Abstract
Changes were evaluated in cell wall components, hygroscopicity, and thermodynamic properties of Pinus sylvestris L. wood that had been in contact with salt (NaCl) for 300 years in a former saltworks storehouse in Gerri de la Sal (Lérida, Spain), comparing it with recently felled wood (new wood) of the same species. The wood in contact with salt (salt-covered wood) had higher equilibrium moisture content values except in the first interval of the adsorption isotherm, probably because of mechanical blocking of polar sites by salt crystals. The high fibre saturation point values of this wood are caused by the salt dissolved in water above water activity of about 0.75. More energy is invested in the desorption process, and the values are higher in the salt-covered wood.
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Changes in Cell Wall Components of Pinus sylvestris L. Wood after 300 Years in Contact with Salt (NaCl)
Alberto García-Iruela,a,* Luis García Esteban,a Paloma de Palacios,a Francisco García Fernández,a Raquel Martín-Sampedro,b and María E. Eugenio b
Changes were evaluated in cell wall components, hygroscopicity, and thermodynamic properties of Pinus sylvestris L. wood that had been in contact with salt (NaCl) for 300 years in a former saltworks storehouse in Gerri de la Sal (Lérida, Spain), comparing it with recently felled wood (new wood) of the same species. The wood in contact with salt (salt-covered wood) had higher equilibrium moisture content values except in the first interval of the adsorption isotherm, probably because of mechanical blocking of polar sites by salt crystals. The high fibre saturation point values of this wood are caused by the salt dissolved in water above water activity of about 0.75. More energy is invested in the desorption process, and the values are higher in the salt-covered wood.
Keywords: Dynamic vapour sorption; Hygroscopicity; Isotherms; Thermodynamic properties; Sorption; Impregnated wood
Contact information: a: Departamento de Sistemas y Recursos Naturales, Cátedra de Tecnología de la Madera, Escuela Técnica Superior de Ingeniería de Montes, Forestal y del Medio Natural, Universidad Politécnica de Madrid, Ciudad Universitaria, 28040, Madrid (Spain); b: Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria, INIA, Carretera de la Coruña, km 7.5, 28040, Madrid (Spain); *Corresponding author: agiruela17@gmail.com
INTRODUCTION
Over time and in certain ambient conditions, structural wood can be subjected to the action of biotic agents (bacteria, algae, fungi, lichens, insects, molluscs and crustaceans) or abiotic agents – whether chemical (acids, alkalis, oxygen, ozone, salts, aerosols, pollution gases) or physico-chemical (solar radiation, nuclear radiation, thermal radiation or fire) – and physico-mechanical factors (low temperatures, physical action of water, cyclic moisture, sustained loads, periodic loads, impact loads, rubbing) (Wazny 1993). These agents can degrade the lignocellulosic complex in the cell wall, mainly the polysaccharides (Fengel 1991), which in turn modify the hygroscopic response and thermodynamics of wood. Studying the hygroscopic behaviour of naturally or artificially aged wood gives a better understanding of the water sorption mechanisms of wood. If the wood has been exposed to unusual conditions or situations that are not easily reproduced in controlled experiments, the results provide very valuable information about sorption by wood.
The combined action of degradation in archaeological wood due to simultaneous action of abiotic and biotic factors is very difficult to assess separately, because both types of factors can degrade the same cell wall components. For example, tunnelling bacteria and some white-rot species preferably attack lignin, eroding all cell walls and degrading the middle lamella (Blanchette 2000). Abiotically, prolonged acidic hydrolysis in the absence of microorganisms can modify not only carbohydrates, but also residual lignin (Blanchette et al. 1991).
In this instance, the action of a chemical abiotic agent (sodium chloride) and the possible presence of a biotic attack when the structure was first erected allow prolonged wood-salt interaction to be studied.
Using the sorption isotherms is a good method to determine the hygroscopic behaviour of wood. Popescu and Hill (2013) studied the effect of ageing in historic Tilia cordata Mill. wood exposed for 110, 160, 170, and 240 years and found that there was no simple correlation between exposure time and the sorption behaviour of wood. The oldest sample showed similar behaviour to that of new wood of the same species used as a control. Other studies reported different behaviour in recently felled wood and samples of the same species in varying conditions and exposure times. Esteban et al. (2006) and Esteban et al. (2008a) showed that the equilibrium moisture content (EMC) of 18th century Pinus sylvestris L. wood subjected to non-aggressive exposure in the beamwork of Aranjuez Palace (Spain) was higher than the EMC of recently felled wood. Esteban et al. (2009) studied another sample of P. sylvestris wood that had been buried and was more than 1000 years old, and Esteban et al. (2010) studied buried Quercus spp. wood aged almost 6000 years in the Ría Villaviciosa area, a marine environment. In the latter two studies, the adsorption-desorption isotherms of the old wood were above those of recently felled wood of the same species, mainly due to the increase in active –OH sites. Simon et al. (2017a) demonstrated polysaccharide degradation due to high temperatures and the high percentage of extractives in Pinus canariensis C. Sm. ex D.C. wood buried in pyroclastic ash after an eruption about 3,000 years ago, resulting in lower EMC in the wood buried in ash than in recently felled wood of the same species.
Few studies have been conducted on the behaviour of wood treated with salt (NaCl). Loughborough in (Kollmann 1959) reported that sodium chloride causes higher EMC and higher fibre saturation points; Lesar et al. (2009) characterised Picea abies Karst. wood artificially impregnated with salt; and Petr and Dejmal (2014) studied the effects of various salt concentrations on Populus alba L. No studies were found on wood naturally impregnated with salt.
Using wood that has been in contact with salt for 300 years enables the study of a material subjected to unique conditions rather than the artificial conditions of typical laboratory procedures or industrial impregnation. It provides a unique opportunity to compare such wood with recent samples of the same species to determine possible chemical changes in the cell wall, variations in the cellulose crystallinity indices, changes in the hygroscopic response, and any differences in the thermodynamic parameters associated with the sorption process.
Because the wood had spent a long time in contact with salt, under cover, and at reasonably constant relative humidity (RH) and temperature, the samples have high scientific value.
The possible biotic and abiotic ageing of the wood, combined with the presence of salt and its osmotic properties, must have influenced the cell wall components, and therefore the wood can be presumed to have different hygroscopic behaviour as a result.
This study analyses the possible changes in the cell wall components and relates them to the hygroscopic and thermodynamic behaviour of recently felled Pinus sylvestris wood and wood of the same species that had been in contact with salt in the salt storehouse in Gerri de la Sal (Lérida, Spain) for 300 years.
EXPERIMENTAL
Materials and Methods
Salt-covered wood
Samples were obtained from three beams forming part of the wooden structure of the Real Alfolí building, in Gerri de la Sal, on the banks of the Noguera Pallaresa river, province of Lérida (Spain). This building is the largest non-religious construction in the Pallars-Sobirá region, and for 300 years it was used to store salt from the saltworks in Gerri.
Salt extraction began to decline in the 1960s, and the 1982 flood in Noguera Pallaresa brought an end to the saltworks and salt storage. However, the entire wood structure still has salt adhered to it. The salt-covered wood has been preserved as testimony to the history of the building, which was designated as a National Heritage Asset (Fig. 1) in 1995 and now houses the saltworks museum. The building has a square ground plan with sides of around 25 metres, rubblework walls and a wooden structure. Documents held by the town hall show that the wood used in its construction was pine from the local area, and except for the recent addition of a staircase, the wood is all original. Two beams removed to install a staircase were calculated to be more than 65 years old from the growth ring count. Following habitual practice among carpenters of the time, only pieces from the centre of the tree were used for the wooden structures, giving an idea of the age of the timber used in the construction.