Bricks

Last updated: November 20, 2009.

What's the easiest way to build a house or a wall? With bricks, of course! They're simple to use, inexpensive, attractive to look at, and they can last hundreds of years. Some of the most famous constructions in history have been made from brick, including parts of the Great Wall of China and many of the structures built during the Roman Empire. Brick is an amazingly versatile construction material. Let's take a closer look at what it is and how it works!

Photo: Bricks are attractive to look at, but some people prefer to add their own decoration all the same! The rough texture of bricks makes them relatively easy to paint.

What is brick?

Stone is a natural building material you can use the moment you dig it out of the ground. Bricks, on the other hand, have to be made from clay before we can build with them. Clay is a naturally occurring ceramic based on the chemical elements aluminum, silicon, and oxygen. If you've ever dug wet, clay-rich soil, you know it's very thick and sticky. To turn this gooey material into hard, durable bricks, we have to cut and mold it into rectangular chunks which are then fired in an industrial oven called a kiln at temperatures of over 1000°C (1800°F).

Bricks are popular as building materials for several reasons. First, clay is available throughout the world in large quantities and brickmaking is a fairly simple process, so bricks themselves are relatively inexpensive. Building bricks are much lighter and easier to work with than stone and sometimes last longer. They're attractive to look at, weatherproof, and—like other ceramics—very good at resisting high temperatures. By using different clays, it's possible to make bricks in different colors. Traditional red bricks take their color from iron in their clay, while yellow bricks have a greater quantity of lime or chalk.

Photo: Most bricks are this distinctive red-brown color because of the iron they contain. This brick pattern is an example of runner bond (see below).

There are essentially two kinds of bricks: ordinary building bricks and refractory bricks:

• Building bricks are made to a standard size (typically 20-22cm long, 9-11cm wide, and 5-7cm high (approx 8-8.5in long, 3.5-4.5in wide, and 2-3in high), with the dimensions varying slightly from country to country). They're made from higher grades of clay and finished on at least one side (face) so they look attractive on houses and walls.
• Refractory bricks are made for high-temperature use for lining such things as industrial smokestacks (chimneys) and household fireplaces, so they tend to be made more crudely and less attractively finished. Unlike ordinary bricks, they're typically made using such raw minerals as fireclay, alumina (aluminum oxide), silica (silicon oxide), and dolomite (calcium magnesium carbonate). Some are designed to survive temperatures over 2000°C (3600°F); the "ceramic tiles" that protect the Space Shuttle from heat when it re-enters Earth's atmosphere from space are actually very thin refractory bricks.

How are bricks made?

Photo: Left: Millions of bricks are made every day. Right: Take a look at the brickworks where those bricks were made (in Dorset, England) and you can see the clay in the ground is pretty much the same reddish-brown color due to the iron it contains.

Brickworks (brickmaking plants) are built in places where there are large supplies of clay available nearby. The first stage in making bricks involves digging the clay from pits in the ground. Raw clay isn't immediately usable as it is: rocks and other impurities have to be removed first by screening and filtering. The clay is then mixed with water and kneaded in machines that resemble giant food mixers or modern breadmaking machines. The now-soft clay mixture is squeezed out through a rectangular-shaped hole (imagine toothpaste squeezing from a tube with a square-shaped hole) in a process called extrusion. Wires cut the lengths of clay into separate bricks, which are then stacked up on trucks and moved into drying rooms where the moisture they contain is allowed to evaporate over a period of about a day or so. Once that process is complete, the trucks are moved again into giant kilns (the ovens that turn the soft clay into hardened bricks ready for building), some of which are over 100m (330ft) long! The firing time and temperature vary according to the type of clay being used and the type of end-product required. Although much more efficient, this process—digging the clay, shaping it, and heating it to harden it—is essentially the way bricks have been made for at least 6000 years. Traditionally, bricks were shaped by hand and left to fire in the sun. Sun-dried adobe bricks are still made this way.

Refractory bricks (also called fire bricks and fireclay bricks) are made by a slightly different process. Since they need to withstand much higher temperatures than ordinary building bricks, the clay they're made from is compressed by hydraulic rams to make a much more dense mass. before the bricks are shaped and loaded into the kiln. That's why refractory bricks and much heavier than ordinary building bricks of roughly the same size.

Bricklaying

Bricks can be laid in any quantity to make all kinds of structures, from walls and smokestacks to churches and bridges. The process of arranging bricks and fixing them together with mortar is called bricklaying—and it's quite a skilful job. The mortar used is a watery mixture of cement, sand, and lime that binds the bricks like an adhesive, holding them firmly together and keeping water out at the same time. Building bricks often have holes bored into them, partly to make them lighter and less expensive but also so the mortar penetrates inside them and holds them more securely.

Bricks are laid end-to-end in a row called a course, with each course slightly offset so the joints do not line up with the ones above and beneath. This arrangement distributes the weight of the wall and any loads it has to support throughout the entire structure, making the whole thing considerably stronger and safer. Bricks can be laid in many different patterns (known as bonds) with their lengths (called stretchers) alternating with their ends (called headers). Examples of bonds include runner (where all the bricks are stretchers), English (where a course of headers alternates with a course of stretchers), American (where several courses of stretchers are spaced with occasional courses of headers), and Flemish (where each course is made up of alternate headers and stretchers and the courses are arranged so the headers of one course are aligned with the centers of the stretchers above and below).

Photo: A bricklayer builds a wall. This is an example of American bond with a number of stretcher courses spaced by occasional header courses. Note the string that's being used to line up the bricks and ensure the wall is straight. Photo by William L. Dubose courtesy of US Navy with annotations by explainthatstuff.com.

Further reading

• How buildings work: How do buildings stay up? Why don't they fall down? We take a peek at the science behind the places we live and work in.
• Dry stone walls: How can you build a strong and durable wall from stones with no mortar whatsoever?