C O N T E X T 1 3 4 : M A Y 2 0 1 4 23 leads to wood that is weaker and less stable to moisture. Microfibril angle (MFA) is therefore a dominant factor in the engineering properties of wood, especially its stiffness3. MFA is not affected by the aging of the wood but it needs to be taken into consideration when replacing wooden timber elements. Microfibril angle varies greatly within a tree, and varies considerably according to how the tree has grown. It has been observed that the MFA correlates well with the mechanical stresses that are present in the different stages of a tree’s growth. For example, juvenile wood needs to be flexible and bend without breaking, and accordingly has a higher MFA than mature wood. Mature wood needs to have lower MFA, giving it the rigidity to withstand the compression forces exerted by the weight of the tree4. During the current project at Glasgow University, mechanical tests have been carried out on both historic and modern wood. Sequences of samples were cut from the centre out to the bark and their resistance to bending and compression were measured on a tensile testing machine.The results revealed great variation in stiffness within each timber beam, due to varying MFA fromwhat had been the centre of the tree to the outside. Ideally, when replacing historic timber, MFA should be considered as well as the timber species. Direct measurement of MFA is difficult and expensive. However, some degree of matching can be obtained by making sure that replacement timber is cut from the same part of the tree as the historic timber.This can be estimated from the curvature of the annual rings. The width of the rings is a guide to how fast a tree has grown, which also affects the MFA5. Matching ring curvature and width, features well known to a skilled carpenter, can help the engineering properties of new timber to be matched to old. Matching new and old timber in this way presupposes that the properties of the timber do not change with age. Such changes are known to occur, especially in oak, but until now they have not been much investigated scientifically.Tests on structural changes in the chemical makeup of new and old wood were carried out on samples cut with a microtome into thin sections only one cell thick, so that their composition could be measured on a Fourier transform infrared (FTIR) microscope. The FTIR results showed that both oak and pine wood lose acetyl groups over historic time. Acetyl groups are a part of the structure of the hemicellulose polymers6.Their removal could imply either that acetic acid was being split off, leading to further breakdown of the wood structure, or that the hemicellulose polymer chains were being broken up and the whole polymer was being lost. Or both might have happened. The formation of acetic acid would also lead to increased corrosion of any iron nails and fixtures that might be attached to the wood. As hardwoods and softwoods have different hemicellulose polymers, they may also differ in their manner of breakdown.There is currently more testing going ahead to discover what effects on wood properties are caused by the loss of acetyl groups over time. One of these tests is to determine if hemicellulose breakdown has any effect on the water absorption by wood when the relative humidity changes.The results show there are some differences between new and historic wood in the amounts of water absorbed.With oak the difference is very small but the historic Scots pine shows unexpectedly large differences from new wood. These are being further investigated. When historic timber structures require repair, it is standard practice to keep the timber components intact where possible. When this is not possible the decayed part of the timber is cut away and a new piece of timber is spliced into place, keeping the species the same to minimise intrusion on the original ‘feel’ of the building7. Ideally, more than just the species should be considered. Changes in humidity shrink timber according to its MFA, and large stresses may be set up within the structure if the new and the old parts of a spliced joint are not matched. The replacements of timber on a like-for-like basis within historic structures is a complicated matter. The timber replacement needs to be chosen selectively, and particular caution is recommended in environments where the relative humidity may not be easily controlled. The microfibril angle: how the cellulose microfibrils are angled compared to the axis of the cell itself (Photo: Mike Jarvis) References 4 Via B et al (2009) ‘Mechanical response of longleaf pine to variation in micro fibril angle, chemistry associated wavelengths, density, and radical position’, Composites Part A: Applied Science and Manufacturing, Volume 40, Elsevier 5 Lasserrea, J et al (2009) ‘Influence of initial planting spacing and genotype on microfibrilangle, wood density, fiber properties and modulus of elasticity in Pinus radiata D Don corewood’, Forest Ecology and ManagementVolume 258 6 Ridout, B (2000) Timber Decay in Buildings: the conservation approach to treatment, E &FN Spon 7 Morton, B (2009) ‘BellFrames under Threat’, Context 110, July. Institute of Historic Building Conservation The FTIR microscope used for the chemical analysis for the historic wood (Photo: Kate Hudson-McAulay) Mike Jarvis is reader of biomaterials at the University of Glasgow and supervisor to this PhD project. Kate HudsonMcAulay is a PhD student researching the structural and mechanical integrity of historic wood, funded by Historic Scotland.
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