Artificial rocks
If you look out of a window, the chances are that you will see a building made of brick or concrete. These two materials are the most important building materials used today. Both are artificial. They do not occur naturally and have to be made. It may seem odd that when the earth's crust can provide us with all the stone we need, we should make other materials instead.

The advantages of using brick are that bricks have stone-like properties but are in a shape which is easy to pack together in a wall, and they are the correct size and weight to be held and lifted in the hand. The advantage of using concrete is that it can be poured to fill almost any shape then sets as hard as rock. This means that a specially shaped piece of concrete can be ready for use quickly. If the same piece were made from rock, it might take weeks to cut it into shape.

Although brick and concrete are artificial the raw materials from which they are made are natural. Bricks are made from clay. The clay is made from tiny particles of rock. They have formed naturally when rock such as granite has been worn away by the weather. Concrete is made from rock fragments. These are much larger and can be the size of pebbles. They have also been made by the weathering of rocks. The rocky fragments in concrete are bound together by cement. Even cement is made from rock. It is made from powdered limestone mixed with clay.

When did people begin to use bricks?
The oldest brick wall that has been found is 12,000 years old. It was built in Jericho, which is in Jordan - a country in the Middle East. The bricks in the wall are made from clay that has been dried in the Sun. They are simply laid on each other like the stones in a dry stone wall. They do not have any mortar to hold them together. About three thousand years later the people of Jericho began to use mortar to hold their bricks together.

What is mortar?
It is a substance, which is used to hold bricks together. It is also used to hold stones together. The first mortars were made of clay or bitumen. Later, the Egyptians used a mortar that contained gypsum. This is the material from which plaster of Paris is made. By Roman times a mortar using sand, lime and water had been invented. A common mortar used today is made from sand, cement and water.

Are bricks still dried in the Sun today?
In some parts of the world where there is little rain, bricks are still dried in the Sun. The disadvantage of Sun-dried bricks is that they do not wear very well and may crumble in time. If they become soaked they can be destroyed. In most parts of the world bricks are heated in a kiln. They may be kept at a temperature of 800 - 1,100°C for between four and ten days. Inside a brick the clay particles are packed together and touch each other in many places. These places where the clay particles touch while the brick is in the kiln, melt and form a glassy substance. When the bricks cool down, the fired clay forms a hard, strong, brittle substance that is more hard-wearing than the Sun-dried brick.

Where do we get clay from for brick making?
Out of the ground below the soil. In some places there are large layers of clay under the soil. When the soil is removed the clay can be dug out and a huge clay pit made. When the clay is taken to the brickworks it is put through a screen which is like a sieve. Small stones in the clay are removed by the screen. Only small particles of clay pass through. The clay may be ground up before it is made into brick. Grinding up the clay makes all the particles the same size so they can push together more closely and make a stronger brick.

How are bricks shaped?
If the clay is thick, it may be pressed into a mould. You can tell a pressed brick. It has a hollow in one side. This hollow is called a frog. If the clay is thinner, it is put in an extruder. This is a machine which pushes the clay through a rectangular slot to make a clay slab. As the slab leaves the extruder it is cut up into bricks by wires that pass through the clay.

Are there different kinds of brick?
Yes. The bricks used on the outside of a building may be coated in sand or have a pattern cut in them. They are called facing bricks and are designed to be attractive and weather-resistant. Bricks for use inside a building are called common bricks. They are designed simply to make a wall. They are not particularly attractive or weather-resistant. as they are usually covered by plasterboard. In factories where heavy machines are used, walls and supports may be built from engineering bricks. These bricks are made from closely packed clay and are very strong to support the weight of the machines.

When was concrete invented?
It was invented in about 200 BC by the Romans. They mixed stone, brick and mortar. A hundred years later they discovered that concrete made from volcanic ash and lime could set and remain strong in lakes, rivers and the sea. In other places the Romans used concrete made from clay, limestone, gypsum, sand, gravel, and crushed stone. They used concrete for making roads, aqueducts and sewers.

Has concrete been widely used in Britain since Roman times?
No. When the Romans left Britain in the fifth century, concrete making ceased. It did not begin again until the eighteenth century. That is why you do not see old houses or churches made of concrete.

Sometimes you see concrete with metal bars in it. Why are they there?
This is called reinforced concrete. The metal is steel and it is used to make the concrete extra strong. You can see it being used in the foundations of buildings and bridges, as well as in floors in tall buildings such as multi-story car parks.