Radiocarbon dating (also known as C-14 dating) is one of the most common methods of dating used in archaeology. It relies on measuring the relative amounts of different types of carbon in the object being dated, as these change in a predictable manner through time. Carbon atoms exist in three sizes (or isotopes), weighing 12, 13 and 14 atomic mass units each. These occur in air (as carbon dioxide) in the proportions
- 12Carbon - 99%
- 13Carbon - 1%
- 14Carbon - one part per billion
14Carbon is weakly radioactive, and so it decays at a predictable rate (the half life). 14Carbon is also formed constantly in the upper atmosphere. It forms carbon dioxide, travels throughout the atmosphere and becomes part of every living being. This means that the proportion of 14Carbon to 12Carbon is stable in every living thing (including plants). When an organism dies, it ceases to breathe, and ceases to take in 14Carbon, so the proportion of 14Carbon to 12Carbon starts to decrease, at a predictable rate. If you know the rate of decay, and measure the proportion of 14Carbon to 12Carbon, you can work out how old the object you are dating is.
While this is not a problem when dating bone, it makes dating wood fairly tricky: the date produced is the date the tree was cut down, not necessarily the date of manufacture of the artefact. Also, large structural timbers are frequently re-used (for example, late Saxon boat timbers were reused in the Medieval quaysides of Kings Lynn), so dates can be artificially old.
The rate of 14Carbon formation is not constant, so radiocarbon dates must be calibrated. Calibration curves are derived from looking at carbon isotope ratios in ice cores and tree rings. These are used to create a calibration curve. For more information on radiocarbon dating and calibration, have a look at the Oxford Radiocarbon Accelerator Unit's webpages, or
Aitken, M. (1990) Science Based Dating in Archaeology Harlow: Longman
Bowman, S. (1990) Radiocarbon Dating London: British Museum Publications Ltd.