22 C O N T E X T 1 3 4 : M A Y 2 0 1 4 MIKE JARVIS and KATE HUDSON-McAULAY The science of replacing historic timber A research project is examining the mechanical properties of historic timber to uncover how its performance changes with age and whether current carpentry repairs are suitable. As an organic material, timber has admirable properties such as strength in tension and bending as well as compression, and remarkable stability through age. These properties are all interlinked1. The aim of a research project at Glasgow University is to examine the mechanical properties of historic timber, to uncover how its performance changes with age and to find out if carpentry repairs currently used in the conservation of historic structures are being done in the right way to keep these buildings alive. Traditional carpentry methods, putting in new wood of the same species as is removed, are the main forms of repair to timber structures.The aim is to maintain the historical integrity of the building, to keep it looking as authentic as possible and not to distract from the original historical elements.There is a presumption that all wood of one species is the same, and that old wood and new wood do not differ in their structural properties. But little is known about the aging of historic wood, because what has survived through time is part of a structure and is rarely available for destructive scientific testing.To assess the natural ageing of wood, this project has had the privilege of access to historical wood samples from the 15th and 16th centuries from AOC Archaeology and Historic Scotland, for destructive testing.The testing methods had to be miniaturised to get as much information as possible out of the small amount of historic wood that it was justifiable to use. Modern wood was tested alongside the historic wood to provide information on how these would coexist in a joint. Modern oak samples were provided by Carpenter Oak andWoodland (Scotland), who have experience in supplying timber for restoration projects. The way in which wood is used in a medieval building is very much like the way a tree functions in nature.The trunk of the tree is under mechanical compression just like a building post holding up rafters, and the rafters act almost like the tree’s branches in withstanding bending stresses. Knees of grown timber connect these members as the grain runs in a tree to connect the branches to the trunk. The frames of a wooden ship are designed similarly, but upside down. That is why it is important to run both compression and bending tests on historic wood samples to discover any changes in these mechanical properties over time. Changes in wood over historic time, unless due to fungal decay or insect attack, are likely to be chemical, as in paper and other organic materials. Thus understanding structural changes in wood polymers is essential if we are to understand what may happen to the mechanical properties of the wood as it ages, and what impact its aging will have on the conservation of historic structures. There are three main polymer types in wood, controlling its chemical and mechanical properties. Cellulose and the hemicelluloses are long-chain carbohydrate polymers. Lignin is also a polymer but it is not a carbohydrate. The lignin and hemicelluloses form a matrix through which run fibres of cellulose called microfibrils. The cellulose microfibrils provide most of the strength of wood. They are wound in a spiral around each wood cell. If the spiral is steep and all the microfibrils are almost aligned with the cells, the microfibril angle is said to be small, and the wood will be strong and stiff along the grain2. High microfibril angle, with the cellulose mainly encircling the cells, Like-for-like conservation repair work carried out by Carpenter, Oak and Woodland (Scotland) (Photo: Carpenter, Oak andWoodland) Top:Thin sections cut using a microtome for analysis (Photo: Kate Hudson-McAulay) Below: Softwood wood cells under magnification (Photo: Mike Jarvis) References 1 Zwerger, K (1997) Wood andWood Joints: building traditions of Europe and Japan, Bikhäuser 2 Bader,T et al (2011) ‘Stiffness properties of the archaeological oak wood from the Oseberg ship’, Proceedings of the European Workshop on Cultural Heritage Preservation Frauenhofer Information Centre for Planning and Building 3 Chaffey, N (2000) ‘Microfibril orientation in wood cells: new angles on an old topic’, Trends in Plant Science, Volume 5, Elsevier
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