Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.Carbon-14 has a half-life of 5,730 ± 40 years— during the succeeding 5,730 years.Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.
This is very useful as a record of the radiocarbon concentration in the past.
Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history).
To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.