Making iron
Have you ever wondered where all the substances on the Earth came from? For centuries, scientists wondered about this too. When they studied the stars they found their answer. There are clouds of gas scattered throughout the universe. These are the places where stars are made. A star is made from two gases. They are called hydrogen and helium. Inside the star hydrogen takes part in an irreversible change, which makes helium. A great deal of heat is released which causes some of the helium to make other substances such as carbon and even iron. When some stars finish changing their hydrogen to helium, they explode and form a super nova. The heat released in this explosion makes all the other substances in the universe. The substances formed in stars make a dust. It mixes with the clouds of gas. When a new star is forming the dust swirls around it and forms into planets. The Earth formed around the Sun in this way.

Humans have existed on the Earth for about two million years. For most of that time they used the natural materials around them such as stone and wood. About five thousand years ago humans discovered that if you heated rock strongly a shiny liquid would flow out which set hard when it cooled. This substance was copper. About two thousand years later humans discovered how to get iron out of certain kinds of rocks. We have been taking iron out of rocks ever since. At first the iron was heated with charcoal in a small furnace. Today we use blast furnaces up to 60 metres high to provide us with all the iron we need. A blast furnace can produce up to ten thousand tonnes of iron a day and work every day for up to ten years.

Rock, like everything on the Earth, is made from substances formed in the stars. These substances are joined together in many different ways. In some rocks there are large amounts of iron and oxygen joined together. They form a substance called iron oxide. Rocks which are rich in iron oxide are called iron ores. In a blast furnace iron is split from oxygen so we can use it. Just as heat was essential for the formation of substances in the stars, it is essential for getting iron out of its ore but the heat does not simply melt the iron. Irreversible changes take place to release the iron.

Iron ore is mixed with coke and limestone before it is tipped into the top of the blast furnace. The furnace gets its name from the ring of pipes which blow or blast air. The pipes are near the bottom of the furnace so the blast of air rises up inside it. The air provides large amounts of oxygen, which makes the coke burn fiercely. The temperature inside a blast furnace can reach 2,000°C. Coke contains carbon and at these high temperatures it snatches oxygen from the iron in the iron ore. When the carbon and oxygen leave the top of the furnace in the hot gases, they leave joined together as carbon dioxide. The melting point of iron is 1,535°C. This is well below the temperature of the furnace so the metal melts. It flows down the furnace and collects at the bottom. The rocky pieces of the iron ore join with the limestone to make slag. This is a rock-like substance, which floats on top of the liquid iron. There are two taps at the bottom of the blast furnace. One is above the other. The slag is removed through the upper tap and the molten iron flows away through the lower one.

Next time you see an iron fence or gate think of the journey the metal has made. It formed billions of years ago in a star now dead, floated across the universe, became part of a rocky planet, then was dug out of the ground and heated twenty times higher than a boiling kettle before it was cooled, shaped and set up by a road.

What caused the heat in the star?
People used to think that stars burnt like lumps of coal. If something is to burn like coal it needs oxygen. There is no oxygen in space as there is in the air. The heat is produced in a different way. A star is packed with hydrogen and helium atoms. The hydrogen atoms in the centre of a star are squashed so much that they stick together, take part in an irreversible change and become helium atoms. In this change huge amounts of energy are released. A large amount of this energy is heat energy. In the centre of the Sun, which is an average sized star, four million tonnes of hydrogen is turned to helium every second and the temperature is about 14,000,000 degrees.

What causes the heat in a blast furnace?
The burning of the coke. Coke contains carbon. Oxygen in the blast of hot air joins with some of the carbon in the burning process and heat is released. This is an irreversible reaction. The heat provides the energy for the oxygen in the iron oxide to be moved away from the iron and join with carbon to form carbon dioxide. This is not the burning process. It is another kind of irreversible change, which is usually called a chemical reaction.

How many different substances are produced in the stars?
About a hundred and eighteen. These substances are called elements. Some examples of elements are oxygen, helium, and iron, aluminium, copper, gold, carbon and silver.

How many different kinds of substances are there?
Millions. Elements join together in a huge number of different ways. For example carbon and oxygen can join together to form carbon dioxide and carbon monoxide. In carbon dioxide one atom of carbon joins with two atoms of oxygen and in carbon monoxide a carbon atom joins with just one atom of oxygen.

What happens to the iron after it leaves the blast furnace?
The iron may be made into cast iron. This is used to make the metal covers you see in the road. Some iron is made into wrought iron and made into chains and gates and fencing. Most of the iron is turned into steel.

How is steel made?
The iron straight from the blast furnace contains some carbon. The carbon has come from the coke. Steel is made by blowing jets of oxygen into the molten iron to remove some of the carbon. The oxygen combines with the carbon and forms carbon dioxide. Steels are made with different amounts of carbon in them. If the jet of oxygen is only blown for a short time only a small amount of carbon will be removed and a high carbon steel is produced. This is known as hard steel. It is brittle and is used to make knives and scissors. Medium steel has less carbon than hard steel and is used for making springs and rails for railways. Mild steel has less carbon than medium steel and is used for making girders. The steel with the lowest amount of carbon in it is called soft steel. It is used to make metal cans, wire and car bodies.