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Chapter 1: Ancient Metals

Iron (Fe)

Iron is a metal element, symbol Fe (from Latin "ferrum" ), atomic number 26.

It's the most common metal on Earth, and supposedly the 10th most common element in the universe.

Meteorites were ancient people's first source of iron. About 500 meteorites land on Earth every year, almost always smaller than a soccer ball, and about 1% of those get found. There are different kinds of meteorites; about 5% are made of iron and iron-nickel alloys, and a further 1% are iron and rock mixes. Large meteorites are more likely to be iron-nickel because that sort of meteor is less likely to break up in the atmosphere than rock or rock/ice meteors.

Wouldn't it have taken a long time to collect iron from meteorites? Not necessarily. Just one good meteorite strike could provide a lot of iron. For example, over 1,500 meteorite fragments have been recovered from the area surrounding the Odessa Meteor Crater in Texas, with the largest one over 130 kg. (That crater is about 170 m across and was originally 30 m deep, now just 5 m.)

Meteoric iron wouldn't need to be from recent strikes, that people saw come down, either. The Willamette meteorite (pictured at right) in Oregon, USA, was 14 tonnes, and had been known of by American Indians for centuries (and who may have seen it come down, as their name for it means "the visitor from the sky").

The Cape York meteorite was originally around 50 tonnes, and was used for tools and harpoons by the Inuit of that part of Greenland for centuries (so a big meteorite could last a long time). Trade resulted in pieces of it being used 2,200 km away.

The biggest known meteorite, and also the largest natural lump of iron, is the Hoba meteorite in Namibia, which is thought to have been 66 tonnes when it was first discovered in 1920, now about 60 tonnes. It was found when a farmer was ploughing his field with an ox when the plough made a metal-on-metal scratching sound, then came to a sudden stop. Poor ox. Its rectangular shape is very unusual for meterorites – 2.95 x 2.84 m and between 75 and 122 cm thick – and there is no crater. The meteorite consists of 82.4% iron, 16.4% nickel and 0.76% cobalt.

The second and eighth biggest iron meteorites are both in Campo del Cielo, Argentina, South America. The natives claimed they fell from the sky.

Iron ore is commonly found in New Zealand as magnetite (or titanomagnetite when it is combined with titanium). Magnetite is the most magnetic of all the minerals on Earth. In New Zealand we call it iron sand.

Cast iron is made by heating iron ore to 1,150-1,200 °C with carbon. The resulting molten alloy is 96.5% iron and 3.5% carbon.

Wrought iron is the result of further refinement and contains little or no carbon. An outdoor iron pillar in India has survived 1,600 years without corroding away. Described as "one of the world's foremost metallurgical curiosities" it is made of wrought iron with impurities such as phosphorus (up to 1%) that may have protected it from rusting. These photos taken by my father (click for larger versions):

Steel is an alloy of iron and up to 2.1% carbon (by weight).

Glass (for ordinary windows etc) is coloured green because of iron impurities in sand.

Construction uses a lot of iron, especially in the form of reinforcing steel, or rebar for short. Concrete has huge compressive strength but needs to be confined for that property to be fulled used. Reinforcing helps confine concrete so the two complement one another very nicely. Exposed steel beams will soften in the event of a fire and lose their strength, so they need to be protected with special paint that expands when it gets hot. In its expanded state the special paint insulates the steel and helps keep it strong.


Combustion is when a substance burns - the substance combines with oxygen. For example, when hydrogen burns it makes water (as steam since it's hot).

2H2 + O2 → 2H2O + heat

When carbon burns in a charcoal fire it combines with oxygen from the air to make carbon dioxide.

C + O2 → CO2 + heat

If something combusts extremely rapidly it's called an explosion. Often the reaction rate of combustion varies according to the pressure it's occuring under. In other words, black powder (gunpowder) will burn if in a pile but explode if contained).

Combustion is a kind of reaction called oxidation. Oxidation can also happen slowly. For example, our bodies breathe in oxygen and breathe out carbon dioxide - the same reaction as above.

Another example of slow oxidation is iron rusting or corroding. It slowly combines with oxygen from the air or from water. However, iron can also combust, or burn, if the particles are small enough. Either way the chemical equation is the same:

2FE + 3O2 → 2Fe2O3

or 3FE + 2O2 → Fe3O4

Oxygen doesn't have to be supplied from the air. An example of iron burning quickly, and without oxygen from the air is a fireworks sparkler. A sparkler uses potassium nitrate to supply the required oxygen. They are hard to light because the reaction needs a lot of heat to get going, but it supplies enough heat of its own to keep going once it is alight.

Compare the three equations below for aluminium burning in air, and the combustion of thermite, which is powdered aluminium combined with either magnetite (black/blue iron oxide, Fe3O4) or hematite (red iron oxide, rust, Fe2O3):

4Al + 3O2 → 2Al2O3 + heat

Fe2O3 + 2Al → Al2O3 + 2Fe + heat

3Fe3O4 + 8Al → 4Al2O3 + 9Fe + heat

Thermite, a combination of iron oxide and aluminium, is used to weld railway lines together to form seamless rails, since one of the end products is liquid iron. An ordinary fireworks sparkler (which burns at 1,000-1,100 °C) can be used to light thermite but because themite burns so hot:

  1. It's nearly impossible to put out once alight.
  2. Liquid iron may flow all over the place.
  3. If ignited on damp ground it may cause a steam explosion, throwing burning thermite or molten iron around.

It also emits lots of UV light, so the reaction should not be viewed directly, and sunglasses should be worn.

The different steel alloys used to make cutlery are discussed in The Physics in Your Fork, an article that talks about the crystaline stucture of steel and how it is affected by different steel compositions.

Lead (Pb)

Lead is a dense metal element, symbol Pb (from Latin "plumbum"), atomic number 82.

Lead is possibly the earliest-known metal, and is mentioned in Exodus 15:10 "But you blew with your breath, and the sea covered them. They sank like lead in the mighty waters."

Lead attribute Makes it suitable for
Soft (just 1.5 on the Mohs scale – fingernails are 2.5) and very malleable Sealing of roofs and roof edges.
Poor electrical conductor (compared to other metals) ?
Low melting point Solder (as an alloy with tin).
High resistance to corrosion Lead-acid batteries (and can be used to contain sulphuric acid).
High density and low cost Fishing sinkers.
Keels of yachts.
High-velocity lead poisoning (bullets).
Radiation shielding.
Chemical properties Lead glass is made with up to about 30% lead oxide (unlike ordinary glass). The lead oxide increases the index of refraction and so increases sparkle, and also blocks X-rays. Lead glass is used in television screens, crystal ornaments, etc. Lead crystal is simply lead glass that has been cut with facets.

Fine wine glasses are often made of lead crystal but it's not a good idea to keep wine in lead crystal overnight because the acid in wine can leach the lead out from the crystal. See discontinued uses below – lead acetate.

Lead is poisonous, and so needs to be used with caution. It's also a heavy metal, so when it gets into the body it's hard to get out again. There is no known use of lead in the body, but can mimic other important metals – calcium, iron, and zinc. Apart from workplace dangers, most lead poisoning occurs in children under 12 years old, because they are most sensitive to it.

Discontinues uses of lead include:

  • Pencil leads. For ages (like centuries) they've actually been graphite – a form of carbon – mixed with clay.
  • White, yellow and red pigments in lead paint – especially bad on babies' cots, as the paint may taste sweet, encouraging sucking/teething on it.
  • Tetraethyl lead as an additive in petrol. Thanks to lead in car exhaust, the lead concentration along some New Zealand roads supposedly got high enough that some countries would have been mining it.
  • Plumbing in ancient Rome. (We get the word plumbing from the Latin word for lead, plumbum.)
  • Lead acetate to sweeten wine. Many Roman Emperors possibly went crazy because of this. A physician in Germany in the 1600s noticed that monks were healthier because they didn't drink wine sweetened with an oxide of lead.

Recycling is an important source of lead, as more than half of the lead people use comes from recycled lead. In other words, dead car batteries are worth money.

Tin (Sn)

Tin is a metal element, symbol Sn, (from Latin "stannum"), atomic number 50.

Tin is a silver coloured metal, is malleable and ductile, is not easily oxidized in air and resists corrosion. It is found in many alloys and is used to coat other metals to prevent corrosion.

Tin attribute Makes it suitable for
Malleable Tin foil (now replaced by aluminium foil).
Resistant to corrosion Tin plating on cans etc – the small amount of tin in tinned foods is not harmful to humans.
Makes useful alloys Bronze: 60% copper and 40% tin.
Pewter: fine pewter for eatingware 96-99% tin, 1-4% copper; other grades used to include up to 15% lead, but lead is no longer used to make pewter.
Solder: 60% tin, 40% lead, which gives a melting point under 190 °C.

Temperature affects the properties of tin. Above 13.2 °C tin exists as metallic white tin, but under that temperature it slowly changes into gray tin, with a different atomic structure. Tin becomes a superconductor below 3.72 K (-269.44 °C).


Tin plating experiment: If pineapple is left in its can after opening and put into a fridge it "goes yucky" with a grey powder coming off the inside of the can, discolouring the pineapple and juice. Other sorts of canned fruit doesn't do this. Is it a result of acidic pineapple juice or tin plating? We can test it by finding out if the can without any pineapple or pineapple juice behaves as tin would behave when it gets cold.

Empty the can (perhaps the best part of this experiment!), take off the label (and keep it for experiment records), then cut the can into two parts using tin snips. Use a permanent marker to label the two parts however you want, then put each section in a container like a yoghurt pot (we don't want to cause a mess).

  1. Put one container in the freezer – a cold dark environment.
  2. Use the other container as a control or comparison, to find out what would happen if no special action was taken. Put it in a drawer – a warm dark place (above 13.2 °C).

After a day or two observe the can sections to see if there are any changes. Has it confirmed our suspicions?

Extension: If there are changes that look like white tin has changed to grey tin in the freezer, maybe we can get the grey tin to change back to white tin by warming it up again. Cut the affected section in half and put each half in its own container.

  1. Put one container back in the freezer.
  2. Put the other container in a warm dark environment like a hot water cupboard, or a hot environment like an oven (on low). Don't put plastic in the oven, though.

If the grey tin was powdery the white tin will probably drop off, so use a white container.

Repeat using a different sort of can, such as baked beans or, better, a different kind of fruit (peaches are nice).

Window glass is often made by floating molten glass on top of molten tin, and thus called float glass. Some tin is absorbed into the lower side of the glass, and under ultraviolet light a sheen can be seen on that side. Sheet glass is made by the older method of drawing molten glass up vertically between rollers and gives imperfect thicknesses.


Pilkington Glass explains the float process step by step.

Description of the different types of iron and steel and the production.