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EVER found a piece of old pottery in a field and wondered who used it, what
they did with it and how long ago that was? There is a way to find out.
Archaeology, the study of the human past, centres on material culture
鈥攖he physical structures and objects (artefacts)鈥攖hat people
have made or used. We can also find out how people lived by looking at
ecofacts鈥攅nvironmental information that directly relates to human
activity. For example, evidence of burning associated with forest clearance
characterises the start of agriculture in Neolithic times.

Whether they鈥檝e found an artefact or an ecofact, archaeologists have tough
questions to answer. Where did Europeans live in the Bronze Age about 6000 years
ago? When did writing begin in China and who was buried in a treasure-laden tomb
in Greece? Scientific methods can help with the basic what, when, where, how and
who class of questions. They can also go some way towards discovering why people
lived as they did鈥攁nd that鈥檚 a far more complicated task.

The archaeologist must first find a site worth investigating. A site
may turn out to be a metropolis, an isolated burial mound, a Roman villa or
an ancient rubbish tip. But faced with a landscape, how do you find where people
lived and went about their daily business in ancient times?

The first step is a survey, pinning down where to explore further. Field
walking鈥攁n organised stroll across a likely site鈥攎ay be
low-tech, but it is also highly effective. A team will cross and recross bare
fields, usually ploughed, to spot surface finds. You might find a clue: perhaps
a few pottery fragments. Plotting a rough map of such finds could turn up
clusters or something that鈥檚 worth investigating further. For example, on a
large-scale field walk in Greece, a 鈥渉alo鈥� of pottery fragments was found to
encircle sites where people had lived. Local historical records may hold other
clues: a steep-sided mound marked on a map of England may indicate an early
Norman fortification or an Anglo-Saxon meeting place. It could, however, have a
far more modern origin鈥攕uch as waste from a mine.

Taking to the air gives you an overview of the area, dramatically improving
your chances of spotting subtle structures almost obliterated by erosion or
human intervention. If you fly over a potential site when the Sun is low in the
sky and casts long shadows across the landscape, you may be able to see ditches,
mounds or even medieval plough furrows. These can then be recorded by means of
aerial photography. These photos can show not only shadows, but also
variations in the growth of ground cover. Crops or grass growing above
stone foundations or walls receive less water from their roots, and from the air
these will appear as a light, parched crop mark in the field. Where a
ditch or well was cut into the surface in ancient times, vegetation may grow
more vigorously, and here a darker crop mark will be visible. Characteristic
shapes can then be seen, such as the rounded rectangle of a Roman fort, the
circle of ditches around a barrow鈥攁ncient burial mound鈥攐f the
European Bronze Age, or the banjo-shaped depressions marking the remains of a
British Iron-Age settlement from 500 BC.

Soil marks鈥攄ifferences in colour of the surface鈥攐ften show
in ploughed sites. In addition, aerial remote-sensing devices can detect
structures that don鈥檛 show up on photos. Thermal imaging, for instance,
reveals large stone structures hidden under the surface. Two images are taken
from a plane, one at night and one during the day, using infrared
thermal-imaging cameras and special film. Because stone cools more slowly than
the thin layer of soil over it, comparing the two images will reveal the
location of former buildings. Similarly, using various wavelengths from infrared
to ultraviolet lets you pick out other differences in soil or vegetation.

Some archaeological prospecting methods even show what鈥檚 lurking beneath the
surface by 鈥渟eeing鈥� through it. Ground-penetrating radar can be used at
ground level by towing a portable device over the surface to build up a
3-dimensional profile of what lies beneath the ground. Short pulses of
electromagnetic energy are sent through the soil from an antenna and records the
differences between the returning signals. Computer programs analyse and image
the data as a series of 2D horizontal time slices. These are then built
up into a 3D picture of the site.

Resistivity surveying is another surface-based method that can
supplement information gathered from aerial photography. Buried walls or
artefacts are located by measuring the soil鈥檚 resistance to the flow of a small
electric current through a specific area. If pieces of wall, mosaic tiles or
other stony material are present in one part of the soil, it will be drier than
a stone-free area, and so won鈥檛 conduct a current as easily. Two pairs of wires
are pushed into the ground and a current passed between them. The soil鈥檚
resistance is measured and plotted on a grid over the entire site. The resulting
pattern of greater and lesser resistance will reveal the presence of many
different features, such as storage pits, ditches and structures for further
investigation.

DIGGING IN TO HISTORY

Know your soil

When a structure or site has been found, excavation may begin
(Figure 1).
The principle behind excavation is that soil builds up over the years,
sealing the remains of activities and structures beneath it. So by digging a
trench, you can reach the past. Geological information about the area under
excavation helps unravel which of the features that emerge are natural, and
which result from disturbances caused by people living or working there. To pick
out natural features archaeologists have to know how soil forms through erosion
(see Inside Science No. 123). Accumulated rubbish and waste can also build up
and alter the terrain. In the Middle East, for example, one way to find an
ancient settlement is to look for tells鈥攎ounds that rise up above
the flat plains. These have formed over thousands of years, as houses built of
sun-dried clay bricks have been built and rebuilt. The bricks crumble and form a
layer of soil, providing a platform on which the next house is built.

The first step in excavation is to strip away the topsoil. As the trench is
dug, each change in soil colour or consistency is noted. In a chalk soil, for
example, dark areas may mark the place where a wooden post was driven into the
ground to support a roof. When the wood rotted away, it discoloured the lighter
soil. As the trench is dug, the different layers鈥攕trata鈥攁re drawn
onto plans. Stratigraphy鈥攖he order and relative position of strata
and their relationship to the geological timescale鈥攑rovides the context
for anything found, from animal bones to bits of pottery. Every artefact found
in the same layer is likely to have lain on the surface at the same time in the
past. A pottery drinking mug found at the same level as iron nails from ships,
charcoal fragments, loom weights and animal bones could show that the person who
drank from that mug wore clothes that had been woven, ate meat, had a fire and
probably had something to do with boats. The artefacts in our example could have
been deposited during the Iron Age, late antiquity or medieval
times鈥攑erhaps even more recently. And, of course, some things may have
been buried deliberately.

The gradual accumulation of finds from different levels of a trench enables
archaeologists to build up successive pictures of the past. Each find must be
identified, and some may be used to help to date the site.

SHOWING THEIR AGE

How artefacts are dated

A battery of tests is available to extract information from excavated objects
(Figure 2).
Pottery is a key find. It鈥檚 the most commonly found artefact from
the past, and is often all that distinguishes one age from another. Science may
be needed for an accurate date, although style鈥攄istinctive shapes and
decoration鈥攊s one clue to a pot鈥檚 age and the culture that produced it.
For example, some Roman vessels are more closely dated from stylistic features
than from any hard science.

Thermoluminescence can date a clay artefact using the natural weak
radioactivity it emits. Unfired clay emits electrons at a known rate. The
electrons become trapped within the clay鈥檚 irregular lattices, until the clay is
fired above 500 掳C and the electrons are driven out, resetting the clay鈥檚
鈥渃lock鈥� to zero. As time passes, electrons are trapped again in irregularities
within the clay. To date a clay pot, a small sample is ground to powder, and
then subjected to a sudden, intense blast of heat. This releases the trapped
electrons, causing a flash of light. Measuring the intensity of this flash
allows researchers to determine how many electrons were released, and so
calculate how long it is since the pot was fired.

The number of electrons can also be measured without using heat. In
electron-spin resonance, a ground-up sample is held in a magnetic field and
bombarded with different wavelengths of electromagnetic radiation. Because the
sample does not have to be heated, it can be used to date, for example, a tooth
that would be destroyed if heated. Measuring which wavelengths make the
electrons vibrate and how much energy they absorb enables researchers to
estimate the sample鈥檚 age.

As well as helping to find the site, magnetism is used to date
artefacts. Intense heat makes the magnetic moments (dipoles) of the clay
molecules line up in the same orientation as the Earth鈥檚 magnetic field. During
a magnetic survey, such an orderly array will show up as a tiny variation from
the local magnetic field pattern. To find its age, the strength and direction of
the magnetic field of a fired sample from a kiln may be compared with historical
records of magnetic fields that go back about 300 years. You can even tell when
the kiln itself was last used. But how do you find the age of other kinds of
objects?

In 1960, the American chemist Willard Libby was awarded a Nobel prize for his
part in developing the precise dating of archaeological remains, known as
radiocarbon dating (see Box 1). Twelve years earlier, he had announced a way
to measure how much time had elapsed since the death of a living organism. His
method was the first absolute dating technique for organic material. Animals and
plants absorb carbon-14 from the atmosphere while they are alive but, when they
die, the process ceases. Radioactive isotopes decay at a regular rate, and
carbon-14 is known to have a half-life of about 5700 years, changing from the
unstable carbon-14 isotope to a stable form of carbon. So measuring the amount
of carbon-14 in organic remains indicates how long ago they died. The technique
revolutionised archaeology.

Radiocarbon dating yields age estimates that are correct within a range,
rather than an actual year before the present. It may show that a sample is
3000卤100. This does not mean that it is between 2900 and 3100 years old. Rather,
the researchers are 67 per cent sure that the real date is in this range. In
statistical language, the quoted uncertainty is one 鈥渟tandard deviation鈥� (see
Inside Science No. 82). Having only one sample dated by this method does not
help the archaeologist much. The date may be anomalous and the artefact could
have come from an atypical period or place. But having a group of dates falling
within the same probability range shows that the broad date is correct.

Libby鈥檚 assumption of a steady level of atmospheric carbon-14 was challenged
by observations showing that changes in solar activity and in the Earth鈥檚
magnetic field affected the amount of carbon dioxide in the atmosphere. There
had to be a way to check radiocarbon dates against something else. And there
was. Hans Suess, an American chemist, produced the first calibration curve for
radiocarbon鈥檚 inaccuracies by means of tree rings of known age. Suess knew the
ages of the tree rings through dendrochronology, a technique that
American astronomer Andrew Douglass pioneered in 1901. Suess鈥檚 results showed
that radiocarbon dates could be out by as much as 800 years, depending on the
period (see Box 2).

A slice across a tree trunk shows growth rings, two per year. Each
marks a season. If the weather was inclement鈥攁 cold winter following a
poor summer, say鈥攖hen the tree doesn鈥檛 grow much and a narrow ring forms.
Sun, rain and high temperatures produce a broad ring. Reading the sequence of
growth rings is like checking a bar code: a pattern of narrow and broad bands
marks the growth across a group of years. Counting back across the years reveals
the age of the tree at felling. Another sample of wood may match some or all of
a particular growth-ring pattern. The wood鈥檚 carbon-14 鈥渃lock鈥� began ticking
when the tree was cut down, so the dates from radiocarbon tests on pieces of
tree whose age is known are used as a marker
(Figure 3).

But there is more to artefacts than their age. How do you find out about the
people who owned them, for example? Various chemical tests will give you
a window on what people ate, how they made glass for bead necklaces, and where
they kept their animals. Tests can be performed on finds excavated from the
soil, or on the soil itself. Most look for telltale chemical traces of objects,
food or other organic matter. For example, large quantities of urine will raise
soil鈥檚 nitrogen level. This chemical signature endures for several thousand
years, and can reveal where animals were kept. Fat molecules, or lipids, can
also survive for many years鈥攆or example, as a greasy residue in a pot.
Although lipids swiftly oxidise in the atmosphere, the compounds they make can
be compared with modern fats to see what animal they came from.

Gas chromatography-mass spectrometry marries two instruments to
analyse organic substances. The first gives an indication of the quantities of
material present, while the second ionises its components, separates them by
mass and identifies them. Together they can reveal what species of animal a fat,
say, came from. Similar tests on traces of plant matter can show which
vegetables were cooked, or even which plant resin was burnt as incense. For
example, during the reign of the 14th-century BC Egyptian pharaoh Akhenaten,
people used the resin from a species of pistachio tree.

Chemists have also provided archaeology with key information about the
preservation of organic matter. The condition of such remains varies with the
soil鈥檚 chemistry, especially its acidity. The humic acid of a peat bog, produced
by the decay of plants in water, reacts with calcium in bones to form a soluble
compound. So the bones and teeth of bodies that end up in such a bog, such as
people ritually drowned during the Iron Age in northern Europe, will dissolve,
but the skin and clothing may survive because the lack of oxygen in the water
halts bacterial action鈥攍eaving a 鈥渂ag of skin鈥�.

Metals can be identified in several ways. The simplest is by eye. Copper, for
example, reacts with air to form characteristic green oxides, called
corrosion products. Iron in the form of a sword or nail reacts with air and
water to form a reddish bloom of corrosion. These metal salts may be all that
remains of an ancient metal object. Often the encrusted corrosion is so thick
that only an X-ray can reveal the object鈥檚 shape. X-rays may also show how
something was made鈥攆or example, by revealing the joins where a hollow
bronze statue was pieced together.

Compositional analysis of metals can also reveal their origins, and
ultimately point to trade patterns of the past. Metal ores are rarely pure, and
the isotopic signature of the metal can be used to trace it to a particular mine
or area, perhaps even trace trade routes.

Physical methods can also tell us a lot about the plants, weather and other
environmental factors in ancient times. Botanists can identify fragments of
plant seeds, grasses and nuts found in the soil. At Poverty Point, a bluff above
the Mississippi in Arkansas covered with giant mounds and rings of earthworks,
this showed that people ate a varied diet, went fishing and harvested pecan nuts
from nearby woods.

For a more detailed picture of the past environment, soil from the
archaeological site is sampled for pollen (palynology). From the variety
of pollen grains it is possible to deduce the species and the ratio of each
plant or tree growing in a particular period. Swedish scientist Lennart von Post
tracked pollens through time using samples of soil. He showed that when the
climate was warmer, different tree species would take over a region, whereas
birches, for example, would thrive in cooler climates. Analysing pollens found
during a dig shows not only what trees people could have used for building or
for firewood, but also reveals the climate of their times. Palynology can also
show when people began to change their environment, for example by coppicing
woodlands for building materials.

What is the future of this science of the past? Instead of investigating a
striking feature, such as a temple, a tomb or single medieval village,
archaeologists now look at an area as a whole. How does the village 鈥渇it鈥� into
the region? What is the pattern of settlement or trade around it? Landscape
archaeology reflects the ideas of ecologists and environmental scientists,
because it is more interesting and useful to discover interactions between
species and their environment than to consider a species in isolation. The
information is richer.

But as we鈥檝e seen, physics and the other sciences are at work in archaeology,
too鈥攖he discipline being a great collaborative effort that also draws on
history, anthropology and many other disciplines. Most of the scientific
techniques that archaeologists use to find ancient sites and date or identify
objects have been developed only in the past 70 years or so. And more are on the
horizon.

In radiocarbon dating, samples of organic remains such as charcoal or bone
are burnt in pure oxygen to yield carbon dioxide, which makes it possible to
measure the different carbon isotopes. This is then placed in a type of Geiger
counter surrounded by heavy shielding to prevent background radiation or any
stray cosmic rays from affecting the signal. The amount of beta particles
(electrons) emitted by the sample is measured and this gives the amount of
carbon-14 present. The residual carbon-14 indicates how old the sample
is鈥攖he lower the amount, the older the sample. As this method requires a
large sample, it cannot be used with small rare objects.

Refinements led to the direct counting of carbon-14 atoms, known as
accelerator mass spectrometry. This is advantageous because it鈥檚 easier and
uses far smaller samples, so it鈥檚 less destructive of finds. Different isotopes
have different masses and, in a magnetic field, they will be deflected at
different angles. So a tiny carbon sample can be turned into graphite, a form of
carbon, then bombarded with caesium ions to release carbon ions.These are
accelerated through a curved tube lying in a magnetic field. The carbon isotopes
are deflected by an angle proportional to their mass. Carbon-14 is guided to the
tube鈥檚 outlet, where it can be counted. Only tiny samples are needed.

But there are problems with both methods. Is carbon-14 forming at a steady
rate in the upper atmosphere? The strength of the Earth鈥檚 magnetic field
changes, attracting more or fewer cosmic rays towards the planet. More than
10,000 years ago, the levels changed when deep water in the ocean鈥攚here
most carbon-14 was held鈥攚elled up towards the surface.These events have to
be taken into account in radiocarbon dating.

Dating wood used to make artefacts or to build dwellings is possible using
tree-ring dating or dendrochronology. Recording the distinctive pattern of rings
in one tree trunk, and counting back the years to check its age, gives a basis
for trying to match part or all of that pattern to another, older piece of wood.
If this sample matches the first, but has more rings, for example showing an
extra 200 years of growth, then another step back into the past has been found.
Testing samples of known ages using radiocarbon dating gives an independent
check on radiocarbon results. Thus a model chronology is built up.

This overlapping work on rings from wood such as oak found preserved in bogs
in northern Europe has given us a growth pattern reaching back more than 7000
years for the region. Counting growth rings on the world鈥檚 oldest tree鈥攖he
bristlecone pine of North America that Suess studied鈥攇ives another
chronology that reaches back 11,000 years.