Wood
by Chris Woodford.
Last updated: November 12, 2022.
There's plenty of it, it's relatively cheap (or even free), it's
environmentally friendly, it looks great, it's warm and cozy, it's
super-strong, it lasts hundreds or even thousands of years, and you
can use it for everything from building bridges to making
paper or heating your home. It's wood—and it's quite possibly the most useful
and versatile material on the planet, with many thousands of
different uses. So what is it that makes wood so good? Let's take a
closer look!
Photo: Wood really does grow on—or rather in—trees. Who'd have thought you could make a lovely coffee table or a fruit bowl from a gnarled old specimen like this? The outer part of a tree trunk might look dead, but it's very much alive: tree trunks grow outward (getting wider) as well as upward (getting higher).
What is wood?
You often hear people grumbling about money and all kinds of other things that "don't grow on trees"; the great thing about wood is that it
does grow on trees—or, more specifically, in their trunks and branches.
Structure of wood
Photo: This fence pole was once a tree—and you can still clearly see the annual growth rings if you look down on it from above.
Take a tree and peel off the outer "skin" or bark and what you'll find
is two kinds of wood.
Closest to the edge there's a moist, light,
living layer called sapwood packed with tubes called xylem
that help a tree pipe water and nutrients up from its roots to its
leaves; inside the sapwood there's a much darker, harder, part of the tree
called the heartwood, which is dead, where the xylem tubes
have blocked up with resins or gums and stopped working. Around the
outer edge of the sapwood (and the trunk) is a thin active layer
called the cambium where the tree is actually growing outward
by a little bit each year, forming those famous annual rings
that tell us how old a tree is.
Slice horizontally through a tree,
running the saw parallel to the ground (perpendicular to the trunk),
and you'll see the annual rings (one new one added each year) making
up the cross-section. Cut vertically through a tree trunk and you'll
see lines inside running parallel to the trunk formed by the xylem
tubes, forming the inner structure of the wood known as its grain.
You'll also see occasional wonky ovals interrupting the grain called
knots, which are the places where the branches grew out from
the trunk of a tree. Knots can make wood look attractive, but they
can also weaken its structure.
Photo: Every piece of wood is unique. Burls (knotty deformed growths) are often used to make highly decorative things like this staircase at Twin Pines Lodge in Dubois, Wyoming. Photo by Carol M. Highsmith, courtesy of Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive, US Library of Congress Prints and Photographs Division.
Hardwoods and softwoods
Wood is divided into two distinct kinds called hardwood and softwood,
though confusingly the names don't always refer to its actual
hardness or softness:
- Hardwoods typically come
from broad-leaved (deciduous) trees (those that drop their leaves
each fall, also known as angiosperms because their seeds are encased
in fruits or pods). Examples include ash, beech, birch, mahogany, maple,
oak, teak, and walnut.
- Softwoods typically come from evergeen
(coniferous) trees (those that have needles and cones and retain them
year-round, also called gymnosperms. Examples include cedar, cypress, fir,
pine, spruce, and redwood.
Photo: Left/above: Hardwood comes from deciduous trees like this oak. Its leaves (inset) drop off in the fall and new ones grow in spring.
Photo: Right/below: Softwood comes from evergreen conifers, like this pine, which has needles that stay on all year and cones (inset).
It's generally true that hardwoods are
harder than softwoods, but not always. Balsa is the best-known
example of a hardwood that is actually very soft. Hardwoods have
lovely, attractive grains and are used for such things as making fine
furniture and decorative woodwork, whereas softwoods often come from very
tall, straight trees, and are better suited for construction work (in the form of planks, poles, and so on).
Chemical composition
Look at some freshly cut wood under a microscope and you'll see it's made up of
cells, like any other plant. The cells are made of three substances
called cellulose (about 50 percent), lignin (which
makes up a fifth to a quarter of hardwoods but a quarter to a third
of softwoods), and hemicellulose (the remainder). Broadly
speaking, cellulose is the fibrous bulk of a tree, while lignin is the
adhesive that holds the fibers together.
What's wood like?
The inner structure of a tree makes wood what it is—what it looks like,
how it behaves, and what we can use it for. There are actually
hundreds of different species of trees, so making generalizations
about something called "wood" isn't always that helpful: balsa
wood is different from oak, which isn't quite the same as hazel, which is
different again from walnut. Having said that, different types of
wood have more in common with one another than with, say,
metals,
ceramics, and plastics.
Strength
Photo: Trees are strong partly because of roots like these that
anchor them in the ground, but also because wood is inherently a strong material.
This is the famous Moreton Bay Fig Tree in Santa Barbara, California dates from 1876
and is around 24m (80ft) tall. Photo courtesy of The Jon B. Lovelace Collection of California Photographs in Carol M. Highsmith's America Project,
Library of Congress,
Prints and Photographs Division.
Physically, wood is strong and stiff but, compared to a material like
steel, it's
also light and flexible. It has another interesting property too.
Metals, plastics, and ceramics tend to have a fairly uniform inner
structure and that makes them isotropic: they behave exactly
the same way in all directions. Wood is different due to its
annual-ring-and-grain structure. You can usually bend and snap a small, dead, tree branch with
your bare hands, but you'll find it
almost impossible to stretch or compress the same branch if you try
pulling or pushing it in the opposite direction. The same holds when
you're cutting wood. If you've ever chopped wood with an ax, you'll
know it splits really easily if you slice with the blade along the
grain, but it's much harder to chop the opposite way (through the
grain). We say wood is anisotropic, which means a lump of wood
has different properties in different directions.
Photo: Wood is a traditional building material, as popular today as ever. Because wood is anisotropic, natural wooden beams work better resisting vertical, squashing forces (where they are in compression) than horizontal, bending ones (where they are in tension). Here, strong diagonal beams add further strength. Photo by Carol M. Highsmith from the Carol M. Highsmith Archive, courtesy of US Library of Congress Prints and Photographs Division.
That's not just important to someone chopping away in the woodshed: it also
matters when you're using wood in construction. Traditional wooden
buildings are supported by huge vertical poles that transmit forces
down into the ground along their length, parallel to the grain.
That's a good way to use wood because it generally has high
compressive strength (resistance to squeezing)
when you load it in the same direction as the grain. Wooden poles are much weaker placed
horizontally; they need plenty of support to stop them bending and
snapping. That's because they have lower tensile strength
(resistance to bending or pulling forces across the grain). Not all woods are the same,
however. Oak has much higher tensile strength than many other woods,
which is why it was traditionally used to make the heavy, horizontal beams
in old buildings. Other factors such as how well seasoned (dry) a
piece of wood is (as discussed below) and how dense it is also affect
its strength.
Chart: Wood can be very weak. In tension (for example, stretched horizontally in struts or beams), it's one of the weakest of all common materials. That's why it's more likely to be used in compression (in vertical beams), where it's very much stronger. (Concrete suffers from the same problem, which it's why it's often reinforced with steel.)
All woods are different, and vary with atmospheric conditions, but typically they're 10–30 times stronger when compressive forces push or pull them in the longitudinal direction compared to when tensile forces act in the radial direction (see the inset picture of a tree trunk for an explanation of these terms).
Durability
One of the best things about wood is how long it lasts. Browsing through the
daily news, you'll often read that archeologists have unearthed the
buried remains of some ancient wooden article—a wooden tool,
perhaps, or a simple rowboat or the remnants of a huge building—that
are hundreds or even thousands of years old. Providing a wooden
object is properly preserved (something else we discuss later), it
will easily outlast the person who made it. But just like that
person, a wooden object was once a living thing—and it's a
natural material. Like other natural materials, it's subject to the
natural forces of decay through a process known as rotting, in
which organisms such as fungi and insects such as termites and
beetles gradually nibble away the cellulose and lignin and reduce
wood to dust and memories.
Photo: Under attack! The big problem with wood is that it's a natural material subject to attack
from other natural things, notably fungi and insects. This is what Formosan subterranean termites can do to wood.
Photo by Scott Bauer courtesy of US Department of Agriculture/Agricultural
Research Service.
Wood and water
Wood has many other interesting characteristics. It's hygroscopic,
which means that, just like a sponge, it absorbs water and swells up
in damp conditions, giving out the water again when the air dries and
the temperature rises. If, like mine, your home has wooden windows,
you'll probably notice that they open much more easily in summer
than in winter, when the damp outdoor conditions make them swell
into the frames (not necessarily such a bad thing, since it helps to
keep out the cold). Why does wood absorb water? Remember that the
trunk of a tree is designed to carry water from the roots to the
leaves: it's pretty much a water superhighway. A freshly cut piece of
"green" wood typically contains a huge amount of hidden water,
making it very difficult to burn as firewood without a great deal of
smoking and spitting. Some kinds of wood can soak up several times
their own weight of water, which is absorbed inside the wood by the
very same structures that transported water from the roots of the
tree to the leaves when the tree was a living, growing plant.
Wood and energy
What other properties does wood have? It's a relatively good
heat insulator (which comes in handy in building construction), but dry
wood does burn quite easily and produces a great deal of heat energy
if you heat it up beyond its ignition temperature (the point at which
it catches fire, anywhere from around 200–400°C, 400–750°F). Although wood can absorb sound very
effectively (another useful property in buildings, where people value
sound insulation shutting out their neighbors), wooden objects can
also be designed to transmit and amplify sounds—that's how musical
instruments work. Wood is generally a poor conductor of electricity
but, interestingly, it's piezoelectric (an electric charge
will build up on wood if you squeeze it the right way).
Environmentally friendly
Photo: Wood fuel (a type of biomass) can be an environmentally
friendly form of renewable energy. This is a
power plant in
Burlington Vermont that burns 76 tons of wood chips per hour (left) to make electric power.
The wood is mostly grown within 100km (60 miles) of the plant, and a lot comes from wood industry offcuts
and logging waste. Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).
Wood was one of the first natural materials people learned to use, and it's
never lost its popularity. These days, it's particularly prized for
being a natural and environmentally friendly product.
Forestry is a rare example of something that has the potential to be completely
sustainable: in theory, if you plant a new tree for every old tree you
cut down, you can go on using wood forever without damaging the
planet. In practice, you need to replace like with like and forestry
is not automatically sustainable, whatever papermakers like us to
believe. A brand new tree has much less ecological value than a
mature tree that's hundreds of years old so planting a thousand
saplings may be no replacement for felling just a handful of
ancient trees.
Logging can be hugely environmentally damaging, whether it involves clearcutting a tropical rainforest or selectively
felling mature trees in old-growth temperate woodland. Some of the
processes and chemicals used in forestry and woodworking are also
environmentally damaging;
chlorine, used to bleach wood fibers to make paper,
can cause water pollution in rivers, for example.
But on the positive side, growing trees remove carbon dioxide from the atmosphere and planting more of
them is one way to reduce the effects of climate change. Trees also
provide important habitats for many other species and help to
increase biodiversity (the wide range of living organisms on
Earth). Practiced the right way, forestry is a good example of how
people can live in perfect harmony with the planet.
Another great advantage of wood is that it naturally biodegrades.
Where plastics
can persist in the environment for hundreds of years, wood naturally recycles itself.
Wood can start to rot away in a matter of months when it gets wet and
starts to be eaten away by fungi. As we'll see in a moment, this can be a major problem:
wooden buildings and structures, such as fences, will rot away in time
unless you take steps to preserve them.
Wood is also easy to reuse and recycle.
Wood originally used for one purpose (old railway track sleepers or scaffold boards) can easily be cleaned up, restored, and used elsewhere (as raised beds in gardens, building cladding, garden chippings, or whatever).
Using wood
How does wood get from the tree to the roof of your house, your bookshelf, or
the chair you're sitting on? It's a longer and more complex journey
than you might think that takes in harvesting, seasoning, preserving
and other treatment, and cutting. Here's a brief guide.
Harvesting
Photo: Chopping down a longleaf pine is only the start of the fun: now you've got to get it home preferably
without damaging the rest of the forest in the process. That's where this skidder machine comes in,
lifting up the logs with a hydraulic crane
and dragging them away with a powerful diesel engine.
Photo by Randy C. Murray courtesy of US Army.
Growing plants for food is called agriculture; growing trees for human use is silviculture—and
the two things have a great deal in common. Wood is a plant crop that
must be harvested just like any other, but the difference is how long
trees take to grow, often many years or even decades. How wood is
harvested depends on whether trees are growing in plantations (where
there are hundreds or thousands of the same species, generally of similar age) or in mature
forests (where there's a mixture of different species and trees of
widely differing ages).
Planted trees may be grown according to a
precise plan and clear-cut (the entire forest is felled) when
they reach maturity. A drastic approach like that makes sense if the
trees are a fast-growing species planted specifically for use as
biomass fuel, for example. Individual trees can also be
selectively felled from mixed forests and either dragged away by machine or
animal or even (if it makes economic and environmental sense) hauled
upward by helicopter, which avoids damaging other nearby trees.
Sometimes trees have their bark and small branches removed in the
forest before being hauled away to a lumber yard for further
processing, though they can also be removed intact, with the entire
processing done offsite. It all depends on the value of the tree, the growing
conditions, how far away the lumber yard is, and how easy the tree is
to transport. Another interesting form of forestry is called
coppicing, which involves removing long, thin, low-growing
branches from trees such as hazel and willow in a careful and
respectful way that does no long-term damage.
Photo: These cottonwood trees might look too spindly for making poles or planks, but they'll not be used
for either. They're part of a fast-growing plantation that produces biomass,
a type of renewable energy burned in power plants.
Biomass is better for the environment because the trees take in as much carbon dioxide when they grow
as they give out when they're burned; leaving aside the energy wasted in harvesting and processing,
a biomass plant produces no overall carbon dioxide emissions, unlike a traditional power plant fueled by oil or coal.
Other "energy crops" include willow, poplar, and eucalyptus. Photo by Warren Gretz courtesy of
US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
Seasoning
A freshly cut tree is a bit like a sponge that comes presoaked in
water, so it has to be completely dried out or seasoned before
it can be used. Dry wood is less likely to rot and decay, it's easier
to treat with preservatives and paint, and it's much lighter and
easier to transport (typically, half a freshly felled tree's weight
may come from water trapped inside). Dry wood is also much stronger
and easier to build with (it won't shrink so much) and if a tree is
destined for burning as firewood (or an energy crop), it will burn
more easily and give out more heat if it's properly dried first.
Typically wood is dried either in the open air (which takes anything
from a few months to a year) or, if speed is important, in vast
heated ovens called kilns (which cuts the drying time to days or
weeks). Seasoned wood is still not completely dry: typically its
moisture content varies from about 5–20 percent, depending on the
drying method and time.
Preserving and other treatment
In theory, wood might last forever if it weren't attacked by bugs and
bacteria; preservatives can greatly extend its life by preventing
rot. Different preservatives work in different ways. Paint,
for example, works like an outer skin that stops fungi and insects
penetrating the wood and eating it away, but sunlight and rain make
paint crack and flake away, leaving the wood open to attack
underneath. Creosote (historically, the most popular wood preservative) is a
strong-smelling, oily brown liquid usually made from coal-tar. Unlike paint,
it is a fungicide, insecticide, miticide, and sporicide: in other words, it works
by stopping fungi, insects, mites, and spores from
eating or growing in the wood. There is some controversy about its
potential environmental effects and there are many alternatives.
Photo: A fence before (right) and after (left) treatment with wood preservative.
Different kinds of treatment help to protect and preserve wood in other ways.
It's a great irony that wood can be used to build a fine home that
will last many decades or burn to the ground in minutes. Wood is so
plentiful and burns so well that it has long been one of the world's
favorite fuels. That's why fire-protection treatment of wooden
building products is so important. Typically, wood is treated with
fire retardant chemicals that affect the way it burns if it catches
fire, reducing the volatile gases that are given off so it burns more
slowly and with greater difficulty.
Cutting
There's a big difference between a tree and the table it might become, even though
both are made from exactly the same wood. That difference comes
mainly from skillful cutting and woodworking. How much cutting
a tree needs depends on the product that's being made. Something like
a utility pole or a fence post is not much more than a tree stripped
of its branches and heavily treated with preservatives; that's an
example of what's called roundwood. Trees need a bit more work
in the sawmill to turn them into lumber, timber, or
sawnwood (the three names are often used interchangeably,
though they can be used with more specific meanings). Flat pieces of
wood can be made from trees by cutting logs in two different
directions. If you cut planks with the saw running in lines parallel to the length of the trunk,
you get plainsawn (sometimes called flatsawn) wood (with ovals or curves
on the biggest flat surface of the wood); if you fell a tree, cut the trunk into quarters, then slice each quarter
into parallel planks, you get quartersawn wood (with the
grain running along the biggest flat surface in broadly parallel stripes).
Photo: Left/above: Plainsawn wood is parallel to the trunk, revealing the annual rings as curves or ovals. Right/below: Quartersawn wood is first quartered and then sawn, revealing a pattern of roughly parallel lines.
See how attractive those patterns look? Not surprisingly, wood that's destined for furniture and other decorative
uses has to be cut much more thoughtfully and carefully with regard
to what's called its figure. This is the way a particular tree
is cut to show off the growth patterns it contains in the most
attractive way in the final piece of wood. The figure can also depend
on which part of a tree is used. Wood cut from near the stump of a
tree will sometimes produce a more attractive figure than wood cut
from higher up.
Other wood products
Photo: Particle board is made from offcuts of wood stuck together and coated with an attractive veneer or other surface layer (perhaps plastic or a laminate). This is what an Ikea Billy bookcase looks like if you peer round the back. You can see the veneer on the extreme left and a hardwood backing on the right.
Roundwood and sawnwood are what you might call natural wood products, because
they involve using cut pieces of tree more or less in raw form. There
are many other ways of using trees that involve greater amounts of
processing. Some woods are very rare and expensive, while others are cheap and plentiful,
so a common technique is to apply an outer layer of expensive and attractive wood to a core of
cheaper material. Veneer is a thin decorative layer applied to cheaper
wood made by turning a log against a blade, much like
peeling an apple. Using veneer means you can get an attractive wooden
finish at much lower cost than by using a solid piece of expensive
wood. Plywood is made by taking layers of wood (or plies) and
gluing them together with an outer coating of veneer. Typically each
ply is placed at 90 degrees to the one underneath so the grains
alternate. That means a piece of plywood is usually much stronger
than a piece of the natural wood from which it's made. Laminated
wood is a weaker kind of plywood in which the grain of each layer
runs in the same direction. Particle board (often called
chipboard) is made by taking the waste chips, flakes, and
sawdust from a mill and forcing it under high pressure, with glue, in
a mold so it sticks together to make planks and panels. Low-cost and
self-assembly furniture is often made this way. Fiber-board is
similar, but made with wood-pulp fibers instead of wood chips and
sawdust. Hardboard is a thin sheet of wood made from wood fibers in much
the same way.
Not all wood products are immediately recognizable as such. A great deal of
the paper and cardboard people use is made by turning cellulose from
trees into a fibrous pulp, for example. Lignin (the other main
chemical inside wood) also has many uses, including making plastics
(such as the celluloid used in old-fashioned photographic film),
paints, turpentine, and yeast products.
