New scientific evidence of the materials and craft of Hard yellow paper in Tang Dynasty collected by Gansu Provincial Museum

doi:10.21203/rs.3.rs-3971567/v1

Hard yellow paper is one of the few surviving Chinese famous paper from more than a thousand years ago. In particular, the documents found in Dunhuang are important collections in major museums around the world. Although there has been some research before, the production process is still a lot of controversy. In order to find out the development of materials and making craft, such as the fiber, dyeing and coating, 24 paper relics of Hard yellow paper in the Tang Dynasty from Gansu Provincial Museum have been observed and tested comprehensively by spectroscopy and spectrometry Most of the paper is handed down from generation to generation so they are in good condition of preservation. Raw materials and different surface coating materials of paper are observed by ultra-depth microscope surface observation. The organic composition of paper coating and fiber dyeing can also be accurately obtained by micro-infrared spectrometer and ultra-liquid chromatography-mass spectrometry. a series of new evidence has been found, which provides the possibility to solve the current questions and disputes. According to the results, the hemp, ramie and bark fibers are considered as raw materials; the raw materials of paper’s surface may contain plant glue and animal glue. Berberine, palmatine and phellodendrine are detected from fragments which were considered as the dye of paper. Finally, the Hard yellow paper will be upgraded by hammering, calendaring and surface coating in the Tang Dynasty.

Hard yellow paper

fiber analysis

papermaking process

micro infrared

dye analysis

Ancient handmade paper was born from 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. The invention and popularization of paper made the change of the carrying material of calligraphy from silk to paper.

Chinese paper cultural relics are important collections in major museums around the world, and have been widely concerned by scholars. According to the records of “the Paper Chronicles” [1], “Changwu Chronicles” [2], and “Xingge Ancient Key Points” [3], papermaking technology spreads to Xinjiang with cultural exchanges and trade. The neighboring countries such as Goryeo (today's Korean Peninsula) and Fuso (today's Japan) introduced the papermaking method and produced paper. Especially the Jin and Tang Dynasties were the important periods in the development of Chinese paper. In this period of time, the new technologies of paper manufacturing and processing had produced a variety types of paper which have heavy cultural significance and historical value. Their production technology was very mature, and a large number of famous papers widely praised in history appeared. But unfortunately, most of these papers didn’t remain.

In the 26th year of Guangxu's reign in the Qing Dynasty (1900), the Dunhuang sutra collection cave had been discovered, in which there were more than 40,000 precious documents from the Jin Dynasty to the Song Dynasty (AD 4th century to the 11th century). The documents were Buddhist scriptures and about 90 percents of them were written in Chinese. In addition, there were various documents in ancient Tibetan, Uighur, Khotan and Sanskrit. At this point, the classical paper relics had attracted worldwide attention and become the key type of the critical collection types in various museums [4–6]. The writing of the Dunhuang Sutra was elaborate and extremely large in number. The Hard yellow paper (Yinghuang paper) was commonly used in sutra writing and considered as a typical representative of high-grade paper due to its sophisticated processing technologies, great strength, smoothness, and dignified color.

The contents of the Hard yellow paper of the Tang Dynasty reflect the social and historical background of the period, the life of the different classes, and the mentality of the people of faith. Therefore, it is important to study the gradual changes in the beliefs of different classes of people during the Tang Dynasty. The cultural exchanges of the sutras reflect the cultural characteristics of northern China and epitomize the exchange of Buddhist culture between ancient China and surrounding regions. The Hard yellow paper with Buddhist scriptures is the most important carriers. Therefore, it is still necessary to do deeper research to get a comprehensive judgement by using some additional methods, such as infrared spectroscopy. It is an important method for the exploration of the historical value, artistic value, and scientific value of the Hard yellow paper. A thorough study of the records on the processing process of paper in ancient Chinese paper documents is helpful to restore the ancient papermaking technique and paper processing process.

Scholars also made a lot of scientific and technological analysis of significant handwritten materials in libraries and archives. The British Library had commissioned by the China Paper Research Institute to analyze and study the paper patterns of the Sui and Tang dynasties of Dunhuang in their collections [7]. In 1991, Berndt et al. had analyzed a batch of Chinese linen documents of the 12th century by infrared spectroscopy, and the results showed that the particles covered on the surface of the cultural relics were polysaccharides similar to gum Arabic, which is speculated to be plant gum applied by ancient Chinese artisans on the surface of the paper [4]. In 2000, Bell et al. used resonance Raman to identify the yellow dye of the Vrajna Prajna Sutra that was dated to 868 AD, and the results showed that it mainly contained berberine hydrochloride and flavorin [5]. 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 Library [6]. In 2008, Helman Wazny et al. used polarizing microscope and Herzberg staining method to conduct fiber analysis on Dunhuang documents in the Tibetan Special Collection of the British Library. The results showed that five of samples completely or partially contained ramie fiber, two of the samples contained the bark fibers entirely or partly, and one sample had been added the fibers of hemp or jute probably [8]. This conclusion had been considered by Li XC et al. who investigated more than ten pieces of Dunhuang manuscript paper collected in Gansu Province's museum collections from the Northern Wei Dynasty to the Tang Dynasty, and the analysis showed that most of the Dunhuang scrolls were ancient paper produced by papermaking, with both starch sizing and wax coating technique [9]. Chinese researchers have also carried out a series of studies on paper cultural relics, mainly focusing on technology of making paper, changes of processing technology, and communication channels of documents [10–13]. So, the achievements of the preceding sages provided an important reference for advancing the research of ancient the Hard yellow paper.

Even though, many problems still not been solved effectively because the number of research samples was too small and the method was not comprehensive enough. Firstly, there is no literature to accurately record the production methods of the Hard yellow paper during the Tang Dynasty, and its details of materials and processes were not really cleared. Some records of paper medicine to maintain the paper being smoothly, and some records of wax on the surface of the paper to maintain the rigidity of the paper, but there was no exact scientific and technological literature records. Secondly, in view of the complexity of the Hard yellow paper production technology, especially the characteristics of the surface of ancient paper cultural relics had been greatly different from the initial state. The existence of the coating layer was not properly confirmed from the surface topography, and similar XRD analysis of the elements only explained the composition of the substance, and couldn't reflect its chemical structure. Thirdly, if there was no wax detected, how did the Hard yellow paper ensure a smooth and firm surface? With the continuous development of modern science and technology, the detection methods are processing constantly, which could deeply and comprehensively study the ancient papermaking technology and analyze its scientific principle.

Gansu Provincial Museum has the advantage of collection, and 24 cultural relics were analyzed this time. The paper cultural relics that passed from generation to generation have important historical, artistic and scientific value. The analysis results provide a scientific basis for answering the above questions. A variety of analysis techniques was utilized comprehensively in this project. Such as ultra-depth of field microscopy and bio-optical microscopy were used to observe the paper body and micro-damage detection. Ultra-high performance liquid mass spectrometer and micro-infrared spectrometer were applied in getting dye and surface element with a tiny size of paper relics falling paper scraps (1*2mm) as the research object. 24 Hard yellow paper relics of Tang Dynasty from Gansu Provincial Museum collection were analyzed by relatively comprehensive scientific method. The raw materials of Tang Hard yellow paper were explored from the identification of the paper fibers. The paper making process revealed from the aspects of surface gloss, copying, and staining. Based on the above analysis results, we would have a clear understanding of the processing techniques and the specific use of Hard yellow paper in the Tang Dynasty.

Samples of Hard yellow paper relics of the Tang Dynasty (AD 6th century to the 9th century) were selected as the main research objects (see Table 1, Fig. 1) from Gansu Provincial Museum. The average size of a single sheet of paper was 24.5-28.1cm in width and 34.3-55cm in length. There was a special sample No. 13079 with a size of 27.4cm*143.8cm, 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 paper scraps. The size of largest fragment was 1*2mm.

Table 1 Information of 24 paper relics

Relic NO.	Title	Size(cm)	Grade	Thickness (μm)
02381	Lotus sutra Vol.6	Width:25,Length:468	Ⅱ	166
02481	Kai Shu Nirvana Sutra Vol.24	Width:28.8,Length:852.5	Ⅰ	98
10406	Sutra	Width:25.5, Length:49.2	Ⅲ	/
10413	Golden light Sutra ZantanpinVol.4	Width:27, Length:186.5	Ⅲ	114
10415	Dacheng Aparimitayur sutra	Width:31, Length:344	Ⅱ	141
10419	Nirvana Sutra Vol.28	Width:25.8, Length:95.3	Ⅲ	139
10424	Diamond Sutra	Width:26.2, Length:49	Ⅲ	175
10427	Sutra of Buddhas’ Names Vol.6	Width:24.8, Length:21.3	Ⅱ	74
10430	Si yi fan tian suo xiang sutra Vol.1 and Vol.2	Width:28.1, Length:987.4	Ⅱ	145
10433	Golden light Sutra	Width:25.5, Length:58.5	Ⅲ	146
10437	The Great Prajna Paramita sutra Vol.99	Width:29.5, Length:43.9	Ⅱ	150
10440	Golden light Sutra	Width:25.8, Length:104	Ⅲ	85
10456	Kai shu Upasai Sutra Vol.10)	Width:25.7, Length:459	Ⅰ	79
10464	Lotus sutra Shoujipin Vol.6 , Huachengyupin Vol.7	Width:26.4, Length:784	Ⅱ	117
10471	He Bu Golden light Sutra Vol.6	Width:25.9, Length:34	Ⅲ	61
10475	Da fang guang fo hua yan sutra Vol.14	Width:26.8, Length:138.5	Ⅲ	71
10480	The Great Prajna Paramita sutra Vol.52	Width:26, Length:886	Ⅱ	133
10481	Daji sutra Vol.6	Width:39.7,Length:1869	Ⅱ	87
13079	Mahayana Nirvana sutra Vol.19	Width:26.8, Length:456.7	Ⅱ	57
13083	Saddharmapundarika sutra Vol.836	Width:24.9 Length:538	Ⅲ	62
13093	Xiaopin Prajna Paramita sutra Vol.1	Width:26.2, Length:95	Ⅲ	87
20951	The great Prajna Paramita sutra Vol. 157 Xiangjifo Pin Vol.10	Width:26.4, Length:674	Ⅱ	152
10426	Saddharmapundarika sutra	Width:25.5, Length:356.5	Ⅱ	126
10434	The great Prajna Paramita sutra Vol.13	Width:26.2, Length:785	Ⅱ	134

/：There are mounting process that cannot be measured

2.1 Materials and Chemicals

Samples of Hard yellow paper got from the Chinese Tang Dynasty (6th to 9th century). The reagents were used as follows: ZnO(Guangnuo, Shanghai, China), I₂(Ghtech ,Guangdong, China), KI(Fangzheng, Tianjin, China), methanol (MeOH) (Fisher Scientific Pittsburgh, PA, USA), ultrapure water (18.3MQ) (Millipore, Bedford, USA), formic acid (FA) (Aladin), ethylenediaminetetraacetic acid (EDTA) (Fuchen Chemical Reagents Factory, Tianjin, China). Data of paper fibers were obtained using the microscope(Zeiss Axio Lab.A1, Germany), the polarizing microscope(Zeiss Scope-A1, Germany), the ultra-depth of field microscope (KEYENCE VHX-6000, Japan) and the micro-infrared spectrometer(Thermo Scientific Nicolet IN10, USA). Spectra were collected in the range of 550–4000 cm^− 1. Data of dyeing was obtained by using UPLC–Q-TOF MS (Waters Xevo G2-XS Q-TOF MS, USA).

2.2 Dyestuff Extraction

MeOH/ FA/ 10 mM EDTA aqueous solution (85:5:10, v/v/v) was used as extraction solvents to extract dyestuff from the sample paper. The extraction procedure was performed as follows: 1–2 mm pieces of paper were placed in sealed tubes containing 1.0 mL extraction solvent, sonicated at 75℃ 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 paper sample were performed with the same extraction procedure, which was used as blank sample.

2.3 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; fibre furnish analysis; part 3: Herzberg staining test”. The image processing system can provide fiber length, width and other information, combined with fiber atlas [11], to determine the type of fiber.

2.4 Analytical Method

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.

Extracts of dye components from paper sample were separated by ultra-high performance liquid chromatography (ACQUITY UHPLC, 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. Elution gradient was started at 5% A holding for 0.5 min, and a liner 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 liner elution in 0.1 min and equilibrated for 1.4 min before starting next injection. Column temperature was 30℃, the injection volume was 1 µl.

Mass spectrometry (MS) analysis was accomplished by using a Xevo G2-XS Q-Tof mass spectrometer (Waters Corp, Milford, MA, USA) equipped with the ESI ion source. The acquisition mode was MS^E 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 were obtained in positive and negative mode. The source temperature was 120℃, cone voltage was 40 V, desolvation temperature was 450℃, flow rate of the desolvation gas (N₂) was 800 L/h and nebulizer gas flow rate was set at 10 L/h.

3.1 Paper fiber analysis

Paper fiber dyeing could distinguish the hemp fiber and bark fiber, which were main materials of the paper (as shown in Fig. 2-Fig. 4). The gelatinous membrane on the surface of the fiber, also known as the gelatin coat, was one of the main characteristics of mulberry bark fibers [14, 15]. The bark of conifers had a distinct transverse articulation, the cell cavities were distinct, some of the fibrous cavities were large, some of them were minute, the ends of the fibers were often branched, and ending in a minute round ball. The average length was generally 4.65 ~ 16.94 mm, and the average fiber width was 18.37 µm. The analysis of the fibers of sample 10471 showed that the fibers were cylindrical and wine-red after dyeing, with irregular transverse joints in the outer walls of the fibers. There were a distinct transparent gelatinous coat attached to the outer walls, and a large amount of pale yellow glue attached around the fibers.

According to the fiber’s width and color, we considered the fiber of 13093 could be bark (as shown in Fig. 2), and fibers from 10413 could be hemp (as shown in Fig. 3). In this experiment, there were ten samples that contain 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 hemp [16–18].

As shown in Fig. 4, the plant fibers were tightly interwoven. The interweaving between the flatting fibers were more closely, and the interweaving points become more obvious. In this way, the morphological characteristics of the fibers could be linked to the paper processing, such as hammering with the mallet or rolling and rubbing paper with smooth stones, which were called hammering or calendering.

3.2 Composition analysis of surface coating and filler

According to the observation of the surface morphology from Hard yellow paper samples by ultra-depth microscope, the relevant information on the processing technology of ancient paper become clear. If the paper was coated with mineral pigments, a large number of mineral particles could be observed between the fibers under the microscope. While the paper was coated with rubber and other processing could be observed on the surface of the film layer. Taking 10480 as an example (Fig. 5), the surface of the paper is relatively clean and compact by using the ultra-depth of field microscope. Starch particles after staining were clearly recognizable in Fig. 6. After staining, the clumpy blue-purple color observed under a biological microscope was generally attributed to the reaction of starch particles meeting iodine, and it was determined that the starch filling process used in the paper combined with polarimetric observations (Fig. 7). Therefore, it was inferred that the Hard yellow paper samples in Tang Dynasty were processed by sizing or waxing with the paper surface.

However, it was difficult to determine the other composition of the surface coating raw materials by microscopic observation alone. Therefore, it was necessary to obtain relevant information of the type of raw material of the surface coatings from Hard yellow paper samples. Near infrared spectroscopy has rich structure and composition information, and the wavelength and intensity of near infrared absorption of different groups or the same group in different chemical environments are obviously different. Based on these characteristics, infrared spectrometer was very suitable for the measurement of the composition properties of hydrocarbon organic materials, and the measurement of paper materials and cementing materials involved in paper cultural relics [19]. The following representative samples were prepared for the specific analysis. 10464, 13079, 13093, 10413 were selected as examples for the coating of paper for infrared detection in order to obtain an accurate classification of coatings.

According to the infrared spectrum of 10464 as shown in Fig. 8, the strong and wide absorption peak near 3294cm^− 1 was the stretching vibration absorption peak of N-H. Around 2961cm^− 1 and 2931cm^− 1 were the antisymmetric and symmetric stretching vibration absorption peaks of methylene. The absorption peak of carbonyl C = O stretching vibration near 1654cm^− 1 belonged to the amide Ⅰ band, the C-N stretching vibration and the C-N-H bending vibration near 1543cm^− 1 belonged to the amide II band, C-N stretching vibration and the C-N bending vibration near 1240cm^− 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 [19, 20], which reflected that the surface coating of the sample contains protein substances. The absorption peak near 1400cm^− 1 was attributed to C-H bending vibration [21]. From the fiber staining analysis micrograph of 10464 samples, it could be seen that the yellowish glue aggregates around the fibers. In summary, it was speculated that animal glue exists in the sizing material of this sample.

The sample of coating raw material containing plant glue was taken as example 13079. According to the infrared spectrum Fig. 9, the strong and wide absorption peak at 3344cm^− 1 was generated by the stretching vibration of polysaccharide compound O-H. The absorption peak of -CH₂ stretching vibration was near 2917cm^− 1. The O-H bending vibration absorption peak was near 1640cm^− 1, and it could contain the carbohydrate carbonyl stretching vibration absorption peak. The absorption peaks in the range of 1427 ~ 1318cm^− 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 1033cm^− 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 sample [22, 23]. As shown in Fig. of the fiber dyeing analysis of 13079 (Fig. 3), a large number of transparent yellow colloidal substances were clustered around the fibers. Based on the infrared spectrum analysis results, it could be inferred that the sizing raw material of this sample may contain plant colloidal materials.

The sample of coating raw material consisting of plant gum and animal glue was taken from 13093. It could be seen from the infrared spectrum Fig. 10 of the sample that the strong and wide absorption peak of 3336cm^− 1 was the overlapping absorption peaks of O-H and N-H stretching vibration, indicating the presence of hydroxyl and amino groups. The two narrow cusp absorption peaks at 2919cm^− 1, 2850cm^− 1 were -CH₂ antisymmetric and symmetric stretching vibration absorption peaks. The characteristic absorption peak at 1647cm^− 1 was caused by the stretching vibration of amide-carbonyl C = O, which was the amide-I band. The peak at 1542cm^− 1 was the amide II band, which was the characteristic absorption peak of C-N stretching vibration and C-N-H bending vibration. It indicated the existence of protein substances and was presumed to be animal gum [20]. The spectral peak in the range of 1160 ~ 1033cm^− 1 was the characteristic absorption region of polysaccharide, and the C-O stretching vibration absorption peak near 1060cm^− 1 belonged to polysaccharide, indicating that polysaccharide might exist in the sample, which was presumed to be plant gum [23]. According to the fiber dyeing analysis micrograph Fig. 2, it could be found that a large number of light yellow glue was gathered around the fiber, which indicated that this sample might be processed with raw materials containing animal glue and plant glue. As recorded in some ancient Chinese documents, processing techniques of the Hard yellow paper of the Tang Dynasty was placing a paper on a hot iron and coat with yellow wax until smooth. Combined with the hydrophobicity of wax and infrared test results of Hard yellow paper, other plant gums were more likely to be used as major additives rather than wax.

According to the infrared spectrum Fig. 11 of 10413, the wide and strong absorption peak near 3344cm^− 1 was generated by the O-H stretching vibration of the polysaccharide compound. The absorption peak near 2923cm^− 1 was caused by -CH₂ antisymmetric stretching vibration. Around 1647cm^− 1, the O-H bending vibration absorption peak may also contain the carbohydrate carbonyl stretching vibration absorption peak. In the range of 1430 ~ 1319cm^− 1, the variable Angle vibration absorption peaks were caused by carbohydrate CH₃, CH₂ and CH. The characteristic absorption region of polysaccharide was in the range of 1160 ~ 950cm^− 1, and the C-O stretching vibration absorption peak of polysaccharide is near 1060cm^− 1, indicating that polysaccharide is contained in the surface coating of the sample. In combination with the microscopic Fig. 4 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 appearing blue when they encounter iodine [22, 23], it was speculated that starch existing in paper sample.

Through careful observation and analysis of both sides of the samples, it could be 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 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 easily broken by aging, will additionally affect the accuracy of qualitative analysis of the dyes composition. As a result, in the process of dye analysis, the analytical methods of minor sampling, 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 high performance liquid chromatography-four-pole time-of flight mass spectrometry was used to analyze the dyes. In this experiment, the dye components 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. 12).

As shown in Fig. 12.2 (total ion flow diagram of 10481 sample dye extract under ESI⁺ of PLC-QTOF-MS), in ESI⁺ mode, characteristic components related to dyes were detected in 10481 sample dye extract at 3.71min, 4.51min and 5.15min. The intensity of 5.15min chromatographic peak is higher, which indicates that this substance should be the main component of dyeing raw materials. Analysis was carried out by the mass spectrometry of the corresponding compound. At 5.15min, a molecular ion peak of m/z 336.136 was detected, and the fragment ions in the high energy channel contained m/z 320.101, m/z 292.105 and m/z 278.091 (Fig. 12.3). Through structural analysis and related literature, we can see that the compound should be berberine [24–26]. In addition, a molecular ion peak with a mass-charge ratio of m/z of 352.130 existed at 4.51min, and its corresponding fragment ions included m/z336.095, m/z320.101 and m/z308.100 (Fig. 12.4). Based on the fragmentation information and reference to relevant literature reports, the substance was inferred to be palmatine [26]. At 3.71min, 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.

Table 2

The compounds in relics
Samples	Retention Time (min)	[M + H]+ m/z	Fragment ion m/z	Compound	Molecular formula
10481	5.15	336.136	320.101/292.105/278.091	Berberine	C₂₀H₁₈NO₄⁺
	4.51	352.130	336.095/320.101/308.100	Palmatine	C₂₁H₂₂NO₄⁺
	3.71	342.179	192.107/177.084	Phellodendrine	C₂₀H₂₄NO₄⁺
13079	5.15	336.140	302.104/292.108/278.091	Berberin	C₂₀H₁₈NO₄⁺
	4.5	352.130	336.099/320.101/308.104	Palmatine	C₂₁H₂₂NO₄⁺
	3.71	342.179	192.107/177.084	Phellodendrine	C₂₀H₂₄NO₄⁺
10427	5.25	336.128	320.096/292.101/278.084	Berberine	C₂₀H₁₈NO₄⁺

According to the data in the literature, it inferred that the substance should be cupreine [27–30]. The plant containing berberine were Phellodendron amurense Rupr., Coptis chinensis Franch, yellow vine, etc. Berberine, palmatine and phellodendrine were the main components of berberine dye, and berberine and palmatine were the main hair color components. So it could be judged that the sample should be made of berberine as a dye (as shown in Table 2).

The analysis and identification of the dyeing components of ancient paper cultural relics can help us to understand ancient culture at that time. Even we can also get the types of source of the dye and the dyeing process, which could speculate more information on social, technological and cultural development, aesthetic orientation and trade exchange in ancient times.In addition, due to the great susceptibility of organic artifacts, especially organic dyes and organic relics are easily fading and aging under the external factors such as light, temperature, and humidity, as well as the formation of harmful gases. Therefore, it is great significance to clarify the types of raw materials and dyes of ancient dyed paper. It is useful for the restoration and reproduction of the appearance and color of ancient paper relics, as well as for the restoration and conservation work in later periods.

In order to obtain the relevant information of processing characteristics, the basic properties and composition of Hard yellow paper samples collected in Gansu Provincial Museum were analyzed systematically in this study. Through the analysis of the surface microscopic morphology, fiber types, dyeing and coating materials samples, the 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 in 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 aesthetic orientation in ancient society. In addition, transition papermaking materials is from using hemp, to the mixture of bark and hemp, gradually to full bark paper. The analysis results of paper production process show that the vast majority of these Hard yellow paper in Tang Dynasty have been dying by yellow tiller and double-sided coating, the types of rubber materials were animal glue or starch. It is not like the records in ancient books that Hard yellow paper was all finished with worm wax. The cause of smooth paper may be the effect of other physical processes, such as calendering or striking with stick.

Based on the rational understanding of the sample of Hard yellow paper in Tang Dynasty, we can understand the processing of the Hard yellow paper deeply. From a broader point of view, it also provides a relatively scientific representation of the Hard yellow paper processing technology which helps us to understand the ancient wisdom contained in the traditional paper processing process. In addition, identification of the raw materials and coating materials of ancient dyed paper dyes has a great significance for us to restore and reproduce the appearance color of ancient paper cultural relics. As well as it is useful for the restoration and protection work in the later period.

Acknowledgements

Thanks are given to Wu Yi-qian, He Meng-jun, Kong De-sheng and Lei Yong. They kept and screened these relics, also help us perform testing with dye and wax. This research could not have been completed without their help.

Authors’ contributions

Zhou XJ performed most of the analytical work and is a major contributor in writing the manuscript. Gong YC, and Zhao RX have observed the microstructures of fibers by optical microscopy and found their characteristics. Zhang Y and is the conservator of HPLC-MS. Wang Q provided us with the opportunity to study Hard yellow paper. Qv YJ provided analysis and discussion of the results of IR. Gu A introduced information about the ancient craft of Hard yellow paper. All authors read and approved the final manuscript.

Funding

This research was supported by the National Key R&D Program of China, No. 2019YFC1520300.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations Competing interests

The authors declare that they have no competing interests.

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No competing interests reported.

New scientific evidence of the materials and craft of Hard yellow paper in Tang Dynasty collected by Gansu Provincial Museum

Status:

Version 1

Abstract

Figures

Introduction

1 Information of paper relics

2 Experimental

3 Results and Discussion

4 Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

New scientific evidence of the materials and craft of Hard yellow paper in Tang Dynasty collected by Gansu Provincial Museum

Status:

Version 1

Abstract

Figures

Introduction

​1 Information of paper relics

2 Experimental

3 Results and Discussion

4 Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

1 Information of paper relics