Welcome to “Walking Woods Edinburgh”, a self-guided audio tour around the centre of Edinburgh that takes a closer look at the history of the city through the eyes of a wood scientist. The tour was developed as part of the 20th annual meeting of the Northern European Network for Wood Science and Engineering (WSE), but it is meant to be interesting for wood scientists and laypeople alike.
Go to the route map and list of stops
Download transcript of the episode.
Anne and I talk about the Great Hall of Edinburgh Castle, specifically its roof. Here is “the forest of timbers” that Anne describes:
Anne talks about how she was able to date the timbers in the roof and in many structures along the Royal Mile with the help of dendrochronology. For this, she took “cores” with a special core driller. This does not damage the structure too much, but allows scientists to see the growth rings clearly. Just to quickly recap how the dating works:
A more detailed explanation: Explore the Science of Tree Ring dating
You can see that we need the bark edge to know exactly when the tree was felled. But there are many reasons why the bark edge might not have survived to the present day:
When cutting timber, we usually want square edges. This makes things easier for bearing and connections. The round shape of the tree’s circumference is usually just cut off. Sometimes a little of it remains, and we call that wane. In modern construction timber only little wane is normally permitted, but historically it was more common to use waney timber. Obviously, it was a lot of work to square-edge timber, and so the non-visible edges, even in prestigious structures like the castle, might still have wane,
Anne also mentions that the outside of the tree was sometimes removed for fear of infestations. This is because the outside of the tree, the sapwood, is always susceptible to fungal decay, and in may species can be attacked by insects as well. Meanwhile the heartwood of certain species is somewhat resistant to decay. The heartwood of oak specifically is resistant to insects and moderately durable to fungi according to EN 350. So, to avoid attracting these organisms, the sapwood might have been removed before construction.
Where this was not the case, of course, the sapwood might also just have decayed over the long life of a structure. For this to happen, the moisture content of the wood needs to have been above 18% for extended periods of time. Nowadays this condition is mostly given in weather-exposed applications, but in unheated historic structures (in Scotland, no less) wood was at risk of decay even indoors, so some of the sapwood might have disappeared over the years.
All the better that Anne found some well-preserved timbers in the roof of the Great Hall and could date the timbers to 1484-1509. Here is Anne’s paper that gives much more detail on the work: The Late-medieval Roof over the Great Hall in Edinburgh Castle.
Anne and I also start talking about cheap timber imports. Find more about that (and hear more about “deals”) at the next stop.
Next stop: Castle terrace