Evaluation of Deterioration Degree of Archaeological Wood from Luoyang Canal No. 1 Ancient Ship

Abstract

This study provides a detailed investigation of archaeological wood samples from the Luoyang Canal No. 1 site, focusing on wood species identification, physical properties, mechanical property analyses, and morphological examination. The identified wood species, belonging to the Ulmus genus, exhibited a 43% decline in compressive strength in waterlogged environments. Further, the wood exhibited increased moisture content, higher porosity, reduced basic density, and elevated shrinkage rates, indicating a mild level of degradation. X-ray diffraction was employed for the observation of cellulose structure, and Fourier transform infrared spectroscopy (FT-IR) demonstrated significant removal of cellulose and hemicellulose components. These findings emphasize the importance of understanding wood degradation mechanisms to evaluate structural integrity and durability in guiding the development of effective preservation strategies for archaeological wood artifacts. Continued research and conservation are crucial to deepen our knowledge of wood deterioration processes and enhance the implementation of preservation techniques.

1. Introduction

Wood is an important natural material and was one of the earliest substances utilized by humans [1,2,3]. Numerous wooden objects discovered during archaeological excavations embody the cultural legacy of past generations, possessing substantial scientific, historical, and artistic importance [4]. Burial environments are usually wet or aquatic, which results in all of the pores of wood remains, including vessels, fibers, and micro capillaries, being entirely filled with water [5,6]. These archaeological woods can be extremely fragile due to various deteriorating factors, such as microbial attacks [7,8,9,10,11], temperature, humidity [7,9], oxygen, and chemical pollutants [7,11]. Hence, their preservation is imperative for maintaining the integrity of wooden artifacts [12,13]. Since the mode of wood degradation exhibits significant variability across different archaeological locations [14,15], a comprehensive understanding of the extent of deterioration in archaeological wood is essential for implementing scientific, accurate, and rigorous restoration and preservation plans [16].

Luoyang No. 1 is a wooden merchant ship that was discovered in September 2013 at the bottom of an ancient canal. Carbon-14 analyses of excavated porcelain, iron products, wooden artefacts, and soil confirmed that the shipwreck occurred during the early Qing Dynasty (1644–1902). The ship was uncovered after seven months of excavation and was transferred to the Luoyang Museum for further conservation [17].

This study focuses on the archaeological wood of the Luoyang Canal No. 1 ancient ship, beginning with wood species identification. Using recent healthy wood of identical species as a control group, the assessment of deterioration involved the evaluation of physical characteristics such as maximum moisture content, fundamental density, porosity, and rate of shrinkage. Furthermore, mechanical properties evaluation and chemical characterization techniques were utilized to assess wood degradation. The use of scanning electron microscopy (SEM) allowed for the observation of wood fiber structure and analysis of degradation across its various components. The findings of this research are expected to offer valuable insights for the restoration and conservation of the Luoyang Canal No. 1 ancient ship, while also providing a benchmark for evaluating the extent of degradation.

2. Materials and Methods

2.1. Materials

The wood samples gathered from the archaeological site of the Luoyang Canal No. 1 ancient ship were procured and relocated to the museum for further analysis. The samples consisted of more than 20 pieces of wood, varying in length from 12 to 26 cm, characterized by their lightweight, frayed edges, and dry surfaces coated with sediment. Subsequent examination revealed that these specimens were classified as hardwood, exhibiting ring-porous characteristics. According to the excavation staff, the wooden specimens were probably part of the keel of the ancient ship discovered at the Luoyang Canal site. The wood underwent a preservation procedure at the excavation location, which included treatment with a 4% boron-based WP-1 water solution and a PEG composite solution consisting of 7% urea, 21% dimethyl urea, and 10% PEG 4000 [17]. The concentration of the PEG composite solution was incrementally raised until the saturation point of the woods was achieved, with a peak concentration of 40%. Further information on the ancient ship can be found in a prior study [17]. To assess the extent of deterioration in archaeological wood, recent healthy wood from the same species was selected for comparative analysis.

2.2. Microscopic Identification

The methodology employed in this study for determining the species of the archaeological wood primarily adhered to the sectioning protocol delineated in ISO 13061-1 (2014) [18]. Firstly, the archaeological wood was cut into 10 small blocks with dimensions of 5 mm × 5 mm × 10 mm (radial × tangential × longitudinal), which were subsequently boiled in distilled water until they sank. After cooling, the blocks were sliced using a Leica manual rotary microtome (Histo Core Biocutr, Ernst Leitz Company, Witzler, Hesse, Germany) to produce consistent thin sections (10–20 μm) from tangential, radial, and cross-sectional surfaces. These sections were then treated with a 1.0% safranin solution for 30 min, followed by two rounds of dehydration using 50%, 70%, and 95% ethanol. After the treatment, the samples were extracted with forceps and mounted on glass slides. Subsequently, the prepared archaeological wood specimens were examined for their microstructure using an Axio Scope A1 Zeiss polarizing microscope (Carl Zeiss Company, Jena, Germany) to identify the species, considering the macroscopic features of the wood blocks.

2.3. Physical Properties

The evaluation of physical properties of archaeological wood entailed a comparison of different parameters between the archaeological wood (AW) and recent healthy wood (RHW) of the same species. Each set of AW and RHW samples were prepared as 20 × 20 × 20 mm (radial × tangential × longitudinal) small blocks, with 10 blocks in each group. Surface moisture was removed with a towel every 24 h, and the blocks were promptly weighed (accurately to within 0.001 g). The data were recorded until the mass variance between successive measurements fell below 0.2%, signifying the point at which the wood reached saturation. The specimens were dried to a constant weight in an oven at 103 ± 2 °C, following ISO 13061-1 (2014) [18].

The maximum moisture content (MMC) and basic density (Db) can be calculated as follows [19,20]:

$$\mathrm{M}\mathrm{M}\mathrm{C}=\frac{\mathrm{m}-{\mathrm{m}}_0}{{\mathrm{m}}_0}\times 100\%$$

(1)

$$\mathrm{D}\mathrm{b}=\frac{{\mathrm{m}}_0}{{\mathrm{a}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\times {\mathrm{b}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\times {\mathrm{l}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}=\frac{{\mathrm{m}}_0}{{\mathrm{v}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}$$

(2)

where, m and m₀ are the masses of water-saturated and dry wood (g); and a_max, b_max, and l_max represent radial, tangential, and longitudinal dimensions of the waterlogged wood, respectively (cm).

Porosity (P), the maximum shrinkage rate of wood in three characteristic directions (ShD), and the total volume (ShV) were also calculated according to the formulae [21]:

$$\mathrm{P}=(1-\frac{{\mathrm{m}}_0}{{\mathsf{\rho }}_{\mathrm{w}\mathrm{s}}\times {\mathrm{a}}_0\times {\mathrm{b}}_0\times {\mathrm{c}}_0})\times 100\mathrm{\%}$$

(3)

$$\mathrm{S}\mathrm{h}\mathrm{V}=\frac{{\mathrm{V}}_{\mathrm{m}}-{\mathrm{V}}_0}{{\mathrm{V}}_{\mathrm{m}}}\times 100\mathrm{\%}$$

(4)

$$\mathrm{S}\mathrm{h}\mathrm{D}=\frac{{\mathrm{l}}_{\mathrm{a}}-{\mathrm{l}}_{\mathrm{b}}}{{\mathrm{l}}_{\mathrm{a}}}\times 100\mathrm{\%}$$

(5)

where a₀, b₀, and c₀ represent the radial, tangential, and longitudinal dimensions of the dried specimens (in cm), respectively, ${\mathsf{\rho }}_{\mathrm{w}\mathrm{s}}$ means the density of wood substance (g/cm³), which is generally 1.54 according to Ugolev (1986) [22], and l_a and l_b are the lengths of the waterlogged and dry wood (in cm).

The results of these physical properties were then examined in SAS (version 9.4, SAS Institute, Cary, NC, USA) to conduct a comparative study of different wood types through t-tests at a significance level of p = 0.05.

2.4. Mechanical Properties and Morphological Characteristics

Specimens of AW and RHW with dimensions of 20 × 20 × 30 mm (radial × tangential × longitudinal) were prepared for the assessment of mechanical properties, with 15 specimens assigned to each group. A destructive loading test was performed with a Shimadzu universal testing instrument AGS-X (Shimadzu Corporation, Kyoto, Japan) following ISO 13061-17 (2017) [23]. The specimens were placed at the center of the spherical movable support of the testing machine, loaded at a uniform speed, and broken within 1 to 5 min. The mean forces of five samples were recorded, and tensile strength was calculated by the formula:

$${\mathsf{\sigma }}_0=\frac{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}{\mathrm{b}\mathrm{t}}$$

(6)

where σ₀ represents the longitudinal compressive strength of the dried specimens (MPa), P_max is the maximum destructive load (N), and b and t represent the width and thickness of the wood specimens (in mm), respectively.

The microstructure of the AW and RHW specimens was analyzed using a scanning electron microscope (SEM) Hitachi S-3400N II (Hitachi Ltd., Tokyo, Japan). To enhance the conductivity of the wood samples and obtain clearer images, a gold palladium SEM annular sputtering target 2″ ID × 3″ OD × 0.1 mm Anatech was applied to the surface of the specimens.

2.5. Chemical Properties

For the quantification of holocellulose and acid-insoluble lignin in wood, 500 g each of air-dried AW and RHW samples were ground to 40–60 mesh. Holocellulose and acid-insoluble lignin contents were determined using the TAPPI T204 cm-97 [24] and TAPPI T22 om-02 standards [25].

The crystal structures of the archaeological wood and recent healthy wood were investigated by X-ray diffractometer. Powdered samples that went through a 200 mesh were examined using Ultima IV X-ray diffractometry (Rigaku Corporation, Tokyo, Japan). The diffracted intensity of Cu Ka radiation (under a condition of 40 KV and 30 mA) was measured in a 2θ range between 5° and 60° and at a scan speed of 2°/min. The crystallinity index (C_rl) was calculated by comparing the intensities of the crystalline and amorphous peaks [26]:

$${\mathrm{C}}_{\mathrm{r}\mathrm{l}}=({\mathrm{I}}_{002}-{\mathrm{I}}_{\mathrm{a}\mathrm{m}})/{\mathrm{I}}_{002}\times 100\mathrm{\%}$$

(7)

where C_rl represents the crystallinity of the wood cellulose (%), I₀₀₂ is the maximum intensity of the diffraction peak of the (002) crystal plane near 2θ = 22°, and I_am stands for the intensity of the diffraction peak in the amorphous region near 2θ = 18°.

To examine the chemical deterioration of the archaeological wood, powdered samples from oven-dried wood were processed into 200 mesh KBr pellets for FT-IR analysis. The infrared spectra of the AW and RHW wood specimens were recorded in KBr tablets on a Bruker ALPHA FT-IR spectrometer (Bruker Corporation, Billerica, MA, USA), with a resolution of 0.01 cm⁻¹ using the 4000–500 cm⁻¹ spectral range. For spectral processing, the OriginPro 2021 software (OriginLab Corporation, Northampton, MA, USA) was used. Five recordings were performed for each analyzed sample, and the average spectrum obtained was used for the evaluation.

3. Results and Discussion

3.1. Microscopic Identification

In wood identification, both the macroscopic characteristics and microscopic structures of wood provide important information [27]. In this research, after removing impurities from the surface of the selected samples, the cross-section clearly illustrated vessels and distinct growth rings. The drastic change in vessel size from earlywood to latewood indicates that the archaeological wood was a ring porous hardwood. Additionally, the large ray cells on both radial and tangential sections are prominently visible.

The microscopic image depicted in Figure 1a illustrates that in the earlywood zone of the cross-section, the vessels are elliptical in shape. Most individual vessel pores in the earlywood area are arranged continuously, forming groups of 2–3 pores aligned radially. The latewood vessel pores are small to medium in size, typically forming clusters of 4–10 vessel cells, with some arranged in a wave-like tangential pattern. Certain small vessels exhibit localized overlapping spiral thickenings on their walls. These vessels feature single perforations, either circular or oval, arranged alternately with bordered pits. Axial parenchyma cells are sparsely connected to the vessels, creating tangential bands in the earlywood region. The end walls of parenchyma cells show distinct or indistinct thickening, devoid of crystals but containing gum. Wood fibers exhibit varying thicknesses in their walls, with bordered pits present. Rays are non-storied, typically comprising 2–6 rows and measuring 8–20 cells in height. Ray tissues consist of multiseriate rays, occasionally with uniseriate rays, characterized by circular, oval, and polygonal outlines of ray cells. These cells contain gum, possess thickened end walls, and lack crystals. Based on the macroscopic structure and microscopic features [28,29], in conjunction with the distribution of forest resources and tree species at the excavation site, it has been concluded that the archaeological timber originated from Ulmus parvifolia Jacq.

3.2. Physical Characterization

Typically, waterlogged archaeological wood subjected to microbial attack undergoes varying degrees of cellulose and hemicellulose degradation [30,31]. The hydrophilic small molecules produced during deterioration increase the water absorption capacity of the wood, which explains the much higher MMC and lower density of AW samples compared to the RHW samples.

Table 1. Mean values of maximum moisture content (%), basic density (g/cm³), porosity (%), and shrinkage rate (%) of archaeological wood (AW) and recent healthy wood (RHW). p-value less than 0.001 indicates a statistically significant result.

Wood Type → Parameters ↓		Archaeological Wood	Recent Healthy Wood	p-Value
Maximum moisture content (%)		218.66 ± 7.96 1	102.61 ± 8.51	<0.001 2
Basic density (g/cm3)		0.396 ± 0.015	0.765 ± 0.016	<0.001
Porosity(%)		68.80 ± 1.18	43.89 ± 1.26	<0.001
Shrinkage rate (%)	Tangential	8.67 ± 0.43	5.84 ± 0.46	<0.001
	Radial	8.46 ± 0.56	3.80 ± 0.60	<0.001
	Longitudinal	2.23 ± 0.97	1.16 ± 0.91	0.063
	Volume	17.46 ± 0.92	11.44 ± 0.99	<0.001

Note: ¹ Arithmetic mean of ten values ± standard error; ² Wood type was used as a factor, p value for the significance analysis via t-test was at the 0.05 level.

presents the maximum moisture content (MMC), basic density (Db), porosity (P), and shrinkage rate (Sh) of AW and RHW specimens for physical characterization. The maximum moisture content of 218.66% in AW is 2.13 times of the 102.61% observed in RHW. Referring to De Jong’s criteria [32,33], archaeological wood with MMC ranging from 135% to 225% should be classified as the first grade of deterioration, which means the AW samples employed in this study are mildly degraded. The comparison reveals that the AW samples exhibit a fundamental density of 0.396 g/cm³ and a porosity of 68.80%, while the RHW samples demonstrate values of 0.765 g/cm³ and 43.89%, respectively. These results suggest a reduction in basic density and porosity resulting from degradation, which considerably weakens the mechanical strength of AW cell walls [34]. The statistical analysis (p < 0.001) further validates notable differences in MMC, basic density, and porosity between AW and RHW samples.

Affected by prolonged deterioration, archaeological wood exhibits weakened cell walls due to the tension resulting from water evaporation and subsequent cell wall contraction [35,36,37,38]. Therefore, shrinkage rates are critical indicators of wood degradation. For the archaeological wood in the current study, the shrinkage rates in the tangential, radial, and longitudinal directions were measured at 8.67%, 8.46%, and 2.23%, respectively, higher than typical values. These results are in line with the volumetric shrinkage displayed, where the AW samples are 1.48 to 2.23 times higher than those of the RHW samples. It is important to stress that these multiples are significantly lower compared to the levels of severely degraded wood documented in prior research [39,40,41]. Consequently, the archaeological wood analyzed in this research should be considered as mildly degraded.

3.3. Mechanical Properties and Morphological Characteristics

Mechanical property assessments play a crucial role in determining the structural integrity and long-term durability of archaeological wood [42]. As illustrated in Figure 2, the compressive strength of the archaeological wood (AW) samples was recorded at 36.12 MPa, a notable decrease compared to the 62.91 MPa found in the modern healthy reference wood (RHW). This decline can be primarily attributed to the deterioration of wood cell walls and the breakdown of the fiber structure [9]. Additionally, the increased presence of pores in the archaeological wood diminishes its ability to withstand pressure, resulting in compromised mechanical properties that render it more susceptible to damage when subjected to external forces.

According to the scanning electron microscope images (Figure 3), the growth rings and vessels are clearly characterized on the cross section. Despite the accumulation of wood fragments within vessels, the cell lumens of the archaeological wood samples retain a relatively intact microscopic morphology. This suggests that the internal tissues have maintained a degree of mechanical strength sufficient to resist collapse during the drying process. The structural characteristics of the wood rays in the radial section remain discernible (the arrows), regardless of a noticeable relaxation in the arrangement of the wood fiber cells. The image of the tangential section reveals clearly identifiable fiber cells and rays, indicating that the level of deterioration in this wood sample has not reached a critically severe stage.

3.4. Chemical Properties

The changes in the physical and mechanical properties of archaeological wood are attributed to alterations in its chemical composition.

Table 2. The content of holocellulose and acid-insoluble lignin in AW and RHW.

Chemical Components	Archaeological Wood	Recent Healthy Wood
Holocellulose (%)	48.09 ± 2.96	77.83 ± 3.51
Acid-insoluble lignin (%)	39.41 ± 2.15	23.12 ± 1.16

presents the content of holocellulose and acid-insoluble lignin in AW and RHW. For archaeological wood, holocellulose accounts for 48.09% of the composition, which is only 61.78% of that present in the RHW samples. This finding is consistent with previous studies [43,44,45,46], underscoring the importance of holocellulose degradation and the inherent chemical and biological resistance of lignin in the decay mechanisms of archaeological wood.

Diffractometric analysis has been employed to evaluate the crystallinity index (CI) of cellulose, which is the sole crystalline constituent in wood, given that hemicellulose and lignin are completely amorphous.

Figure 4 displays the X-ray diffraction patterns of both the AW and RHW, demonstrating essential similarities in their profiles. In the recent wood spectra, the characteristics peaks at 2θ = 15.2°, 22.2°, 24°, and 30° can be observed, corresponding to crystallographic planes of cellulose [47]. The XRD spectrum of AW presents additional diffraction peaks at 2θ = 21°, 26.7°, and 27.8°, which can be ascribed to crystalline compounds due to waterlogging. Notably, the diffraction intensity of the RHW is marginally higher than that of the AW, suggesting a decline in cellulose content in the AW samples used for analysis. According to the formula outlined in Section 2.5, the cellulose crystallinity of archaeological wood was determined to be 38.21%, which is slightly lower than the 42.46% found in recent healthy wood. These results indicate that the cellulose in the AW had become degraded to some extent [48,49].

Fourier transform infrared (FT-IR) spectroscopy is an essential technique for assessing the degradation or transformation of wood components, as changes in specific characteristic peaks, including their emergence, disappearance, enhancement, or weakening, indicate alterations in the wood’s chemical composition [50,51,52,53]. Figure 5 represents the FT-IR spectra of the AW and RHW, revealing a fundamental similarity between them. Prominent peaks at 3400 cm⁻¹, 2925 cm⁻¹, 2855 cm⁻¹, and 1025 cm⁻¹ are visualized [54], corresponding to the stretching vibration of hydroxyl (-OH), C-H asymmetrical stretching, C-H symmetrical stretching, and -C-O-C- stretching vibration, respectively.

Significant changes are observed in the spectral comparison between AW and RHW in the range of 1800–1200 cm⁻¹. One of the most striking differences is seen at the peak 1730 cm⁻¹, associated with the C=O stretching of acetyl or carboxylic acid [55]. This peak is nearly absent in the AW spectra, indicating a notable deterioration of cellulose and hemicellulose within the archaeological samples. However, the two characteristic peaks associated with lignin at 1597 cm⁻¹ and 1505 cm⁻¹ in AW remain intact and exhibit slightly higher intensities compared to those in RHW [56]. This feature agrees with the higher relative content of acid-insoluble lignin presented in AW as outlined in Table 2. Furthermore, slight weakening of the peaks at 1370 cm⁻¹ and 1223 cm⁻¹ in the archaeological wood suggests cellulose degradation [57]. Chemical property analyses reveal that the primary mode of degradation in archaeological wood involves the breakdown of cellulose and hemicellulose. Specifically, cellulose undergoes considerable degradation, whereas lignin exhibits relative stability with a slight increase in its proportion.

4. Conclusions

The comprehensive analysis of the physical, mechanical, morphological, and chemical properties of archaeological wood samples, identified as belonging to the Ulmus parvifolia Jacq, has provided valuable insights into the degradation processes and characteristics of the wood. The identification of the tree species underlines the historical significance of Ulmus wood as a crucial material for shipbuilding during the Qing Dynasty. Examination of parameters such as maximum moisture content, basic density, and porosity confirmed mild degradation levels in the wood from the Luoyang Canal No.1 ancient ship. Additionally, detailed chemical property analyses revealed that the degradation of the archaeological wood primarily stemmed from the breakdown of cellulose and hemicellulose, while lignin demonstrated a relatively stable nature. These results not only deepen our comprehension of the wood’s condition, but also provide critical values for the consequent restoration and preservation. They stress the urgent need for conservation strategies to protect this historical artifact for future generations.