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Wooden plinth


This wooden plinth was discovered during the excavation of an Iron Age farmstead near Liverpool. Although the site was occupied between 2,100 and 2,300 years ago, the age of the plinth was unknown. Attempts to use tree-ring dating failed because the plinth doesn't have enough rings to measure.

Radiocarbon dating showed the plinth to be 2,800 to 3,000 years old, much older than the site itself. The plinth was important enough to be handed down from generation to generation. It is now thought that it may have been used to support the statue of a god

Accession number 1997.109

Radiocarbon dating

Scientists can use radiocarbon dating to estimating the age of organic remains such as bone or wood. Carbon can exist in different forms called isotopes. One of these is radioactive and decays into a different element - nitrogen. It takes 5,730 years for half the original radiocarbon atoms in an sample to decay. By counting the number of radiocarbon atoms left in a sample, scientists can estimate its age.

The average adult human has about 400,000,000,000,000,000,000,000,000,000 atoms of carbon in their body; 400,000,000,000,000 of these will be radiocarbon.

Dendrochronology - another method of dating wooden objects

The study of tree-ring dating is called Dendrochronology. Dendrochronologists can tell the age of a tree or wooden object by counting and measuring the widths of the annual rings. From looking at different trees, sampling historic wooden objects and examining wood preserved on archaeological sites, an overlapping record of continuous consecutive ring widths (called a reference chronology) can been built up.

The reference chronology may cover several thousand years for some species of trees. Ring patterns from wooden objects of different types, (including paintings on panel) can be compared against this master chronology record, and if the patterns of ring widths match up, an estimate of their likely age can be given.

For an oak tree trunk section felled at a known date, its age can be determined by simply counting the rings back from the last one under the bark. For many objects made from wood, however, dating is not this straightforward. If the sequence of measured ring widths in the object can be matched to a dated chronology, only then can we hope to establish a date.

In most cases the bark edge no longer remains on wooden objects, as it would be trimmed off when the trunk was cut into planks. Many of the outer rings might be missing. Also, we may have no idea when the tree was cut down or how old the wood is, other than a general interpreted date from what the item is – for instance if it is part of a mediaeval barn roof, or an 18th century piece of furniture.

When an oak tree was cut up and made into things, both the bark and sapwood (still present on the tree trunk section) would be removed, so even if a match for the measured ring widths can be established with a dateable chronology, there is no surviving outer ring under the bark giving a reference to when the tree was actually felled. An estimate must be made of the number of missing rings. Once this is done, we can suggest an earliest felling date for the tree from which the items were made. This has to be estimated for wood from a particular region at a particular time, and a likely date range is all that can be estimated.

As an added complication, we may not know how long the felled wood was stored for before it was used. Each year as a tree grows it forms a new ring of water conducting pipes under its bark. In oak the pipes are larger in spring than later in the year. The rings are thicker in good growth years and thinner in poor growth years.

Did you know?

The world’s oldest living tree is thought to be a bristlecone pine in the White Mountains of California, USA. It is 4,723 years old. It was nicknamed ‘Methusela’ by Dr Edmund Schulman, the scientist who identified its age.