The structural and mechanical integrity of historic wood

Little is known about historic wood as it ages naturally. Instead, most studies focus on biological decay, as it is often assumed that wood remains otherwise stable with age. This PhD project was organised by Historic Scotland and the University of Glasgow to investigate the natural chemical and physical aging of wood. The natural aging of wood was a concern for Historic Scotland as traditional timber replacement is the standard form of repair used in wooden cultural heritage; replacing rotten timber with new timber of the same species. The project was set up to look at what differences could exist both chemically and physically between old and new wood, which could put unforeseen stress on the joint between them. Through Historic Scotland it was possible to work with genuine historic wood from two species, Oak and Scots pine, both from the 1500’s, rather than relying on artificial aging. Artificial aging of wood is still a debated topic, with consideration given to whether it is truly mimicking the aging process or just damaging the wood cells. The chemical stability of wood was investigated using Fourier-transform infrared (FTIR) microscopy, as well as wet chemistry methods including a test for soluble sugars from the possible breakdown of the wood polymers. The physical properties assessed included using a tensile testing machine to uncover possible differences in mechanical properties. An environmental chamber was used to test the reaction to moisture of wood of different ages, as moisture is the most damaging aspect of the environment to wooden cultural objects. The project uncovered several differences, both physical and chemical, between the modern and historic wood which could affect the success of traditional ‘like for like’ repairs. Both oak and pine lost acetyl groups, over historic time, from their hemicellulose polymers. This chemical reaction releases acetic acid, which had no effect on the historic oak but was associated with reduced stiffness in historic pine, probably due to degradation of the hemicellulose polymers by acid hydrolysis. The stiffness of historic oak and pine was also reduced by decay. Visible pest decay led to loss of wood density but there was evidence that fungal decay, extending beyond what was visible, degraded the S2 layer of the pine cell walls, reducing the stiffness of the wood by depleting the cellulose microfibrils most aligned with the grain. Fungal decay of polysaccharides in pine wood left behind sugars that attracted increased levels of moisture. The degradation of essential polymers in the wood structure due to age had different impacts on the two species of wood, and raised questions concerning both the mechanism of aging of wood and the ways in which traditional repairs are implemented, especially in Scots pine. These repairs need to be done with more care and precision, especially in choosing new timber to match the old. Within this project a quantitative method of measuring the microfibril angle (MFA) of wood using polarised Fourier transform infrared (FTIR) microscopy has been developed, allowing the MFA of both new and historic pine to be measured. This provides some of the information needed for a more specific match when selecting replacement timbers for historic buildings.

A petrographic investigation into the durability of common replacement sandstones to the crystallisation of de-icing salts

Scottish sandstone buildings are now suffering the long-term effects of salt-crystallisation damage, owing in part to the repeated deposition of de-icing salts during winter months. The use of de-icing salts is necessary in order to maintain safe road and pavement conditions during cold weather, but their use comes at a price. Sodium chloride (NaCl), which is used as the primary de-icing salt throughout the country, is a salt known to be damaging to sandstone masonry. However, there remains a range of alternative, commercially available de-icing salts. It is unknown however, what effect these salts have on porous building materials, such as sandstone. In order to protect our built heritage against salt-induced decay, it is vital to understand the effects of these different salts on the range of sandstone types that we see within the historic buildings of Scotland. Eleven common types of sandstone were characterised using a suite of methods in order to understand their mineralogy, pore structure and their response to moisture movement, which are vital properties that govern a stone’s response to weathering and decay. Sandstones were then placed through a range of durability tests designed to measure their resistance to various weathering processes. Three salt crystallisation tests were undertaken on the sandstones over a range of 16 to 50 cycles, which tested their durability to NaCl, CaCl2, MgCl2 and a chloride blend salt. Samples were primarily analysed by measuring their dry weight loss after each cycle, visually after each cycle and by other complimentary methods in order to understand their changing response to moisture uptake after salt treatment. Salt crystallisation was identified as the primary mechanism of decay across each salt, with the extent of damage in each sandstone influenced by environmental conditions and pore-grain properties of the stone. Damage recorded in salt crystallisation tests was ultimately caused by the generation of high crystallisation pressures within the confined pore networks of each stone. Stone and test-specific parameters controlled the location and magnitude of damage, with the amount of micro-pores, their spatial distribution, the water absorption coefficient and the drying efficiency of each stone being identified as the most important stone-specific properties influencing salt-induced decay. Strong correlations were found between the dry weight loss of NaCl treated samples and the proportion of pores <1µm in diameter. Crystallisation pressures are known to scale inversely with pore size, while the spatial distribution of these micro-pores is thought to influence the rate, overall extent and type of decay within the stone by concentrating crystallisation pressures in specific regions of the stone. The water absorption determines the total amount of moisture entering into the stone, which represents the total amount of void space for salt crystallisation. The drying parameters on the other hand, ultimately control the distribution of salt crystallisation. Those stones that were characterised by a combination of a high proportion of micro-pores, high water absorption values and slow drying kinetics were shown to be most vulnerable to NaCl-induced decay. CaCl2 and MgCl2 are shown to have similar crystallisation behaviour, forming thin crystalline sheets under low relative humidity and/or high temperature conditions. Distinct differences in their behaviour that are influenced by test specific criteria were identified. The location of MgCl2 crystallisation close to the stone surface, as influenced by prolonged drying under moderate temperature drying conditions, was identified as the main factor that caused substantial dry weight loss in specific stone types. CaCl2 solutions remained unaffected under these conditions and only crystallised under high temperatures. Homogeneous crystallisation of CaCl2 throughout the stone produced greater internal change, with little dry weight loss recorded. NaCl formed distinctive isometric hopper crystals that caused damage through the non-equilibrium growth of salts in trapped regions of the stone. Damage was sustained as granular decay and contour scaling across most stone types. The pore network and hydric properties of the stones continually evolve in response to salt crystallisation, creating a dynamic system whereby the initial, known properties of clean quarried stone will not continually govern the processes of salt crystallisation, nor indeed can they continually predict the behaviour of stone to salt-induced decay.