One of the dating methods most people think of when they talk about archaeology is radiocarbon dating. This is one of the absolute dating methods that archaeologists use to date an artifact. Only organic materials can be dated using this method, but archaeologists can also use it for inorganic artifacts sometimes too.  If an inorganic artifact, like pottery or stone tools, comes from the same layer of soil as an organic artifact, like plant remains or bone tools, archaeologists can use the age of the organic artifact to assign an approximate age to the inorganic artifacts as well.

Even though radiocarbon dating is a pretty well known technique not all archaeologists that have organic samples are able to do it, or perhaps more importantly, the funds to do it. It can cost over $600 to run these kinds of special scientific tests so sometimes archaeologists need to rely on other dating techniques instead.

The science behind it all…

All organic materials contain carbon. During its lifetime, a plant or animal aborbs carobon (¹⁴C and ¹²C) from the atmosphere. Once it dies, its radiocarbon clock beings to tick down as the amount of ¹⁴C begins to decay into ¹²C. This happens at a constant rate, known as the half-life. Radiocarbon dating is based on the half-life of carbon isotope 14 (written as ¹⁴C) as it undergoes radioactive decay into the carbon isotope 12 (¹²C), which is stable and does not decay. Physicists can measure the ratio of ¹⁴C to ¹²C and calculate when the organism died.

The half-life of ¹⁴C is 5730 years, which means that after 5730 years half of the ¹⁴C will have decayed into ¹²C.  The thing is the process isn’t that exact!

Not as accurate as hoped…

• “A.D. 550 +/-50”: the real date of this artifact is between A.D. 500 and A.D. 600 because the results of radiocarbon dating are always estimates.
• We can only date objects up to 100,000 years old this way (But that captures a large span of the human experience!)
• “A.D. 600 cal”: This shows a calibrated date that scientists have calculated since the ratio of ¹⁴C to ¹²C in the atmosphere is not steady through time
• Nothing that has died after 1950 can be dated using the radiocarbon method. Nuclear tests in the 1950s and 1960s threw off the natural ratio of carbon isotopes.

How its done…

Radiocarbon dating methods have really improved over the decades.  In the 1940s physicists need really large samples to test radiocarbon—they would use a Geiger counter to literally count the rate of decay and it wasn’t very accurate. But in the 1970s a new method that used a fancy piece of equipment known as an Accelerator Mass Spectrometer (AMS) was discovered.  Radiocarbon tests that use AMS are a lot faster, more accurate, and can date even very tiny samples, even corn kernels!

In addition to radiocarbon dating being expensive and sometimes inaccurate, it also takes time, and most labs have long wait lists. It can be a long time before archaeologists are able to get the results of their tests. Regardless, radiocarbon dating has become a media and movie darling, making it one of the most well known techniques for dating artifacts.

Want to learn more about dating methods?  Be sure to check out the series introduction or follow the links below!

References
Paul Bahn, and Colin Renfrew. Archaeology Theories, Methods, and Practice. 5^th ed. London: Thames & Hudson. 2008.