they see an appreciable degradation of their gems.

The structure of graphite is radically different from diamond. It consists of planes of carbon atoms connected in a hexagonal pattern. Each plane is an extremely strong and stable structure, and the bonds between the carbon atoms are stronger than those in diamond—which is surprising, given that graphite is so weak that it is used as a lubricant and as lead in pencils. The crystal structure of graphite. The conundrum can be explained by noting that within the graphite layers each carbon atom has three neighbors with which it shares its four electrons. In the diamond structure, each carbon atom shares its four electrons with four atoms. This gives the individual graphite layers a different electronic structure and stronger chemical bonding than diamond. The flip side, though, is that each atom in graphite has no electrons left over to form strong bonds between its layers. Instead, these layers are held together by the universal glue of the material world, a weak set of forces generated by fluctuations in the electric field of molecules, called van der Waals forces. This is the same force that makes Blu-Tack sticky. The upshot is that when graphite is put under stress, it is the weak van der Waals forces that break first, making graphite very soft. This is how a pencil works: as you press it on the paper you break the van der Waals bonds and layers of graphite slide across one another, depositing themselves on the page. If it weren’t for the weak van der Waals bonds, gra...

(although these days we call graphite ...

in the diamond structure, all four electrons in each carbon atom are partnered up with a corresponding electron. In this way, all atoms in the lattice are strongly held in a bond, and there are no “free” electrons. This is the reason why diamonds do not conduct electricity, because there are no electrons free to move within the structure to carry the electric current. In the graphite structure, on the other hand, the outer electrons do not just bond with a counterpart electron in a neighboring atom but rather form a sea of electrons within the material. This has several effects, one of which is to allow graphite to conduct electricity, since the electrons can move around like fluid. Graphite was used by Edison for the first light bulb filaments because it also has a high melting point, which allows it to glow white hot without melting when a high current passes through it. Meanwhile, the sea of electrons also acts as an electromagnetic trampoline for light, and this reflection of light is what makes it appear shiny like other metals.

Clay is a mixture of finely powdered

minerals and water. Like sand, these mineral powders are the result of the eroding action of the wind and water on rocks, and are in fact tiny crystals. Clays are formed often in river beds, where these eroded minerals are washed down from mountains, settle on the river bed, and form a squidgy, soft dough. Different mixtures of