Analyzing ancient Chinese handmade Lajian paper exhibiting an orange-red color
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KeywordsLajian paper Manufacture Orange-red Non-destructive Turmeric dyestuff
X ray fluorescence spectroscopy
attenuated total reflection flourier transformed infrared spectroscopy
An old Chinese saying states, “Paper lasts one millennium.” As the birthplace of papermaking, China is the nation with the longest history of making traditional handmade paper. Over the centuries, papermaking technology has been constantly refined, and different papers have been produced for different purposes. Lajian paper—a very famous traditional handmade paper used primarily for imperial edicts, scripture, calligraphy, and painting—exhibits different colors, structure, and fine textures. Sometimes, the paper was decorated with gold or silver flecks for aesthetic effect, and these decorated papers were used by royal courts in ancient China. Lajian paper appeared in the Sui-Tang period and was produced widely during the Qing dynasty [1, 2, 3]. After waxing, the paper would be smooth, waterproof, anti-mildew, and insect-resistant. The use of this coating technology in China precedes the technology’s use in Europe by more than 1000 years. However, the ancient method of making ancient Lajian paper has disappeared because of its time-consuming, painstaking nature, and the emergence of modern mechanical techniques at the end of the Qing dynasty [2, 3, 4]. Fortunately, some museums and private collectors in China still have some Lajian paper as well as Chinese calligraphy and paintings that have Lajian as the base material.
As Lajian is a famous traditional handmade paper that is an important part of China’s cultural heritage, concerns have been raised about the manufacture of Lajian paper in recent years [2, 3, 4, 5]. Some people have attempted to reconstruct the traditional technique; however, these attempts have not yielded very satisfying results because of a lack of comprehensive and scientific understanding of the paper’s production process until now. It has been reported that Lajian paper manufacturing typically included coloring and waxing the paper to form a coating that protected it from humidity and mold growth [2, 3, 4]. The results of our previous analysis in which a piece of single-layered pink Lajian paper was examined revealed that the paper’s composition and structure differed slightly from assumptions derived from traditional folklore . Our previous study indicated that single-layered Lajian paper was produced from bamboo fibers mixed with wheat and mulberry fibers. The front of the paper was manufactured with kaolin and then polished with wax that was mixed with cochineal dye, minium, and animal glue to form a purple-red color. The same treatment, except for the use of cochineal dye, was administered to the back.
A small piece of orange-red Lajian paper (1.5 cm × 2 cm) decorated with small flecks of silver and gold on the top layer (Fig. 1) and dating from the Qing dynasty was provided by the Sichuan Museum. Visual observation revealed that the Lajian paper was constructed from two layers of paper.
Very small segments of dyed fibers were extracted from the Lajian paper to conduct fiber characterization and SERS analysis. To minimize the risk of damage to the paper, partially protruding or detached fibers were extracted from the edge of the paper.
Cross-section sample preparation
Cross-section samples were selected and mounted in methacrylate light-curing embedding resin (Technovit® 2000 LC+) and cured in a blue light “oven” (Technotray CU/Heraeus Kuler, Germany). The cross-section surface was polished by hand with Micro-Mesh polishing cloths of various grades up to the smallest, 12,000 grit (Micro-mesh, USA).
Sample treatment for SERS procedure
Silver colloids were prepared using microwave-assisted reduction of Ag2SO4 in the presence of glucose and sodium citrate . Samples of concentrated colloids were prepared for SERS analysis by centrifuging the colloid solution and replacing it with ultrapure water.
The fibers’ structural and compositional characterizations were determined according to the procedures described previously .
The paper’s micro-structure and elements were characterized using a high-resolution Hitachi Fields S-4800 SEM micro-analysis instrument equipped with EDS for elemental analysis; the machine was operated at 5 kV. The cross-sectional samples were obtained by cooling samples in liquid nitrogen for 10 min and immediately breaking the frozen sample. All samples were sputter-coated with carbon.
Optical microscopy (OM) observation
Cross-section photomicrographs were captured using a Zeiss Axiotch 100 HD polarizing microscope under ultraviolet and white reflected light at different magnification levels.
X-ray photoelectron spectroscopy experiments
X-ray photoelectron spectroscopy (XPS) measurements were obtained using an ESCALAB Mark II X-ray photoelectron spectrometer (XPS, VG Scientific, UK) in an ultrahigh vacuum. Mg Kα (1253.6 eV) radiation was utilized as the excitation source. High-resolution scans of core-level spectra were set to 15 eV of pass energy and recorded with an energy step of 0.05 eV. The binding energy was corrected by referencing the C1 s peak at 284.6 eV.
A Bruker Senterra™ dispersive Raman spectrometer was used to perform micro-Raman spectroscopy. The instrument was operated at 785 nm with an integration time of 30 s and resolution of 3–5 cm−1. The laser beam was focused with a 50 × long working distance objective; the surface laser power was limited to 1 mW to avoid not only degradation or heat-induced physical changes to the paper sample, but also heat-induced decomposition of lead white in case the latter was used as a coating pigment. Spectra were acquired and processed using OPUS 7.0 Raman software.
ATR-FTIR measurements were performed using a Thermo Nicolet iZ10 FTIR spectrophotometer equipped with a Thermo-Nicolet iN10 infrared imaging microscope. All samples were analyzed with a smart orbit single reflection diamond attenuated total reflectance (ATR) mode from 4000 to 650 cm−1 for 128 scans with 4 cm−1 spectral resolution. The FTIR microscope was equipped with an internal pressure sensor, a precise motorized X–Y state, and an MCT detector, and was cooled with liquid nitrogen. ATR correction was not applied to the measured FTIR spectra before analysis.
SERS measurements of organic dye on paper
Samples for SERS analyses were obtained by removing fragments of single color fibers (~ 100 μm in length) from the paper using a tungsten needle. The samples were analyzed following a multistep approach described previously [6, 7]: (1) selected paper fibers were briefly placed in a polyethylene micro-vial cover and then exposed to HF vapor. (2) After approximately 5 min, a 2 μl Ag colloid was deposited on the sample, followed by 0.4 μl of 0.1 N KNO3 solutions to induce colloid aggregation.
SERS analysis was conducted by placing the micro-vial directly under the objective for observation using the previously mentioned Raman instrument. Spectra were acquired by exciting the specimen at 488 nm through a 20 × long working distance objective microscope at a resolution of 3–5 cm−1 and integrating the signal twice for 15 s. Laser power did not exceed 0.4 mW.
pH measurements were conducted in accordance with Gomaa  using a flat-surfaced pH electrode (METTLER TOLEDO S400 instrument, Switzerland). A drop of distilled water was placed on the paper surface, and a pH electrode was placed on the paper. The results are represented as an average of five repetitions of this procedure.
Results and discussion
Micro-structural and fiber characterization
The SEM image (Fig. 3a) of the cross-section revealed that the thickness of the paper was approximately 100 μm. The thickness of the upper and lower layers was approximately 60 and 40 μm, respectively. Figure 3b shows some needle-like and flaked materials on the fiber surface. The EDS results indicated that the main elements for these needle-like materials were C and O (the atomic percentage was greater than 95%); these results may be ascribed to organic dyes. The flaked materials may be kaolinite (Al2O3·2SiO2·2H2O) as the main elements were Al, Si, and O. According to SEM–EDS results, kaolinite may have been used as powder or coating pigment while needle-like material may have been used in dyes.
Traditional Chinese paper was made from plant fibers. Optical and microscope identification are effective tools for identifying fiber morphology [10, 11, 12, 13, 14, 15]. Figure 5 shows the fiber structure under SEM observation. SEM images showed long tubular fibers with uneven surfaces and holes or crevices. Bast fibers typically exhibit these characteristics [10, 11]. Other fibers with many shallow grooves may be bamboo fibers [10, 11]. According to SEM test, the raw materials used in the Lajian paper are likely bast fiber and bamboo fiber, which were the characteristic components of traditional Chinese paper with significant consistency and strength.
Natural organic dyes and their corresponding lake pigments were important materials in the artist’s palette from ancient times until the second half of the nineteenth century, when the industrial production of synthetic dyes began. Turmeric—which is made from the rhizome of Curcuma longa L. (family Zingiberaceae)-is a main ingredient of curry powders. Turmeric is well known for its color, flavor, and digestive properties. Some researchers have reported that turmeric was used as a dye in ancient China [27, 28, 29]. Curcumin is the main colorant in turmeric, which is used to produce yellow-orange colorants. Previous research demonstrated that turmeric was typically stable, pH sensitive, and exhibited a yellow, red, or dark orange color under acidic, alkaline, or neutral conditions, respectively . The paper acidity test showed that the pH value of the paper surface was 7.1–7.4. This is why the organic dyestuff exhibited a dark orange color under UV illumination (Fig. 2). Likewise, Suo  reported that turmeric dye has a needle-like micro-structure, as shown in Figs. 3 and 4. According to these results, turmeric may be confirmed as an organic dye in Lajian paper.
In this study, a small piece of orange-red Lajian paper dating from the Qing dynasty was evaluated by combining SERS microanalysis with non-destructive and micro-destructive SEM, XPS, micro-Raman, and ATR-FTIR techniques. Micro-observations revealed that the raw fiber materials were bast fiber mixed with bamboo fiber. The evidence derived from SEM, XPS, Raman, ATR-FTIR, and SERS analyses demonstrated that minium and turmeric dyes were used as colorants to obtain an orange-red color while kaolinite might have been used as a coating pigment. The process used for the upper layer of the Lajian paper may have included (in sequence) dyeing the paper by brushing coating pigment and organic dyes on the surface, brushing inorganic red pigment on the coated surface, waxing the paper with Chinese insect wax, and then polishing the surface to form a smooth wax coating. Animal glue was used as a binding medium during dyeing, but further experiments are needed to identify the type of animal glue. The same treatment, except for the intentional use of inorganic pigment, was applied to the backing layer. Neutral conditions are conducive to preserving this type of paper. The wax coating surface and turmeric and minium colorant could protect the paper from both humidity and mold growth.
The authors gratefully acknowledge financial support from the High Level Research Team Building Plan of Social Sciences in Sichuan Province (Sichuan Federation of Social Science Association  43-2) and the Sichuan University Research Cluster for Regional History. Yanbing Luo gratefully acknowledges Marco Leona, the Director of Scientific Research Department of Metropolitan Museum of Art and his co-workers for their help and assistance with the Raman and SERS analyses during Yanbing’s visiting scholar program at MET.
Data were collected by YBL, JLC and YFH. YBL and CY prepared and revised the manuscript. All authors read and approved the final manuscript.
The research was supported by the Science & Technology Program of Sichuan, China under Grant 2019YFS0494; the Key Projects of Social Science Planning in Sichuan Province under Grant SC18A013.
The authors declare that they have no competing interests.
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