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Radiocarbon
Radiocarbon dating is perhaps the best-known of all scientific dating techniques in archaeology. Also known as
carbon-14 (C-14) dating (after the carbon isotope which it measures) it relies on techniques developed in nuclear
physics to measure the time which has elapsed since the death of plants or animals, whose remains have been
recovered archaeologically.
Radiocarbon dating has developed into an essential technique in archaeology since its early development by American
nuclear scientist Willard Libby and others in the 1940's and 1950's. A tiny fraction (around 1 in a million million)
of carbon atoms are of the mildly radioactive carbon 14 isotope. All organic matter contains this small fraction of
C-14 as part of its normal growth. Unlike other carbon isotopes, C-14 'decays' (i.e. changes to another element) in
a steady and measurable way. When animals or plants die, their tissues are no longer incorporating fresh carbon
(including this small amount of C-14) so the fraction of this isotope in their remains gradually declines. Samples
are processed in a laboratory and the carbon is extracted. The content of C-14 is measured against other carbon
isotopes to find its relative amount, from which the age of the sample can be calculated. A large and expensive
apparatus known as an accelerator, which separates the isotopes, is used to carry out the analysis. Radiocarbon
dating can measure dates up to 10,000 years ago with a reasonable degree of reliability, and is capable of
contributing useful dating information up to 40,000 years ago. However, even for the last ten thousand years it
still has fairly wide margins of error - it is only usually accurate to within 50 to 100 years.
Unfortunately the fraction of C-14 atoms in the environment varies over time and this affects the age calculations.
To produce calendar years, results must be 'calibrated' against another independent dating technique -
dendrochronology is the most common. A series of measurements using both techniques have been taken on a number of
samples and this has provided a reference chart (or 'curve'). Uncalibrated dates are usually written using bp
(before present = 1950), bc and ad, whereas calibrated dates use capitals: BP, BC and AD and often Cal is added to
confirm that the dates are calibrated.
Return to Dating Methods.
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