Bonekickers

History behind ‘Army of God’: Dendrochronology

Dendrochronology, or tree-ring dating, is a reliable method to obtain precise dates. It relies on the principle that annual variation in the seasons is never the same, so the size of each annual ring will be different, giving a unique pattern. Around 50 rings are required to provide a good match, and the sample is compared against known chronologies. In Britain oak is the most reliable species, although elm has also been successfully dated. In the Middle East, reliable chronologies have been published for cedar and juniper, from about 3,000 BC.

Obtaining a good date requires that sapwood is preserved on the sample. This is formed as the outer rings, and if the edge of the sapwood survives, then it may be possible to provide the exact year of felling - sometimes even the season. The pattern of rings varies by region so it is also possible to work out where the wood came from. The rings are measured by taking a core through the timber, and then using a microscopic device to measure the intervals between the rings.

Dendrochronology is used to date buildings, works of art, archaeological wood and even fossilized bog oak.