The cell wall characteristics
The results of surface area, total pore volume and shrinkage of recent and archaeological oak wood dried under different conditions are presented in Table 1. It is evident that drying conditions correspond to wood shrinkage and thus, its porosity, regardless of the degree of wood degradation. The smallest surface area (SA) and the lowest total pore volume (TPV) were obtained with the most shrunken oven-dried wood, while less distorted samples dried at critical point had the highest SA and TPV values. A similar relationship between the different drying methods (resulting in shrinkage level due to collapse of micropores) and the surface area was observed, i.e. for eucalyptus wood flour by Kang et al. (2018), for pine wood flour and holocellulose by Papadopoulos et al. (2003), wood pulp by Weatherwax and Caulfield (1971) or spruce wood by Stone and Scallan (1965). The relationship between the degree of degradation and wood porosity is also visible, irrespective of the drying method. The lowest SA and TPV were obtained with air-dried sound recent oak. For comparison, with less dense oven-dried spruce wood, surface areas ranging between 0.6 and 0.8 m2 g−1 were determined using the same measurement technique (Hill and Papadopoulos 2001). For degraded wood, irrespective of the drying method, the values were only slightly higher for slightly degraded heartwood and significantly higher for severely decayed sapwood. The results are in line with the chemical and physical characteristics of the same archaeological oak that has previously been published (Broda et al. 2019b). It is important to stress that the results for oven-dried or air-dried waterlogged wood do not represent the real porosity of waterlogged wood due to its shrinkage. Only the data obtained with wood dried at the critical point can be considered as closest to the original waterlogged wood fully saturated with water.
More detailed characteristics of pore size and distribution in waterlogged archaeological wood are presented in Fig. 2. Generally, three pore size classes are distinguished: micropores (< 2 nm), mesopores (2–50 nm) and macropores (> 50 nm) (Ormondroyd et al. 2017b). However, it should be mentioned that the nitrogen absorption method does not give reliable results for pores with the diameter below 4 nm. Generally, in the method used, emptying of the pores depends on the diameter of the meniscus of the liquid nitrogen at the filled pore throat. The larger the diameter, the higher the pressure at which the pore will empty, which is determined using the Kelvin equation. In the case of a pore diameter less than 4 nm, nitrogen does not behave as a classical liquid and all the pores tend to empty spontaneously at the pressure P/Po = 0.4. In this case, the Kelvin equation is not valid anymore. Nevertheless, the method can still be used for making a comparison between the tested wood samples.
According to the above-mentioned classification, it is clear that in the tested wood, regardless of the drying method, the prevailing fractions are micropores and small mesopores typical for wood (Fig. 2a, b). However, in the case of sapwood, the presence of larger macropores is also visible, and a distinction in the pore distribution pattern and the cumulative pore volume between sapwood and heartwood can be noticed. This is the result of the different degree of degradation of the particular wood layers mentioned above. As shown in Fig. 3, cryo-SEM micrographs give an insight into the microstructure of hydrated waterlogged oak wood. From Fig. 3a, b, a significant difference in the decay extent between sapwood and heartwood is evident. The sapwood cell walls are very thin (Fig. 3a, c, e) and consist mainly of middle lamellae (that indicates an advanced decay by soft-rot and/or erosion bacteria) which are riddled with smaller and larger holes (with a diameter of about 2 µm and smaller) which will increase their porosity (Fig. 3e). Apart from that, the cell lumina are filled with insubstantial, unstructured networks of heavily degraded S2 debris, which may affect the surface area and porosity and also explain the presence of larger pores in the highly degraded sapwood structure. In comparison, the cell walls of heartwood are still thick and composed of all the layers (Fig. 3b, d, f). However, signs of moderate decay are clearly visible in the form of numerous cavities, crevasses and detachments within the heartwood cell walls, which explains the pattern of pore distribution shown in Fig. 2.
It is noticeable that the cumulative pore volume decreases with the shrinkage due to the stringent drying conditions (Fig. 2, Table 1), which is characteristic for lignocellulosic materials in general (Kang et al. 2018; Papadopoulos et al. 2003; Park et al. 2006; Stone and Scallan 1965). However, the different pattern of changes in cell wall pore size distribution between highly degraded sapwood and moderately decayed heartwood dried under various conditions can be observed. In heartwood, the volume of micro- and mesopores is just diminishing as the wood shrinkage increases. In sapwood, the reduction of the cell wall micro- and smallest micropores (< 10 nm) is also evident but, due to the collapse of the biggest macropores upon air- or oven-drying, a new fraction of mesopores appears and the amount of macropores significantly decreases (Fig. 2a, c). Considering the fact that the S2 layer debris are visible in cp- (Fig. 3c, e) and not in air-dried sapwood (US in Fig. 4) along with the patterns of pore distribution in cp- and air-dried samples, it could be suggested that these structures are related to the presence of macropores in highly decayed waterlogged sapwood.
By comparing the patterns of pore distribution and cumulative pore volume of recent and waterlogged wood (Figs. 5a and 6a), the difference resulting from wood degradation is clearly visible. With sound recent oak, the quantity of pores (mainly micro- and small mesopores) and cumulative pore volume in the cell walls is noticeably lower than with lightly degraded heartwood and significantly reduced in comparison with severely decayed sapwood, which is furthermore characterised by the presence of macropores.
As reported before, MTMS treatment of archaeological waterlogged oak resulted in its dimensional stabilisation (Broda et al. 2019b). This effect was particularly notable in the case of archaeological oak sapwood, where MTMS limited wood shrinkage almost completely (from 28% shrinkage seen for untreated wood) (Table 2). The stabilising effect is clearly visible in the macroscopic picture of sapwood samples (Fig. 1c), as well as in the SEM micrograph (Fig. 4, sample TS), where the cell walls are not collapsed and the cells retained their regular shape. In the case of waterlogged oak heartwood, shrinkage was reduced from about 7% (untreated wood) to about 3%. The difference between the silane anti-shrink efficiency for sapwood and heartwood resulted from its WPG (Table 2) that was dependent on the degree of wood degradation (thus wood density) (Broda et al. 2019b). The chemical modification by silane was also reflected in significant changes in the cell wall porosity when comparing the results of treated samples with untreated wood dried at critical point, as only this result represents real porosity of waterlogged wood (Table 1). In the case of heartwood, the surface area decreased more than nine times and the pore volume decreased about four times. For sapwood samples, SA decreased even more, nearly 32 times and TPV decreased by 44 times. The pore size and distribution patterns of treated and untreated heartwood are similar (Fig. 5b); however, the decrease in pore volume after treatment is discernible (Figs. 5b, 6b). In the case of sapwood, the difference is even more pronounced (Figs. 5c, 6b) as the silane treatment resulted in the significant reduction in the total volume of pores of all sizes making the pattern of pore size and distribution similar to that of the recent oak or treated waterlogged heartwood.
The observed reduction in surface area and total pore volume of treated waterlogged wood can be explained by a mechanism of wood modification by the silane. It has been shown that small MTMS molecules can penetrate and encrust the cell wall pores creating a uniform coating on the cell wall surface (Donath et al. 2004; Mai and Militz 2004). In the case of degraded wood, the encrustation seems particularly effective as the pores in its cell walls are larger. The results obtained indicate the veracity of this mechanism as reinforcement of the cell wall was observed indicated by the dimensional stabilisation and reduction of the surface area and total pore volume of the treated archaeological wood. It should be noted, however, that the effect of chemical modification on the cell wall was clearly visible despite the differences in shrinkage between untreated wood dried under various conditions (Table 1). Untreated wood shrank significantly upon oven- or air-drying (especially sapwood) due to the cell wall collapse caused by the internal forces of hydrogen bonding between polysaccharide components, which disrupted its porous nature thus reducing surface area and total pore volume. Therefore, the values of the measured surface area and the micropore network were smaller than the expected real values for waterlogged wood (drying at critical point allowed to maintain wood structure almost unchanged, and the porosity values obtained for these samples can be considered as real porosity of hydrated waterlogged wood). In the case of treated wood, the shrinkage was reduced and the spatial structure of the cell wall remained “developed” (similar to the original structure of fully waterlogged wood). Theoretically, when considering only the cell wall collapse and wood shrinkage without the coating effect of the silane, we would expect that the porosity of the treated and almost unshrunken samples should be similar to untreated wood dried at the critical point and higher than for untreated and shrunken air- or oven-dried ones. The observed opposite effect of porosity reduction in the treated wood clearly indicates that the silane was embedded in the pores and thus reduced the porosity and preserved the spatial structure of the cell wall.
Figure 7 presents the sorption isotherms reported as EMC against RH and the absolute hysteresis for the heartwood and sapwood of the air-dried degraded waterlogged oak wood and for sound recent oak. Although for the detailed moisture sorption characteristics two adsorption/desorption cycles are usually recorded, it was decided to focus on the single cycle, as the aim of this research was to compare the moisture properties of sound and degraded wood treated and untreated with silane. For sound recent oak, the maximum EMC of 21.8% was at 92.5% RH. The adsorption and desorption isotherms of archaeological oak sapwood and heartwood almost coincide, with the maximum EMC of about 23% at around 88% RH.
The observed increase in moisture sorption of degraded wood is in line with the data published by, for example, Hoffmann (1986), Schniewind (1990), Ljungdahl and Berglund (2007), Esteban et al. (2009, 2010) or Guo et al. (2018). They observed increased hygroscopicity of decayed archaeological wood of different species, even though it might be expected that degradation of the most hydrophilic wood hemicelluloses should reduce hygroscopicity due to preferential removal of the amorphous polysaccharide component, which dominates the hygroscopic behaviour. Hoffmann (1986) explains this phenomenon by an increased porosity of the degraded cell wall resulting in greater capillary condensation (and more pronounced capillary forces for water absorption). This is consistent with the results of the present research as higher porosity of the cell wall of archaeological wood (regardless of the drying method and resulting shrinkage) was observed in comparison with recent wood (discussed above). As wood is a dynamic/swelling material, the observed phenomenon might also be a result of a reduction in mechanical resistance of the degraded cell walls due to the internal swelling pressure exerted by the sorbed water molecules. However, there are a number of alternative hypotheses. For example, Esteban et al. (2009, 2010) and Guo et al. (2018) explain increased hygroscopicity of waterlogged wood by changes within the polysaccharides. They showed that, although the process of decomposition of archaeological wood caused degradation of hemicelluloses, it also decreased the crystallinity index of the cellulose and the length of the crystalline regions, which resulted in the emergence of the new sorption sites. Thus, the equilibrium moisture content and the hysteresis observed were higher than for recent wood. Furthermore, according to Ljungdahl and Berglund (2007), the oxidation of chemical groups in wood polymers during degradation also increases hygroscopicity. This observation agrees with the results of FT-IR and XRD analyses for the same oak wood by Broda and Popescu (2019). It was shown that archaeological oak sapwood was characterised by high degradation of polysaccharides and low crystallinity level, while oak heartwood only displayed slight decay of polysaccharides with a crystallinity similar to recent oak. Additionally, different moisture properties between sound and degraded wood could result from the different content of extractives. Skaar (1984) concluded that hygroscopicity of wood with high extractive content is generally lower than this without extractives and in the case of one-thousand-year old waterlogged oak, a very low extractives concentration could be expected. The sorption isotherm is also dependent on the mechanical properties of the cell wall—the weaker (more degraded) the cell wall is, the more water it absorbs, since the equilibrium is affected by a balance of the swelling pressure of the absorbed water and the ability of the cell wall to resist this pressure. Brown rot fungal decay has been shown to sequentially degrade hemicelluloses during initial stages of decay, leading to considerable strength loss of wood at the macroscale (Curling et al. 2002). It makes logical sense that any degradation of the hemicelluloses would have a similar effect on the strength of the cell wall. In the case of highly degraded wood, it is possible that the hygroscopicity is determined by more than one of these effects.
It is interesting that although the degree of degradation and porosity of archaeological sapwood is higher than of heartwood, their sorption isotherms are similar. The possible reason is that the high shrinkage of the highly degraded sapwood decreased its surface area, thus, decreasing its potential hygroscopicity to the level similar to less degraded heartwood.
The absolute hysteresis of air-dried degraded waterlogged oak wood and sound recent oak presented in Fig. 7b is relatively similar, but the peak in the degraded sapwood is offset to a lower moisture content. On looking at the sorption section of the curve (Fig. 7a), UH sorbs more readily on initial increase in relative humidity than US, while desorption occurs at a similar rate throughout. This may indicate that the sapwood sorption sites, which closed due to hydrogen bonding between degraded internal surfaces, are slower to reopen in the presence of moisture vapour than the less degraded heartwood.
It is clear from Fig. 8a, c that MTMS treatment of archaeological oak wood resulted in a notable decrease in hygroscopicity of the newly formed wood/silane entity as a whole, compared to an untreated sample. The maximum EMC for heartwood decreased from 24% for untreated to 19% for treated wood. In the case of sapwood, the maximum EMC decreased even more significantly from 23 to 10%. This could be explained by the reduction in the surface area and total pore volume of the treated wood as discussed above (a bulking mechanism that reduces room for water molecules) or a chemical modification of wood by the applied silane that potentially could react with wood hydroxyls reducing the number of sorption sites. However, when considering chemically modified wood it is vital to take into account the presence of the applied chemical agent. Therefore, the isotherms were corrected for the weight of silane inside the wood structure (Fig. 9a, c). These isotherms reflect the moisture properties of the modified wood component itself. From this, it can be seen that for heartwood, the difference in maximum EMC between modified and unmodified samples is not significant. In the case of sapwood, the difference in maximum EMC is slightly greater as it decreased from 24 to 21%. Such small differences between maximum EMC of untreated and treated wood point rather at a bulking mechanism than chemical modification by silane.
The difference in sorption behaviour between treated archaeological sapwood and heartwood could be explained by the amount of silane accumulated in wood during the impregnation process. As WPG of the sapwood was about 111% in comparison with 21% for heartwood (Table 2), the lower hygroscopicity of the former is not surprising.
The impregnation also resulted in a reduction in hysteresis between treated and untreated wood (Fig. 8b, d). It was particularly notable for archaeological oak sapwood as a result of higher level of silane absorption in comparison with heartwood. However, looking at the plots when silane content is taken into account (Fig. 9b, d) then it can be seen that the difference in levels of hysteresis is reduced. However, irrespective of whether silane weight is used, a marked shift in the humidity at which the peak hysteresis occurs can be observed with treated wood (Figs. 8b, d and 9b, d). This shift moves the hysteresis plot so that it appears similar to that for the undegraded recent wood, with peak hysteresis seen at approximately 60% RH rather than at 70 to 80% RH for the untreated wood. It can result from an increased rigidity of the cell wall due to the silane presence (Hill et al. 2012a, b; Keating et al. 2013) as it was shown that silane treatment improves mechanical properties of fibre and wood composites (Bengtsson and Oksman 2006; Bisanda and Ansell 1991; Hafezi et al. 2016).
Previous DVS experiments revealed a chemical instability of the MTMS-waterlogged elm wood system (Broda et al. 2018). During the experiment, the mass of the MTMS-treated sample systematically decreased below the initial dry mass during the desorption stage. This effect was explained by gel ageing and the resulting mass loss of evaporating by-products of the sol–gel process, which takes place upon alkoxysilane polymerisation, and the effect was neutralised only after 12 cyclic sorption stages. The present experiment did not demonstrate such instability of the silane. It can be explained by the lower concentration of MTMS in treated oak in comparison with treated elm (WPG of 111% and 21% for oak sapwood and heartwood and 242% for elm) and the resulting full alkoxysilane polymerisation. This observation shows that an additional stage of wood seasoning to stabilise the MTMS inside its structure is required only in the case of particularly high silane absorption (which takes place when wood is highly degraded and permeable), which seems to be important from the wood conservation perspective.