by Chris Woodford. Last updated: February 10, 2017.
The major periods of
civilization, such as the Stone Age and Bronze Age, were named for the
materials that dominated them, and it may seem surprising that there
never been a "Ceramics Age." Yet almost any age might have qualified
this title. Archeologists have found evidence of primitive ceramic
dating back to around 24,000 B.C.E., but those most modern of
the silicon chip and the catalytic converter, are also examples of
The modern era is as much a ceramics age as any other. Let's find out more about ceramics
and how they work!
Photo: A ceramic Percy Pig piggy bank. It started life as a soft piece of clay molded to shape,
fired hard in a pottery kiln, then painted with bright colors.
What are ceramics?
Photo: Porcelain plates are very familiar examples of ceramics, but there
are other, much more surprising uses of ceramics too.
Ceramics once referred purely to pottery and to articles made by
firing materials extracted
Today, the term has a much broader definition. Ceramics are
generally thought of as inorganic and nonmetallic solids with a range
including very high hardness and strength, extremely high melting
and good electrical and thermal insulation.
The best-known ceramics
are pottery, glass, brick, porcelain, and cement. But the general definition of a ceramic—a nonmetallic and inorganic solid—is so broad that it
a much wider range of materials. At one end of the scale, ceramics
simple materials such as graphite and diamond, made up from different
arrangements of the element carbon. But at the other end of the scale,
complex crystals of yttrium, barium, copper, and oxygen make up the
ceramics used in so-called high-temperature superconductors (materials
with almost no electrical resistance). Most ceramics fall somewhere
these extremes. Many are metal oxides, crystalline compounds of a metal
element and oxygen. Others are silicides, borides, carbides, and
respectively made from silicon, boron, carbon, and nitrogen. Some of
most advanced ceramic materials are combinations of ceramics and other
materials known as ceramic matrix composites
Properties of ceramics
Ceramics are best known as brittle solids particularly suited for withstanding high
but, in fact, the different materials used in ceramics can give them a
wide range of properties. The classic properties of ceramics include
strength and brittleness, high electrical and thermal resistance, and
ability to withstand the damaging effects of acids, oxygen, and other
because of their inertness (chemical unreactivity). But not all
behave in this way. For example, graphite is a very soft ceramic and
electricity well, whereas diamond is a
very good conductor of heat.
called ferrites are particularly good conductors of electricity and
have almost no electrical resistance at all. Ceramic matrix composites,
made by embedding fibers of a strengthening material in what is known
as a ceramic matrix, are not at all brittle.
Photo: Silicon and carbon fuse to form silicon carbide power (left),
which can be made into a hard and hard-wearing ceramic called silicon carbide that can survive
high temperatures. It has many applications, from drills and cutting
tools to components (middle, right) that can withstand high temperatures in gas-turbine engines
that would melt ordinary metal parts. Ceramic components are also used in ordinary car engines for the same
reason. Picture by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (NREL) (picture id 6307388).
a particular ceramic depend not just on the materials from which it is
made but also on the way they are joined together—in other words, on
crystalline structure. Diamond is strong because all of its carbon
atoms are bonded tightly to other carbon atoms. Graphite (such as that used
pencil "leads") shears because it is made up from different layers.
the carbon atoms are tightly bonded within a given layer, the different
layers are held together only by much weaker bonds. China clay (also
kaolin) behaves in a similar way to graphite, with its constituent
silicon, oxygen, and hydrogen atoms tightly bonded into flat sheets.
the weak bonds between those sheets are easily broken when water
surrounds them and it is this that makes wet clay so easy to mold. When china
clay is fired, heat removes the water, and the chemicals inside the clay
themselves into crystals of aluminum silicate tightly bonded by
glass, which is overall very much stronger.
How ceramics are made
Photo: Ceramic floor tiles get their hardness and durability from being fired. Picture by Michael Sandberg courtesy of US Navy.
Firing is the process by which ceramics have traditionally been made; indeed, the word
can be traced back to a Sanskrit word meaning "to burn." Simple
such as bricks and certain types of glass are still made by processes
would be recognized by people who lived thousands of years ago. Just as
in ancient times, today's pottery is made by digging clay from the
mixing it with water to make it flexible, shaping it on a
wheel or in a
mold, and then firing it in a kiln. Some of today's processes are more
sophisticated than the techniques of past times. Machines have long
used in processes such as extrusion (forcing a material into shape by
it like toothpaste through a shaped tool), jiggering (laying the
material automatically into a rotating mold), or hot pressing (forcing a
form of the ceramic into a mold then simultaneously heating it and
it to fuse the material into shape).
The latest industrial ceramics sometimes demand more advanced production processes. Extremely
tough ceramics made of silicon nitride are made by a method called
reaction bonding. This involves forming silicon powder into the desired shape
then heating it with nitrogen gas. Because the silicon powder already
occupies the same volume as the finished product, grains of silicon nitride can
form only by fusing together tightly.
Types of clay and their common ceramic uses
The US Geological Survey lists six types of clay mined in the United States: common clay, kaolin (China clay), bentonite, ball clay, fuller's Earth, and fire clay, and each has a number of different uses:
- Common clay is mostly used for bricks, cement, and aggregate.
- Kaolin is widely used for making glossy paper.
- Bentonite has a variety of industrial uses, including drilling mud and foundary sand, and is also found in household products that absorb pet waste.
- Ball clay is a high quality clay prized for its use in ceramics, sanitaryware, and wall and floor tiles.
- Fuller's Earth is also used for pet-waste products.
- Fire clay is used in refractory (high-temperature) bricks and cement.
Bricks—a closer look
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.
Photo: Most bricks are this distinctive red-brown color because of the
iron they contain. This brick pattern is an example of what's called runner bond:
all the bricks are pointing the same way but the bricks in one course run over the joins in the course beneath.
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. As we've already seen above, 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
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.
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
- 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 protected the Space Shuttle from heat when it re-entered Earth's atmosphere from space
were actually very thin refractory bricks.
How are bricks made?
Photos: Left/above: Millions of bricks are made every day, but why are they this color? Right/below: Take a look at the brickworks where those bricks were made (near Swanage, Dorset, England) and you can see the clay in the ground is pretty much the same reddish-brown color due to the iron
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. 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.
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
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.
- Brick: A World History by James W. P. Campbell and Will Pryce. Thames & Hudson, 2003. A fascinating, comprehensive history of how humans have used brick from neolithic times to the present day. Lavishly illustrated.
- Brick in the Landscape: A Practical Guide to Specification and Design by Rob W. Sovinski. John Wiley and Sons, 1999. An unusual book that explores the use of brick, as a traditional material, for hard landscaping in gardens and other outdoor areas.
- Bricks and Brickmaking by Martin Hammond. Shire, 2009. A short (32-page) booklet explaining why bricks have been so popular for so long. Focuses mainly on British architecture.
The world of modern ceramics
Photo: Toilets are still largely made from ceramics (though the lid and seat are typically made of plastic or wood).
It is difficult to think of an area of modern life that has not been touched by ceramics.
Our homes are made from brick walls, held together by cement made from
calcium silicates, and glass windows, also
made from silica. Inside,
walls are plastered with ceramic gypsum, porcelain bathrooms are
with tiles made of clay and talc, and kitchens stacked with pottery and
glass have decorative ceramic floor tiles. Clay pipes link our homes to
the sewage system and ceramic insulators are essential in connecting
to the electricity grid. Back inside the house, that electricity flows
through television sets that contain
more ceramic insulators,
such as capacitors and resistors made from ceramics, computers based on
silicon chips, magnetic ceramics used in the
electric motors of appliances such
as vacuum cleaners and food blenders,
and piezoelectric ceramics used
in tiny headphones
Telephone calls and cable television
may be piped to the home through glass
fibers, while other kinds of
fibers keep heat inside the walls and the roof. That heat may itself be
provided by a heated ceramic filament, just as lighting comes from
bulbs or fluorescent tubes.
not just proved useful in everyday situations. The properties of
ceramics have made them important for some much more extraordinary
For example, the toughened silicon carbide used in hip replacements is
designed to be porous so that it stimulates natural bone growth and
formation around the artificial joint. Ceramic engine components are
in "lean burn"
car engines that combust fuel more cleanly.
which convert air pollution into less harmful gases, are made from light
strong aluminosilicate ceramics that can withstand the high
generated in car exhausts. The latest generation of lightweight,
submersibles are being built not from
steel, like their predecessors,
from ceramics originally made for defense purposes. One of the most
uses of ceramics is a new kind of paint made from a piezoelectric
Like other piezoelectric materials, this produces a tiny electric
when it undergoes stresses and strains and its Japanese inventors
it could be used to detect metal failures or even earthquakes.
Ceramics in action!
When NASA's Space Shuttle returned from space, thousands of heat-resistant tiles
protected its exterior from overheating due to friction generated by Earth's
atmosphere. The now-retired Shuttle used different tiles made from carbon,
ceramics, and silica composites. But future reusable space planes are expected to
use a new, slimline ceramic material made from hafnium and zirconium
metals that will enable it to be both more aerodynamic and withstand
temperatures up to 4300°F (2400°C).
Picture courtesy of NASA on the Commons.
that have practically no electrical resistance—were discovered in 1911,
but found few practical applications because they became
only at temperatures close to absolute zero (–459.67°F or
–273.15°C). But in the 1980s,
scientists invented new types of ceramic superconductors that showed
at temperatures as high as –292°F (–180°C). High-temperature
such as this are expected to find many new applications, including
magnetic levitation ("maglev") trains
and super-fast computers.
Maglev train picture courtesy of US Department of Energy.
such as rubber and plastic are relatively poor conductors, but they
will conduct electricity if extremely large voltages are placed across them.
This makes them unsuitable for use in high-power applications, such as
in electrical generators, and transformers. Ceramics made from alumina
(aluminum oxide) and porcelain are widely used in insulators that
electrical equipment outdoors. Ceramics have been used as the
insulation in automobile sparking plugs since the early 20th century.
Power plant picture courtesy of US Department of Energy.