Rusting and tarnishing
About two and a half thousand years ago a group of people set up a camp on a hillside. They built a group of wooden huts with turf roofs and put a tall fence around them. Near the centre of the camp they built a furnace. This was made of stone to withstand the heat. The furnace was about a metre in diameter and had a chimney nearly three metres high. At one side was a small tunnel and further round was a channel, which led into a shallow pit.
In another part of the camp some people made charcoal. This is formed from a pile of logs which are covered with turf then set on fire. As only a small amount of oxygen in the air can reach the logs, they only burn slowly and incompletely. When the charcoal-making process is over, the turf is removed and rods of black, brittle charcoal are produced. Charcoal is produced by an irreversible change.

One of the men has returned with some red-looking stones. He places them in the furnace with some charcoal. The charcoal is set on fire and, as it burns down, more is pushed into the furnace through the small tunnel. In time, it becomes so hot inside the furnace that another irreversible change takes place. The red-looking stones contain iron, which is locked up with oxygen to form a substance called iron oxide. The charcoal contains a substance called carbon. In the heat of the furnace the carbon snatches the oxygen from the iron and eventually forms carbon dioxide. This gas escapes with the smoke from the top of the furnace. The iron is so hot that it melts and flows down the channel into the shallow pit. Here it cools and forms a lump of solid metal.

Later the metal lump is heated and melted. It is poured into a mould and made into an axe head. This is fixed to a wooden handle and propped up against a hut ready for use.
The axe is used for many years in the woods around the camp. Three generations of people at the camp use it to cut down trees to make charcoal or repair huts. Often the wood is used just to make a fire to cook food.

One day the people at the camp decide that it is time to move on. They pack up their things, gather their sheep, goats and cattle and set off through the woods to find another home. The huts, furnace and broken pots are left behind. In a corner of one hut lies the axe. Someone has forgotten to pack it. By the time they remember they have walked for many days. It is too late to turn back and collect it.

Over the years, the hut rots away and falls over the axe. The turf from the roof covers it and makes it damp. For centuries a slow irreversible change takes place. Oxygen from the damp soil air joins with iron on the axe's surface. A substance called iron oxide is made. It forms brown flakes, we call rust, which fall away from the axe. More iron is exposed to the air and more rust is made and the axe becomes smaller and smaller.

One day, only a few years ago, a group of archaeologists visited the campsite. There was not much to see. Even the stones from the furnace had been scattered and there were only mounds where the huts and fence had been. One archaeologist took a trowel and dug down into the soil inside a hut. When she peered into the hole she gasped. The rust had not completely destroyed the axe. Some of it remained to go on show at a local museum. Perhaps you have seen it.

Do all metals rust?
No, only iron and steel. In fact, there is a kind of steel which does not rust. It is called stainless steel. It is used to make cutlery and to make kitchen sinks and draining boards. If ordinary steel were used the damp conditions in the sink and on the draining board might lead to rusting.

How can rusting be prevented?
There are two things which make iron and steel rust. They are oxygen and water. Both must be present for rusting to take place. If one or both are kept away from the metal then rusting will not take place. For example, a steel nail in a door inside the home will not rust. Although it is surrounded by air, which contains oxygen, the nail will remain shiny because water or dampness is not present. If the nail were outside in a fence it would be in contact with both oxygen and water. This would make it rust.

Does painting metal stop it rusting?
Yes, it does. The paint forms a coating over the surface of the metal and keeps oxygen and water from it. Car bodies are made of steel and are exposed to wet weather yet they will not rust provided the coat of paint is not broken. Sometimes, chippings from the road make holes in a car's paint work and expose the surface of the metal. When this happens, the steel will rust unless it receives another coat of paint.

Some oily pieces of iron and steel do not rust. Why is this?
The oil forms a coating over the metal. Oxygen and water cannot get through it so the metal under the oil remains rust free.

You can sometimes see streams of orange water. Is this rust?
It is a substance very similar to rust and is made by oxygen joining with iron. Many rocks contain minerals, which have iron in them. When oxygen reaches these minerals it snatches the iron from them and forms iron oxide. This substance is not in the form of rusty flakes but its particles make the water a rusty colour.

Can iron oxide give colour to a rock?
Yes, it can. Sand contains small amounts of iron oxide, which give it a yellow colour. Sandstone is made from grains of sand, which are stuck together. They get their brown or red colour from the iron oxide in the sand grains.

Mars is called the red planet. Is this due to iron oxide?
Yes, it is. At one time there was enough oxygen on the planet to join with the iron in its rocks and make iron oxide. This has produced red sandstones so that when you look through a telescope at the planet it has a red colour.

An important building like a museum may have a copper dome but it looks green. Why?
Copper is a bright shiny brown metal. You use it in work on electricity. It is used to make wires. Copper is sometimes made into sheets and used as a roofing material. When the copper roof is first put in place the copper stays brown but in five to ten years it turns grey green owing to an irreversible change which takes place between the copper and the substances in the air. The grey green substance is called verdigris.

Why does silver go brown. Is this rust?
No. Silver can take part in an irreversible change with a substance in the air called sulphur. When this happens, a substance called silver sulphide forms. It makes the brown coating on a silver object. The silver sulphide can be removed by polishing it. If silver is polished regularly silver sulphide will not form on it and the silver will remain untarnished.