- "When we date buildings correctly, we add new chapters to the story of Norway," says Hanna Geiran, Director of National Heritage.
For many years, researchers and companies have used tree rings on timber to date buildings. The challenge has been that there have been no common guidelines for sampling and how the samples have been interpreted. The new guide has been created in close collaboration with Vestfold and Telemark county councils, with good contributions from Agder county council, as both have worked extensively and well with this topic.
- "The timber can provide us with important information about both our collective history and about individual buildings," Geiran says. "The dating of buildings is key information for how authorities and private individuals should relate to our old buildings. Dendro dating, or tree-ring dating, can contribute to new knowledge that affects how we understand our building heritage and how we manage it.
When a building is older than 1537, it is automatically listed. Buildings dating from between 1537 and 1649 are assessed by the National Heritage Board, and the result may be that they are declared listed. Here, tree-ring dating is a decisive factor. More than 400 buildings have either been declared listed or are in the process of being declared listed, and most of these are based on dendrochronological dating. A preservation order has consequences for the owner, such as what they are allowed to do with their building, and that they can apply for grants from the state for restoration.
- "With this guide, we want to help ensure that everyone dates correctly the first time," says Geiran. "We're keen to ensure that dating takes place in a consistent and verifiable way, so we need a common standard for how these dates should be done.
Why is dating important?
Around Norway, there are «undiscovered» buildings that may be automatically protected. The Cultural Heritage Act states that all buildings from before 1537, and parts of them, are automatically protected. The same applies to Sami cultural monuments from 1917 or earlier. Standing buildings from before 1650 can be declared automatically protected.
- "We can find these buildings using dendro dating," says Geiran, "and thus ensure the lasting protection of unique building heritage and important knowledge material.
In addition, dendro dating can reveal older, inaccurate dates based on building practices and building types. "For example, these can be dates based on the type of lintel," explains Geiran. "For a long time we assumed that a particular type of lintel called findalslaft was from before 1349, but control dating has shown that this type of lintel was also used at the end of and after the Middle Ages. In this way, dendro dating can contribute to corrections, and in some cases it can also lead to a change in conservation status after a building has been dated.
In order for the guide to be easy to use in the practical implementation of the dating work, the content is adapted to field work and the sampler's needs in a separate document called «field guide». It can be used as a checklist in connection with sampling.
How to date with tree rings
Dendrochronological dating is usually done by extracting drill samples from the timber of the building. This involves a physical intervention in the building.
The basic principle of dendrochronology is to use annual variations in tree ring patterns to derive information about the growth period, climate variations, time of felling and so on. This can be used to determine the year the timber was harvested.
There are also methods that do not involve physical intervention in the building, but they are less commonly used. These are photographs of a suitable, clean cut surface with good resolution, or CT X-rays, which are relatively expensive.
