Archaeology Dating Methods

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accelerator mass spectrometer, machine used to measure the carbon 14 decay in a sample

Historical records only go back to around 2000 B.C. Dating of ancient Egypt, and Sumer was done by examining written records of the durations of reins of kings. By working backwards archaeologists came up with the date of 3000 B.C. when those civilizations first evolved.^^

Ancient icons are indispensable archaeological tools. Minted to record a major historical event or the deification of an emperor of major figure, they can help scholar date objects and layers with some precision and provide important information on trade and political ties. Coins are used in a similar fashion. The oldest coins with dates to around 700 B.C.

There are a number of problems in the various proposed chronologies of ancient Mediterranean cultures. One that most students might notice is a diversity in the dates given for Egyptian and Mesopotamian states, periods, and monarchs. The variety of methods used to ascertain dates leads to rival schemes with dates which vary by nearly a century. There is also a much large inconsistency claimed by credentialed scholars whose motives seem, however, to derive from Biblical literalism. R. Wright explains the difference between absolute and relative chronology.

Sometimes geologists can help date objects by measuring the amount of rain wear in cracks, alluvial deposits by streams and glaciers and layers of silt in lakes. Lichenometry is useful in measuring lichens from 100 and 9,000 years old. Alexander Marshak identified the Ishango Bone. Afterwards he investigated scratched pebbles and other materials from the last Ice Age in Europe and discovered inconclusive evidence of time-keeping [time factoring] as far back as about 45000 years.

Carbon 14 Dating

Carbon is useful in measuring minerals, shell, bone, wood, teeth and charcoal between 0 and 40,000 years old. After a living things dies that ratio of carbon 14 isotopes to carbon 12 isotopes decays at a known rate. By measuring the change in ratio of unstable carbon 14 to stable carbon 12 an organic material can be dated. Fossils over 40,000 years have so little carbon 14 left the method is no longer accurate.

Radiocarbon 14 dating was pioneered in the 1940s by University of Chicago chemist Willard Frank Libby, who won the Nobel Prize for his work. Carbon 14 is an unstable radioactive isotope produced when cosmic particles from space slam into nitrogen atoms in the upper atmosphere. Living plants and animals absorb Carbon 14 and other kinds of carbon from carbon dioxide in atmosphere.

When a plant or an animal dies it can no longer absorb carbon. The carbon 14 then begins to revert back to regular carbon (Carbon 12) at a known rate. The amount of Carbon 14 in a sample can thus be used to determine the date the sample died, or in other words when it stopped absorbing carbon.

Carbon 14 has a half life of 5,730 years. This means that after 5,730 years the amount of carbon 14 in an object is reduced by half as the carbon 14 changes to nitrogen 12. After another 5,730 years half of the remaining amount of carbon 14 decays by half again so that only one quarter of the original amount is left — and so on until about 40,000 years when only negligible amounts of carbon 14 is left.

With Carbon 14 dating, a sample of the material to be dated — for example, charcoal from a hearth, a piece of wood from a ship beam, a seed in a strata of soil — is burned and reduced to pure carbon. The ratio of Carbon 14 to Carbon 12 can be measured with a high energy mass spectrometer, revealing the date. Carbon dating can be used to date even minute samples of something such as residue on pots or pigments.

Problems with Carbon Dating

taking a sample of a mummy's tooth

David Silverman of Reed College wrote: “Carbon 14 dating tends not to be very helpful to archaeologists (more so to geologists and the like). Carbon-14 dating only works on items which contain carbon, such as wood or coal. And in some cases C-14 dating can tell you only when an object's raw material first came into existence, as opposed to what you really want to know, which is when the material was shaped into its current form. [Source: David Silverman, Reed College, Classics 373 ~ History 393 Class ]

Over the millennia the amounts of Carbon 14 in the atmosphere have not been constant. This means that in certain period a given organism can absorb more or less Carbon 14 depending on how much is in the atmosphere. Fortunately some trees, such as bristlecone pines in California live a long time, and their ages can be accurately measured using tree rings and Carbon 14 levels can be measured in each tree ring and thus a table has been produced that compensates for fluctuations in Carbon 14 levels during each year.

Short-lived plants such as grain are the best for dating. With wood there is always the problem that the tree was cut down long before it was incorporated into a site. In addition, Carbon 14 levels often vary greatly depending on which part of the tree the sample came from: deep inside the tree or on the outside.

Samples can also be contaminated with younger or older carbon brought it by groundwater, earthquakes or carbonate rocks such as limestone. Contamination can usually be eliminated with careful cleaning before the dating process begins. Even when all goes well, the dating does not produce a date but rather a probability that the sample falls within a certain range of dates.

Nondestructive Radiocarbon Dating

Nikhil Swaminathan wrote in Archaeology magazine, “Precisely dating archaeological artifacts is not as easy or harmless as it might seem. The most common method, radiocarbon dating, requires that a piece of an organic object be destroyed — washed with a strong acid and base at high temperature to remove impurities, and then set aflame. The resulting release of carbon dioxide is fed to an accelerator mass spectrometer, which measures the decay of radioactive carbon 14 — the more the carbon 14 has decayed, the older the object is. [Source: Nikhil Swaminathan, Archaeology magazine, January/February 2011]

Over the past 20 years, chemist Marvin Rowe of Texas A&M University has developed a nondestructive method for carbon dioxide extraction. In his process, an object is placed in a vacuum chamber and a supercritical fluid — a hybrid gas/liquid — is applied as a solvent (as in dry cleaning). Next, Rowe passes plasma — an “electrically excited ionized gas” — over the artifact, which selectively strips carbon from the sample. “It’s essentially like slowly burning the sample, so we can just oxidize a little off the surface and collect that carbon dioxide,” explains Rowe. This year he further refined the method so it will work on objects coated in sticky hydrocarbons, such as the resins that cover Egyptian mummy gauze.

Thus far, he’s dated samples of wood, charcoal, animal skin, bone from a mummy, and ostrich eggshell. “Everything so far that we’ve tried to do with the nondestructive technique has agreed statistically with regular radiocarbon dating,” Rowe says, “and you basically don’t see any change in the sample.” R. E. Taylor, a radiocarbon expert at the University of California, Riverside, says Rowe’s technique may have limitations, as items older than 10,000 years will have impurities that the technique may not be able to purge. Archaeologists, meanwhile, are hailing the discovery as one of the most important in decades, particularly for issues surrounding the repatriation of human remains from Native American burials, which modern tribes don’t want to see harmed.

Rowe’s refinement of carbon dioxide extraction dovetails with an update to the radiocarbon calibration curve, which increases the accuracy of radiocarbon dating by accounting for past fluctuations in carbon 14. According to researchers at Queen’s University of Belfast, the new curve doubles the accuracy of dating as well as the age of artifacts on which it can be used, from 25,000 to 50,000 years.

Normally carbon samples are converted to acetylene gas by combustion in a vacuum line. The acetylene gas is then analyzed in a mass spectrometer to determine its carbon isotopic composition.

Thermolumiscence and Optically Simulated Luminescence

Thermolumiscence counts the number of electrons trapped in the microscopic crystal structure of a burned flint tool or other objects that to have been exposed to early-man-produced heat. By measuring the trapped electrons, the time when an object was last heated can be estimated. The method is useful in measuring minerals and natural glass between 0 and 500,000 years old.

The science behind thermolumiscence is the following: When minerals and natural glass are heated to a certain point radioactive atoms surrounding and buried inside crystals can release particles than can knock electrons out of their orbits. The released electrons sometime get stuck in defects in the crystal structure and over time the crystal fills with electrons at a regular, measurable rate. The trapped electrons are measured by reheating the material. As the trapped electrons escape they release light. By measuring that light scientists can count the trapped electrons and determine the age of the material.

Optically simulated luminescence is used to determine when minerals such as quartz are buried under sand or sediments by determining when they were last exposed to sunlight. This method operates under the same principals as thermolumiscence: by measuring the trapped electrons, the time when an object was last exposed to sunlight can be estimated. The trapped electrons are measured by firing beams of photons at the object. As the trapped electrons escape and return to their atoms they release heat. My measuring the heat scientists can count the trapped electrons and determine the age of the material.

Optically simulated luminescence is useful in measuring minerals and natural glass between 0 and 500,000 years old. The trick is to find objects that have not been exposed to sunlight and prevent them from being exposed to sunlight. Just of few seconds of exposure to sunlight can cause the trapped electrons to break from the crystals and return to their original state. Scientists who rely on this methods can not look for fossils and objects in the normal way in the sunlight. They hammer hollow, stainless steel cylinders into the sand and capped them and later examine their finds in a darkroom and fire beams of photons their samples to release the trapped electrons.

Dendrochronology (Tree Ring Dating)

bristlecone tree rings

Dendrochronology is useful in measuring tree rings from zero to 12,000 years old. Tree rings from bristlecone pines provide scale for correcting radio-carbon dating. Every year trees lay down two layers — one broad and light colored during the growing season, the other dark in the winter when little growth occurs. The rings are not only useful in measuring age but also offer clues to what happens in given years. Each cross section of tree trunk has a distinct pattern of tree rings. Sometimes the rings have unusual growth patterns that occur during a particularly hot winter or cold summer or an event like a volcanic eruption.

With pieces of wood from trees that have been dead for centuries or millennia archaeologists often look for unusual patterns that indicate an event — like a volcanic eruption — that occurred on a known date and then count the tree rings from that event to determine the date of the artifact.

Artifacts dated using tree rings include logs used in the tomb of King Midas of the Phrygians, cut in 718 B.C.; timbers from an Egyptian shipwreck that revealed a gold scarab with Queen Nefertari's name, cut in 1316 B.C.; and logs of bronze age houses in Acemboyuk and Kultepe Turkey, cut in 1752 B.C. and 1810 B.C.;

Uses of Dendrochronology

Many objects from the Mediterranean and Middle East have been dating by looking at structural timbers from trees that grew during the catastrophic eruption on the volcanic island of Thera in the Aegean Sea that occurred around 1500 B.C. and hurled vast amounts of material in the atmosphere and caused a cooler and wetter summer worldwide. Bristlecone pines from California and oak tress from Irish bogs have tree rings that are more tightly packed after the volcano erupted.

Dendrochronology is useful in measuring tree rings from zero to 12,000 years old. Tree rings from bristlecone pines provide scale for correcting radio-carbon dating. To determine when wood buildings were built, Stuart Manning, an archaeologist and dendrochronology expert at Cornell University, used a technique called tree-ring radiocarbon wiggle matching. This involved measuring the amount of radioactive carbon in samples drawn from a number of rings in wood. [Source: Daniel Weiss, Archaeology Magazine, November/December 2021]

Given that the baseline amount of radiocarbon in the atmosphere fluctuates over time based on factors such as the level of sunspot activity, the amount of radiocarbon in a sequence of tree rings does not decline at a steady rate that corresponds to the rings’ ages. Instead, a graph of the rings’ radiocarbon content includes a series of “wiggles,” or fluctuations in the amount of radiocarbon. By comparing these wiggles with measurements of radiocarbon in trees whose precise ages are known, Manning was able to estimate the age of the wood from the tanks in northern Italy the lower tank was built around 1444 B.C. and the upper tank was built about 12 years later, around 1432 B.C. There is some uncertainty in the measurement of radiocarbon levels, but both estimates are thought to be accurate to within around four years.

Dendrochronology also offers information on climate at certain specific times. In February 2023, in a study published in Nature, researchers led by Manning said an examination of trees alive at the time of the Hittites showed that three consecutive years of severe drought may have caused crop failures, famine and political-societal disintegration that led to the Hittite Empire’s collapse. Reuters reported: The researchers examined long-lived juniper trees that grew in the region at the time and eventually were harvested to build a wooden structure southwest of Ankara around 748 B.C. [Source: Will Dunham, Reuters, February 9, 2023]

The trees offered a regional paleoclimatic record in two ways: patterns of annual tree-ring growth, with narrow rings indicating dry conditions; and the ratio of two forms, or isotopes, of carbon in the rings, revealing the tree's response to water availability. They detected a gradual shift to drier conditions from the 13th century B.C. into the 12th century B.C. More importantly, both lines of evidence indicated three straight years of severe drought, in 1198, 1197 and 1196 B.C., coinciding with the known timing of the empire's dissolution.

Pottery Analysis

Almost every archaeological site yields thousands of pieces of broken pottery known as pottery shards, or potsherds. Because pottery was cheap and easy to make, ancient people thought nothing of throwing it away. Archaeologists are adept at deciphering pottery shards. They can often date a site, gauge its level of sophistication and establish trading patterns solely by examining pottery shards.

There are tables and guides that archaeologists can refer to identify potsherds. Different kinds of pottery can be differentiated by the designs, pottery making process and the composition of clay, glazes and pigments used to make it. Large data bases of over 7,000 kinds of pottery from different ages and different sites around the Mediterranean alone can be used to identify the date and origin of the objects. The analysis is based on the type of clay used, pottery type and ornamentation.

Pottery can be dated measuring the effects of radiation via thermolumiscence, by carbon dating food remains or dating the sediments of the layers in which the pottery was found. A “clay atlas” has been created for “fingerprinting” pottery for 35 trace elements. This atlas is especially useful with small pottery shards.

Determining the Age of Wine by Examining Traces on Potsherds

The earliest evidence of grape wine was found in clay pottery from Khramis Didi Gora, Georgia dated to to 6,000 B.C. Ashifa Kassam and Nicola Davis wrote in The Guardian: “A series of excavations in Georgia has uncovered evidence of the world’s earliest winemaking, in the form of telltale traces within clay pottery dating back to 6,000 B.C. [Source: Ashifa Kassam and Nicola Davis, The Guardian, November 13, 2017 +/]

“To explore whether winemaking was indeed a part of life in the region, the team focused on collecting and analysing fragments of pottery from two neolithic villages, as well as soil samples. Radiocarbon dating of grains and charcoal nearby suggested the pots date to about 6,000–5,800 BC. In total, 30 pottery fragments and 26 soil samples were examined, with the inside surface of the pottery ground down a little to produce a powder for analysis. While many of the pieces were collected in recent excavations, two were collected in the 1960s; researchers have long suspected they might bear traces of wine.The team then used a variety of analytical techniques to explore whether the soil or the inner surface of the vessels held signs of molecules of the correct mass, or with the right chemical signatures, to be evidence of wine. +/

“The results, published in the Proceeding of the National Academy of Sciences, reveal that for eight of the fragments, including the two previously unearthed, the team found traces of tartaric acid – a substance found in grapes in large quantities. Tests on the associated soils largely showed far lower levels of the acid. The team also identified the presence of three other acids linked to grapes and wine. Other evidence indicating the presence of wine included ancient grape pollen found at the excavated sites – but not in the topsoil – as well as grape starch particles, the remains of a fruit fly, and cells believed to be from the surface of grapevines on the inside of one of the fragments. +/

“While the team note that it is possible that the vessels were used to store something other than wine, such as the grapes themselves, they note that the shape of the vessels is suited to holding a liquid and that grapes or raisins would have degraded without trace. Moreover, there are none of the telltale signs that the pots were used for syrup-making, while grape juice would have fermented within a matter of days. +/

accelerator mass spectrometer schematic for radiocarbon dating

Determining the Age of Cheese by Examining Traces on Potsherds

A study published in Nature in 2012 said that the earliest solid evidence of cheese-making comes from the chemical analysis on fragments from 34 pottery sieves discovered in Poland dated to 7,500 years ago. Maria Cheng of Associated Press wrote: “Though there is no definitive test for cheese, Richard Evershed at the University of Bristol and colleagues found large amounts of fatty milk residue on the pottery shards compared to cooking or storage pots from the same sites. That suggests the sieves were specifically used to separate fat-rich curds from liquid whey in soured milk in a crude cheese-making process. "It's a very compelling forensic argument that this was connected to cheese," Evershed said. "There aren't many other dairy processes where you would need to strain." He and colleagues weren't sure what kind of milk was used, but said there were lots of cattle bones in the region. [Source: Maria Cheng, Associated Press, December 13, 2012]

“"This is the smoking gun," said Paul Kindstedt, a professor of nutrition and food sciences at the University of Vermont and author of "Cheese and Culture." He was not involved in the study. "It's almost inconceivable that the milk fat residues in the sieves were from anything else but cheese," Kindstedt said, adding that many experts suspected cheese was being made in Turkey up to 2,000 years earlier than this latest finding in Poland but that there was no definitive proof.

Nature reported: “Peter Bogucki, an archaeologist at Princeton University in New Jersey, was in the 1980s among the first to suspect that cheese-making might have been afoot in Europe as early as 5,500 B.C. He noticed that archaeologists working at ancient cattle-rearing sites in what is now Poland had found pieces of ceramic vessels riddled with holes, reminiscent of cheese strainers. Bogucki reasoned that Neolithic farmers had found a way to use their herds for more than milk or meat. [Source: Nature, December 12, 2012 -]

“In a paper published in Nature, Bogucki and his collaborators now confirm that theory, with biochemical proof that the strainers were used to separate dairy fats. Mélanie Salque, a chemist at the University of Bristol, UK, used gas chromatography and carbon-isotope ratios to analyse molecules preserved in the pores of the ancient clay, and confirmed that they came from milk fats. “This research provides the smoking gun that cheese manufacture was practiced by Neolithic people 7,000 years ago,” says Bogucki. -

Strontium Isotopes

Isotopic studies of strontium and other chemicals found in the teeth and bones of Neolithic humans are helping archaeologists better track the movement of ancient peoples across the landscape. Strontium signatures last not just a lifetime, but potentially thousands of years as tooth enamel, the densest tissue in the body, resists decomposition and contamination after death. It is now commonly used by archaeologists to determine if an individual was local or foreign to the place where their remains were discovered.

According to a study of 5,000-year-old skeletons, being born to affluent parents meant something even in Neolithic times. Maev Kennedy wrote in The Guardian: “Hereditary wealth and privilege date back to the earliest days of farming in the Neolithic, according to researchers who have studied hundreds of ancient human skeletons. They found evidence that the wealth children were born into persisted right up to death and that rich people lived cheek-by-jowl with the poor – who scraped an existence from whatever they could find. "It seems who your parents were mattered even then," said Dr Penny Bickle of Cardiff University, one of the international team of researchers whose findings are reported in the journal Proceedings of the National Academy of Sciences. [Source: Maev Kennedy, The Guardian, May 28, 2012]

“The study looked at levels of strontium isotopes, which can reveal the diet eaten in childhood, in more than 300 skeletons dating from the Neolithic period, around 7,000 ago, from sites across central Europe. Some of the male skeletons were buried with stone adzes – cutting and chopping tools – which were often beautifully polished and made from carefully selected stone, and so were probably also symbols of status and wealth. An analysis of the strontium isotopes in their tooth enamel showed these individuals had lived on food grown in "loess", the most fertile and productive soil. ^=^

“Because strontium markers are laid down in tooth enamel in childhood, it seems they hadn't earned but inherited this richer diet, and the fact that they were buried with the adzes suggests that they died as they had lived: privileged to the end. "This strongly suggests that access to the best soils was being passed on between generations," Bickle said. "Thus, while I think it's not news that status differences and subsistence specialisms date to the Neolithic, this is perhaps the first time we've been able to show that inheritance was a large part of this." ^=^

Dating of a Variety of Materials from Different Ages

The following is a group of rocks and materials that have dated by various atomic clock methods (Approximate Age in Years, Sample material): 1) Cloth wrappings from a mummified bull, samples taken from a pyramid in Dashur, Egypt. This date agrees with the age of the pyramid as estimated from historical records 2,050, Charcoal) ; 2) Sample, recovered from bed of ash near Crater Lake, Oregon, is from a tree burned in the violent eruption of Mount Mazama which created Crater Lake. This eruption blanketed several States with ash, providing geologists with an excellent time zone (6,640, Charcoal); 3) Sample collected from the "Marmes Man" site in southeastern Washington. This rock shelter is believed to be among the oldest known inhabited sites in North America (10,130, Spruce wood); 4) Sample from the Two Creeks forest bed near Milwaukee, Wisconsin, dates one of the last advances of the continental ice sheet into the United States (11,640, Bishop Tuff). 5) Samples collected from volcanic ash and pumice that overlie glacial debris in Owens Valley, California. This volcanic episode provides an important reference datum in the glacial history of North America (700,000, Volcanic ash).

6) Samples collected from strata in Olduvai Gorge, East Africa, which sandwich the fossil remains of Zinjanthropus and Homo habilis — possible precursors of modern man (1,750,000, Monzonite); 7) Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon. Utah (37,500,000, Quartz monzonite); 8) Samples collected from Half Dome, Yosemite National Park, California (80,000,000, Conway Granite); 9) Samples collected from Redstone Quarry in the White Mountains of New Hampshire (180,000,000, Rhyolite); 10) Samples collected from Mount Rogers, the highest point in Virginia (820,000,000, Pikes Peak Granite).

11) Samples collected on top of Pikes Peak, Colorado (1,030,000,000, Gneiss); 12) Samples from outcrops in the Karelian area of eastern Finland are believed to represent the oldest rocks in the Baltic region (2,700,000,000, The Old Granite); 13) Samples from outcrops in the Transvaal, South Africa. These rocks intrude even older rocks that have not been dated (3,200,000,000, Morton Gneiss); 14) Samples from outcrops in southwestern Minnesota are believed to represent some of the oldest rocks in North America (3,600,000,000).

Image Sources: Wikimedia Commons

Text Sources: National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Nature, Scientific American. Live Science, Discover magazine, Discovery News, Ancient Foods ; Times of London, Natural History magazine, Archaeology magazine, The New Yorker, Time, Newsweek, BBC, The Guardian, Reuters, AP, AFP, Lonely Planet Guides, “World Religions” edited by Geoffrey Parrinder (Facts on File Publications, New York); “History of Warfare” by John Keegan (Vintage Books); “History of Art” by H.W. Janson (Prentice Hall, Englewood Cliffs, N.J.), Compton’s Encyclopedia and various books and other publications.

Last updated June 2024

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