Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
One of the most widely used is potassium–argon dating (K–Ar dating).
Potassium-40 is a radioactive isotope of potassium that decays into argon-40.
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).
Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating.
This technique is based on the principle that all objects absorb radiation from the environment.
This process frees electrons within minerals that remain caught within the item.
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.