Introduction

The earliest known examples of handmade paper were produced in China. The invention, development, and popularization of ancient paper were inextricably related to the Chinese civilization, which had a profound impact on the development and inheritance of civilization.

Chinese paper cultural relics constitute significant collections in major museums around the world and have been the subject of extensive scholarly interest. Especially, the Jin and Tang Dynasties were important periods in the development of Chinese paper. During the Tang Dynasty, the newly developed technologies of papermaking and papermaking led to the production of a wide variety of papers of great cultural significance and historical value. The technology used to produce paper was highly sophisticated, resulting in the creation of many famous papers that were widely praised throughout history. It is unfortunate, however, that most of these papers have not survived.

In the 26th year of Guangxu’s reign in the Qing Dynasty (1900), the Dunhuang Sutra Collection Cave was discovered, containing more than 40,000 precious documents dating from the Jin Dynasty to the Song Dynasty (4th to 11th centuries AD). The documents were Buddhist scriptures, and about 90% of them were written in Chinese. There were also various documents in ancient Tibetan, Uighur, Khotan, and Sanskrit. Dunhuang served as the first gateway for the introduction of Buddhism into China, and it became a major center for Buddhist sutra translation. It was clear that not all temple patrons who wished to donate or reproduce these sacred texts had the skills to write. As a result, in an era before the advent of printing, the growing popularity of Buddhism led to a significant increase in the demand for handwritten copies of the sutras. Meanwhile, the art of calligraphy reached its zenith in the handwritten copies of the sutras. By this time, classical paper relics had attracted worldwide attention and had become the main type of critical collections in various museums1,2,3.

These handwritten copies not only provided a glimpse into ancient Buddhist scriptures but also served as invaluable artefacts and historical records documenting the development of Chinese calligraphy. The color of the paper was yellow, often associated with dignity and solemnity. It had special significance in ancient culture. The use of yellow paper to copy sutras reflected respect for religious classics. The hard yellow paper is tough after special treatment. Then the paper was dyed with phellodendron bark to protect the sutras or calligraphy from being damaged by moths or worms. Therefore, the hard yellow paper (Yinghuang paper) was widely used in sutra writing, which was considered a typical representative of high quality paper due to its sophisticated processing technologies, great strength, smoothness and dignified color4,5,6,7.

Therefore, further research on hard yellow paper was required to obtain a comprehensive assessment, which can be achieved by using additional analytical techniques such as Raman spectroscopy, infrared spectroscopy, and multidimensional gas chromatography. Some research has focused on ancient paper additives8,9,10, and various analytical methods have been developed to identify paper fibers11,12,13. Gong et al. used scanning electron microscopy and other methods to analyze the compositional characteristics of writing pigments, paper fillers, and protective layers. They suggested that beewax was present in the protective layer of the paper14. A comprehensive study of representative samples of ancient paper can help us understand the characteristics of ancient paper itself, so that we can infer the ancient paper processing technology, which has a positive impact on the protection of ancient paper itself and traditional papermaking technology. Scholars have conducted extensive scientific and technological analysis of important manuscript materials in libraries and archives15. The British Library was commissioned by the China Paper Research Institute to analyze and study samples of Sui and Tang dynasty paper from Dunhuang in its collections. In 1991, Berndt had analyzed a batch of 12th-century Chinese linen documents using infrared spectroscopy. The results showed that the particles coated on the surface of the cultural relics were polysaccharides, similar to gum arabic. It was speculated that the polysaccharides were plant glues applied to the surface of the paper using a sizing process5. In 2000, resonance Raman spectroscopy was used to identify the yellow dye of the Vrajna Prajna Sutra, which was dated to 868 AD. The results showed that it contained mainly berberine hydrochloride and flavonoid8. Japanese scholars Kazuyuki Jiangnan et al. used scanning electron microscopy and X-ray fluorescence analysis to analyze the composition of twenty Dunhuang manuscripts collected by Stein in the British Library16. In 2008, a polarizing microscope combined with the Herzberg staining method was used for fiber analysis of Dunhuang documents in the Tibetan Special Collection of the British Library. The results showed that five of the samples contained all or part of ramie fiber, both samples contained all or part of bark fibers, and one sample had probably added fiber of hemp or jute17,18,19. The paper collections of the Dunhuang manuscripts in the Gansu Provincial Museum from the Northern Wei Dynasty to the Tang Dynasty were analyzed in 2013. The scholar agreed that most of the Dunhuang scrolls were ancient paper produced by papermaking using both starch sizing and wax coating techniques20. Chinese researchers have also conducted a series of studies on paper cultural relics, mainly focusing on the technology of papermaking, changes in processing technology, and communication channels of documents21,22. Thus, the achievements of the previous sages have provided an important reference for advancing the research of ancient hard yellow paper.

However, many problems could not be solved effectively because the number of research samples was too small and the method was not comprehensive enough. Firstly, there was no literature that accurately recorded the production methods of hard yellow paper during the Tang Dynasty, and the details of the materials and processes were not really clarified. According to ancient literature (Table 1) such as ‘Records of Travel during Official Career’, ‘Collection of Famous Paintings during Past Dynasties’, and ‘Zi Tao Xuan miscellany prose’, the processes used to achieve the smooth, glossy surface were sizing, waxing, and calendering. However, there is no complete scientific conclusion about whether waxing was the key step, and there are no accurate scientific and technological records in the literature. Secondly, given the complexity of the technology used to produce hard yellow paper, the surface characteristics of the ancient paper cultural relics, in particular, were very different from their original state. The existence of the coating layer was not properly confirmed by the surface topography. Thirdly, if no wax was found, how did the hard yellow paper ensure a smooth and firm surface? With the continuous development of modern science and technology, detection methods are constantly being refined, which could deeply and comprehensively study the ancient papermaking technology and analyze its scientific principles.

Table 1 Ancient literature on the waxing process
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The paper cultural relics in the Gansu Provincial Museum, which have been handed down from generation to generation, are of great historical, artistic, and scientific value. The analysis results provide a scientific basis for answering the above questions. This project made extensive use of a variety of analytical techniques. For example, ultra-deep field microscopy and bio-optical microscopy have been used to study the surface and fibers of paper relics. An ultra-high performance liquid mass spectrometer and micro-infrared spectrometer were used to obtain dye and surface elements with a tiny size of paper relics, falling paper scraps (1 × 2 mm) as the research object. Twenty-four hard yellow paper relics were analyzed by a relatively comprehensive scientific method. The paper manufacturing process was revealed from the aspects of surface gloss, paper surface morphology, and dyeing. Based on the above analysis results, we would have a clear understanding of the processing techniques and specific use of hard yellow paper in the Tang Dynasty.

Method

Information about paper relics

Samples of hard yellow paper relics of the Tang Dynasty (AD 6th century to the 9th century) were selected as the main research objects (Table 2) from the Gansu Provincial Museum. The average size of a single sheet of paper was 24.5–28.1 cm in width and 34.3–55 cm in length. There was a special sample No. 13079 with a size of 27.4 cm × 143.8 cm, which was also the longest single sheet of cultural relics that we had actually measured and reported in literature so far. The chromatographic samples were dropped on paper scraps. The size of the largest fragment was 1 × 2 mm.

Table 2 Description of 24 paper relics
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When selecting samples, it is common to choose samples with a smooth, shiny surface and a yellow color. However, the texture of the samples varied and in order to have a universal range of samples, both thin and thick samples were selected, as well as samples that felt tough and slightly brittle.

Chemicals

Methanol (MeOH) and acetonitrile (ACN) were purchased from Fisher Scientific (Pittsburgh, PA, USA). Ultrapure water (18.3 MQ) was produced by a Milli-Q system (Millipore, Bedford, USA). ZnO, I2, KI, formic acid (FA), and dimethyl sulfoxide (DMSO) were purchased from Aladin (Shanghai, China). Ethylenediaminetetraacetic acid (EDTA) was purchased from Fuchen Chemical Reagents Factory (Tianjin, China).

Fiber analysis

The paper fibers were analyzed by Iodine-Zinc Chloride staining. Herzberg dye was prepared according to the method in international standard ISO 9184–3:1990 “Paper, board and pulps; fiber furnish analysis; part 3: Herzberg staining test”. The image processing system can provide the fiber length, width, and other information, combined with fiber atlas11, to determine the type of fiber. Data on paper fibers were obtained using the microscope (Zeiss Axio Lab.A1, Germany), the polarizing microscope (Zeiss Scope-A1, Germany), and the ultra-depth of field microscope (KEYENCE VHX-6000, Japan).

FTIR analytical method

Data of substance was obtained using the micro-infrared spectrometer (Thermo Scientific Nicolet IN10, USA). The micro transparent substance on the surface of the samples was placed on the diamond sample pool for tablet processing. OMNIC Specta software and IRUG infrared spectrum database were used to identify the mixture spectra. Spectra were collected in the range of 550–4000 cm−1.

UPLC–MS analytical method

MeOH/FA/10 mM EDTA aqueous solution (85:5:10, v/v/v) was used as an extraction solvent to extract dyestuff from the sample paper. The extraction procedure was performed as bellow: 1–2 mm pieces of paper were placed in sealed tubes containing 1.0 mL extraction solvent, sonicated at 75 °C for 50 min, centrifuged and took the supernatant, then dried up with a gentle nitrogen stream. The residue was redissolved with 50 µL acetonitrile/water (1:1, v/v) for determination. The extraction solvents without the paper sample were performed with the same extraction procedure, which was used as a blank sample.

Extracts of dye components from the paper sample were analyzed by ultra-performance liquid chromatography/mass spectrometry (ACQUITY UPLC, Waters, USA). Separations were carried out using a Waters BEH C18 (1.7 µm, 2.1 × 100 mm), then eluted with water (A) containing 0.1% formic acid and acetonitrile (B). The flow rate was 0.25 mL/min. The elution gradient was started at 5% A, holding for 0.5 min, and a linear gradient was applied to increase the A to 95% in 8 min and was held at 95% A for 2 min. The column was reset to its original condition with a linear elution in 0.1 min and equilibrated for 1.4 min before starting the next injection. Column temperature was 30 °C, the injection volume was 1 µl.

Mass spectrometry (MS) analysis was accomplished by using an Xevo G2-XS Q-Tof mass spectrometer (Waters Corp., Milford, MA, USA) equipped with the ESI ion source. The acquisition mode was MSE with the low energy (LE) of 4 eV and high energy (HE) ramping from 20 to 40 eV to acquire the MS data. Data acquisition, processing, and instrument control were performed using Masslynx 4.1 software. Mass spectra was obtained in positive and negative mode. The source temperature was 120 °C, the cone voltage was 40 V, the desolvation temperature was 450 °C, the flow rate of the desolvation gas (N2) was 800 L/h, and the nebulizer gas flow rate was set at 10 L/h.

Result

Physical properties of paper relics

By analyzing the data in Table 1 and correlating the gloss with the thickness of the samples, it is possible to deduce a general tendency: Samples with higher gloss are generally less than 80 μm thick. According to the calculated thickness and grammage, it can be seen that the grammage and thickness of the curve of the trend are basically the same; the higher the grammage of the paper, the greater the thickness of the following will be. If the apparent density (g/m3) is calculated and correlated with the thickness, it can be seen that most of the thicker papers have lower apparent densities, e.g., 10424, 10481, and 10455, while the thinner papers have higher apparent densities, e.g., 10415, 10427, and 13079.

According to simulation experiments, the gloss of the paper is reduced after sizing. However, the few samples with high gloss and low thickness are calculated to have a high surface density, so the smooth surface should have been obtained by physical pressing after sizing, e.g., calendaring. As opposed to waxing, we consider that processes such as calendering should be the key process for paper smoothing.

Paper fiber analysis

Based on the width, color, and morphology of the fibers, the preliminary assessment of the 24 paper cultural relics of the papermaking fiber type was completed. Paper fiber dyeing could distinguish the hemp fiber and bark fiber, which were the main materials of the paper. Different fiber types have different end morphologies. The end of the fiber shows obvious fibrillation; it is likely to be hemp fiber, and the width of the hemp fibers was 20–60 μm. According to the literature20, the gelatinous membrane on the surface of the fiber, also known as the transparent membrane, was one of the main characteristics of mulberry bark fibers (red arrow in Fig. 1).

Fig. 1: Figures of the dyed fibers.
figure 1

Fibers dyeing (a 13093, b 10481, c 13079, d 10413).

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The longest length of the fiber from sample 10427 which could be measured was 7.2 mm. The widths of the fibers were shown in Table 3.

Table 3 Information of fibers from 24 samples
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The fibers were cylindrical and wine-red after dyeing, with irregular transverse joints in the outer walls of the fibers. There was a distinctive transparent gelatinous coat attached to the outer walls, and a large amount of pale-yellow glue attached around the fibers. Combined with literature, the samples 13093 and 10481 could be bark fiber (as shown in Fig. 1a, b), and fibers from 10413 and 13097 could be hemp (as shown in Fig. 1c, d). In this experiment, there were 8 samples that contained bark fibers. Thus, in the Tang Dynasty, bark fiber began to be more commonly used in paper making. This changed the previous perception that hard yellow paper was made by hemp23. All 24 samples were examined with the fibers dyeing. There were 12 samples of hemp paper, 10 samples of bark, and 2 samples of mixed materials (Table 3).

Papermaking process

Combined with the gloss and thickness data from Table 1, it was clear that there was a proportional relationship between thickness and gloss value. The gloss value of sample 10471, with the thinnest thickness, was relatively high, while the gloss values of papers with greater thicknesses were generally lower than 4. Thinner thicknesses of paper had higher gloss values. There were exceptions, such as samples 10456 and 10475, which were thin and low gloss. So the three samples, 10471 (high gloss and thin), 10475 (low gloss and thin) and 10433 (low gloss and thick), were selected as typical samples for the surface process based on their respective gloss levels and paper thicknesses. In conjunction with the fiber staining results, there was some glue or starch was observed in the fiber staining results of. The ultra-depth microscope was used to observe the fiber distribution on the surface of the paper and the fiber distribution under light transmission conditions. The morphology of fibers from the three different papers is shown in Fig. 2.

Fig. 2: High-magnification photographs of paper surfaces.
figure 2

Backlighting photography and Surface of samples. (a, b sample 10471, c, d sample 10433, e, f sample 10475).

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The fiber interweaving of 10471 was compacted (Fig. 2a, b). And the morphology of the surface fibers was also different from that of the other samples, it looked clear. The interweaving between the flattened fibers was tighter and the points of interweaving became more obvious under light transmission conditions.

As shown in Fig. 2c, d, there were small particle and light yellow additives were observed around the fibers of 10433. The fibers were not tightly woven and layer was loose under light transmission conditions. However in Fig. 2e, f, the fibers of sample 10475 were sparsely interwoven, but each layer was tightly compacted. The points of interweaving became more obvious under light transmission conditions.

In this way, the morphological characteristics of the fibers could be correlated with the paper processing techniques. The smoothness of the surface of the hard yellow paper was not significantly influenced by additives or sizing, but was the result of an improved stamping and calendering process.

Composition analysis of surface coating and filler

After observing the surface morphology of hard yellow paper samples with an ultra-deep microscope and dyeing the fibers, relevant information about the processing technology of ancient paper became clear. If the paper was stained with mineral pigments, colorful mineral particles should be seen. Although some of the paper surface visible white mineral particles, presumably, should be added to the paper-making process, other additives, such as limestone, dolomite, chalk, shells, and other calcium carbonate content of the material, are used to adjust the pH value of the paper and the softness and hardness24.

Starch particles, after staining, were clearly visible in Fig. 3. After staining, the blue lump observed under a biological microscope was generally attributed to the reaction of wheat starch particles with iodine (Fig. 3a)22. Under polarimetric observations, it was determined that the starch sizing process used in the paper (Fig. 3b, d). As shown in Fig. 3c, some dark or colorless granules were observed after fiber dyeing of sample 13101. In addition, some of the starch granules after Herzberg’s staining may not be blue or purple, but light colored. As shown in Fig. 3c, d, the light color particles (↗) also showed an obvious polarimetric crossing phenomenon. Therefore, it was concluded that the Tang Dynasty hard yellow paper samples were treated by sizing with wheat starch.

Fig. 3: Wheat starch particles in paper samples.
figure 3

Wheat starch particles in paper samples (a fiber dyeing of sample 10415, b polarizing microscope 90° of sample 10415, c fiber dyeing of sample 13101, d polarizing microscope 90° of sample 13101).

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However, it was difficult to determine the other composition of the surface coating materials by microscopic observation alone. Therefore, it was necessary to obtain relevant information on the type of raw material of the surface coating from hard yellow paper samples. Fourier transform infrared spectrometer (FTIR) has rich structure and composition information, and the wavelength and intensity of different groups or the same group in different chemical environments are obviously different. Based on these characteristics, the micro-infrared spectrometer was very suitable for analyzing the composition properties of hydrocarbon organic materials, paper raw materials, and cementing materials in paper cultural relics. For the IR analysis, 15 samples were selected, which could be subjected to micro-infrared detection. Because cellulose is one of the polysaccharides, we use a scalpel to gently peel off the tiny top layer of starch during sampling. Avoid mixing fiber samples as much as possible. Samples 10464, 13079, 13093, and 10413 were selected as examples for infrared detection, which obtained clear and typical spectra. The characteristic peaks in the infrared spectra are shown in Table 4.

Table 4 Characteristic infrared absorption band of the material of the surface coating
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According to the infrared spectrum of 10464 as shown in Fig. 4a, the strong and wide absorption peak near 3294 cm−1 was the stretching vibration absorption peak of N–H. Around 2961 cm−1 and 2931 cm−1 were the antisymmetric and symmetric stretching vibration absorption peaks of methylene.

Fig. 4: Infrared spectrograms of samples.
figure 4

Infrared spectrograms of samples (a 10464, b 13079, c 13093, d 10413).

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The absorption peak of carbonyl C=O stretching vibration near 1654 cm−1 belonged to the amide I band, the C–N–H bending vibration near 1543 cm−1 belonged to the amide II band, C–N stretching vibration and the C–N bending vibration near 1240 cm−1 belonged to the amide III band. The above peaks were consistent with the characteristic peak of the amide group (–CONH2) of the animal compound, which reflected that the surface coating of the sample contains protein substances. The absorption peak near 1400 cm−1 was attributed to C–H bending vibration25,26. From the fiber staining analysis micrograph of 10464 samples, it could be seen that the yellowish glue aggregates around the fibers. By comparing the IRUG database, there was a high spectral similarity between Fig. 4a and egg white; it was speculated that animal glue would be egg white, exists in this sample.

The sample of coating raw material containing plant glue was taken as example 13079. According to the infrared spectrum Fig. 4b, the strong and wide absorption peak at 3344 cm−1 was generated by the stretching vibration of polysaccharide compound O–H. The absorption peak of –CH2 stretching vibration was near 2917 cm−1. The O–H bending vibration absorption peak was near 1640 cm−1, and it could contain the carbohydrate carbonyl stretching vibration absorption peak. The absorption peaks in the range of 1427–1318 cm−1 were generated by variable-angle vibration of CH3, CH2, and CH. In the infrared spectrum, there were characteristic absorption regions of polysaccharides ranging from 1157 to 1033 cm−1, in which multiple acromial peaks were generated by C–O–C and C–OH stretching vibrations in polysaccharides, indicating the presence of polysaccharides on the surface coating of the sample27,28,29. As shown in Fig. 4a, b, there were obvious differences between the peaks in the range 1650–1033 cm−1. In Fig. 1a, many yellow transparent colloidal substances were clustered around the fibers. Based on the infrared spectrum analysis results, it could be inferred that the sizing raw material of sample 13079 may contain plant glue.

The sample of coating material, consisting of glue, was taken from 13093. It could be seen from the infrared spectrum Fig. 4c of the sample that the strong and wide absorption peak of 3336 cm−1 was the overlapping absorption peaks of O–H and N–H stretching vibration, indicating the presence of hydroxyl and amino groups. Compared with Fig. 4b, the band observed at 1647 cm−1 was strong and sharp, probably because of the water adsorbed in the amorphous region of starch granules30. The characteristic absorption peak of C–N stretching vibration and C–N–H bending vibration indicated the existence of protein substances and was presumed to be animal glue22. The spectral peak of polysaccharide indicated that polysaccharide might exist in the sample, which was presumed to be plant glue31. It contained polyhydroxy compounds with many hydrogen bonds between the molecules, but the molecules or hydrogen bonds were broken after heating. This procedure would make the gradual decrease of the intensity of the O–H stretching vibration. It can be assumed that plant-derived polysaccharides such as wheat starch or peach gum were added as papermaking additives in the pulping process. According to the fiber dyeing analysis (Fig. 1a), a large number of yellow transparent gelatinoids had accumulated around the fibers from sample 13093, indicating that this sample might have been processed with raw materials containing glues. As recorded in some ancient Chinese documents, processing techniques of the hard yellow paper of the Tang Dynasty involved placing a paper on a hot iron and coating it with yellow wax until smooth. Combined with the hydrophobicity of wax and the infrared test results of hard yellow paper, other plant glues such as starch or peach gum were more likely to be used as major additives than wax.

According to the infrared spectrum Fig. 4d of 10413, the absorption peak indicates that polysaccharide is contained in the surface coating of the sample. In combination with the microscopic Fig. 1d of the fiber dyeing analysis of 10413, it could be seen that there are a lot of purple amorphous substances and transparent yellow colloids around the fibers after dyeing. According to the rule, which was that amylopectin would appear purple and amylose would appear blue when they encountered iodine, it was speculated that wheat starch existed in the paper sample.

Composition analysis of dye

Through the careful observation and analysis of both sides of the samples, it was found that most of the surface coloring components of the samples were very uniform and compact, and no granular mineral pigments were found on the surface of the samples under the microscope. This showed that the hard yellow paper samples of the Tang Dynasty should be made by dye. However, numerous problems remain in the analysis of ancient paper dyeing components. First, the available samples of ancient paper artifacts are particularly valuable, and the number of samples available for analysis is limited. Secondly, the complex and unstable composition of organic dyes, which is sensitive to breakage with age, will additionally affect the accuracy of qualitative analysis of the dye’s composition. As a result, in the process of dye analysis, the analytical methods of microsampling, high sensitivity, and strong accuracy should be adopted to obtain relatively reliable analysis results.

Due to the limitation of the amount of relic samples, ultra-performance liquid chromatography-four-pole time-of flight mass spectrometry was used to analyze the dyes. In this experiment, the dyes extracts of 13079, 10481 and 10427 in the hard yellow paper samples were analyzed, and the results showed that the dye components were from phellodendron (Fig. 5).

As shown in Fig. 5b, in ESI+ mode, characteristic components of dyes were detected in 10481 at 3.71 min, 4.51 min, and 5.15 min. The intensity of the 5.15 min chromatographic peak is the highest among the peaks, which indicates that this substance should be the main component of the dye. The mass spectrum shows that the peak at 5.15 min is the compound of m/z 336.136, with the fragments of m/z 320.101, m/z 292.105, and m/z 278.091 in the high energy channel (Fig. 5c). Through structural analysis and related literature, the compound should be identified as berberine10,31,32. In addition, a molecular ion peak of m/z 352.130 existed at 4.51 min, with its corresponding fragment ions of m/z336.095, m/z320.101, and m/z308.100 (Fig. 5d) could be identified as palmatine33. At 3.71 min, a molecular ion peak with a mass-charge ratio of m/z 342.179 existed, and the corresponding fragment ions in the secondary mass spectrum were m/z192.107 and m/z177.084, which is phellodendrine34,35.

Fig. 5: UPLC-QTOF-MS analysis of dye.
figure 5

a Total ion flow diagram of the extraction of dye from sample 10481 under ESI+ of UPLC-QTOF-MS. b Molecular ion peak (a) and corresponding fragment ion peak (b) from sample 10481 in 5.15 min. c Molecular ion peak (a) and corresponding fragment ion peak (b) from sample 10481 in 4.51 min. d Molecular ion peak (a) and corresponding fragment ion peak (b) from sample 10481 in 3.71 min.

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According to the data in the literature36,37,38, the plant containing berberine was Phellodendron amurense Rupr., Coptis chinensis Franch, yellow vine, etc. Berberine, palmatine, and phellodendrine were the main components, and berberine and palmatine contain the main chromophore groups. So, it should be dyed by phellodendron (as shown in Table 5).

Table 5 The compounds in relics
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The analysis of the dyes used in ancient paper cultural relics can help to understand the ancient culture at that time. Even the dye’s origin and the dyeing process could reflect more information on social, technological, and cultural development, aesthetic orientation, and trade exchange in ancient times. Therefore, it is of great significance to clarify the types of raw materials and dyes of ancient dyed paper. It is also valuable for the color restoration and conservation of ancient paper relics in the future.

Discussion

In order to obtain the relevant information on processing characteristics, the basic properties and composition of hard yellow paper samples collected in the Gansu Provincial Museum were analyzed systematically in this study. Through the analysis of the surface microscopic morphology, fiber types, dyeing, and coating materials samples, important information about the basic properties and characteristics of the hard yellow paper in the Tang Dynasty was obtained. It could provide rich basic data for promoting the understanding of the processing technology of the hard yellow paper from the Tang Dynasty.

According to the analysis and identification of the dyeing components of ancient paper cultural relics, the dyeing processing process can be speculated, which helps us understand the source types of dyes used by ancient people at that time, and even we can further obtain relevant information about the development of science, technology, culture, and esthetic orientation in ancient society. In addition, the raw materials for writing paper in the Tang Dynasty were also diverse, including hemp, bark, and hemp mixture, and bark paper. The variety of raw materials may have been due to the changing cultural centers at the time, and the great demand for paper led craftsmen to use local materials, further promoting the development of the papermaking process. The analysis results of the paper production process show that most of these hard yellow paper relics in the Tang Dynasty were colored by yellow dye and double-sided coating, the most common type of material in the sizing process was wheat starch, besides some were peach gum and animal glue (egg white). The surface coating of the samples gave little information about the wax.

However, the gloss of the hard yellow paper samples is generally higher than that of other types of writing paper of the same period, presumably because the hard yellow paper has undergone surface treatments such as sizing. In this study, there is variability in the gloss level of the hard yellow paper samples, which may be due to differences in the thickness and uniformity of the coating and the degree of refinement in calendering. In fact, the hard yellow paper was subjected to a surface treatment such as sizing, supplemented by fine polishing and calendering, which had a significant effect on improving the gloss level, reflecting the more sophisticated processing of the hard yellow paper samples.

A comprehensive study of representative samples of ancient paper can help us understand the characteristics of ancient paper itself, so that we can combine the documentary records to speculate and confirm a part of the ancient paper processing technology. The work has had a positive impact on the restoration and protection of ancient paper and traditional papermaking techniques in the later period.