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The effect of traditional processing craft on the hygroscopicity of palm leaf manuscripts

Heritage Science volume 12, Article number: 280 (2024) Cite this article

Abstract

Palm leaf manuscripts, which are crucial carriers of historical, religious, scientific, and artistic information in East and Southeast Asia, specifically encapsulate significant aspects of Buddhist culture and thus require comprehensive research and preservation efforts. The base material of palm leaf manuscripts is processed palm leaves, which are hygroscopic and profoundly affected by environmental humidity. Currently, there is a research gap regarding the impact of traditional processing crafts and natural aging on the hygroscopicity of palm leaf manuscripts. Utilizing dynamic water vapor sorption (DVS), the hygroscopic properties of palm leaves from various years were assessed before and after traditional processing in Yunnan Province, China. The results show that traditional processing slightly increases the equilibrium moisture content (EMC) in environments with 0 to 60% relative humidity (RH), but significantly lowers EMC in high humidity environments, with reductions up to 19.01%. Additionally, hysteresis doubled post-processing, indicating enhanced stability under fluctuating humidity conditions. Sorption models suggest that traditional processing increases the number of adsorption sites while reducing physical adsorption or capillary condensation. FT-IR (Fourier-transform infrared spectroscopy) analysis indicates that the relative contents of cellulose and hemicellulose were reduced by 39.90% and 3.97%, respectively. Degradation occurring in both the crystalline and amorphous regions of cellulose. After natural aging, the hygroscopicity of processed palm leaves improved across the entire humidity range of 0 to 95%, and there was a slight increase in hysteresis. This is due to the increase in both adsorption sites and physical adsorption capabilities. FT-IR results also indicate that the relative contents of cellulose and hemicellulose were decreased by 57.52% and 19.83% after nature aging, respectively. These findings confirm that traditional processing improves the writability and humidity resilience of the leaves, while natural aging enhances their overall hygroscopic properties. This research contributes to our understanding of how humidity damages palm leaf manuscripts. aids in determining optimal RH ranges for storage, and assesses the effectiveness of consolidation treatments in their long–term preservation.

Introduction

Historical palm leaf manuscripts are enduring records of the legacies of ancient civilizations from South Asia and Southeast Asia, particularly reflecting Buddhist teachings and practices. These manuscripts not only contain historical texts, including literature, philosophy, and scientific knowledge, but also artifacts of significant cultural and religious importance within Buddhist traditions [1,2,3]. Scientific evaluations of these manuscripts provide essential references for historical research on local Buddhist religion and culture, and are crucial for the restoration and protection of the manuscripts themselves.

In the past century, researchers have reviewed the processing steps for palm leaf manuscripts, which include cutting, steaming and boiling, polishing, flattening, drying, and perforating [4]. These procedures are designed to enhance the manuscripts' writability and longevity. Current research on preserving these manuscripts includes classifying the types of damage these palm leaf manuscripts can sustain [5,6,7], and studying the relationship between mechanical properties, chemical composition, and physical structure [8]. Based on this, conservators have undertaken a series of protective measures and conducted efficacy evaluations to address issues such as insect damage and mechanical aging inherent in the manuscripts [9]. These efforts include studying suitable insect prevention methods [4], cleaning agents [10], and consolidation treatments [11] for palm leaf manuscripts. The conservation of palm leaf manuscripts is intricately linked to environmental conditions. Addressing the impact of environmental factors on the physical integrity of these manuscripts is crucial for identifying the origins of material deterioration [12]. As a natural cellulose material, palm leaf manuscripts are hygroscopic. Relative humidity (RH) significantly impacts the long-term preservation of palm leaf manuscripts, affecting the proliferation and growth of microorganisms [13, 14], the preservation of inscriptions, and the mechanical properties of the manuscripts [5]. Researchers have studied the effects of different storage environments on the preservation state of these manuscripts [12] and have explored strategies to use the more favorable humidity conditions of coastal regions to delay the brittleness of the manuscripts [5]. These studies provide insights into selecting optimal environmental conditions for storing these manuscripts.

However, there is a notable gap in comprehensive studies on the role of environmental factors in the long-term preservation of these manuscripts, and the effectiveness of traditional processing techniques in achieving desired preservation outcomes still requires validation [10]. This calls for investigations on how environmental factors, especially the RH affects the long-term preservation of these historical manuscripts and further exploration of how traditional processing crafts can improve the environmental stability of palm leaf manuscripts. Researches comparing the hygroscopicity of cellulose-based artifacts, such as paper [15], waterlogged archaeological wood [16] and bamboo [17], before and after consolidation treatments has been reported. Researches in the context of cultural heritage conservation has demonstrated the necessity of investigating the hygroscopic properties of palm leaf manuscripts. Conducting an in-depth study of the sorption behavior of palm leaf manuscripts is imperative to further develop and refine conservation strategies. Given the invaluable nature of historical palm leaf manuscripts, the application of non-destructive analytical and diagnostic techniques for testing the physical properties of these manuscripts holds significant positive implications for their preservation. This approach not only enhances our understanding of their material characteristics but also aids in crafting more effective preservation methods that safeguard their integrity for future generations.

The relative humidity (RH) playing a significant role among environmental factors, significantly impacts the proliferation and growth of microorganisms, the preservation of inscriptions, and the mechanical properties of palm leaf manuscripts [5]. Furthermore, research comparing the hygroscopicity of cellulose-based artifacts, such as paper, waterlogged archaeological wood and bamboo, before and after consolidation treatments has been reported. Consequently, conducting an in-depth study of the sorption behavior of palm leaf manuscripts is imperative to further develop and refine conservation strategies. Moreover, given the invaluable nature of historical palm leaf manuscripts, the application of non-destructive analytical and diagnostic techniques for testing the physical properties of these manuscripts holds significant positive implications for their preservation [18]. This approach not only enhances our understanding of their material characteristics but also aids in crafting more effective preservation methods that safeguard their integrity for future generations.

The use of dynamic water vapor sorption (DVS), a non-destructive method, to assess the hygroscopicity of palm leaf manuscripts emphasizes the potential and importance of non-invasive techniques in the preservation of these invaluable hygroscopic cultural artifacts [18]. The application of DVS in measuring the hygroscopic properties of these manuscripts demonstrates considerable potential, underscoring the feasibility and significance of this approach within the broader scope of cultural heritage conservation. By focusing on hygroscopic properties, this study not only aims to contribute to the development of effective preservation techniques but also to enhance the understanding of the environmental factors that influence the longevity of palm leaf manuscripts.

Materials and methods

Processing palm leaves

Talipot palm leaves (Corypha umbraculifera L.) were collected from various regions in Yunnan Province, China. The leaves were processed following traditional crafts, which include the following steps:

  • Selecting robust palm leaves and shaping them into the requisite dimensions for palm leaf manuscript production.

  • Carefully arranging the palm leaves in a vessel and subjected to a controlled boiling process, utilizing a solution comprising fermented rice water and the extract derived from the fruit of tamarind (Tamarindus indica L.). This procedure lasts approximately 10 h. Fermented rice water and tamarind extract were selected to provide a weakly acidic environment, aims to remove components of the leaves that are prone to decay, thus increasing the flexibility and longevity of the leaves. (Fig. 1a)

  • Removing the boiled palm leaves and brush them to achieve a smooth surface texture. This aims to make leaves suitable for writing (Fig. 1b)

  • Air-drying these palm leaves until they attain a pale green hue.

Fig. 1
figure 1

The boiling (a) and polishing (b) of processing process of palm leaf manuscripts

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These processed samples of palm leaves are referred to as sound palm leaves (SPL). SPL samples from different years were aged under identical natural conditions in Yunnan for several years following processing, after which these samples were transferred to a laboratory in Beijing. To mitigate the short-term impacts of storage environment changes and the transportation process on the physical properties of the samples (such as initial moisture content and hygroscopicity), all samples were seasoned in the laboratory environment (25 °C, 60% RH) for over six months. This conditioning ensured that the samples reached a stable state in the laboratory environment.

SPL samples processed in 2021 were designated as SPL1 (Fig. 2a), while those processed in 2015 were labeled SPL2 (Fig. 2b). Unprocessed palm leaf of the same species was selected as the reference, designated as fresh palm leaf (FPL) (Fig. 2c). The FPL and SPL samples were finely ground to a particle size of less than 200 mesh. Approximately 25 mg of the resulting powder from each sample was utilized for hygroscopicity analysis.

Fig. 2
figure 2

The SPL1 (a) samples, SPL2 (b) and FPL (c) samples

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Simultaneous DVS

The equilibrium moisture contents (EMCs) of both FPL sample and SPL samples under different relative humidity (RH) steps were measured using simultaneous DVS (SPSx, Germany). The measurement procedure can be summarized as follows: During the adsorption process at 25 °C, the RH inside the sample chamber was incrementally increased from 0 to 95% in steps of 10% (i.e., from 0 to 90% RH in increments of 10%), followed by a subsequent desorption process with the humidity decreasing in the same manner. For each mass change (dm/dt), the equilibrium condition for each step was set to be less than 0.001% per minute [18].

Sorption models

The following two classical models were fitted to further illustrate the hygroscopicity of FPL and SPL samples.

The GAB Model

The parameters of the GAB model were calculated as follows with Origin 2022 software (OriginLab Corporation, USA) [19].

The GAB model equation is as follows:

$$\text{EMC}=\,\frac{{M}_{m}\cdot {C}_{\text{GAB}}\cdot {K}_{\text{GAB}}\cdot \text{RH}}{(1-{K}_{\text{GAB}}\cdot \text{RH})\cdot (1-{K}_{\text{GAB}}\cdot \text{RH}+{C}_{\text{GAB}}\cdot {K}_{\text{GAB}}\cdot \text{RH})} \times 100\%$$
(1)

where EMC (g/g) is the EMC; RH (%) is the RH; Mm is the monolayer capacity; CGAB (%) is an equilibrium constant related to monolayer sorption; and KGAB (%) is an equilibrium constant related to multilayer sorption.

The H–H model

The parameters of the H–H model were calculated as follows with Origin 2022 software (OriginLab Corporation, USA).

The H–H model equation is as follows:

$$\text{EMC}=\,{M}_{h}+{M}_{s}=\frac{1800}{w}\bullet \frac{{k}_{1}\bullet {k}_{2}\bullet \text{RH}}{100+{k}_{1}\bullet {k}_{2}\bullet \text{RH}}\times 100\%+\frac{1800}{w}\bullet \frac{{k}_{2}\bullet \text{RH}}{100-{k}_{2}\bullet \text{RH}} \times 100\%$$
(2)

where EMC (g/g) is the EMC; RH (%) is the RH; Mh is the monolayer moisture content (%); Ms is the multilayer moisture content (%); w is the molecular weight of the wood at every adsorption site; and k1 and k2 are equilibrium constants in the sorption process [18].

FT-IR

The FPL and SPL samples were ground into powder finer than 200 mesh, mixed with KBr at a mass ratio of 1:100, and pressed into pellets. Fourier-transform infrared spectroscopy (FT-IR) analysis was conducted using a Nicolet iN10 Fourier Transform Infrared Microscope (Thermo Fisher, USA), recording the absorption spectra within the range of 800–4000 cm−1 during the analysis process. Baseline correction was applied at 4000, 1530, 1890 and 864 cm−1 [20].

Results and discussion

Colors of palm leaf samples

Figure 2 displays the color differences between the FPL and SPL samples. It is evident that traditional processing craft remove chlorophyll from the palm leaves, changing the color from the deep green of FPL to the pale green hue of SPL1. This alteration enhances the writability and readability of the palm leaf manuscripts. Moreover, the SPL2 sample, which have undergone longer periods of natural aging, appear yellower compared to SPL1, reflecting the characteristic aging features of palm leaf manuscripts.

Sorption isotherms

The relationship between the palm leaves' equilibrium moisture content (EMC) and ambient relative humidity (RH) at a constant temperature is described as a sorption isotherm [21] (Fig. 3). Initially, the sorption isotherms of palm leaf samples adhere to the IUPAC Type “II” sorption isotherm [22], indicating that after processing, the hygroscopicity of SPL samples retains the typical characteristics of cellulose-based materials. During the adsorption process, within the 0 to 60% RH range, the EMC of SPL2 sample was higher than that of SPL1 and FPL samples (9.61%, 8.33%, and 8.52% at RH = 60%, respectively). Above 60% RH, the EMCs of the FPL sample exhibited a significant increase, especially above 80% RH, where it substantially surpassed that of SPL samples. For instance, the EMC of FPL sample was 40.53% at RH = 95%, whereas SPL1 and SPL2 samples had EMCs of 21.52% and 25.56%, respectively. Among SPL samples, SPL2 sample exhibited higher EMC across the entire RH range compared to SPL1 sample.

Fig. 3
figure 3

Sorption isotherms of the FPL and SPL samples versus RH ranging from 0 to 95%

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During the desorption process, within the 80–95% RH range, the EMC of FPL was higher than that of SPL samples, (16.21%, 13.14% and 15.95% respectively at RH = 80%). And at RH = 70%, the EMC of FPL sample began to fall below that of SPL2 but remained higher than that of SPL1. When RH dropped below 60%, the EMCs of SPL1 exceeded that of FPL sample but continued to be lower than that of SPL2. Overall, throughout the hygroscopic process, the EMCs of SPL2 sample consistently exceeded those of SPL1 sample, with FPL sample displaying higher EMCs in high humidity conditions but lower in 0% to 60% humidity conditions. This pattern suggests that the longer the SPL samples had been naturally aged, the greater their hygroscopicity. Additionally, the processing craft substantially diminished the hygroscopicity of palm leaves in high humidity environments.

According to the sorption isotherms, long-term preservation of palm leaf manuscripts should avoid relative humidity above 70%. The isotherms indicate that the moisture content of the samples increases significantly above 70% RH, causing fluctuations in humidity that can lead to uneven expansion and contraction. Many monks still need to read these precious historical manuscripts, which require certain physical and mechanical properties. It is known that increasing humidity is an effective way to maintain the physical and mechanical properties of palm leaf manuscripts. However, changing humidity affects the moisture content and dimensions of the manuscripts, leading to problems such as microorganisms and internal stresses, which affect their long-term preservation. Therefore, the adsorption isotherms provide insights into selecting appropriate and economical environmental humidity, as well as controlling the range of humidity fluctuations.

Hysteresis

Regarding moisture hysteresis (Fig. 4), the hysteresis values of the SPL samples exhibited a pattern of initially increasing and then decreasing with changes in RH, whereas the hysteresis values of the FPL sample remained relatively stable across different humidity levels. Moreover, the hysteresis values of sound palm leaves were more than double those of fresh leaves, particularly within the 20%-60% RH range. The hysteresis value in moisture adsorption is related to the difference in free energy gap [23], indicating enhanced moisture stability and more efficient adsorption.. Therefore, it can be deduced that the sample from SPL2 exhibit greater ease of moisture adsorption compared to those from SPL1. Additionally, the difference in free energy between adsorption and desorption process in FPL sample is larger than that in SPL samples, indicating the less hygroscopicity of FPL sample.

Fig. 4
figure 4

Hysteresis values of FPL and SPL samples versus RH

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Although higher hysteresis values typically indicate greater hygroscopicity, they do not necessarily disadvantage the preservation of historical palm leaf manuscripts. A larger hysteresis value means that the leaves absorb water more slowly during humidity increases and likewise release moisture slowly when humidity decreases. Such characteristics can confer greater stability to palm leaf manuscripts during environmental humidity fluctuations, thereby reducing the physical stresses caused by rapid changes in internal moisture levels. These physical stresses can lead to cracking or warping of the manuscripts. After all, this is only a conjecture from the principle of hysteresis of cellulose materials. The relationship between hysteresis and the swelling and shrinkage of palm leaf manuscripts still needs further quantitative research.

Sorption models

The Guggenheim-Anderson-de Boer (GAB) model and the Hailwood–Horrobin (H–H) model were chosen for this study due to their proven applicability in analyzing the hygroscopicity of archaeological materials, such as waterlogged archaeological wood. As natural cellulose materials with similar properties, this study attempts to use these adsorption models to further analyze the reasons for the changes in the hygroscopicity of palm leaf manuscript samples. These models help explain the number of adsorption sites and the type of water binding occurring in the material, which are crucial for understanding the hygroscopic behavior and its impact on preservation. A high coefficient of determination (R2 > 0.99) confirms the efficacy of these models. The parameters utilized for these calculations are detailed in Table 1.

Table 1 Coefficients of the Guggenheim–Anderson–de Boer (GAB) and Hailwood–Horrobin (H–H) models for FPL and SPL samples
Full size table

In this table, Mm is the monolayer capacity, CGAB is the equilibrium constant associated with monolayer sorption, KGAB is the equilibrium constant related to multilayer sorption, SGAB (m2/g) is the internal specific surface area. w is the molecular weight of wood at every adsorption site, k1 and k2 are equilibrium constants in the hygroscopic process, Mh is the monolayer moisture content (%), Ms is the multilayer moisture content (%). P represents the number of adsorption sites in wood, mainly hydrophilic groups (–OH and C=O).

GAB model

The coefficient of determination (R2) for the GAB model demonstrates a high level of accuracy in fitting, confirming that the GAB model is well–suited for describing the sorption behavior of FPL and SPL samples [19]. During the adsorption process, the monolayer capacity (Mm) values represent the amount of water required to form a stable monolayer on the surface of the material, Mm values of the three samples were recorded at 3.69, 4.17 and 4.87, respectively. This data suggests that the FPL sample has fewer monolayer adsorption sites than the processed samples, indicating that processing increases the number of adsorption sites (mainly the hydrophilic groups on cellulose and hemicellulose) in palm leaves. The higher Mm value for SPL2 compared to SPL1 suggests that more adsorption sites are exposed as natural aging progresses. Investigations on the hygroscopicity of wooden artifacts suggest that this exposure could result from the degradation of holocellulose [24] and the transformation from crystalline to amorphous regions [25], cause a higher Mm also points to a lower crystallinity index [26] (related to the degree of order [27] and crystal size of the cell wall substance). The internal specific surface area accessible for moisture adsorption, denoted by the SGAB parameter and inversely proportional to Mm, was recorded at 271.08, 205.38 and 239.84 for the three samples, respectively. The highest SGAB value for FPL may relate to the chemical and physical structure of cuticle on the surface of FPL sample [28]. The intact upper and lower cuticle of epidermis provides more hydroxyl groups and microstructure for water adsorption [29].

H–H model

The fitting parameters in the H–H model (i.e. w, k1, and k2) exhibited a high fitting accuracy (R2 > 0.99) for all samples, affirming that the H–H model effectively describes the experimental sorption data for palm leaves. [30]. w represents the polymer mass of wood per mole of adsorption sites. k1 and k2 are parameters related to the adsorption of water in monolayer and multilayer forms, respectively. The P value indicates the number of accessible water adsorption sites, mainly hydrophilic groups (–OH and C=O) related to cellulose and hemicellulose. SPL2 sample exhibit the highest P value, while FPL sample exhibited the lowest (2.05, 2.32 and 2.71, respectively), suggesting that the naturally aged SPL2 sample have a greater number of accessible water adsorption sites, and that the traditional processing of palm leaf manuscripts increases the number of accessible water adsorption sites in palm leaves.

According to the H–H model, the total adsorbed water content can be divided into water adsorbed in the monolayer (Mh) and the multilayer moisture content (Ms) [31]. Figure 5 illustrates the variations in Mh and Ms during the adsorption process. Generally, in environments with RH ranging from 0 to 40%, the EMC of cellulose materials is predominantly driven by monolayer adsorption. As the RH increases and monolayer adsorption sites become occupied, multilayer adsorption plays an increasingly significant role in the EMC, a pattern observed in both FPL and SPL samples (Fig. 5a). During the adsorption process, the Mh values for SPL samples are consistently higher than those for FPL sample (with maximum values of 3.47, 3.82 and 4.40, respectively), indicating that the processed sound palm leaf samples have more monolayer adsorption sites than FPL sample. Higher Mh values of SPL2 sample compared to SPL1 sample indicate that the water molecules tightly bound to the primary adsorption sites increases with the duration of natural aging. The Ms values reflect the palm leaves’ capability to adsorb moisture through physical adsorption and capillary action in higher humidity environments, primarily associated with multilayer adsorption and water condensation within microporous structures (Fig. 5b). In the 0 to 60% RH range during the adsorption process, the Ms values of the three samples are very similar, yet at 70% RH, the Ms value of the SPL1 is marginally lower than those of the FPL and SPL2 samples (7.48%, 6.07%, and 7.19%, respectively). However, in high humidity environments (RH > 80%), the Ms values of FPL sample exceed those of SPL samples. Among processed palm leaves, older SPL2 sample displayed higher Ms values than SPL1 (36.87%, 17.19% and 20.66% at 95% RH, respectively). These difference can be attributed to extended natural aging, which enhances porosity [16]. The elevated Ms values in the FPL sample may result from the natural cuticle on the leaf surface, which both chemically and physically boosts moisture adsorption in high humidity conditions [32, 33].

Fig. 5
figure 5

Mh (a) and Ms (b) of FPL and SPL samples in adsorption process

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During the desorption process, a pattern similar to that observed during the adsorption process is evident: the Mh values for the SPL2 sample are higher than those for SPL1, and the Mh values for SPL1 exceed those of the FPL sample across the entire RH range (Fig. 6a). The Ms values of the FPL sample remain higher than those of SPL samples (Fig. 6b). Among the SPL samples, the older SPL2 sample exhibit higher Mh and Ms values. This indicates that the patterns of monolayer and multilayer water content during the desorption process in palm leaf samples are consistent with those observed during the adsorption process. However, the curves show that the Mh values for the FPL sample displays almost no difference from those in the adsorption process, while the Mh values for all SPL samples have increased (Figs. 5a and 6a). The Ms values of all palm leaf samples exhibit almost no difference from the Ms values observed during the adsorption process (Fig. 5b and Fig. 6b). This suggests that the difference in hysteresis values (the difference in EMC between adsorption and desorption processes) between SPL and FPL samples is primarily determined by the monolayer water content. Based on the principles of hysteresis [34], it can be inferred that the amorphous regions of SPL samples contain more adsorption sites (primarily hydroxyl groups) than those in the amorphous regions of the FPL sample, primarily due to the degradation of the crystalline regions. [25].

Fig. 6
figure 6

Mh (a) and Ms (b) of FPL and SPL samples in desorption process

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FT-IR

The FT-IR spectra were used to investigate the compositional changes caused by traditional processing craft and its effect on the hygroscopicity of palm leaf manuscripts. As depicted in the Fig. 7a, with each curve representing the average of three parallel tests a semi-quantitative analysis was performed using the peak intensity method, with the characteristic peak for hemicellulose identified at 1730 cm−1, attributed to the stretching vibrations of the carbonyl groups on acetyl hemicellulose. The peak at 1505 cm−1 was selected as the characteristic peak for lignin, corresponding to the skeletal vibrations of syringyl propane in syringyl-based lignin. The peak at 1370 cm−1 was chosen to represent cellulose [35,36,37]. Furthermore, the peaks at 1334 cm−1 and 1318 cm−1 are associated with the in-plane bending vibrations of hydroxyl groups in the amorphous regions of cellulose and the out-of-plane wagging vibrations of methylene groups in the crystalline regions, respectively, and were used to represent the amorphous and crystalline regions of cellulose [38, 39]. The semi-quantitative analysis of the average spectra produced relative peak intensity ratios as shown in the Fig. 7b. The data reveals that the I1730/I1505 peak intensity ratio for SPL1 samples is 60.10% of that for FPL samples, while it is only 42.48% for SPL2 samples. The I1370/I1505 peak intensity ratio for SPL1 samples is 96.03% of that of FPL sample, and 80.17% for SPL2 samples. This suggests that the processing of palm leaves leads to the degradation of holocellulose and especially hemicellulose within the cell wall. With aging, the relative content of cellulose and hemicellulose decreased in different degrees. Previous studies have indicated that the degradation of hemicellulose within the wood cell walls can diminish the mechanical properties of the fibers, particularly causing reductions in toughness and tensile strength, leading to brittleness and other related disorders in palm leaves [40].

Fig. 7
figure 7

FT-IR absorbance spectra (a) and histograms of peak intensity ratios (b) of FPL and SPL samples

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The lowest I1318/I1334 peak intensity ratios in SPL1 indicate that the degree of degradation in the crystalline regions of cellulose is greater than in the amorphous regions, implying that the processing leads to a transformation from crystalline to amorphous regions, exposing more hygroscopic sites and enhancing the hygroscopicity of the palm leaves. However, the higher I1318/I1334 peak intensity ratio in SPL2 samples compared to SPL1 samples suggests that in the natural aging process, the degradation of the amorphous regions of cellulose exceeds that of the crystalline regions. This is consistent with findings from prior research [8].

Conclusion

The sorption isotherms and hysteresis values of fresh palm leaves (FPL) and sound palm leaves (SPL) samples indicate that traditional processing methods reduce the hygroscopicity of palm leaves under high humidity conditions and enhance the stability of palm leaf manuscripts in fluctuating humidity environments, with minimal impact on the equilibrium moisture content (EMC) of palm leaves in low humidity environments. Conversely, the hygroscopicity of SPL samples increases with prolonged natural aging. Adsorption models suggest that sound palm leaves have more adsorption sites than fresh palm leaves. Additionally, with natural aging, the number of these sites increases, as does the amount of moisture retained through pore condensation or physical adsorption. FPL samples have a larger internal hygroscopic specific surface area, retaining more moisture through physical adsorption or pore condensation under high humidity conditions compared to sound leaves. FT-IR analysis of palm leaf manuscript samples revealed chemical composition and crystalline structure changes caused by traditional processing methods and natural aging. Both processes lead to the degradation of holocellulose, particularly hemicellulose. These changes affect the number of adsorption sites, thereby influencing the hygroscopicity of the samples.

These findings highlight that traditional processing methods significantly reduce the EMC of palm leaves under high humidity conditions, which is crucial for the long-term preservation of palm leaf manuscripts in the humid climates of East and Southeast Asia, where these manuscripts are primarily stored. However, it is important to note that the increased hygroscopicity of palm leaf manuscripts with age, along with changes in chemical composition and physical structure, may lead to reduced dimensional stability, further chemical degradation, and potential microbial infection. The SPL samples in this study underwent natural aging. However, historically significant palm leaf manuscripts are stored in boxes or wrapped in cloth, and the hygroscopicity of manuscripts stored under such conditions remains to be studied. Additionally, it is important to recognize that variations in traditional processing methods and environmental conditions could lead to different impacts on the hygroscopic properties of palm leaves.

Therefore, future research should focus on:

  • Investigating the effects of different traditional processing crafts and preservation condition on the hygroscopicity of palm leaves.

  • Exploring the humidity response mechanisms of degradation in historical palm leaf manuscripts through simulated aging test to further understand and mitigate these effects.

  • Utilizing advanced synchronous testing methods such as Dynamic Mechanical Analysis to further investigate the relationship between the physical and mechanical properties of palm leaf manuscripts and relative humidity.

  • Developing and testing new preservation methods, ensuring that these methods do not significantly affect the physical properties, such as hygroscopicity and chromaticity.

In conclusion, this study provides valuable insights into the preservation of palm leaf manuscripts. By understanding the hygroscopic properties and the effects of traditional processing, we can develop better conservation strategies to ensure the longevity of these culturally significant artifacts.

Availability of data and materials

All data generated or analysed during this study are included in this published article and its supplementary information files.

Abbreviations

DVS:

Dynamic water vapor sorption

EMC:

Equilibrium moisture content

RH:

Relative humidity

FPL:

Fresh palm leaf

SPL:

Sound palm leaf

GAB model:

Guggenheim–Anderson–de Boer model

H–H model:

Hailwood–Horrobin model

FT-IR:

Fourier-transform infrared spectroscopy

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Acknowledgements

The authors express gratitude to Xuelian Chen from Administration Office of Potala Palace, Tibet and Mingzhou Chi from University of Science and Technology Beijing for their assistance with this study.

Funding

This research was funded by the National Key Research and Development Program of China, grant number 2022YFF0903902 and 2022YFF0903905.

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Authors and Affiliations

  1. Institute for Cultural Heritage and History of Science & Technology, University of Science and Technology Beijing, Beijing, China

    Dehao Yu, Hong Guo, Hao Luo & Liuyang Han

  2. State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’ An, 710072, China

    Xuanhua Li & Jinmeng Zhu

  3. Queen Mary University of London Engineering School, Northwestern Polytechnical University, Xi’ An, 710072, China

    Si’ao Sun

  4. China Academy of Cultural Heritage, Beijing, 100029, China

    Liusan Li & Shan Wang

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  1. Dehao Yu
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Contributions

Conceptualization: DY and LH; Methodology: DY, HG, LH; software: DY; Investigation: DY and LH; Experiment: DY; Formal analysis: SS; Resources: HL; Writing—original draft preparation: DY; Writing—review and editing: DY and LH; Visualization: Project administration: XL, JZ, LL and SW; Funding acquisition: LH. All authors read and approved the final manuscript.

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Correspondence to Liuyang Han.

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Yu, D., Li, X., Sun, S. et al. The effect of traditional processing craft on the hygroscopicity of palm leaf manuscripts. Herit Sci 12, 280 (2024). https://doi.org/10.1186/s40494-024-01402-2

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