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Article

Changes in Moisture Characteristics of Waterlogged Archaeological Wood Owing to Microbial Degradation

by
Huimin Cao
1,2,
Xin Gao
1,*,
Jiabao Chen
1,
Guanglan Xi
3,
Yafang Yin
1 and
Juan Guo
1
1
Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China
2
College of Material Science and Technology, Beijing Forestry University, Beijing 100083, China
3
National Center for Archaeology, Beijing 100013, China
*
Author to whom correspondence should be addressed.
Forests 2023, 14(1), 9; https://doi.org/10.3390/f14010009
Submission received: 25 November 2022 / Revised: 15 December 2022 / Accepted: 18 December 2022 / Published: 21 December 2022
(This article belongs to the Special Issue Wood as Cultural Heritage Material)
Browse Figures
Versions Notes

Abstract

:
Studying waterlogged archaeological wood moisture characteristics can provide strong support for the safe dehydration and preservation of waterlogged wooden artifacts. Herein, six waterlogged archaeological wood samples with moisture contents of 154%–968% chosen from two ancient Chinese shipwrecks, Nanhai No. 1 and Changjiangkou No. 2, and six non-degraded modern wood samples of the relevant corresponding species were selected to study the moisture characteristics by low-temperature nuclear magnetic resonance (NMR) and the dynamic sorption of water vapor (DVS). It was found that the six waterlogged archaeological wood samples exhibited three deterioration states: slightly, moderately, and seriously deteriorated. Wood deterioration caused significantly increased fiber saturation point (FSP) values for waterlogged archaeological wood. This is mainly owing to changes in the pore size distribution of cell walls. Moreover, NMR is more accurate than DVS in obtaining FSP for severely degraded samples. Additionally, moisture content was positively associated with wood deterioration. Maximum water content (MWC), free water content, and bound water content exhibited an exponential relationship with the basic density (BD). The contribution of bound water in waterlogged archaeological wood was less than that of modern wood and decreased with deterioration.

1. Introduction

Wood is one of the most common biomass materials and is widely used in handicrafts, furniture, building, and ship manufacturing [1,2,3,4]. Therefore, a lot of wooden artifacts have been excavated around the world; some of them have been excavated from underwater (i.e., rivers, lakes, and seas). Compared with wood products in other environments, wood products unearthed underwater are saturated with water (or seawater). Some wood products have even been submerged in water for centuries, e.g., the shipwrecks of the Roman ship San Rossore, the warship Vasa in Stockholm, Sweden, the Mary Rose in Portsmouth, UK, the Bremen Cog in Germany, and the Nanhai No. 1 in China [5,6,7,8,9]. These recovered artifacts reflect the level of scientific and technological productivity under their specific historical periods and are thus precious resources.
The most common form of microbial attack in waterlogged environments is erosion bacteria [10,11]. Erosion bacteria enter the cell lumen through wood rays and pits, starting from the S3 layer to digest cellulose and hemicellulose in the cell wall, then attacking the S2 layer. In the late stage of decay, lignin residues are left in the S2 layer, while the lignin-rich composite intermediate layer remains intact [12]. Waterlogged wood retains its original appearance after being attacked by bacteria, but has significantly reduced physical and mechanical properties and even becomes very soft and easily deformed or collapses when dry [13]. Since drying waterlogged archaeological woodwork is a necessary condition for museumization [14], this poses a significant challenge for protecting waterlogged archaeological wood from dehydration. The effective and safe removal of moisture is the key and prerequisite for wooden cultural relics to be separated from degradation environments, for cultural information extraction, and storage environment preservation. The conservation treatment of waterlogged archaeological wood is not complete until it has been dried to remove any residual moisture, a prerequisite for the collection [15].
The moisture characteristics of waterlogged archaeological wood are fundamental to the development of dehydration preservation technologies. Degradation of the waterlogged archaeological wood results in a large number of pores in the secondary wall, reducing the strength of the cell wall [12,16]. The resulting pores cause more water to enter the wood [5,10,17]. Until now, waterlogged archaeological wood has been studied quite extensively and comprehensively and has mainly focused on deterioration state assessment [18,19,20], microbial reactions [21,22,23], sediment reactions [24,25,26], chemical degradation [27,28,29], physical degradation [18,30], sorption behaviors [31,32], and the synthesis and characterization of fixatives [33,34,35,36,37]. However, little information has been reported on the moisture characteristics of waterlogged archaeological wood.
However, understanding the moisture characteristics of waterlogged archaeological wood is essential for its conservation. During the post excavation phase of storage and restoration, the presence of oxygen makes waterlogged archaeological wood extremely vulnerable to biodegradation hazards [38]. Therefore, waterlogged archaeological wood must be protected as soon as it is unearthed. However, reinforcement and proper drying procedures must be implemented to prevent irreparable collapse and damage [39]. Hence, knowing the moisture characteristics of waterlogged archaeological wood can help in choosing the appropriate drying scheme.
It is generally accepted that water in wood exists in two basic forms, bound water in the cell wall and free water in the lumen. The fiber saturation point (FSP) is defined as the water content of the cell wall when it is saturated with bound water and without free water in the inner lumen [40]. FSP values correspond to a turning point in many properties of wood, including dimensional changes [41,42]. For non-degraded wood, below the FSP, reductions in moisture content led to significant changes in wood dimensions [43,44]. For waterlogged archaeological wood, this change may be more severe. However, FSP may differ depending on the deterioration states of waterlogged archaeological wood. Unknown FSP values impede further understanding of the moisture characteristics of waterlogged archaeological wood, because they correspond to the boundary between free water and bound water.
For non-degraded wood, the FSP is determined by extrapolating the measured adsorption isotherm to 100% equilibrium moisture content (EMC), and the extrapolation method thus obtains an FSP of about 30% [45]. In contrast, the FSP obtained by solvent drainage, pressure plate application, differential calorimetry, centrifugal dehydration, and nuclear magnetic resonance (NMR) methods can reach about 40% [46]. This discrepancy is because the wood specimen, after equilibrium treatment at 100% relative humidity (RH), will still swell further after immersion in water [42]. Therefore, the bound water content determined by extrapolation is lower than the bound water content in the swollen cell wall [47,48]. This difference may be tremendous for waterlogged archaeological wood owing to the decrease in cell wall strength. Therefore, the present research utilized two types of methods to determine the FSP of waterlogged archaeological wood.
As a type of non-destructive and non-invasive analysis technique, TD-NMR has been widely used to study the relationship between wood and water, such as determination of moisture content [49], qualitative analysis of bound water in cell walls and free water in lumens [50], determination of FSP of wood [51], and determination of cell wall pore size distribution [48]. Additionally, 2D NMR and magnetic resonance imaging have been used to study moisture transfer within wood [52,53]. Recent studies have also shown the possibility of using single-sided NMR to analyze waterlogged archaeological wood [54]. Since seriously deteriorated waterlogged archaeological wood is saturated and soft, it cannot thus be allowed to be dried and stressed. Comparatively, other methods are unavoidably practically destructive and invasive or require drying of the sample, such as mercury intrusion, gas adsorption (N2, or CO2), a scanning electron microscope (SEM), and atomic force microscopy (AFM) [55,56,57,58]. Although, supercritical drying or freeze-drying can better maintain the original pore structure of wood. However, the removal of water will still cause the change of pore structure. [59]. Thus, they are not conducive to the analysis of the pore structure of waterlogged archaeological wood in a water-saturated state.
Herein, waterlogged archaeological wood in different states of deterioration were selected as research objects. First, the state of degradation of waterlogged archaeological wood was assessed, and then the FSP of waterlogged archaeological wood was determined by the time-domain NMR technique and dynamic sorption of water vapor (DVS) adsorption isotherm method. NMR is used to determine the actual moisture characteristics in the saturated state. DVS is used to determine moisture characteristics after slow drying and after moisture absorption. Moreover, we analyzed the relationship between the content of different types of moisture in waterlogged archaeological wood and the basic density (BD) as well as the contribution of bound and free water to maximum water content (MWC). Finally, based on the differences in cell wall pore size distributions, we explain the reasons for the change in water characteristics of waterlogged archaeological wood. This work is valuable for understanding the moisture characteristics of waterlogged archaeological wood.

2. Materials and Methods

2.1. Samples

The Nanhai No. 1 shipwreck is one of the oldest, largest, and most well-preserved shipwrecks in the world. It is a wooden merchant ship in the early Southern Song Dynasty, with a history of more than 800 years, and it was discovered in 1987 in the South China Sea of Yangjiang, Guangdong Province. It provides a typical specimen for the study of ancient Chinese shipbuilding, navigation technology, and the history of the Maritime Silk Road [60]. The Changjiangkou No. 2 shipwreck is a shipwreck from the middle and late Qing Dynasty and was discovered in the estuary of the Yangtze River. The waterlogged wood used in this study is part of the hull material that was salvaged in the early archaeological work. The salvage and archaeological work will officially start in 2022, and the preliminary study can provide a reference for subsequent related work. Five samples and one sample of waterlogged archaeological wood were taken from the Nanhai No. 1 shipwreck and the Changjiangkou No. 2 shipwreck, respectively. Modern wood samples corresponding to the genera of waterlogged archaeological wood samples were obtained from the specimens of the Wood Collection of the Chinese Academy of Forestry (Beijing, China). A previous study had identified species information for waterlogged archaeological wood [61]. Images of the original states of the six waterlogged archaeological wood samples are shown in Figure 1. Information on waterlogged archaeological wood samples and the corresponding modern samples wood is listed in Table 1. Before the experiments, all the waterlogged archaeological wood samples were stored in their saturated state before the experiment. The modern wood samples were saturated with distilled water by a vacuum-pressure saturation device (Jiangsu Huaxing Scientific Instruments Company, Nantong, China).
Before the experiment, three 15 mm blocks were cut from the waterlogged archaeological wood and modern wood with a single-sided blade, respectively. The block sample was then cut into cylinders with a diameter of about 8 mm. In this paper, the values with error bars are the average values of three experimental tests and calculations.

2.2. The Maximum Water Content and Basic Density

Tests on the masses and volume of the moist wood were performed before the NMR test. After the NMR test, the samples were dried at 103 ± 2 °C to obtain the mass of dry wood.
The MWC can be calculated according to Formula (1).
MWC = 100 × ( m w − m d ) m d
Here, mw and md are moist wood and dry wood masses, respectively.
The BD can be calculated according to Formula (2).
BD = m d v w
Here, vw is the volume of the moist wood determined by the immersion method [16].

2.3. Micromorphology Characteristics

In order to obtain the microscopic morphology of the waterlogged archaeological wood to determine the degree and type of degradation, the pairs were examined using SEM and optical microscopy. Cross-sections of waterlogged archaeological wood and modern wood were obtained using slicers. SEM and microscopic morphology observation use samples taken close to NMR sampling sites to minimize variability. Freeze-drying was used to remove the moisture in the SEM samples while maintaining the original morphology of the samples. Then, the samples’ anatomical structure was analyzed using a scanning electron microscope (Gemini 300; ZEISS, Oberkochen, Germany). Prior to SEM examination, the samples were sputter-coated with an ultra-thin layer of platinum before imaging. Waterlogged archaeological wood slices with a thickness of 10 μm were obtained using a microtome, and the microstructure of the samples was observed with an optical microscope (BX51, Olympus, Tokyo, Japan).

2.4. Nuclear Magnetic Resonance Test

The NMR experiments were performed on a 23 MHz mini-NMR instrument (NMRC-010V, Niumag Instruments, Suzhou, China). The sample chamber of the NMR instrument was equipped with temperature control accessories. The temperature in the sample chamber can be adjusted from −45 °C to 30 °C with an accuracy of 0.1 °C. A fiber-optic temperature sensor was placed in the sample tube to ensure that the sample reached the set temperature. In this experiment, the sample was fist frozen to −40 °C. The temperature of the sample is ensured to be stable through a fiber-optic temperature sensor and a software program. When the test temperature is low (below −10 °C), a test temperature is set at every five degrees, and when the temperature is higher than −10 °C, a test point is set at every one degree. It takes about 1 h to reach the lowest temperature point (−40 °C) from room temperature, and then the temperature control program starts to heat up according to the set temperature. Whether the temperature is stable or not is judged by the program according to the change of the temperature. The temperature fluctuation of the temperature sensor is set to 0.2 °C, and the temperature control program conducts a temperature inspection for one minute for a total of 20 inspections. After reaching the set temperature, the temperature control program continues to maintain a constant temperature for 30 min.
The T2 relaxation time of the sample was obtained using the CPMG sequence (3) at each temperature [62]. In order to eliminate the influence of temperature on NMR signal amplitude, the signal amplitude was multiplied by the coefficient T/T273, and T273 was the reference temperature selected in this study.
{ [ 90 ° − τ − 180 ° − 2 τ ] n − r t } m
where rt is the time delay, and time delay was 10 s. The echo time was 2τ, τ = 0.05 ms. n was the total number of the echo time, and n = 15,000. m was the number of accumulated scans, and m = 64.
The T2 relaxation time of ice amounts only to 6 μs [51]. The T2 relaxation time of solid wood decays rapidly to zero in tens of microseconds [63]. The echo time in the sequence was 0.1 ms; the macromolecular as well as frozen water (ice) with T2 < 0.1 ms were not detectable in these T2 measurements.

2.4.1. Fiber Saturation Point Calculation

In this study, FSP is defined as the water content at which the cell wall is saturated with bound water with no free water in the lumens [64]. The critical temperature is the temperature at which the water in the lumen is frozen, while the water in the cell wall remains liquid. Unfrozen water below the critical temperature is defined as bound water, and freezing water is defined as free water. At the critical temperature, the free water is frozen, and the NMR signal originates from the bound water of the cell wall, so the FSP can be calculated according to Equation (4). The FSP was calculated based on the critical temperature calculation and the NMR signal intensity at room temperature [51].
FSP = M M C · S c S n
where MWC is the maximum water content, Sc is the NMR signal amplitude of the sample at the critical temperature during the NMR experiment, and Sn is the signal amplitude measured at room temperature.

2.4.2. Pore Size Distribution

The relationship between pore diameter information and NMR signal amplitude was determined by the Gibbs–Thomson Equation (5). According to the Gibbs–Thomson equation, the melting point of water in nanopores is significantly different from that of water in micropores, ΔTm is inversely proportional to the diameter (D) of the pore. Therefore, the freezing of the bound water in different pore size can be realized step by step by controlling the temperature of the sample chamber. Similarly, different pore size proportions can be obtained according to NMR signals.
Δ T m = T m − T m ( D ) = 4 σ T m cos θ D Δ H f ρ
where Tm is the melting point of macroscopic water, Tm (D) is the melting point of pore water with d diameter, ΔTm is the difference between the melting point of pore water and macroscopic water, σ is the free energy, θ is the contact angle, Hf is the specific heat of fusion of water, and ρ is the density of water [65]. The above values were set to Tm = 273.15 k, σ = 12.1 mJm−2, θ = 180 °, Hf = 333.6 Jg−1, ρ = 1.0 × 103 kgm−3 [66]. Considering that the layer of water molecules directly in contact with the wood substrate has a special structure and is non-freezing, it is necessary to increase the thickness of two water molecules (0.6 nm) based on the relationship between temperature and pore size [53,67,68], then the relationship between temperature drops in melting point and pore diameter shown in Table 2.
The relationship between the pore size and the melting temperature of liquid in pore size is established by Gibbs–Thomson equation. Therefore, in NMR experiments, the control of temperature is the division of aperture size. The pore diameter distribution (PDD) proportion within a certain range can be determined by Equation (6).
P D D = S a − S b S c
where Sa, Sb, and Sc are the NMR signal amplitude of the sample at a temperature a, b, and critical temperature, respectively. Sn represents the total number of pores in the cell wall. The total number of pores in the cell wall in any range was (Sa − Sb)/Sc. In this study, 243 K is the lowest temperature, the corresponding pore size is 1.9 nm, and the proportion of pore size less than 1.9 nm is S243K/Sc.

2.5. Dynamic Sorption of Water Vapor

The sampling position of the sample for DVS analysis was adjacent to NMR samples. A blade was used to obtain 4 × 4 × 1 mm flakes of 10–20 mg. The EMC of the samples at 20 °C and different RH values were determined (DVS Advantage ET85, Surface Measurement Systems Ltd., Wembley, UK). The samples were initially dried at a partial water vapor pressure of p/p0 = 0.95 for 2000 min to ensure complete removal of free water and they were then exposed to descending p/p0 steps ranging from 0.95 to 0 for desorption and subsequently ascending in the same manner for absorption. RH considered: 0.95, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95. The equilibrium at each step was defined by the mass change per time (dm/dt) being less than 0.0005%/min per step.
The Guggenheim–Anderson–de Boer (GAB) sorption Equation (7) was used to simulate wood isothermal adsorption. Origin 2019 (OriginLab Corporation, Northampton, MA, USA) was used to fit the adsorption isotherm by least squares regression. The measured adsorption isotherm was extrapolated to 100% RH to determine the FSP [69].
G A B = V m C × K × R H ( 1 − K × R H ) ( 1 − K × R H + C × K × R H )
where GAB is the volume of water vapor adsorbed per unit mass of absolute dry wood after a certain temperature and humidity environment is balanced (cm3·g−1), and Vm is the adsorption water content of monolayer molecules (cm3·g−1), C is the constant related to the difference between the free enthalpy of monolayer water and multilayer water, and K is the constant related to the difference between the free enthalpy of multilayer water and pure water. RH is the relative humidity of the isotherm adsorption process.

2.6. Statistical Analysis

FSP determined by NMR and DVS significant differences were analyzed by statistical analysis with Microsoft Excel (Microsoft, Redmond, WA, USA). Differences between the FSP values determined by MNR and DVS were analyzed by t-test, and significant differences were assessed at a threshold of p < 0.05.

3. Results

3.1. The Deterioration States of Waterlogged Archaeological Wood

In this study, referring to De Jong’s criteria (Table 3) [70], combining SEM images and microscope images, the state of deterioration of archaeological wood is classified into three grades. The MWC of less than 225% is classified as mildly degraded, the MWC between 225% and 350% is classified as moderately degraded, and the MMC greater than 500% is classified as severely degraded.
The MWC of the six waterlogged archaeological wood samples and the corresponding modern wood samples are shown in Figure 2. The MWC values of the modern and archeological wood samples ranged from 101% to 231% and 154% to 968%, respectively. The SEM images of the six modern wood and six waterlogged archaeological wood are shown in Figure 3 and Figure 4, respectively. It can be seen from the figures that the cell walls of modern wood are well preserved, while the cell walls of waterlogged archaeological wood have been degraded to varying degrees. In addition, optical microscope pictures are given in Figure 5.
The MWC values of A3 (Chinese fir) and A6 (teak) samples were 154% and 206%, respectively, lower than 225% in De Jong’s criteria. Moreover, the cell walls showed slight degradation (Figure 4c,f). Therefore, A3 and A6 samples were classified as slightly deteriorated. As can be seen from the microscope images, A3 showed almost no decaying cells, but A6 showed signs of microbial degradation (Figure 5c,f). The MWC value of the A1 (pine) sample was 447%, between 350 and 500% in De Jong’s criteria. The MWC reached as much as four times that of modern wood. Sample A1 showed microbial degradation in the SEM image, with the secondary walls being porous and separated from the intercellular layer (Figure 4a). Invasion by fungi and bacteria was evident from the microscope images, and the cell walls were granular (Figure 5a). Therefore, A1 was classified as moderately deteriorated. The A2 (Sweetgum), A4 (Persimmon), and A5 (Babian) samples have MWC values ranging from 709% to 968%, which is higher than 500% in De Jong’s criteria; the MWC reaches five to nine times that of modern wood. These samples show higher degrees of deterioration based on the SEM images compared to A1. Pores appear in the secondary wall and separate from the intercellular layer (Figure 4b,d). The secondary wall may disappear in more advanced stages of deterioration (Figure 4e). Bacterial and fungal attack was evident and widespread in the severely degraded samples by microscopy images (Figure 5b,d,e). In addition, microbial degradation products were found in the ray cell cavity in A2, the cell wall of A4 is granular and amorphous, and the disappearance of the S2 layer was found in A5. Therefore, A2, A4, and A5 samples were determined to be seriously deteriorated.

3.2. FSP Determined by NMR and DVS

In this study, Sc is the total integrals measured at 271 K (−2 °C); Sn is the total integrals measured at 283 K (10 °C). The critical temperature is determined by the change of NMR signal at different temperatures; when the MNR signal increases dramatically it means free water is present. Changes in NMR signals and NMR signals differentiation across different temperatures of waterlogged archaeological wood (A1–6) and the modern wood (M1–6) are shown in Figure S1. At the temperature of 270–271 K, the NMR signal amplitude increases greatly. Combined with Gibbs–Thomson equation, the melting point drop is inversely proportional to the pore size, and wood extractives dissolved in water may reduce the melting point, which is generally 0.1–2 K lower than that of water [66,71]. Hence, the critical temperature of this study is 271 K.
The FSP values of waterlogged archaeological wood and the corresponding modern wood are shown in Figure 6. The FSP of modern wood ranges from 30% to 33%, close to the traditionally assumed value of 30% [42], while the FSP of the waterlogged archaeological wood ranges from 38% to 110%, which is significantly higher than that of modern wood. For example, in the seriously deteriorated A2, A4, and A5 samples, the FSP even reached three times that of modern wood. In addition, the magnitude of the FSP value is associated with the deterioration states. The FSP values were 38% and 45% for slightly deteriorated waterlogged archaeological wood samples, 69% for moderately deteriorated samples, and over 100% for seriously deteriorated samples. The significant difference in the FSP between waterlogged archaeological wood and modern wood can be attributed to the deterioration caused by microorganisms. Wood-degrading bacteria can degrade the cell wall, causing cellulose and hemicellulose degradation, thereby creating new pores [5,10,17]. These pores allow more water to enter, causing MWC and FSP to increase. It can also be seen from the SEM images that as the degree of deterioration increased, the pores appearing on the secondary wall increase substantially. Although these pores are at the micron level, their moisture content should be attributed to free water. It can be inferred that these micron-level pores are developed gradually from nanoscale pores. Because erosion bacteria attack cellulose and hemicellulose and produce degradation products, lignin is retained [12]. In addition, FSP increased with the deterioration state, which is due to the different severity of microbial attack on wood. In addition, wood deterioration caused by microorganisms dramatically reduces the strength of the cell wall and causes a reduction in the cell wall’s resistance to swelling caused by water [37].
The adsorption isotherms were measured by DVS and fitted with the GAB model. The results of the isothermal adsorption line fit for all waterlogged archaeological wood samples. The fitting results are shown in Figure S2. The coefficient of determination (R2) was greater than 0.99, indicating that the GAB equation accurately described the relationship between the RH and EMC of different waterlogged archaeological wood samples. The values of GAB coefficients obtained from the regression of the EMC at various RH values are listed in Table S1.
The DVS-determined FSP values of waterlogged archaeological wood samples are shown in Figure 7. The FSP values measured during the adsorption process ranged from 23% to 32%, while those during the desorption process ranged from 36% to 86%. Moreover, the difference between the desorption and adsorption process FSP values was smaller for the samples that were slightly deteriorated than those that were moderately and seriously deteriorated. This difference is due to moderately and seriously deteriorated waterlogged archaeological wood hemicellulose fibers and cellulose being more degraded and having lower cell wall strength [29]. Therefore, the FSP determined by the adsorption process is not suitable for FSP analysis of waterlogged archaeological wood.
The NMR- and DVS-determined FSP values of the waterlogged archaeological wood are shown in Figure 8. There was no significant difference in FSP values between the two methods for the slightly and moderately deteriorated samples. However, there were significant differences for the seriously deteriorated samples. The difference is attributed to the fact that seriously deteriorated samples possess lower cell wall strength, such that the resistance of the cell wall to wetting and swelling is reduced. Consequently, cell wall contraction and deformation caused by capillary tension may occur. It should be noted that when seriously deteriorated samples of waterlogged archaeological wood are dehydrated, even if the moisture content is higher than the FSP, the wood may still be deformed. This is because the dimensional changes in archaeological wood are caused by both shrinkage and collapse. Shrinkage occurs on the cell walls, while collapse is when the cell walls fold over each other. Collapse occurs when capillary drying tension exceeds FSP. Cell wall shrinkage occurs below the fiber saturation point, a dimensional change resulting from desorption [72]. In a previous study [48], the difference in FSP between the two methods measured for modern plantation wood was about 5%–14%. This is partly attributed to the presence of free bound water, which is present only in the relatively large pores [47,51,53]. When wood contains free water, it causes the cell walls to swell, allowing these large pores to hold more water [42]. However, in this study, there was no significant difference in the FSP determined by the two methods for slightly and moderately deteriorated samples. This can be attributed to the waterlogged archaeological wood having higher hygroscopicity [32].

3.3. Relationship between the Moisture Content of Samples at Different Satates of Deterioration and Their Basic Density

BD is also an effective way to assess the deterioration states of waterlogged archaeological wood [16]. The relationship between the MWC, free water, and bound water and the BD is plotted in Figure 9. The relationship between the content of different types of water and the BD was fitted to the equation y = axb. In waterlogged archaeological wood and modern wood, the relationship between free water and the BD was highly consistent with the relationship between MWC and the BD with a coefficient of determination (R2) greater than 0.98. However, bound water and BD do not have such a relationship in modern wood (Figure 8f). Because the bound water content of modern wood is around 30%, it does not change significantly with changes in BD. Instead, this relationship emerges in waterlogged archaeological wood. The coefficient of determination (R2) between bound water and BD is 0.87, which is still highly correlated. In previous studies, the MWC of waterlogged archaeological wood was observed to be highly correlated with the BD [16,31]. The present study found that the correspondence between bound water, free water, and the BD of waterlogged archaeological wood was consistent with the relationship between MWC and BD. This suggests the possibility of using other moisture content measures to assess the deterioration states of waterlogged archaeological wood. For example, the ratio of free water to bound water can be obtained by time-domain NMR, so that the purpose of evaluating the deterioration state of waterlogged archaeological wood can be achieved without destroying the sample and without drying. In addition, all the samples from different species show the same behavior, suggesting there is little effect of wood species or even status as a hardwood or softwood. The differences between species were weakened during the prolonged degradation of waterlogged archaeological wood [16].

3.4. The Ratio of Bound Water to Free Water in Waterlogged Archaeological Wood and Modern Wood

The contribution ratios of bound and free water to the MWC of waterlogged archaeological wood and modern wood are shown in Figure 10. The contribution ratio of bound water to the MWC of modern wood was 13%–32%. Except for M3 (Chinese fir), the contribution ratio of bound water of all samples exceeded 23%. The bound water contribution ratio to the MWC of waterlogged archaeological wood ranged from 10% to 24% and decreased with the increased deterioration of waterlogged archaeological wood. For example, the contribution of bound water was 25% and 22% for samples of A3 and A6 with slight deterioration, respectively. Additionally, the contribution of bound water was reduced to 15% for sample A1 with moderate deterioration. For the seriously deteriorated samples A2, A4, and A5, the contribution of bound water was 15%, 15%, and 10%, respectively. In addition, the relationship between the ratio of the bound water contribution and BD for archaeological and modern wood is shown in Figure 11, fitted to the equation y = a + xb. Thus, the contribution of bound water was correlated with the BD of both archaeological and modern wood. The contribution of bound water to MWC decreased as waterlogged wood deterioration increased. This is because the MWC is mainly controlled by the free water present in the macropores and the capillary water affected by mesopores [59].

3.5. The Pore Size Distribution in the Cell Wall of the Waterlogged Archaeological Wood and Modern Wood

Changes in the pore structure of waterlogged archaeological wood are responsible for changes in its moisture characteristics. The pores created by degradation can allow more water to enter the wood [5,10,17]. The pore size distribution in the cell wall of the waterlogged archaeological wood and the corresponding modern samples are presented in Figure 12. For ease of comparison, the pore size distribution is divided into four intervals: <1.9 nm, 1.9–4.6 nm, 4.6–10.5 nm, and 10.5–20.4 nm. For modern wood cell walls, the proportion of pore sizes smaller than 1.9 nm was about 70%, while the proportion larger than 10.50 nm was only 7%–17%, which resembles previously published results [45,48,73]. The pore size distribution of waterlogged archaeological wood is significantly different from that of modern samples. The proportion of pores with a diameter smaller than 1.9 nm was significantly lower than that of modern wood. The percentage of pores smaller than 1.9 nm decreased as the deterioration increased. For the slightly deteriorated samples, the proportion of pore sizes less than 1.9 nm for samples A3 and A6 were 7% and 8% smaller, respectively, than the corresponding modern wood. That of the moderate deteriorated sample A1 was 13% smaller than that of the corresponding modern wood. For the seriously deteriorated samples, A2, A4, and A5, the percentage of pore sizes smaller than 1.9 nm was reduced to less than 50%, which was 20% less than that of the corresponding modern wood. The increased proportion of large pores (1.9–20.4 nm) implies that the cell walls can hold more bound water. This is due to microbial degradation affecting the pore structure of water-soaked archaeological wood. A similar phenomenon was found in previous studies, with an elevated ratio of mesopores [29,59]. Since pores below 20.4 nm were considered in this study, the ratio of pores <1.9 nm was reduced. This is due to the partial cleavage of the polysaccharide backbone and cellulose hydrogen bond network in the cell wall and the complete degradation of the acetyl side chains of hemicellulose, which leads to the loss of cell tissue integrity and the formation of many cavities, and the number of hygroscopic sites and their accessibility increases. In addition, the high water content is mainly the result of increased free water [74]. Therefore, the FSP value of waterlogged archaeological wood increases. In addition, the pore distribution of the waterlogged archaeological wood can guide the appropriate choice of reinforcing agents.

4. Conclusions

In this study, the moisture characteristics and pore structure of waterlogged archaeological wood excavated from two ancient Chinese shipwrecks, Nanhai No. 1 and Changjiangkou No. 2, were analyzed. The six waterlogged archaeological samples were classified into three deterioration states. The FSP of waterlogged archaeological wood was significantly greater than that of modern wood and increased with the state of deterioration. This is mainly due to the degradation of hemicellulose and cellulose resulting in a change in the pore size distribution of the cell wall. The NMR-determined FSP was higher than the DVS-determined FSP for the seriously deteriorated samples, while the difference was not significant for the slightly and moderately deteriorated samples. This is because the removal of free water can also cause shrinkage and collapse of the wood for severely deteriorated samples; this results in the adsorption isotherms not being able to correctly obtain bound water information. The MWC, free water content, and bound water content were positively associated with the BD. The contribution of bound water in waterlogged archaeological wood was less than that in modern wood and decreased with increases in deterioration states. This is because the influence of free water in macropores and capillary water in mesopores is the main factor controlling MWC, while bound water in cell walls only accounts for a small proportion. Insights from this study will provide certain reference values for understanding the moisture characteristics of water-soaked archaeological wood, and they are of great significance for the evaluation and protection of water-soaked archaeological wood.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f14010009/s1. Figure S1: Changes in nuclear magnetic resonance (NMR) signals across different temperatures of waterlogged archaeological wood (A1–6) and the modern wood (M1–6). Figure S2: Adsorption isotherm of waterlogged archaeological wood samples (A1-6). (EMC, equilibrium moisture content; RH, relative humidity). Table S1: Guggenheim–Anderson–de Boer (GAB) equation fitting constants.

Author Contributions

H.C.: data curation, investigation, and writing—original draft. X.G.: supervision and writing—review and editing. J.C.: visualization. G.X.: formal analysis. Y.Y.: formal analysis. J.G.: writing—review and editing, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

The work was supported by “National Key R&D Program of China” (2020YFC1521803).

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank Yaya Ma from Shiyanjia Lab (www.shiyanjia.com) for the SEM analysis on 31 July 2021.

Conflicts of Interest

The authors have no competing interest to declare that are relevant to the content of this article.

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Figure 1. Images of the original states of the waterlogged archaeological wood samples. ((A1–A5) were collected from Nanhai No. 1; (A6) was collected from Changjiangkou No. 2).
Figure 1. Images of the original states of the waterlogged archaeological wood samples. ((A1–A5) were collected from Nanhai No. 1; (A6) was collected from Changjiangkou No. 2).
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Figure 2. The maximum water content (MWC) of waterlogged archaeological wood samples (A1–6) and modern samples (M1–6).
Figure 2. The maximum water content (MWC) of waterlogged archaeological wood samples (A1–6) and modern samples (M1–6).
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Figure 3. Scanning electron micrographs of modern wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
Figure 3. Scanning electron micrographs of modern wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
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Figure 4. Scanning electron micrographs of waterlogged archaeological wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
Figure 4. Scanning electron micrographs of waterlogged archaeological wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
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Figure 5. Microscope image of waterlogged archaeological wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
Figure 5. Microscope image of waterlogged archaeological wood ((a), mason pine; (b), sweet gum; (c), fir; (d), persimmon; (e), banyan; (f), teak).
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Figure 6. The nuclear magnetic resonance (NMR) determined fiber saturation point (FSP) of waterlogged archaeological wood samples (A1–6) and modern wood samples (M1–6).
Figure 6. The nuclear magnetic resonance (NMR) determined fiber saturation point (FSP) of waterlogged archaeological wood samples (A1–6) and modern wood samples (M1–6).
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Figure 7. The dynamic sorption of water vapor (DVS) determined fiber saturation point (FSP) of waterlogged archaeological wood.
Figure 7. The dynamic sorption of water vapor (DVS) determined fiber saturation point (FSP) of waterlogged archaeological wood.
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Figure 8. The fiber saturation point (FSP) determined by nuclear magnetic resonance (NMR) and dynamic sorption of water vapor (DVS). (A1–6, waterlogged archaeological wood).
Figure 8. The fiber saturation point (FSP) determined by nuclear magnetic resonance (NMR) and dynamic sorption of water vapor (DVS). (A1–6, waterlogged archaeological wood).
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Figure 9. The relationship between the content of different types of moisture and the basic density ((a–c), waterlogged archaeological wood; (d–f), modern wood). Note: the values in the figure are the average values of three experimental tests and calculations.
Figure 9. The relationship between the content of different types of moisture and the basic density ((a–c), waterlogged archaeological wood; (d–f), modern wood). Note: the values in the figure are the average values of three experimental tests and calculations.
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Figure 10. The ratio of bound water and free water to the maximum water content (MWC) in waterlogged archaeological wood (A1–6) and modern wood (M1–6). Note: the values in the figure are the average values of three experimental tests and calculations.
Figure 10. The ratio of bound water and free water to the maximum water content (MWC) in waterlogged archaeological wood (A1–6) and modern wood (M1–6). Note: the values in the figure are the average values of three experimental tests and calculations.
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Figure 11. The relationship between the bound water contribution ratio and the basic density ((a), waterlogged archaeological wood; (b), modern wood). Note: the values in the figure are the average values of three experimental tests and calculations.
Figure 11. The relationship between the bound water contribution ratio and the basic density ((a), waterlogged archaeological wood; (b), modern wood). Note: the values in the figure are the average values of three experimental tests and calculations.
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Figure 12. The pore size distributions of the cell wall in waterlogged archaeological wood (A1–6) and modern wood (M1–6). Note: the values in the figure are the average values of three experimental tests and calculations.
Figure 12. The pore size distributions of the cell wall in waterlogged archaeological wood (A1–6) and modern wood (M1–6). Note: the values in the figure are the average values of three experimental tests and calculations.
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Table
Table 1. The sample information.
Table 1. The sample information.
Samples Wood SpeciesOrigin
A1 (Mason pine)Pinus sp.Nanhai No. 1 Shipwreck
A2 (Sweet gum)Liquidambar sp.Nanhai No. 1 Shipwreck
A3 (Fir)Cunninghamia sp.Nanhai No. 1 Shipwreck
A4 (Persimmon)Diospyros sp.Nanhai No. 1 Shipwreck
A5 (Banyan)Ficus sp.Nanhai No. 1 Shipwreck
A6 (Teak)Tectona sp.Changjiangkou No. 2 Shipwreck
M1 (Mason pine)Pinus massonianaFujian Province, China
M2 (Sweet gum)Liquidambar formosanaHunan Province, China
M3 (Fir)Cunninghamia lanceolataFujian Province, China
M4 (Persimmon)Diospyros kakiJiangsu Province, China
M5 (Banyan)Ficus microcarpaHainan Province, China
M6 (Teak)Tectona grandisYunnan Province, China
Table
Table 2. The relationship between temperature drops in melting point and pore diameter.
Table 2. The relationship between temperature drops in melting point and pore diameter.
Tm (D) (K)D (nm)
431.92
2482.18
2532.58
2583.24
2634.56
2688.52
26910.5
27013.8
27120.4
Table
Table 3. De Jong’s deterioration state criteria.
Table 3. De Jong’s deterioration state criteria.
MMC RangeDecay Grade
<135%0
135%~225%1
225%~350%2
350%~500%3
>500%4
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Cao, H.; Gao, X.; Chen, J.; Xi, G.; Yin, Y.; Guo, J. Changes in Moisture Characteristics of Waterlogged Archaeological Wood Owing to Microbial Degradation. Forests 2023, 14, 9. https://doi.org/10.3390/f14010009

AMA Style

Cao H, Gao X, Chen J, Xi G, Yin Y, Guo J. Changes in Moisture Characteristics of Waterlogged Archaeological Wood Owing to Microbial Degradation. Forests. 2023; 14(1):9. https://doi.org/10.3390/f14010009

Chicago/Turabian Style

Cao, Huimin, Xin Gao, Jiabao Chen, Guanglan Xi, Yafang Yin, and Juan Guo. 2023. "Changes in Moisture Characteristics of Waterlogged Archaeological Wood Owing to Microbial Degradation" Forests 14, no. 1: 9. https://doi.org/10.3390/f14010009

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