Libraries of tree rings of different calendar ages are now available to provide records extending back over the last 11,000 years.
The trees often used as references are the bristlecone pine (Pinus aristata) found in the USA and waterlogged Oak (Quercus sp.) in Ireland and Germany.
Radiocarbon dating laboratories have been known to use data from other species of trees.
In principle, the age of a certain carbonaceous sample can be easily determined by comparing its radiocarbon content to that of a tree ring with a known calendar age.
The most popular and often used method for calibration is by dendrochronology.It is also called “radiocarbon” because it is unstable and radioactive relative to carbon-12 and carbon-13.Carbon consists of 99% carbon-12, 1% carbon-13, and about one part per million carbon-14.If a sample has the same proportion of radiocarbon as that of the tree ring, it is safe to conclude that they are of the same age.In practice, tree-ring calibration is not as straightforward due to many factors, the most significant of which is that individual measurements made on the tree rings and the sample have limited precision so a range of possible calendar years is obtained.Very small samples (less than 300 mg) are analyzed by accelerator mass spectrometry (AMS).