Scientific analysis of folk contract documents from Tianshui region: insights of fiber use and preservation state

Abstract

Folk contract documents (FCD) are valuable materials for studying social history, and the paper they use reflects the social realities of different eras and social classes. Research and scientific analysis of numerous FCD samples after the fourteenth century are rare. We conducted a study on 96 Tianshui folk contract documents (TFCD, 107 paper samples) from the Tianshui area of Gansu Province, Northwest China, taking into account both the textual content and the materiality of paper carriers, and interpreted the results from multiple lines of evidence and discussion. Physical performance analysis revealed that the paper used by the northern folk exhibits a lower apparent density, which is not conducive to the long-term preservation of paper. The preservation status investigation, curtain pattern analysis, and fiber analysis show that the paper used in the TFCD differs from traditional cultural paper regarding disease types, production precision, and fiber materials, providing a basis for its protection and restoration. The analysis of chemical components indicated that the aging and yellowing of paper can be correlated with the content of sulfur and carbonyl groups. The feasibility of using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) analysis to quickly identify papermaking fibers in a large number of paper samples was proposed. By utilizing various techniques to inspect the paper of documents, this study helps to enhance the academic understanding of FCD materials. In addition, it expands the knowledge base of Northwest handmade paper.

Introduction

Since the beginning of the last century, the continuous discovery of contract documents has provided valuable new materials for studying traditional Chinese grassroots society (Appendix T1). Academic research on ancient Chinese documents has involved collecting, organizing, and interpreting them. For instance, we can see many collected and organized document images on the International Dunhuang Project (IDP) website (http://idp.nlc.cn/). Scholars have studied culture [1], society [2], religion [3], law [4], history [5], and other aspects of Dunhuang documents. The folk contract documents (FCD) that entered the Ming and Qing dynasties (1368–1912 AD) were mainly organized and interpreted, as was the case for the Chinese Local Historical Literature Database (http://ndfwx.datahistory.cn/). The daily lives of ordinary people recorded in FCD and the complex network of social, economic, and legal relationships that arise from them showcase the social norms of the lower class and interpret macro-institutional policies from a micro perspective. For example, in Fieldwork in Modern Chinese History, FCD are used as a medium to reflect the social reality behind the text's material [6, 7], and using textual content as a material medium will play an important role in promoting research in these areas [8]. The importance of the text of the FCD makes the understanding, protection and restoration, and scientific analysis of its paper carrier materials meaningful.

As representatives of contract documents before the fourteenth century, the Dunhuang and Turpan documents date back to an earlier time and cover a large period. Helman-Ważny summarized early paper research on the Central Asian Silk Road [9] and outlined the early history and technology of papermaking, as revealed by the oldest manuscripts in existence. These early Chinese contract documents were mainly made of rag paper [10], bast paper [11], and bark paper [10]. As traditional carriers of written texts and historical contract document manuscripts, scientific analysis of paper support itself has become an important part of history and archaeological research [10, 12]. The study of paper in FCD [13,14,15,16,17,18] reflects the increasing attention of the academic community on the use of paper after the fourteenth century in folk society; almost all the samples were from southern China. Due to the existence of famous documents such as Dunhuang and Turpan in Northwest China, FCD after the fourteenth century have received little attention, and there are almost no reports of material research.

Material characterization can provide useful information on the paper composition and conservation state [19,20,21,22], such as physical property measurements [15, 17,18,19, 22], fiber identification, and chemical information [12, 23,24,25,26,27]. Among them, Py-GC/MS technology is an effective means of identifying fibers [27,28,29,30,31,32] by using special plant markers (plant sterols, terpenoids, lignin monomer combinations, etc.). However, the technological analysis for identifying FCD fibers only involves microscopic observation [13,14,15,16,17,18], and only a few studies have reported physical property measurements [15, 17, 18]. Frontier characterization techniques and comprehensive investigations have not yet been applied to the study of FCD material samples.

In recent years, scholars [33, 34] have focused on FCD in the Gansu region, Northwest China, which reshaped the social appearance and folk life of the eastern part of the Silk Road in late feudal society. Tianshui (located in Gansu Province, China) is one of the places where ancient Han dynasty paper was discovered (Fangmatan paper), which gives Tianshui a traceable history of paper usage. To explore the history of papermaking technology represented by the region in the use of fibers over a long historical span, this article takes a group of Tianshui folk contract documents (TFCD) as the research object.

TFCD were treated as paper objects for the material investigation to present basic physical information. To gain comprehensive knowledge of the paper used in the TFCD, multi-technology, physical performance measurements, and back transmission observations are used to characterize the basic properties of the paper. Color measurements, Fourier transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF) analysis were used to characterize the aging of the samples. A single method can sometimes cause controversial results [35], and the application of multiple analytical methods could reduce this risk. Therefore, Py-GC/MS analysis combined with fiber microscopic observation was conducted on the paper fiber materials. The combination of multiple technologies has been used for the first time to study multitudinous FCD papers.

Materials and methods

Description of samples: Tianshui folk contract documents (TFCD)

The author Mengfan Ge with a famous calligrapher and collector Yuchun Zhou had visited rural areas in various districts and counties of Tianshui since 2015. We collected folk contract documents from the Qing Dynasty and the Republic of China through visits and investigations. A total of 96 TFCD (107 papers) were studied from samples collected from the Tianshui Gansu region.

Most TFCD are related to land purchase and sale. Figure 1 shows some document images and detailed information. All sample images are listed in Table S1. The term “Tianshui” we will discuss in this research includes the geographical scope of Tianshui in ancient and present-day times (Appendix T2).

Fig. 1

Photo of Tianshui Folk Contract Documents (TFCD) samples, scale:10cm. a 1st.JQ-10. b 1st.YZ-1. c 1st.MG-6. d 1st.XF-4. e 1st.XT-4. f 1st.XF-5

China is a traditional agricultural society where farmland and homesteads are the most basic means of production and livelihood. Therefore, for both the samples of this article and other FCD studies, real estate transaction contracts, including land, homesteads, buildings, and fields, are the main form of FCD. Real estate transaction purchase and sale contracts can be divided into baiqi (白契, e.g., Fig. 1c) and hongqi (红契, e.g., Figures S1 and S2) contracts based on the presence or absence of a deed tax payment (Appendix T3). Most of the samples in this research were baiqi, which reflects the behavior of people not paying tax deeds and relying solely on the parties' trust to complete the transaction and exchange (Appendix T4).

Methods

Physical and chemical examination is a method of attempting to obtain additional data on the production, source, use, and reuse of manuscripts. The basic physical and chemical information of the TFCD, including size, thickness, quantity, grammage, and density, was obtained. Noninvasive color difference analysis instruments were used to characterize paper chromaticity, and X-ray fluorescence (XRF) was used to analyze the elemental composition. The fibers of 107 paper samples were detected using Py-GC/MS, and fiber analysis was used as a supplementary means to observe the fiber composition of individual samples.

Basic properties of the paper and statistical data analysis

The area is calculated based on the measured length and width in units of m². The thickness was measured according to the China National Standards (CNS) GB/T 451.3-2002 (CNS; Paper and board-Determination of thickness), which measures the thickness of the paper sample with a thickness gauge, mm. The grammage density was measured according to GB/T 451.2-2002 (CNS; Paper and board-Determination of grammage), which is quantitative in terms of the mass-to-area ratio, unit g/m². The apparent density, according to GB/T 24328.2-2020 (CNS; Tissue paper and tissue products-Part 2: Determination of thickness, bulking thickness and apparent bulk density and bulk), was calculated as the ratio of grammage to thickness, unit g/cm³.

A fiber optic spectrometer produced by Avantes in the Netherlands was used for the spectrophotometer, a fiber optic spectrometer (AvaSpec-ULS2048CL-EV) with fiberoptic cables (FC-UVIR600-1-ME), a light source (AvaLight-HAL-S-Mini2) and reflection probes (FCR-7UVIR200-2-ME) were used, and optical fibers with a diameter of 600 μm were connected to all the optical parts. The conditions for measuring chromaticity parameters are a D65 light source, a 10-degree field of view, and d/0 measurement geometric conditions, excluding specular reflection. A ceramic whiteboard with a reflectivity of 96% was used as the standard whiteboard (AVANTES WS-2). The data were processed using an AvaSoft 8 instrument equipped with a spectrometer. After specifying the testing conditions, the software calculates the RGB, XYZ, and L*a*b* parameters directly based on the measured spectral data. The CIE L*a*b* color coordinates are as follows: the lightness L* (varying from 0 for black to 100 for white), the chromaticity coordinates a* (varying from -120 for green to 120 for red on the green–red axis), and the b* chromaticity coordinate (varying from -120 for blue to 120 for yellow on the blue-yellow axis). Three in situ measurement points were used for each sample, and the average and standard deviation of each sample were calculated as valid data.

The L*, a*, and b* values were used for Hunter whiteness calculation (Eq. 1), and the maximum value was used as the reference value for color difference calculation. The color differences are calculated based on Eq. (2) [36].

$$W = 100 - \sqrt {(100 - L)^{2} + a^{2} + b^{2} } .$$

(1)

$$\begin{array}{c}\Delta {E}^{*}=\sqrt{\left[{\left({\Delta L}^{*}\right)}^{2}+{\left({\Delta a}^{*}\right)}^{2}+{\left({\Delta b}^{*}\right)}^{2}\right]}.\end{array}$$

(2)

The ΔL*, Δa*, and Δb* are the D-values of the measured samples and reference values (the highest W value sample) of L*, a*, and b*, respectively. The raw chromaticity data, calculated chromaticity difference data, and Hunt whiteness values are shown in Appendix Table S6.

Microscopy and back transmittance observation

The fiber analysis method refers to the third part of GB/T 4688-2002 (CNS; Paper, board and pules-Analysis of fiber furnish) by taking a minimal amount of paper sample fiber, adding ultrapure water, using surgical needles to disperse the fiber on a glass slide, adding a few drops of Herzberg stain (DP0406, LEAGENE), and covering the sample with a cover glass to make a fiber specimen. Microscopic observation and imaging were carried out with an XWY-VIII paper fiber measuring instrument (Beijing Lunhua Technology Co., Ltd., China). The curtain pattern of the paper was observed under the backlight, the paper was captured with a digital camera, the image was stored, and quantitative analysis was performed.

XRF and multivariate statistical analysis

A hand-held XRF instrument (Niton XL3t 950 by Thermo Fisher Scientific, Billerica, USA) was used to analyze the elements, and the soil patterns allowed us to observe more trace elements. The X-ray beam spot on the samples was 3 mm in diameter, which was smaller than the in situ blank paper area, to the extent that accidental misdetection of damage, such as discoloration, was avoided.

The Spearman rank correlation coefficient has strong anti-interference ability [37], and the color data and XRF results in this study sample show a nonnormal distribution. Therefore, we used Spearman correlation analysis to determine the correlation between variables and chose non-curve fitting. There are significant differences in the interpretation of correlation coefficients across different research fields [38]. We use the correlation between chromaticity and elements as a reference here based on the nonlinear distribution of the data. The Boltzmann function was subsequently used to complete the nonlinear curve fitting of variables with significant correlations.

When conducting correlation analysis and fitting analysis, z-score standardization was used for dimensionless processing of L*a*b* data and XRF data, giving each variable the same weight and improving the model accuracy. The normalization of the data and Spearman's correlation analysis were performed with SPSS software, and the results are shown in Appendix Tables S5–S7 for the raw data and processing results. Origin software was used for plotting the results of correlation analysis for data visualization, and nonlinear curve fitting was also implemented.

ATR-FTIR and data preprocessing

The paper sample was analyzed by a Nicolet 6700 (Thermo Scientific, USA) FTIR spectrometer with attenuated total reflection (ATR) attachment. Blank paper without handwriting or disease was placed on the ATR testing platform for spectral collection. Spectra were acquired over the 4000–500 cm⁻¹ range using a resolution of 4 cm⁻¹, with 32 scans per spectrum. OMNIC 8.0 software was used for data acquisition and treatment. The data were baseline corrected, normalized on the vertical axis, and smoothed at 21 points. Subsequently, the peak area of the absorption peak of interest was integrated and calculated.

Py-GC/MS analysis

Pyrolysis gas chromatography–mass spectrometry analysis was performed using a Multi-Shot Pyrolyzer PY-3030D (Frontier Laboratories, Japan) linked with a Quadrupole gas chromatography-mass spectrometer GCMS-QP2020 NX (Shimadzu, Japan). A laboratory analytical balance (BSA224S, 0.1 mg/0.0001 g, Sartorius) was used to weigh 0.2 mg of the sample, which was placed into a sample cup with tweezers. The sample cup was placed in an AS-1020E autosampler (Frontier Laboratories, Japan). The pyrolysis temperature was 500 °C, the inlet temperature was 300 °C, and the injection mode was split (50:1). A stable and continuous carrier gas (high-purity helium, 99.999%) was used. The flow was controlled by a constant flow, the pressure was 112 kPa, and the total flow rate was 79.5 mL/min. The column flow rate was 1.5 mL/min, and the linear velocity was 38.3 cm/s. The chromatographic column used was an Ultra ALLOY + -5 (Frontier Laboratories). The initial temperature of the column oven was 40 °C, maintained for 3 min, heated to 300 °C at 5 °C/min, held for 10 min, heated to 320 °C at 20 °C/min, and held at 5 min. Full-scan acquisition mode was selected, with a scan range of 35–550 u. Real-time analysis systems collected the chromatographic data to generate data files. The GCMSsolution workstation was used to view and analyze these data, and compounds were identified based on NIST 17 library.

Results and discussion

Statistics of the preservation status of the TFCD

Historical paper documents inevitably suffer damage due to the influence of their own materials and external preservation environment, resulting in various diseases of paper cultural relics. The deterioration of paper cultural relics is attributed to several causes, such as microbial contamination, oxidation, acidification, and others. The alteration of paper support upon aging includes physical damage and chemical deterioration. The preservation status and disease status of the samples were analyzed, and the results are shown in Table S2. The types and descriptions of diseases are based on the AIC Wiki as a reference template (https://www.conservation-wiki.com/wiki).

Aging leads to the natural physical weakening and embrittlement of some papers, resulting in wrinkles/creases (Fig. S3e) and defects/damages (Fig. S3d). Wrinkles/increases are fibers in the creased area that may be irreversibly crushed or broken by contraction of the support held under restraint or when the support sheet ruptures along a previously weakened area such as a fold. Defects/damages are areas of support that are physically detached or missing, even when a hole is formed. Chemical deterioration includes stains, discoloration, and some complex types, leading to deterioration. Stains that have a distinct edge or boundary (Fig. S3a, b) include two basic types of stains: water-based and oil-based. Paper manuscripts age under the influence of light, temperature, and humidity, resulting in acidification and yellowing and even weakened mechanical properties. Long-term exposure to dust from the air can also cause paper discoloration. In particular, upon foxing damage (Fig. S3c), the foxing spots are reddish-brown, brown or yellowish, and irregular in shape. The generation of foxing spots is believed to occur from transition metals [39], fungal infections, or dual activity between fungi and iron [40].

Shifts in the color of pigments or dyes generally result from exposure to light, which causes the color to fade (Fig. S4a–c). The physical movement of non-fast colorants is called bleeding. This usually occurs in the presence of moisture and results in a blurred or feathered appearance. Movement may occur laterally or penetrate to the reverse, which is also called sinking. Generally, sizing inhibits the absorption of liquid into the fiber matrix, decreasing the susceptibility of paper to moisture or the feathering of ink and aqueous media. Chinese red stamps also experience pigment changes, and the bleeding problem (Fig. S4f) in red stamps is due to printing paste leakage and insufficient viscosity. Red printing paste is made by mixing cinnabar and vegetable oil, and a high proportion of the raw materials in the printing paste are cinnabar. If the viscosity of the printing paste used is not high, then the cinnabar will sink, and the printing paste will float, resulting in oil leakage [41].

The statistical analysis revealed the distribution of different types of diseases in the sample, as shown in Fig. 2. The characteristic feature of TFCD samples in paper diseases is that up to 95% of the samples have creases, as well as a large proportion of stains, damages, and defects. The pigments are mainly concentrated in red stamps, with 98% of the samples exhibiting red stamps indicating fading and 70% revealing bleeding. This finding is consistent with the statistics of disease investigations of 305 seals from the Ming Dynasty to the Republic of China [41]. The proportion of fading and bleeding in handwritten black ink is much lower than that in black and blue printing. Fading and bleeding depend on both the fillers used in the carrier and the additives used in printing or writing ink. Factors such as moisture intake and other external factors are also worth considering.

Fig. 2

Statistics of investigation results on paper preservation status. a Physical damage and chemical deterioration of paper. b Fading and bleeding of pigment

Basic properties of the TFCD paper

In the general performance of paper, grammage, tightness, and uniformity directly affect the physical properties of paper, and thickness is an important indicator for choosing repair paper. The thickness, weight, quality, and tightness of the TFCD paper samples are shown in Fig. S5a. The grammage of most of the TFCD samples are less than 43 g/m² (except for three samples, No. 87, No. 94, and No. 95), and the thicknesses are between 0.063 mm and 0.178 mm (except for one sample, No. 87). There are large differences in thickness and grammage between samples, which shows that there is no strict requirement for folk contract document paper.

To better describe the differences in FCD in different regions after the fourteenth century, we analyzed and calculated data from existing studies. Six samples of Jingshan (JS) FCD [18], one sample of Xiaoxian (XX) FCD [17], and fifteen samples of Mayang (MY) FCD [15] were included, and their physical performance was compared with that of the TFCD, as shown in Fig. 3a. The TFCD paper is more discrete in thickness and grammage, the compactness is significantly lower than that of the three groups of paper samples that have been studied, and the degree of dispersion is the smallest. The mechanical strength of paper is affected by many factors, such as the raw materials, pulping process, beating process, papermaking process, and storage environment. The apparent density is an important factor reflecting the mechanical strength of paper. The tensile properties, burst strength, tearing resistance, and folding endurance of paper increase with increasing sealing strength [42]. However, for traditional handmade paper, a lower apparent density can cause the paper structure to fail, increase ductility, and increase ink adhesion [43]. The low density of the TFCD paper samples promoted adhesion of the ink so that the handwriting was better preserved (Fig. 2b). A better ductility also increases the ease of folding for carrying and storing paper when used, but a low density reduces the mechanical strength of the paper, which is not conducive to the long-term preservation of TFCD paper. A significant proportion of physical damage occurs on the TFCD paper, as discussed in Sect. “Statistics of the preservation status of the TFCD”.

Fig. 3

a Basic characteristics of the paper. b Chromatic aberration diagram; YG: sample time range in 1723–1735, QL: 1736–1795, JQ: 1796–1820, DG: 1821–1850, XF: 1851–1861, TZ: 1862–1874, GX: 1875–1908, XT: 1909–1911, MG: 1921–1949, 1949: means after 1949. c Visualization results of Spearman correlation analysis between chromaticity and elements (the color block represents the correlation between the variables corresponding to the horizontal and vertical coordinates)

The elements in the TFCD samples were analyzed by X-ray fluorescence (XRF), and the major elements detected were Ca, Fe, K, and S; the minor elements were Mn, Ti, Cu, and W; and the trace elements were Cr, Sr, Zn, Hg, V, Pb, As, Zr, Mo, and Rb. Trace metals such as Rb and Mo can be derived from water or tools used in the papermaking process. When drying paper, workers stick wet paper to courtyard walls or special drying paper walls and dry it with the help of sunlight in northern China [44]. The trace elements Sr and Zr may be introduced into the surface of the paper through the soil wall drying step mentioned above. The presence of trace elements may not necessarily indicate their origin. Old papers included metal elements, e.g., Fe, Ti, and Al [45]. Metal elements were detected in all the paper samples in this study, and transition metals (Cu, Fe, and Co) may be from contaminants from water sources, papermaking materials, or papermaking equipment. Based on our research objectives, we selected the main elements for correlation analysis.

Color changes can be evaluated through the CIE L*a*b* system, and the chromaticity space (Fig. S5b) allows us to observe the trend of changes in paper chromaticity over different age ranges (based on the recording time of the sample text, as shown in Fig. 3). The results of Spearman correlation analysis (Fig. 3c) and nonlinear curve fitting (Fig. S5c) were visualized to characterize the correlation between the elements and chromaticity. The group with a significant correlation between the element concentration and chromaticity value was mainly composed of S, a*, and b* (Fig. 3c, blue color block). There is a strong positive correlation between potassium and both sulfur and chromaticity (Fig. 3c), and the curve fitting results also indicate a positive correlation between sulfur and b* values (Fig. S5c). In previous studies, Ca content and L* value can often be positively correlated [46], while the Fe concentration is negatively correlated [47]. Notably, the observed correlation does not guarantee a causal relationship between the two variables [48]. Therefore, from the perspective of inorganic composition, the sulfur in TFCD may be the main factor affecting paper yellowing. At the same time, we used infrared spectroscopy to conduct correlation analysis on the color changes caused by aging to explore the influence of organic components on yellowing (Fig. 4).

Fig. 4

a Infrared spectra of the paper samples. b Comparison of the area of the carbonyl absorption peak with the W, L*, a*, and b* values

Aging experiments have shown that carbonyl groups at 1700–1600 cm⁻¹ increase with paper aging [49], and the area of the carbonyl region is negatively correlated with whiteness [23]. IR spectroscopy analysis (Fig. 4a) was performed on four randomly selected samples (No. 1st-XT-3, 1st-JQ-6, 1st-DG-3, 1st-JQ-11) and the sample with the highest W value (No.1st-MG-17), and the absorption peak area of carbonyl groups was calculated. The carbonyl content of the TFCD paper samples was negatively correlated with the W and L* values but positively correlated with the degree of yellowing (b* value).

The correlation analysis between inorganic elements and organic functional groups on the color of paper indicates that both the fiber material and external factors undeniably impact its aging. The aging and degradation of fiber materials themselves reduce the degree of polymerization of cellulose, forming oxidation groups containing carbonyl or carboxyl groups, leading to paper aging and yellowing. Equally important, insufficient alkaline substances in the papermaking process and the influence of acidic substances in the external environment can also lead to paper acidification and yellowing.

Observation of the papermaking process

The ShuoWen JieZi records that paper is a layer of floc on the curtain (纸,絮一苫也), and “苫” mostly represents the paper curtain (zhilian, 纸帘). A porous woven net is tied to the mold frame to receive fiber pulp, which is a paper curtain. There are two different technical systems for the production of handmade paper: the pouring method (jiaozhifa, 浇纸法) and the lifting method (chaozhifa, 抄纸法) [50]. Two kinds of papermaking methods lead to obliviously different traits, which is manifested in the presence of curtain lines (lianwen, 帘纹). In the pouring method, the pulp, which consists of fibers that diffuse in a small amount of water, is spread evenly over a cloth pinned across a wooden frame [51], and the sheet of paper is dried under the sun, which involves the absence of curtain lines. The lifting method involves the dipping of curtain lines into a vat of pulp to collect the fiber, which is subsequently removed and shaken to form a sheet. The screen leaves impressions on the finished paper, which can be seen in transmitted light [11].

Due to the difference in the amount of pulp deposited on and among the curtain lines, different light and dark lines are formed on the paper sheet. The impressions left by the lifting method are called “laid” lines; laid lines are a denser line pattern and are generally a horizontal line pattern. Stitching or lacing impressions are called “chain” lines [11], and the distance between two chain lines is relatively wide—generally a vertical line pattern—which links the laid lines. Patterns of chain and laid lines in the paper structure allow us to distinguish handmade woven paper from handmade laid paper characterized by particular numbers of laid lines [10]. The density of the laid lines can be used as a reference factor for judging the quality of the paper. The type of papermaking method used can be used to qualitatively analyze whether the lines are laid and whether the lines are straight or crooked [52]. Quantitative analysis was used to measure the number of laid lines within 1 cm (Fig. 5a) and the distance between two chain lines (Fig. 5b), and the detailed data are recorded in Table S3.

Fig. 5

Photo of TFCD paper samples under backlight. a 1st. MG-11, b 1st. MG-5, c 1st. ZG-2, d 1st. MG-6, e 1st. MG-18, f 1st. ZG-5

The laid line was observed through photos taken under backlight (Fig. 5), most were straight, but some samples exhibited crooked laid lines in TFCD samples. Grass curtains are used in the production of handmade paper in the north, while bamboo curtains are commonly used in the south [53]. In the lifting method papermaking of bamboo curtains, there are obvious straight laid lines on the paper through the backlight; in the lifting method papermaking of grass curtains, the laid lines are often crooked [54].

Owing to variations in the intensity of beating, the nature of fiber raw materials, and the intended applications of paper, various densities of paper curtains are selected for the production of handmade paper, ultimately leading to disparities in the density of the laid circuit. For example, in Fig. 5c, e, fiber deposition is obvious under backlight, and the fiber interweaving distribution is uneven. Large fibers cannot pass through a paper curtain with a small gap, so a paper curtain with a wider laid line is selected. Moreover, paper with sparsely laid lines (Fig. 5c, e) is often accompanied by a combination of wide-chain lines and narrow-chain lines. Table S3 shows that there is almost no consistent interval between the recorded chain lines. For chain lines in Asian papers, the intervals are not consistent, whereas in European papers, they are widely spaced at regular intervals [55]. The TFCD paper is between 6 and 13 laid lines/cm, and the general calligraphy and painting paper used in the Ming and Qing Dynasties was 15 laid lines/cm [56]. The quantity of laid lines reveals that the handmade paper utilized in folk paper differs significantly from the type employed in calligraphy and painting. Specifically, the laid lines in the latter are more densely packed, whereas they appear sparser in the former.

There are a total of 4 documents without curtain lines that display the characteristics of machine-made paper under the observation of backlight transmission, all of which are officially issued documents. Two pieces of 1910 machine-made paper are both zhafu documents (No. 69 and 70 in Tables S1, T3), and two pieces of machine-made paper after 1949 are both land and property ownership certificates (No. 94 and 95 in Table S1).

Fiber identification of the sample (Py-GC/MS analysis and fiber analysis)

Pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) is a commonly used technique in the study of cellulosic materials and has achieved remarkable results in characterizing handmade paper in the East Asian region, such as handmade papers from China, Japan, and Korea [20, 30, 31]. The pyrolysis products of paper can be divided into three regions (Fig. 6a). The compounds eluted from the cellulose fingerprinting region are usually associated with cellulose and lignin pyrolysis products, while the plant marker region allows us to identify fiber types through the eluted plant biomarkers. The second region is at a retention time (RT) between 35 and 50 min, corresponding mainly to fatty acids and their derivatives. A series of 12–24 carbon atoms were detected in this region, mainly hexadecanoic acid and octadecanoic acid. which are linear carboxylic acids and saturated fatty acids [32].

Fig. 6

TIC chromatogram of the TFCD pyrolysis products. a TIC chromatogram of 1st. DG-9 and 1st.MG-1. b EIC chromatogram at m/z 218. c TIC chromatogram of the plant marker region

The plant marker area in the defined region of interest (ROI) after 50 min and the characteristic phytosterols and terpenes in plants can be used as biochemical markers to determine plants [20]. The pentacyclic triterpenes α,β-amyrins and their derivatives are considered characteristic plant markers of mulberry fiber [28, 30, 57]. Many studies have confirmed that the pentacyclic triterpenoid compound amyrin (m/z 218, m/z 203, and m/z 189) can be used as a characteristic compound for characterizing paper mulberry [20, 58]. The compounds eluted from hemp fibers in the total ion flow chart often exhibit low levels [30] or are almost invisible [29].

This study used an extracted ion chromatogram (EIC, Fig. 6b) for sample screening to quickly classify fibers in a large number of samples. The samples with strong EIC signals at m/z 218 at the designated RT position were recorded as group M (bark fiber, mainly mulberry fiber), and group X (bark fiber) had peak values but RT offset under ion selection. The samples without peaks at the specified mass-to-charge ratio are recorded as group B (bast fiber, mainly ramie and hemp fibers). The fiber identification results of 107 paper samples screened for plant markers using EIC are shown in Table S3, and data visualization was conducted to reveal the spatiotemporal distribution characteristics of samples containing bark fibers or bast fibers (Fig. 7). Bast fiber has a wider period in TFCD samples.

Fig. 7

Distribution of the two types of fibers during the sample time (based on the EIC screening results)

Figure 6c shows the total ion chromatogram (TIC) of three randomly selected group M samples and two group B samples to further characterize the compound distribution of bast fibers and bark fibers in the ROI. The corresponding compounds are shown in Table S4. Almost all of the compounds were eluted from the group M samples, including oleanane derivatives (β-amyrone) that can be strongly associated with mulberry bark fibers. Almost none plant markers (phytosterols and terpenes) tends to be bast fibers [30], and group B samples exhibited a low level of characteristic structures in the ROI. Several fiber samples identified as bast fibers were selected for microscopic observation as supporting evidence.

We selected several late-age Group B samples (Fig. 8) for fiber microscopic analysis and the identification of fiber characteristics through microscopic observation [59]. Generally, the main microscopic characteristics of bast fibers in traditional papermaking are pure fibers without nonfibrous cells and brownish-red or purple‒red staining by Herzberg staining. The fibers are long and wide and have longitudinal strips and dense horizontal stripes on their surface. The direction of the stripes depends on the degree of beating of the fibers during the papermaking process. Due to the longer fiber length, the bast fibers are often cut off, and they are scattered at the ends. The scattering of the ends is a sufficient manifestation of processing, but it also causes the fibers to split longitudinally and even causes the filamentous fibers to float around the fibers. Thus, the fiber analysis in Fig. 8 is consistent with the characteristics of bast fibers (ramie or hemp). In addition, the four samples in Table S3 have no curtains under backlight observation (marked as D in Table S3), which are machine-made paper. Two of the machine-made paper samples were property ownership certificates, and we conducted fiber analysis on them (Fig. S6), which also showed obvious characteristics of bast fibers (ramie or hemp).

Fig. 8

Microscopy images of fibers in Group B samples treated with Herzberg stain. a–f 1st. MG-1, 1st. MG-17, 1st. MG-18, 1st. ZG-2, 1st. ZG-3, 1st. ZG-6

The lack of plant markers (phytosterol and terpenes) for Py-GC/MS analysis may allow us to identify bast fibers. When there are numerous paper samples, it may be feasible to use the EIC method for sample screening (Fig. 6) and type distribution (Fig. 7). Notably, conducting standard sample analysis on fibers from various traditional handmade papermaking processes, establishing corresponding plant marker spectra, and conducting fingerprint comparisons and identification will greatly improve accuracy.

In summary, the physical performance analysis revealed that the TFCD paper exhibited a lower apparent density, and the observation of curtain patterns allowed us to track their manufacturing techniques. The investigation of preservation status, chromaticity, XRF, and infrared analysis provides a basis for the aging, discoloration, and protection of paper. The rapid identification of paper-making fibers in a large number of paper samples using Py-GC/MS is feasible.

Temporal variation in the technical history of fiber raw materials

The paper used by ordinary people includes mulberry paper [13, 15], bast paper [17] (ramie [14] or hemp [13]), mixed bast paper [14] (ramie mixed with hemp), bamboo paper [13,14,15,16], mixed bark paper [13, 14] (mulberry mixed with other bark fibers), mixed papers made of bast fibers and herb fibers [18], xuan paper [14] (i.e., a Chinese handmade paper called ‘xuan’ used by artists and calligraphers, wingceltis bark fiber mixed with rice straw fiber) and recycled paper [13] reported by scholars. The raw materials of the contract documents were diverse, and there were differences between folk document paper and cultural calligraphy and painting paper (e.g., bamboo fiber paper and xuan paper were quite popular during the Ming and Qing dynasties [60]).

Until the 1950s, the paper used for TFCD was mainly handmade from bast fibers. Only the land and property ownership certificates uniformly issued by the government were made of machine paper, which is also made from bast fiber. However, exploring why the region continued to use paper made of bast fibers until 1955 requires both chemical analysis and historical records. The distribution and changes in the ancient and modern paper industry during different historical periods, such as differences caused by papermaking production processes and transportation restrictions, involve the dependence on raw materials and consumption areas at different times [61].

The Tianshui and surrounding areas during the Qing Dynasty that recorded the paper industry mainly included Kangxian, Chengxian, Wudu, and Huixian (Fig. S8c). Since the Ming Dynasty, bark has been used for papermaking. According to statistics, in 1914, there were 58 paper-making households in Gansu.¹ In the 1930s, some rough paper in Gansu began to be exported outward,² and the paper produced by adjacent provinces was mainly sold in Gansu using paper made from rag bast fiber.³ A survey conducted during the period of the Republic of China in the local area also showed that the main types of papermaking were bast paper and rag bast fibers (Figure S7, marked in red). According to historical records [62], there are many rural papermaking workshops near Tianshui, and there are rich sources for people to buy handmade paper. During the Ming and Qing Dynasties, the paper industry in the economically underdeveloped areas of Northwest China used bark and bast as raw fiber materials [63]. Xinjiang, located in the northwestern region, also uses these two types of fibers as raw materials for papermaking. A study conducted by the author showed that Xinjiang mulberry paper was still used to make paper currency until the Republic of China [27].

The convergence of raw material selection in papermaking in the northwestern region is closely related to the geographical environment and vegetation conditions. The climate zoning map (Fig. S8a) shows that Tianshui is located in the temperate zone and that the vegetation is mainly composed of deciduous broad-leaved leaves (Fig. S8b). Bamboo plants are distributed in tropical, subtropical to warm temperate regions. From the perspective of climate and vegetation zoning, the eastern part of the Gansu region can contain bamboo resources other than bark (mulberry) and bast (hemp and ramie). The paper industry in neighboring provinces has bamboo paper. In Shaanxi Province, bamboo paper and bark paper are produced [64], while in Sichuan Province, bamboo paper is common. However, even if the climate allows, the eastern part of Gansu Province has not developed bamboo resources for papermaking. With historical changes in climate and temperature, bamboo was no longer cultivated as an economic tree north of the Yellow River in China after the early Ming Dynasty [65]. The border between eastern Gansu and Shaanxi was located in the middle of the Qing Dynasty, and bamboo forests were still extensively cut down and have since disappeared [66].

TFCD still uses only bark fibers and bast fibers, and bast fibers have been used for a longer time instead of the more prevalence bamboo paper of that historical period (Qing dynasty) [60]. We also need to pay attention to materiality research, which involves the use of paper media of class status. Research on other non-contract documents in Gansu Province indicates that bamboo paper is used in the region. Scholars have analyzed realistic painting screens at the Jingchuan Museum in Gansu, and bamboo paper was used [67]. A survey of the ancient book paper collection in the Gansu Library showed that the majority of ancient book paper is made from bamboo paper produced in the southern region but not locally produced. Other books with regional and folk attributes, such as the paper used in Handwritten Chinese Opera Scripts (清末民国西北地方曲艺手抄剧本), are made of bark and bast fibers [68]. This indicated that there is a significant difference between the paper used in books and paintings with high cultural attributes and the paper used in folk documents.

Conclusion

There are 107 paper samples from a batch of 96 folk contract documents in the Tianshui area of Gansu Province, revealing some forms of contract documentation in China after the Qing Dynasty (after 1616 AD). Physical and chemical analysis provides us with necessary references for the use of paper by the common people during historical periods, and it is found that the quality of paper in this region is significantly different from that of paper studied in other regions. Multiple techniques were used to measure and characterize the basic physical characteristics of the paper, and a materiality research model with continuous time attributes and a large number of samples was implemented. At the same time, we focused on the chromaticity of inorganic components and organic functional groups, expanding our understanding of paper aging and yellowing. By harnessing the synergistic power of microscopic analysis and chromatographic techniques for cross-verification, it becomes conceivable to accurately discern a vast array of sample fiber types through the distinctive compounds unique to plants.