by Chris Woodford. Last updated: April 1, 2021.
You started your morning
with ceramics—and they'll dominate your day. Inside your brick, cement, and
you woke to the quartz clock, washed in the tiled bathroom,
breakfasted on pottery cups and bowls. Maybe you worked all day at a
computer (packed with ceramic-based electronic components, like
resistors), before heading back home for a
glass of wine, gobbled down dinner from those same pottery plates,
and sat in front of the liquid-crystal TV (or Gorilla glass
smartphone), before heading for bed and setting the quartz clock,
ready to repeat again tomorrow. Though it's far from obvious, we live
in a ceramic world, just as people have for thousands of years. But
what exactly are ceramics?
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.
Glass, tiles, pottery, porcelain, bricks, cement,
diamond, and graphite—you can probably see from this little list
that "ceramics" is a very broad term, and one we're going to have
difficulty defining. What do all these very different materials have
From a chemical viewpoint, we define ceramics in terms of
what they're not. So you'll find most science textbooks and
dictionaries telling you ceramics are nonmetallic and inorganic
solids (ones that aren't metal
or based on carbon compounds); in other words, ceramics are what we're left with when we
take away metals and organic materials (including
rubber, and anything that was once alive).
Some books also try to define ceramics as
"refractory" materials, which is a technical,
materials science term that
means capable of putting up with everyday abuses like extremes of
temperature, attacks from acids and alkalis, and general
wear-and-tear. It often seems easier to define materials in terms of
their properties (how they behave when we heat them, pass
through them, or soak them in water, for example). But once we start
doing that, things can get confusing. For example, graphite (a form, or
allotrope, of carbon) is considered a ceramic because it's
nonmetallic and inorganic, yet (unlike most ceramics) it's soft,
wears easily, and is a good conductor of electricity. So if you
looked only at the properties of graphite, you wouldn't consider it a
ceramic at all. Diamond (another form of carbon) is also a ceramic
for the same reason; its properties couldn't be more different from
those of graphite, but they're similar to those of other ceramics.
(Like modern ceramics such as tungsten carbide, diamond has long been
used in cutting and drilling tools).
Types of ceramics
People first started making ceramics thousands of
years ago (pottery, glass, and brick are among the oldest
human-invented materials), and we're still designing brand new
ceramic materials today—things like catalytic converters for
today's cars and high-temperature superconductors for tomorrow's computers. There's quite a big difference between age-old,
general-purpose ceramics like brick and glass and modern, engineered
ceramics that are sometimes designed for a single, specific purpose,
such as filtering soot from a truck's dirty
or making a drill bit that lasts five times longer. That's partly why materials
scientists like to divide ceramics into two kinds: traditional, and
advanced (or engineering) ceramics.
Photo: Traditional ceramics: Toilets are a good example,
though the lid and seat are typically made of plastic or wood.
Bricks, pottery, glass, porcelain, tiles, cement,
and concrete are our classic, time-tested ceramics. Although they all have different uses, we can still think of them as general-purpose
materials. Take tiles, for example. We can put them inside our homes
or outside; on the walls, the floors, or the roof; and we can stick
glass in our windows or poke away at it on our smartphone screens—we
can even drink champagne out of it. Ceramics like this are ancient
materials—ones our ancestors would recognize—that
have gradually found more and more uses as the centuries have worn on.
Advanced engineering ceramics
By contrast, advanced ceramics are ones that have
been engineered (mostly since the early 20th century) for
highly specific applications. For example, silicon nitrides and
tungsten carbides are designed for making exceptionally
hard, high-performance cutting tools—though they do have other uses
as well. Most modern engineered ceramics are metal oxides,
carbides, and nitrides, which means they're compounds made by
combining atoms of a metal with oxygen,
carbon, or nitrogen atoms. So, for example, we have tungsten carbide, silicon carbide, and boron nitride, which are hard, cutting-tool ceramics; aluminum oxide
(alumina) and silicon dioxide are used in making integrated circuits
("microchips"); and lithium-silicon oxide is used to make the
heat-protective nose cones on space rockets. High-temperature superconductors are made from crystals of yttrium, barium, copper, and oxygen.
Not all high-tech ceramic materials are simple
compounds. Some are composite materials, in which the ceramic forms a kind of background material called the matrix, which is reinforced
with fibers of another material (often carbon fibers, or sometimes
fibers of a totally different ceramic). A material like this is known
as a ceramic matrix composite (CMC). Examples include silicon carbide
fibers in a silicon carbide matrix (SiC/SiC) with boron nitride at
the interface between them—a material used in cutting-edge
gas-turbine jet engines.
Photo: Advanced ceramics: Silicon and carbon fuse to form silicon carbide powder (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).
What properties do ceramics have?
As we've already seen, the most important general
property of ceramics is that they're refractory: they're
rough-and-tumble materials that will put up with fair amounts of
abuse in the most ordinary and extraordinary situations. Just
consider, most of us tile our kitchens and bathrooms because ceramic tiles are hard,
waterproof, largely resistant to scratches, and keep on looking good
for year upon year; but engineers also put (very different!) ceramic tiles
on space rockets to protect them against heat when they whiz back to Earth.
If we're summarizing their properties, we can say
that ceramics have:
- High melting points (so they're heat resistant).
- Great hardness and strength.
- Considerable durability (they're long-lasting and hard-wearing).
- Low electrical and thermal conductivity (they're good insulators).
- Chemical inertness (they're unreactive with other chemicals).
Most ceramics are also nonmagnetic materials, although ferrites (iron-based ceramics) happen to make great magnets (because of their iron content).
Those are the useful points, but, thinking about traditional ceramics
like glass or porcelain, you'll also have noticed one major
drawback: they can be fragile and brittle, and they'll smash or
shatter if you drop them (subject them to "mechanical shock") or suddenly change their
temperature ("thermal shock").
Why are ceramics like this?
The interesting question is why ceramics behave
like this—and the no-less-interesting answer boils down to
materials science: it's all to do with how the atoms inside are bonded together. That explains how most materials work
In metals, for example, atoms are relatively weakly bonded (which
is why most metals are fairly soft); their electrons are shared
between them in a kind of sea that can "wash" right through them,
which is (simplistically speaking) why they conduct electricity and
A material like rubber, on the other hand, is made of
long-chain molecules (polymers) that are very weakly attached to one
another; that's why raw, white, latex rubber is so stretchy and why black,
vulcanized rubber (like that used in car tires) is harder and
stronger, because heat-and-sulfur treatment makes strong cross-links
form between the polymer chains, holding them tightly together. All
the electrons are locked up in bonds of various kinds (none are free
to carry an electric current), and that's why rubber is generally a
Ceramics are different again. Their atoms are
ionically bonded (like sodium and chlorine in sodium chloride, common
salt), which holds them firmly in place (making ceramics hard and
strong) and locks up all their electrons (so, unlike in metals, there
are no free electrons to carry heat or electricity). Metals can bend,
stretch, and be drawn into wires because their rows of regularly
packed atoms will slide past one another. But in a ceramic, there are
no rows of atoms; the atoms are either locked in a regularly
repeating three-dimensional crystal or randomly arranged to make
what's called an amorphous solid (a solid without a neat and tidy,
internal crystalline structure). If you whack a lump of metal with a
hammer, the mechanical energy you supply is
dissipated as layers
of atoms jump past one another; in other words, the metal bends out
of shape. If you whack a ceramic such as glass, there's nowhere for
that energy to go—no way for the glass to deform and soak up the
blow—so it shatters instead. This explains why ceramics are both
hard and brittle.
As we've already seen, not all ceramics behave
this way. Graphite is soft because it's made of layers of carbon
atoms that will slide and shear (that's why a graphite pencil leaves
lines on paper); diamond is hard because it has a much more rigid
crystalline structure. Clay dug from the ground is soft and pliable
because, like graphite, its atoms are made of flat sheets that can
slip past one another, held together only by weak bonds. When you add
water to clay, the polar water molecules (positively charged at one end,
negative at the other end) help to pull those bonds apart, making the
clay even more malleable. When you fire clay, the water evaporates
and the aluminum, silicon, and oxygen atoms lock into a rigid
structure made from aluminum silicate, bonded together by silicate
glass—and that's why fired clay is so hard.
Artwork: Why do ceramics and metals behave differently? 1) You can bend metals because the atoms inside them can slide past one another fairly easily. 2) In a ceramic, the atoms are tightly bonded. If you apply too much force, the only thing a ceramic can do is break apart: the energy has nowhere else to go. 3) In metals, there are free electrons (blue) to carry heat and electricity. That's why metals are good conductors. 4) In a ceramic, the electrons are all "busy" binding atoms together and there are none spare for carrying electricity and heat. That's why ceramics tend to be good insulators (non-conductors).
What are ceramics used for?
From glass and brick to porcelain and cement,
we've already seen that there are countless different things that can
be described as ceramics; not surprisingly, then, there are literally
hundreds of different applications for ceramic materials in
everything from aerospace to zoo-keeping.
Airplane jet engines, for example, are
examples of machines called gas turbines, which work by burning fuel mixtures at high temperatures to make a fiery exhaust that powers a plane through the air.
The need to cope with incredible temperatures explains why engine components are
often made from ceramics. It was for exactly the same reason that
31,000 ceramic tiles were used on the now-retired Space Shuttle to
protect it from burning up on its way back to Earth from space. Tragically, it was
the failure of a ceramic tile that led to the demise of the Space
Shuttle Columbia as it struggled to return to Earth in February 2003.
(The next generation of reusable space planes is expected to use
higher-performance tiles made from ceramic-matrix composites.)
Photo: Ceramic tiles helped to protect the Space Shuttle from heat when it came
back into Earth's atmosphere. Picture courtesy of NASA on the Commons.
If aerospace is an extraordinary use for
extraordinary ceramics, construction is one of the best known uses
for ordinary, everyday ceramics. Even in our modern age of plentiful
plastics, brick, glass, cement, concrete, porcelain, and tiles of all
kinds are still the raw materials from which most
buildings are made.
The tools used on construction sites are often made with ceramics
too. Whether you're cutting glass, drilling holes in tile, grinding
concrete, or sawing through brick, engineering ceramics like tungsten
or silicon carbide will help you knock more traditional ceramics into
shape, generally working better, for longer, than traditional tools
made of steel.
Ceramics aren't always at the cutting edge; a lot
of the time, we don't notice them at all—especially when they're
hiding inside electrical and electronic equipment.
Anything with an electric motor (that's every chore-busting, electric-powered machine in your home) contains magnets, and quite often they're made from ferrite ceramics. (You'll also find ferrite magnets, or other
kinds of ceramic transducers, in loudspeakers and headphones.) While
we use conducting metals like copper to carry electricity from place
to place, we have to use ceramics to insulate high-voltage
electricity in places like power
plant generators and
Photo: You'll often see ceramic insulators (the stacks of round discs) protecting overhead power lines. They're made from porcelain, glass, or other ceramics. Credit: Photographs in the Carol M. Highsmith Archive,
Library of Congress, Prints and Photographs Division.
Sometimes, ceramics insulate us
from electricity and heat at the same time: heating elements are often built into ceramic holders, electric
cooktops are made from
high-performance ceramic glass, and incandescent lamps have glass bulbs that protect us from heat and electricity while protecting
their filaments from the atmosphere. The most advanced electrical use
of ceramics is probably in high-temperature superconductors
(materials with virtually no electrical resistance). While
traditional superconductors have to be cooled down to near absolute
zero (−459.67°F or −273.15°C), these new ceramics become
superconducting in relatively warm conditions (still a chilly
−292°F or −180°C!), which makes them far more practical for use
in things like floating "maglev" trains and cutting-edge
"Animal, vegetable, or mineral?"—so goes the
famous guessing game; and there's a temptation to see mineral-based
ceramics as artificial, unnatural, and quite apart from the living,
breathing world we all inhabit. It's perhaps surprising, then, to
find so many applications for ceramics in the world of medicine. How
about the piezoelectric transducers
that create ultrasonic waves used
in pregnancy scans? Or what about dentures (false teeth) made from
porcelain or glass eyes? Or bone implants
made from silicon nitride, which are cleverly designed to be
porous so they promote natural bone growth? If you're wondering what
ceramics have to do with zoology (as I suggested up above), you'll
find plenty of dogs that—just like us humans—have had ceramic
implants in their bones and teeth.
Photo: High-temperature superconductors
made from ceramics could allow electricity to flow through things with little or no resistance,
making possible technologies like superfast computers and "floating" Maglev trains. Picture courtesy of US Department of Energy.
How do you make ceramics?
Ceramics generally start with a clay-based
material dug from the ground that's mixed with water (to make it soft
and flexible) and other materials, squashed into shape, then fired at
high-temperature in a large industrial oven called a kiln. Firing is
what most ceramics have in common; the very word "ceramic" originally comes
from Sanskrit and means "to burn" (though, more recently, traces
from the Greek word "keramos," meaning potter's clay). 
These four basic processes—digging the raw material from the ground, adding water,
shaping, and firing—have been used to make ceramics for thousands
Photo: Ceramic tiles get their hardness from being fired. Although that makes them extremely durable, it also means they're relatively fragile and brittle: they crack quite easily.
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,
- Kaolin is widely used for making glossy paper. (It's also used in kaolin and morphine, a medicine for upset stomachs.)
- Bentonite has a variety of industrial uses, including drilling mud and foundry 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.
Each one of these also has numerous different grades and
qualities, so it's probably more accurate to talk about China clays or Ball clays
in the plural. Ball clay, for example, is used to make things like fine
porcelain tableware and bathroom suites, but even within a single
ball clay mine, different grades of clay will be simultaneously
excavated from different areas and kept separate (or blended in
various ways) for different end uses.
Before they're fired, raw ceramics can be shaped in all kinds of ways; different manufacturing processes are used for different end products. So pipes, for example, are made by extrusion (squeezing clay through a hole, a bit like toothpaste from a tube). Glass is made by blowing, molding, or being floated on top of water (the
float-glass process by which large, flat windows are made). Bricks, on the other hand, are almost always made in molds to ensure they're a consistent size and shape for stacking into walls. While a great deal of modern pottery is molded, some is still thrown by hand, on a
foot-powered wheel, in the traditional way. Other ceramic processes include pressing (squeezing powder into a mold), casting, and jiggering (laying raw material into a rotating mold). Advanced engineering ceramics are often made in more advanced ways. For
example, the silicon nitride used in cutting tools is made by
reaction bonding, in which silicon powder is squashed into shape and
heated with nitrogen gas.
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, 2003/2016. 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.