coatings

Communication Non-invasive Optical Technical Identification of Red Pigments on Chinese Paper Notes

Jie Ren 1, Cunjin Gao 1, Jigang Wang 1,* , Yang Shen 1, Jilong Shi 1, Quanxiao Liu 1,* and Wei Chen 2,*

1 Key Laboratory of Printing and Packaging Materials and Technology, Beijing Institute of Graphic Communication, Beijing 102600, ; [email protected] (J.R.); [email protected] (C.G.); [email protected] (Y.S.); [email protected] (J.S.) 2 Department of Physics, The University of Texas at Arlington, Arlington, TX 76019-0059, USA * Correspondence: [email protected] (J.W.); [email protected] (Q.L.); [email protected] (W.C.)

Abstract: Red pigments with bright colors were widely used in ancient Chinese painted pottery, books, antiques, calligraphy, and paintings. Herein, red pigments of traditional paper notes were investigated by non-invasive optical technology in order to enrich the Chinese historical pigments knowledge base. The results of laser Raman spectroscopy tests on five paper notes clearly identified the inorganic mineral pigments including ocher and cinnabar. Infrared spectroscopy measurements indicated that an artificial synthetic magenta was employed as the organic pigment. Inorganic and organic red pigments were applied together on the same samples 2 and 5 which can be speculated to serve an anti-counterfeiting function. In addition, SEM-EDS analysis of sample 5 clearly showed that the red pigment was composed of lead oxides and ZnS was added as color modulator. Combined

 with the abovementioned non-invasive techniques, analysis of printed pigments can provide a  feasible method to authenticate and conserve paper notes.

Citation: Ren, J.; Gao, C.; Wang, J.; Shen, Y.; Shi, J.; Liu, Q.; Chen, W. Keywords: Chinese paper notes; red pigments; non-invasive identification; Raman spectroscopy; Non-invasive Optical Technical optical technology Identification of Red Pigments on Chinese Paper Notes. Coatings 2021, 11, 410. https://doi.org/10.3390/ coatings11040410 1. Introduction Red pigments were some of the most commonly used pigments in ancient China due Academic Editors: Chen Tei-Chen to their bright colors. For example, the inorganic red pigments on painted pottery at the and Aivaras Kareiva Yangshao cultural site and on a Northern Wei Dynasty pottery figurine were demonstrated to be Fe O [1]. A red HgS lacquerware pigment was found at the Zhejiang Yuyao Hemudu Received: 14 March 2021 2 3 site and in the Jiangsu Xuyi Han tomb [2]. The red pigments HgS and Pb O were widely Accepted: 31 March 2021 3 4 Published: 1 April 2021 applied in Dunhuang murals [3–6]. Numerous red pigments in the ancient building materials of the of Beijing included Fe2O3, Pb3O4, and synthetic organic

Publisher’s Note: MDPI stays neutral red pigments [7–9]. Red pigments are also widely used in modern Chinese bills and with regard to jurisdictional claims in stamps, such as the business licenses of the and the Republic of China, and published maps and institutional affil- the imperial China engraved coiling dragon 2-point and 4-point stamps [10]. During our iations. research on the material of stamps, we found that there are many reports on the use of pigments abroad, such as the Hawaiian missionary stamps issued by the Hawaiian Islands in 1851 [11], Mauritius stamps in 1847 [12], stamps and banknotes in the region of Rijeka in Croatia in 1918 [13], and a set of stamps in the Italian Kingdom and Republic [14]. In addition, red pigments are widely used in painting, printing, dyeing, sealing style, and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. other fields. This article is an open access article The Chinese Printing Cultural Heritage Research Center has collected a large amount distributed under the terms and of paper notes. The research methods of scientific and technological history workers conditions of the Creative Commons for cultural relic collections generally involve site visits and investigations, followed by Attribution (CC BY) license (https:// observation with the eyes, hands and other traditional methods. However, these traditional creativecommons.org/licenses/by/ identification methods sometimes have some shortcomings, and the use of scientific and 4.0/). technological means can make up for these shortcomings and increase the accuracy of

Coatings 2021, 11, 410. https://doi.org/10.3390/coatings11040410 https://www.mdpi.com/journal/coatings Coatings 2021, 11, x FOR PEER REVIEW 2 of 8

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technological means can make up for these shortcomings and increase the accuracy of identification. Herein, research on pigments can help identify the time that the paper notes wereidentification. fabricated Herein,and suggest research methods on pigments for preserving can help them. identify Considering the time the that non-regener- the paper ativenotes and were valuable fabricated characteristics and suggest of methodspaper notes, for preservingit is necessary them. to apply Considering in situ non-inva- the non- siveregenerative analytical and techniques. valuable characteristics of paper notes, it is necessary to apply in situ non-invasive analytical techniques. 2. Experimental Samples and Instrumentation 2. Experimental Samples and Instrumentation 2.1.2.1. Experimental Experimental Samples Samples AllAll samples were were collected by by Chinese Printing Cultural Cultural Heritage Heritage Research Research Center, Center, BeijingBeijing Institute Institute of of Graphic Graphic Communication Communication (B (BIGC).IGC). Figure Figure 1a1 showsa shows the the five five stamps stamps (la- beled(labeled from from Sample Sample 1 to 1Sample to Sample 5) analyzed 5) analyzed in this in study this studyset. Figure set. Figure1b shows1b showsthe images the obtainedimages obtained by optical by microscopy, optical microscopy, taking the taking poin thets on points the pictures on the pictures of sample of 3 sample and sample 3 and 5sample as examples, 5 as examples, the points the on points the two on thesample twos samples are photographed are photographed through through an optical an opticalmicro- scope,microscope, and the and magnification the magnification used is used 10×. is 10×.

(a)

(b)

Figure 1. ((a)Historical) Historical Chinese Chinese stamps stamps investigated investigated in in this this study. study. Selected Selected areas areas are are mark markeded in in the the figure figure with with small small (blue) (blue) circles (the dates,dates, fromfrom leftleft toto rightright are are 1884, 1884, 1916, 1916, 1933, 1933, 1939, 1939, 1940). 1940). (b ()b The) The optical optical microscopy microscopy images, images, taking taking the the points points on on the pictures of sample 3 and sample 5 as examples. the pictures of sample 3 and sample 5 as examples.

RedRed marks marks are noticeable on all five five notes. Small dots (marked by small circles) were chosenchosen on eacheach paperpaper notenote for for optical optical measurements, measurements, and and each each was was referred referred to asto “a”as “a” or “b”or (for example, 1-a, and 1-b represent dots “a” and “b” on Sample 1, respectively). Sample

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1 is a bank bill from the 1884 issue, of 23.4 cm × 9 cm size. The body adopted the blue pigment printing, and the center was covered by a red square seal, as shown in area 1-a; the lower right corner of the sample was stamped by a red cross-page seal, as shown in area 1-b. Sample 2 is a 15 cm × 12 cm bill issued in 1916. The bill body adopted red pigment printing and was stamped with purple and black handwriting, as shown in sample 2-a. Sample 3, of 8.8 cm × 13 cm size, is a bill issued in 1933. The bill body adopted blue pigment printing, and the right side of the body was stamped with a red rectangular seal, “In and/or out of the Current Silver Coin”, in area 3-a; the right edge was stamped with a diamond “Guang Yi” red seal in area 3-b. Sample 4, sized at 14.5 cm × 10 cm, is a 1939 issue note. The nominal subject is red in area 4-a, and area 4-b has a red chapter cover on the right side of the seal. Sample 5 is a 1940 print issue note sized at 16.4 cm × 8.8 cm. The nominal subject is blue, the far left in area 5-a is printed with “Do not fill in here” in red letters, the center was stamped by a “transfer paid” rectangular red seal in area 5-b, the central printing of the bill has 2 red vertical stripes in area 5-c, and the lower right corner of a round red seal is affixed in area 5-d.

2.2. Instrumentation A XploRA micro-Raman spectrometer (Horiba Jobin-Yvon, Palaiseau, France), coupled with a BX-51 confocal microscope (Olympus, Tokyo, Japan) was utilized for Raman analysis. All tests were carried out in a dark room (room temperature: 25 ◦C), and the Raman spectrometer’s working temperature was reduced to −70 ◦C. All samples were examined at a 100× microscope objective to locate the areas for Raman analysis. The laser spot size was approximately 1 µm, and the laser power reached approximately 2~3 mW on the samples. The conditions for recording each Raman spectrum are as follows: 50 s exposure time, 2 times of accumulation, and 100~3500 cm−1 spectral range. A JEOL JSM-6610LA SEM-EDS (JEOL, Peabody, MA, USA) was employed to distin- guish elemental content in detail with a secondary electron (SEI) signal, magnification by 50~100; the EDS conditions included a 20 kV accelerating voltage, a 10 mm working distance (WD), and an objective lens diaphragm of 3, the chemical compositions were determined by a JEOL EX-94300S4L1Q Energy Dispersive Spectrometer (JEOL, Peabody, MA, USA). All measurements were carried out at room temperature. FT-IR (Nicolet 6700 FT-IR, Thermo Fisher, Waltham, MA, USA) measurements were also employed. The test range is from 500 to 4000 cm−1. The optical microscope used was a VHX-5000 Confocal Laser instrument (Keyence, Osaka, Japan).

3. Results The XploRA Raman spectrometer was employed to test the red pigments of all the samples. The results are shown in Figure2. Based on the analyses in Figure2, the marked red pigments in areas 1-a and 1-b produced Raman signal bands between 600 and 150 cm−1. The Raman spectra vibrational bands of area 1-a and area 1-b at 284, 402, and 605 cm−1 correspond to symmetric Fe–O bending, and the signals at 218 cm−1 and 228 cm−1 correspond to symmetric Fe–O stretching [1]. The presence of these peaks indicates that the main component of this pigment is ocher [4–6]. Cinnabar was also applied as a coloring material for ancient articles. The characteristic Raman peaks of area 3-a, 3-b, 4-b, and 5-d showed evidence of mercury sulfide (HgS) pigment, with bands at 252, 284, and 344 cm−1 related to the trigonal mercury sulfide structure [12]. Cinnabar’s wide application could be due to economic and technological motivations. Coatings 20212021,, 1111,, 410x FOR PEER REVIEW 4 of 8

Figure 2. Raman spectra of areas 1-a, 1-b, 3-a, 3-b, 4-b, and 5-d.

The characteristic characteristic Raman Raman bands bands were were difficult difficult to todetect detect in areas in areas 4-a, 4-a, 5-a, 5-a,and and5-b. Ac- 5-b. Accordingcording to the to the morphological morphological characteristics, characteristics, it was it was speculated speculated that that the the material material might might be anbe organic an organic pigment. pigment. Based Based on this on logical this logical inference, inference, FT-IR FT-IR spectrometry spectrometry could couldbe a better be a optionbetter optionto test the to test marked the marked area pigments. area pigments. FT-IR absorption FT-IR absorption peaks of peaksthe areas of the 4-a, areas 5-a, and 4-a, 5-b5-a, are and almost 5-b are similar, almost similar,as shown as shownin Figure in Figure3a. These3a. These peaks peaks match match the spectrum the spectrum of a of a C16H12N2 infrared standard in the Spectral Database for Organic Compounds (SDBS) C16H12N2 infrared standard in the Spectral Database for Organic Compounds (SDBS) li- brary,library, and and the the corresponding corresponding structure structure is shown is shown in Figure in Figure 4. Based4. Based on the on red the color red attrib- color attributes of the samples, it can be inferred that the compound of structure C16H12N2 is utes of the samples, it can be inferred that the compound of structure C16H12N2 is similar similar to magenta C18H12N2O6SCa [15], as shown in Figure5. Both of the results have to magenta C18H12N2O6SCa [15], as shown in Figure 5. Both of the results have chromo- chromophores, auxochrome groups, and conjugated structures. To determine whether the phores, auxochrome groups, and conjugated structures. To determine whether the meas- measured pigment is indeed magenta, a standard magenta sample was measured again ured pigment is indeed magenta, a standard magenta sample was measured again by FT- by FT-IR. Absorption peaks at 1600, 1580, 1500, and 1450 cm−1 could correspond to the IR. Absorption peaks at 1600, 1580, 1500, and 1450 cm−1 could correspond to the character- characteristic vibrations of a benzene ring. The stretch at 1554 cm−1 can originate from the istic vibrations of a benzene ring. The stretch at 1554 cm−1 can originate from the C–N C–N stretching vibration. The band at 1340 cm−1 can be assigned to the vibration of a C–N stretching vibration. The band at 1340 cm−1 can be assigned to the vibration of a C–N group. Compared with the magenta standard sample and the FT-IR spectra of areas 4-a, group. Compared with the magenta standard sample and the FT-IR spectra of areas 4-a, 5-a, and 5-b in Figure3a, the characteristic peaks at 3837, 2356, 1874, 1554, 1039, 941, and 5-a, and 5-b in Figure 3a, the characteristic peaks at 3837, 2356, 1874, 1554, 1039, 941, and 848 cm−1 are roughly the same. Therefore, the red pigment in areas 4-a, 5-a, and 5-b might −1 848be the cm same are roughly type of redthe same. organic Therefore, pigment the with red a pigment similar structure in areas 4-a, to magenta. 5-a, and 5-b It canmight be bealso the seen same from type Figure of red3b organic that the pigment FT-IR spectra with a of similar the paper structure and sample to magenta. 5-b are It consistent,can be also seenand thefrom sample Figure 5-b 3b isthat a test the point FT-IR with spectra pigment, of the andpaper its and absorption sample peak5-b are is consistent, much stronger and thethan sample that of 5-b paper, is a test especially point with the absorption pigment, an peaksd its absorption are at 3337, peak 2914, is 1493, much and stronger 1015 cm than−1, −1 thatrespectively. of paper, especially the absorption peaks are at 3337, 2914, 1493, and 1015 cm , re- spectively.The basic composition of paper is plant fibers, and the auxiliary materials are mainly composedThe basic of adhesive, composition filler, of and paper pigments. is plant Rubberfibers, and is applied the auxiliary to resist materials liquid penetration are mainly composed of adhesive, filler, and pigments. Rubber is applied to resist liquid penetration and diffusion. Fillers including talcum powder (Mg3[Si4O10](OH)2), calcium carbonate and diffusion. Fillers including talcum powder (Mg3[Si4O10](OH)2), calcium carbonate (CaCO3), and kaolin (Al2O3·2SiO2·2H2O) impart specific properties such as opacity and (CaCObrightness3), and to thekaolin paper. (Al2O Pigments3·2SiO2·2H are2O) mainly impart usedspecific to dyeproperties and modulate such as theopacity color and on brightnesspaper [16]. to the paper. Pigments are mainly used to dye and modulate the color on paper [16].

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(a((a)a ))

(b((b)b )) Figure 3. (a) IR spectrum of areas 4-a, 5-a, and 5-b and the magenta standard sample. (b) IR spec- FigureFigure 3. 3.3. ( (a(a)) )IR IRIR spectrum spectrumspectrum of ofof areas areas 4-a, 4-a, 5-a,5-a, 5-a, andand and 5-b5-b 5-b andand and the the the magenta magenta magenta standard standard standard sample. sample. sample. (b ( )b(b IR) )IR IR spectrum spec- spec- trum of areas 5-b and paper. trumoftrum areas of of areas 5-bareas and 5-b 5-b paper.and and paper. paper.

FigureFigure 4. 4. Structure of C16H12N2. FigureFigure 4. 4. Structure Structure of of C C161616HHH121212NNN2.2 .2 .

Figure 5. Structure of C18H12N2O6SCa. FigureFigure 5. 5. Structure Structure of of C C181818HHH121212NNN2O2O26OSCa.6SCa.6SCa.

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No Raman scattering effect was observed in the red pigments of area 2-a in sample 2 and areaNo Raman 5-c in sample scattering 5, separately. effect was observedSEM-EDS in can the be red used pigments to investigate of area 2-athe inelements sample of 2 andthe lesser area 5-c components in sample 5,and separately. thin layers SEM-EDS of pigm canents. be Referring used to investigateto the left color the elements spline, the of thecontent lesser is componentshigher in areas and that thin are layers closer of to pigments. the color Referringof the upper to the part. left The color red spline, pigments the contentof area 2-a is higher in sample in areas 2 were that measured are closer by to microscopic the color of observation the upper part. and Theenergy red dispersive pigments ofspectrum area 2-a mapping in sample analysis, 2 were measured as shown byin microscopicFigure 6. The observation paper and andink energyincluded dispersive Al, S, Si, spectrumCa, Cl, Zn, mapping Ba, O, and analysis, Pb. Herein, as shown Si, inCa, Figure and 6Ba. The are papermainly and distributed ink included as filler Al, S, compo- Si, Ca, Cl,nents Zn, in Ba, the O, paper, and Pb.and Herein, the structures Si, Ca, can and be Ba further are mainly inferred distributed as corresponding as filler components to the fillers Al2O3·2SiO2·2H2O, CaCO3, and BaCO3, respectively. The distributions of Al, S, Cl, Zn, Ba, inAl2 theO3·2SiO paper,2·2H and2O, theCaCO structures3, and BaCO can3 be, respectively. further inferred The distributions as corresponding of Al, toS, Cl, the Zn, fillers Ba, Pb and other elements are consistent with red writing, and the elements are mainly dis- AlPb2 Oand3·2SiO other2· 2Helements2O, CaCO are3 consistent, and BaCO with3, respectively. red writing, The and distributions the elements ofare Al, mainly S, Cl, dis- Zn, Ba,tributed Pb and in otherred ink elements as pigments are consistent or fillers. with Pb can red be writing, considered and the the elements only color are element mainly distributedaccording to in the red attributes ink as pigments of the red or pigment, fillers. Pb which can be is considered preliminarily the inferred only color as elementthe lead compounds Pb3O4 or PbO. In addition, zinc sulfide (ZnS) may also be added for color accordingcompounds to Pb the3O attributes4 or PbO. of In the addition, red pigment, zinc sulfide which is(ZnS) preliminarily may also inferredbe added as for the color lead compoundsmodulation, Pbwhich3O4 madeor PbO. the In sample addition, closer zinc to sulfideorange (ZnS)red from may the also sample be added appearance. for color modulation,The red whichpigments made of thearea sample 5-c in closersample to 5 orange were also red frommeasured the sample by energy appearance. dispersive spectrumThe red mapping pigments analysis, of area as 5-c shown in sample in Figure 5 were 7. The also paper measured and ink by included energy dispersive Mg, Al, S, spectrumSi, Ca, Cl, mappingBa, O, and analysis, Pb. In general, as shown Mg, in Si, Figure Al, and7. The Ba paperwere mainly and ink distributed included Mg, as filler Al, components in the paper, and the structures can be inferred as Mg3[Si4O10](OH)2, S,components Si, Ca, Cl, Ba,in the O, andpaper, Pb. and In general, the structures Mg, Si, Al,can and be Bainferred were mainlyas Mg distributed3[Si4O10](OH) as2, Al2O3·2SiO2·2H2O, and BaCO3. The distributions of S, Ca, Ba, Pb, and other elements are fillerAl2O3 components·2SiO2·2H2O, inand the BaCO paper,3. The and distributions the structures of can S, Ca, be inferredBa, Pb, and asMg other3[Si elements4O10](OH) are2, Alconsistent2O3·2SiO with2·2H red2O, andwriting, BaCO and3. The the distributionsabovementioned of S, elements Ca, Ba, Pb, are and likely other mainly elements distrib- are consistentuted in red with ink redas pigment writing, andor fillers. the abovementioned Pb is the only color elements element, are likely according mainly to distributed the attrib- inutes red of ink the as red pigment pigment, or fillers.which Pbis preliminarily is the only color inferred element, to be according the lead tocompounds the attributes Pb3O of4 utes of the red pigment, which is preliminarily inferred to be the lead compounds Pb3O4 theor PbO. red pigment, which is preliminarily inferred to be the lead compounds Pb3O4 or PbO.

2.0 mm IMG1 2.0 mm C K 2.0 mm O K 2.0 mm Al K 2.0 mm IMG1 2.0 mm C K 2.0 mm O K 2.0 mm Al K

2.0 mm Si K 2.0 mm S K 2.0 mm Cl K 2.0 mm Ca K 2.0 mm Si K 2.0 mm S K 2.0 mm Cl K 2.0 mm Ca K

2.0 mm Zn K 2.0 mm Ba L 2.0 mm Pb M 2.0 mm Zn K 2.0 mm Ba L 2.0 mm Pb M FigureFigure 6.6. SEM-EDS mapping imageimage ofof areaarea 2-a.2-a.

300 µm IMG1 300 µm C K 300 µm O K 300 µm Mg K 300 µm IMG1 300 µm C K 300 µm O K 300 µm Mg K

300 µm Al K 300 µm Si K 300 µm S K 300 µm Cl K 300 µm Al K 300 µm Si K 300 µm S K 300 µm Cl K

300 µm Ca K 300 µm Ba L 300 µm Pb M 300 µm Ca K 300 µm Ba L 300 µm Pb M FigureFigure 7.7. SEM-EDS mapping imageimage ofof areaarea 5-a.5-a.

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4. Conclusions The red pigments of samples were analyzed by laser confocal Raman spectroscopy, scanning electron microscopy energy dispersive spectroscopy (SEM-EDS), and infrared spectroscopy. The red pigments of area 1-a and area 1-b in sample 1 are ochre, and the red components of areas 3-a and 3-b in sample 3, area 4-b in sample 4, and area 5-d in sample 5 are cinnabar. The orange red pigments in area 2-a in sample 2 indicate that Al, S, Cl, Zn, Ba, Pb and other elements are mainly distributed in red ink. The red pigments in area 5-c in sample 5 show this red ink contains S, Ca, Ba, Pb, and other elements, and the red pigments in area 4-a in sample 4, area 5-a, and area 5-b in sample 5 may be magenta. Based on the detection and analysis of the red pigments on the Qing Dynasty and the Republic of China bill series, the red pigments on early negotiable paper notes are essentially inorganic pigments, while the red pigments of most modern paper notes are organic pigments. Paper printing materials have evolved from inorganic pigments to organic pigments, which reflect the necessity and practicality of anti-counterfeiting and the influence of the development of industrial technology on the printing industry. The three abovementioned combined technologies are a powerful means to analyze bill pigments, which has broad application potential for the analysis of presswork and historic paper relics.

Author Contributions: Investigation: J.R., C.G., Y.S., and J.W.; funding acquisition: J.W. and W.C.; writing—original draft preparation: Q.L. and J.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China Grant Nos. (52002074, 2016YFA0202302, 61527817, 61845236, 52002074, 51602071 and 11427808), Beijing Social Science Foundation No. 15LSC014, Beijing Natural Science Foundation (4132031), Beijing Postdoctoral Research Foundation, Beijing University Student Research Program, Beijing City College High-level Teachers Team Construction Program for the Young Top Talents Training CIT&TCD201904050. The Open Research Subject of Key Laboratory of Dielectric and Electrolyte Functional Material Province No. HKDE201902. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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