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.  The discovery that the mildly radioactive carbon 14 isotope, which is absorbed from the atmosphere by all living things, declines in a steady measurable way once they have died and stopped absorbing it - was the key to establishing the technique. The radioactive decline of carbon-14 in organic matter from animals, humans or plants (which are represented in archaeology by material such as wood remains or bone) is called a half-life.  When samples are processed in a laboratory, the carbon is extracted and the content of C-14 is measured against other carbon isotopes to find its relative amount, from which the half-life 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.  Even for the last ten thousand years it still has fairly wide margins of error – it is only accurate to within a standard deviation of (usually) between 50 and 100 years.  C-14 half-life years can vary in length, so to convert them into calendar years, they must be calibrated to another dating technique which can provide an alternative chronology – dendrochronology is the most common.  A series of measurements using both techniques are taken on the same samples and the combined results provide a source of chronological reference (known as a curve after their representation in graph form).  If the curve is steep, it means that there is a high chance that C-14 and dendro measurements will produce a unique correspondence and therefore a reliable date.  If the curve is flat (meaning that correspondences are similar over a long period), then the potential for a reliable date is reduced.  This is the case for the British Iron Age.

 

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.  An uncalibrated date may be written: 2950±100 bp, the plus-minus symbol indicating the standard deviation.  This date calibrated would be 1000 ± 100 (Cal) BC.