Elemental Analysis of Chinese Black Inks on Xuan Paper by ArF Laser

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Elemental analysis of Chinese black inks on xuan paper by ArF laser-excited plume fluorescence Yue Cai, Zhengyu Huang, Michelle Hoi Ching Cheung, Vincent MottoRos, Po-Chun Chu, Yuanwei Wang, Haoyi Zhong, Ronald Yuen, Kelvin SzeYin Leung, Judy T.S. Lum, Sut Kam Phoebe Ho, and Nai Ho H. Cheung Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02628 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

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Analytical Chemistry

Elemental analysis of Chinese black inks on xuan paper by ArF laser-excited plume fluorescence

Yue Cai a,b, Zhengyu Huang a, Michelle H.C. Cheung a, Vincent Motto-Ros c, Po-Chun Chu a, Yuanwei Wang a, Haoyi Zhong a, Ronald Yuen d, Kelvin S.Y. Leung e, Judy T.S. Lum e, Sut-Kam Ho f and Nai-Ho Cheung a*

a

Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China

b

ANA Artwork Material Analysis Ltd, Mong Kok, Hong Kong, China

c

The Institute of Light and Matter, Université Claude Bernard Lyon 1 and the CNRS (UMR5306), Villeurbanne CEDEX, France

d

Thermo Scientific, Niton Analyzer Asia, Shatin, Hong Kong, China

e

Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China

f

Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Macau, China

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ABSTRACT: Chemical analysis of Chinese black ink on xuan paper is useful for the authentication of Asian artwork.

The analysis has to be non-destructive and has to

accommodate artworks of all sizes. We apply three analytical techniques, ArF laser-induced plume fluorescence, Fourier transform infra-red (FTIR) spectroscopy and portable X-ray fluorescence (pXRF) to analyze five commercial Chinese black inks on two kinds of xuan paper. The FTIR signal is found to be interfered by the substrate which is inevitable because the pigments diffuse extensively into the xuan fiber network. The XRF signal is shown to be feeble and no signal can be registered until the samples are stacked and when the analytes are present at tens of percent. In contrast, the plume fluorescence technique can detect the minor and trace signature elements. The method is based on a two-laser-pulse scheme performed on a high precision optical setup: the first 355 nm laser pulse ablates a thin layer of the ink to create a plume; the second 193 nm laser pulse induces multi analytes in the plume to fluoresce.

Partial-least-square discriminant analysis of the fluorescence spectra

unambiguously sorts the ink-xuan combinations while the sampled area is not visibly damaged even under the microscope. The laser probe can handle samples of arbitrary size and shape, is air compatible, and no sample pre-treatment is necessary. .


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Today, the demand for Asian artwork of lasting value is fervent. Hundreds of artwork auctions are held every year, bringing in billions of US dollars. 1 Forgery is therefore extremely profitable, especially when imitations can be produced at low cost.2 There is therefore urgent need for the authentication of Asian artwork, but reliable methods are lacking. The predominant method is subjective analysis. For instance, to identify a fake copy, Morellian analysis is commonly applied when a master’s repeated painting detail and style are examined. 3

Provenance is another common method to

discriminate forgeries. It determines the authenticity of the painting by tracking its collecting history.

While both methods are useful, they have their limitations.

The accuracy of

Morellian analysis depends heavily on the examiner and the forgers can modify the provenance record.4 Objective analysis is therefore desirable to complement the subjective approach. Among the various objective methods, chemical analysis of the colorants and the substrate is considered the most reliable. 5 Take the substrate for example; the majority of Chinese paintings and calligraphy are done on xuan paper. Xuan paper is produced from pulped bark fibers of blue sandalwood mixed with rice straw.6- 8 The paper comes in two main types, the 7

unprocessed raw xuan, and the processed ripe xuan. Raw xuan allows water to freely diffuse both laterally across and longitudinally into the paper. Ripe xuan prevents that diffusion by potassium alum treatment.9,10 As we will later show, while the paper may not produce as strong a chemical signal as the colorant in our analysis, it does cause matrix effects. The chemistry of the colorants can be more tale-telling. An important example is Chinese black ink. It consists mainly of black nano-particles coated with collagen.11 Traditionally, the black particle is soot from the burning of wood or oil, but ancient samples may use bone char while modern ones are industrial carbon black.11,12 The collagen coating is derived from animal glue. It makes the nano-particles soluble in water and stay dispersed.11 There are still



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other minor components in the ink, including perfumes and colorants to give individualized accent.11,13- 15 While the volatile components may be lost from the specimen with time, the 14,

remaining fragments can be stable and are identifiable by their vibrational infra-red (IR) spectra. For nondestructive artwork analysis, the transmission version of IR spectrometry is handicapped by substrate interference (see Supp. Info.),16 the diffuse reflectance version has limited sensitivity while the attenuated total reflectance (ATR) version requires optical contact that may not be compatible with xuan (see Supp. Info.).17 For inks that penetrate deeply into the paper, substrate interference can still occur in the ATR mode.18 Elemental analysis can be a more viable option.

It was shown that the elemental

composition correlated with the rich and individualized history of the ink samples. For example, the elemental fingerprint can be used to trace a particular ink manufacturer,19 a particular manufacturing process,11,19,

20

or a particular era.11,12,14,19

For artwork

authentication, the analytical technique has to be sensitive and nondestructive, and be able to produce spectral fingerprints by detecting many elements simultaneously. It also needs to handle specimen of all sizes.

Standard techniques such as laser-induced breakdown

spectroscopy (LIBS) or mass spectrometry are the most notable.11,18,21,22 Yet they can be too destructive or too restrictive in terms of sample size. Particle beam techniques such as particle-induced x-ray emissions and Rutherford back scattering had been applied,19 but the high vacuum requirements may not be compatible with most specimens. X-ray fluorescence (XRF) is nondestructive and the portable version is applicable to artwork of practically any geometry.

It is a convenient tool for initial screening but its sensitivity may be limited.23

For more definitive analysis and to offer higher spatial resolution, our group recently developed an optical microprobe based on ArF laser-induced fluorescence (LIF) of laser plumes. This plume-LIF technique, or PLIF for short, was shown to be highly sensitive.24 We applied it to the analysis of laser printed inks and demonstrated unambiguous sorting of


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ink samples with invisible damage.23 We also elucidated the physical mechanism of PLIF recently.25 Below, we will report the analysis of Chinese inks on xuan paper using portable XRF and PLIF. We will show that ink-on-xuan presents special challenges. We will demonstrate that portable XRF is useful for preliminary screening, and that PLIF complemented with chemometrics can deliver the required discrimination while causing invisible damage. We will also highlight the spectral features that facilitated the sorting.



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EXPERIMENTAL SECTION

Materials. We picked five Chinese black inksticks, all from the ink manufacturing province of Anhui in China. Details are summarized in Table 1. Three of them, coded L, O, and R, represent the three main types of Chinese ink. They are made from the soot of burning lacquer, tung oil, and pine resin, respectively. The fourth is another pine resin ink, coded R’, that is similar to ink R. It is made by the same manufacturer except at a different plant site. The last is made from pure carbon black, coded C, that contains little signature impurities. Inks R’ and C are chosen to test the discriminatory power of our analysis. Each ink solution was prepared the traditional way by grinding the inkstick over an inkstone with water, and was used within 24 hours. Measures were taken to minimize contamination. For example, only distilled water was used, the inkstone and paint brushes were thoroughly rinsed, and all operations were performed with gloved hands. Inks from the first grind was discarded to avoid chemicals from the decorative coating of the inkstick.26 The consistency of the ink concentration was checked by measuring its absorption in the visible spectral range. The mass concentration of the ink solution was determined by drying a known volume of the ink solution and measuring the residue mass. It was 50 ± 5 mg cm-3. Two kinds of xuan paper were used, raw xuan that absorbs water strongly, and ripe xuan that does not. Details are given in Table 1. The thickness was about 90 µm for both. The ink solution was applied evenly with a brush over a 2 cm × 2 cm area of the paper. The darkness of the dried ink spot was compared against a reference grayscale.* It was found that raw xuan absorbed about three times more ink solution than ripe xuan to produce the same

*

The grayscale was prepared by pipetting a known volume of ink solution evenly onto xuan papers to completely stain their graduated dimensions.


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shade of blackness, presumably because of the more extensive ink diffusion. Ten ink-paper combinations were prepared, ranging from ink L on ripe xuan to ink C on raw xuan. These samples were allowed to dry in a dust-free enclosure and stored under controlled humidity until use.

XRF. XRF analysis was performed with a portable XRF probe (Thermo Scientific Niton XL3t GOLDD+). The probe head was brought in contact with the ink surface. The probed area was 8 mm in diameter. The probe was set to TestAll Geo mode which combined the mining mode and the soil mode. Exposure was 80 s for one spectrum, with 20 s under each of the four filters: main, low, high and light.

PLIF. PLIF analysis was carried out using the setup shown schematically in Figure 1a. It was a two-laser-pulse approach. 27 - 29 The first pulse from a Nd:YAG laser (Continuum 28

Surelite II, 3rd harmonic, 355 nm, 10 Hz, 9 ns) was delivered into the optical system through lens L1 (−50 mm f.l.) and L2 ( 100 mm f.l.). It was reflected downwards and focused by lens L3 (75 mm f.l.) onto the surface of the ink sample to produce a plume.

The focusing was

controlled by the motorized translation of lens L1. The focal spot at the target surface was 55 µm radius (1/e2) and the laser fluence was about 180 mJ cm-2. We avoided sampling an area twice by translating the sample in the x-y directions. Eleven µs later, the ablation plume was intercepted transversely by the second laser pulse from an ArF laser (GAM Ex5, 193 nm, 10 Hz, 8 ns). The ArF laser beam was focused by lens L4 (100 mm f.l. uv-grade) to a point 8 mm in front of the plume. The spot size at the plume was 640 µm × 280 µm and its center was 145 µm from the sample surface. The ArF fluence was about 100 mJ cm−2. The ArF laser pulse vaporized the ink particulates in the plume to produce analyte atoms. The trailing portion of the same 193-nm laser pulse induced the various analytes to fluoresce.29,30



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Fluorescence emissions were collected by lens L6 (20 mm f.l.) and imaged by lens L7 (30 mm f.l.) onto the round end of an optical fiber bundle. The bundle was a round to linear type. It consisted of 120 fibers and each with a clear aperture of 100 µm diameter. The linear end was mounted at the entrance slit of a 0.5 m spectrometer (Acton SpectraPro-500) equipped with a gateable intensified charge-coupled device (ICCD, Andor iStar DH734-18F-63). Lenses L1 through L5 were all 25.4 mm in diameter. Lenses L6 and L7 were 12.7 mm in diameter The ICCD was gated on with the firing of the 193-nm pulse and stayed on for 200 ns. Unless stated otherwise, the full width of each captured spectrum was 40 nm and the instrumental resolution was about 300 pm. This resolution was preserved in all off-line spectral smoothing. For minimally destructive single-shot analysis, precise and stable alignments of samples and optical beams are essential else spectral signal fluctuates significantly from shot to shot. 31

As an example of alignment precision in our design, the height z of the target surface

could be adjusted to better than 10 µm relative to a reference level by tracking the cross-hair from a red laser pointer (Figure 1, panel b). Its working principle is illustrated in panel c. At the same time, the in-line and real-time CCD viewing of the probed area and the red crosshair through lens L5 (50 mm f.l.), at 64× magnification, not only revealed the x, y and z position of the target surface but also displayed visually the difference before and after the laser shot.

AA and ICPMS. The elemental composition of the inksticks was analyzed by atomic absorption (AA) and inductively coupled plasma mass spectrometry (ICPMS) to establish analyte concentration for reference. The procedure was reported previously.23

Chemometrics. The chemometric analysis was performed using SIMCA (version 14.0).


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RESULTS AND DISCUSSION

XRF results. When we performed XRF analysis of the ink-on-xuan samples, no reliable signal could be registered. The null signal was due to the low mass density of the xuan substrate. Most of the hard x-ray was transmitted instead of inducing fluorescence.† When we stacked two layers of the sample, the signal became detectable. When we stacked still more layers, signal-to-noise ratio did not seem to improve. We therefore stacked duplicates of the sample and measured the concentration of a range of elements. They included S, Si, Ca, Cl, P, Fe, W, K, Zn, Ti, Cu, Cr, Sr, V, Nb, Mg, Ba, and Sc, roughly in the order of highest to lowest concentrations. In each scan, the probe returned the concentrations and the associated uncertainties of these analytes.‡ Most of the concentrations that we measured were too low to be useful. They were either comparable to the uncertainties (within a factor of two) or below the concentrations in blank xuan. For example, R’-ripe registered 360 µg/g of Cu but the uncertainty was 200 µg/g; and R-ripe registered 99 mg/g of Si but blank-ripe had 120 mg/g of that element. Only two analytes, Ca and S, served useful discrimination purpose. Their concentrations above blank paper in all ten ink-xuan combinations are shown in Figure 2. As can be seen, two out of five inks could be separated, ink L based on [Ca], and ink R based on [S]. Although the sorting is only partial, the fact that the measurement is non-destructive, simple and fast makes portable XRF a useful screening tool. If more discriminatory sorting is required, or when the inked area is less than 8 mm in diameter, or when sample stacking is not possible because duplicates are not available, more sensitive techniques will be necessary. †

The mass per area of the xuan paper is about one-third that of typical 80 g m−2 printer paper.

‡

We did multiple scans of the same sample and noticed that the reported uncertainties were comparable to the standard deviations of the concentrations from the multiple scans.



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PLIF results. As we have seen, ink-on-xuan cannot be readily identified by XRF. This kind of specimen also presents special challenges to laser sampling. Unlike the pigments in oil painting, Chinese black inks diffuse extensively through the fibers of the xuan paper, as evidenced by the sideway blurring and the darkening of the bottom of the paper. This is especially so for raw xuan (see Figure 3). As a result, three complications arise. First, the ink mass in the probed volume is significantly reduced. Second, ink components migrate through the fiber at different rates, akin to paper chromatography. Third, signal from the paper can interfere. The dilution effect of the first complication demands extra sensitivity from the analytical probe. We therefore need to apply the more sensitive PLIF technique. PLIF spectra.

We captured numerous single-shot PLIF spectra of the various ink-xuan

combinations. Figure 4 shows inks L (green), O (red), R (gray), R’ (blue) and C (black) on ripe xuan. The display format is explained in the figure caption. Each trace is the average of 100 single-shot spectra and consists of nine 40-nm-wide regions stitched together. The spectral area of each region is normalized to remove shot-to-shot fluctuations. The prominent spectral features are labeled by arrows at the top. They clearly demonstrated the multianalyte detection capability of PLIF. We should also point out that none of the spectral features was visible when either laser beam was blocked, regardless of the ICCD timing. This testifies to the synergy of the two laser pulses in the PLIF scheme. Based on Figure 4, four observations can be drawn. First, the broad spectral features among the five inks are very similar: C2 from the carbon soot; and Ca, CN, and Na from the animal glue. This similarity illustrates the analytical challenge. Second, careful examination shows that ink L has the most Ca, ink O has the most Cu, and ink R has the most Fe. Using the Cu 324. 8 nm, Fe 344.1 nm and Ca 527.0 nm lines as examples, their prominence can be compared against the error bars drawn against them. Third, although inks R and R’ are made from the same pine soot by the same company, they contain significantly different amounts 10 ACS Paragon Plus Environment

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of Fe. Fourth, ink C has no unique spectral signatures, which is expected from its pure carbon origin. By comparing Figure 4 and Table 2, the four spectral features can be seen to be qualitatively consistent with the AA and ICPMS results. It is also interesting to note that the aluminum doublet at 308.2 and 309.3 nm are equally prominent among all five inks. This suggests that the common alum in ripe xuan rather than the ink was the source of aluminum (see Table 2). We should point out that the agreement between the PLIF spectra and the analyte concentrations is only qualitative. Their quantitative correlation is not strong. For example, the peak height of the Ca 527.027 line before normalization was 11,000 and 5,400 CCD counts for inks L and R respectively but ink L carried a hundred times more Ca (see Table 2 and Figure 2). We should note that PLIF was probing the very top layer but XRF and AA were measuring the bulk. Any kind of chemical segregation with penetration into the xuan paper would give rise to the observed difference. Given the cellulose network of the xuan paper, chromatographic separation probably has occurred.32 Analogous results for ink on raw xuan are plotted in Figure 5. As can be seen, the four observations drawn earlier, (1) the general similarity among the five spectra, (2) L-O-R distinguished by Ca-Cu-Fe, (3) R has more Fe than R’, and (4) C being featureless, are equally applicable to the raw case. There are important differences though. The signals are weaker here, as can be seen from the heights of the error bars relative to the signals. The absolute intensity before normalization is only about 60% of that of the ripe case despite three times more ink was applied. As evident from Figure 3, the ink solution diffused all the way to the back side so the mass of dried ink that could be sampled was much less. Another visible difference is the intensity of the aluminum doublet around 309 nm. This time, the Al emissions are evidently lower for ink O, suggesting that the contribution from the paper is not dominant (see Table 2). A third difference is the poor agreement between the PLIF signal



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intensity and the analyte concentration, even qualitatively. For example, the peak height of the Cu 324.754 nm line before normalization was 8,700 and 12,000 CCD counts for inks O and R’ respectively but ink O carried 47× more Cu (see Table 2). Apparently, the more extensive diffusion in raw xuan accentuated the chromatographic effect. Chemometric analysis. Based on the PLIF spectra of the inks on ripe xuan, we used partial-least-square discriminant analysis (PLSDA) to sort the five inks. For that purpose, we captured two independent sets of spectral data either from different areas of the sample or from sample replicates. One set is used to train the PLSDA model while the other set is used to test the model. Each set consists of 100 single-shot panoramic spectra covering the nine spectral regions (1024×9 pixels) for each of the five inks. In PLSDA language, we have a supervised five-class model with each ink belonging to one class; 1024×9 variables, 100×5 training observations and another 100×5 test observations. The class identity of each of the observations is known. For data pre-processing, we normalized the total intensity of each 1024-pixel single-shot spectrum to 5,000 to remove the shot-to-shot fluctuations. We then mean-centered the data values for each of the 1024×9 variables. The sorting accuracy of the 100×5 test observations is shown in Table 3 in the form of a confusion matrix. As can be seen, the sorting accuracy is 98% or better for all ink types except R which is 96%. As expected, ink R is mainly confused for R’ (4/100). Overall, the sorting accuracy is 98.2%. Similar PLSDA analysis was performed on the raw samples. The confusion matrix is shown in Table 4. As can be seen, the sorting of all inks is still highly reliable, with an average accuracy of 91.8%. The lower accuracy relative to the ripe case is expected given the poorer spectral quality (Figure 5 versus Figure 4). The sorting of R is again the worst,


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this time with more observations confused for C (15/100).

For ink C, a number of

observations is wrongly identified as R’ (7/100) and O (4/100). Coefficient spectra.

In the PLSDA sorting of the ink-on-ripe samples, the weight

coefficients for the 1024×9 variables used by the model are plotted in Figure 6 in the form of coefficient spectra. Several features can be noted. First, for ink L, positive weights are assigned to the various Ca I lines. This is not only consistent with our earlier observation that ink L is rich in Ca; it also illustrates how multivariate chemometrics exploits the advantage of multi-element PLIF analysis. Second, positive weights are assigned to Cu lines for ink O and Fe lines for ink R. This is again in agreement with our earlier findings. Third, ink R’ is sorted based on negative weights on Fe lines which distinguishes it from R. Finally, ink C is sorted based on negative weights on Cu, and ambivalent positive and negative weights on Ca I and Na I lines. The rest of the spectrum is like white noise. All these follow from its featureless spectrum. They also explain its slightly poorer sorting accuracy. Analogous coefficient spectra for ink-on-raw xuan are plotted in Figure 7. The positive weights on the Ca I, Cu I and Fe I lines for inks L, O and R respectively are again evident though not as strong as in the ripe case. Neither is the negative weights on the Fe lines for R’. For ink C, the coefficient spectrum is even blander. This lack of decisive biases explains the poorer sorting accuracy relative to the ripe case, especially for inks R and C. Invisible damage. We investigated the damage to the sample caused by the 355 nm laser ablation. We used ink R’ on ripe xuan as a typical specimen. Micrographs of the craters are shown in Figure 8. The top row shows the damage caused by 355 nm ablation at 180 mJ/cm2 which is the fluence used in the PLIF analysis. Panels (a), (b) and (c) show craters after 0, 1, and 2 shots, respectively. As can be seen, damage was not visible after one shot, and became barely visible after two shots. The bottom row shows the analogous damage when the 355 nm laser fluence was increased to 3.1 J/cm2 in order to produce LIBS spectra of comparable



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signal-to-noise ratio as the PLIF spectra. Evidently, the black inks were stripped from the sampled spot in just one shot. CONCLUSIONS We analyzed five commercial Chinese inks on raw and ripe xuan paper. The five inks included the three major types of soot ink made respectively from lacquer, oil and resin. The other two inks were selected to present unique analytical challenges. We first screened the ink-on-xuan samples using a portable XRF probe. We found that the samples were optically too thin and two layers had to be stacked to produce measurable signal. The lacquer and resin inks could be identified this way. We then analyzed the samples by a two-laser-pulse scheme performed on a high precision optical setup. The first 355 nm laser pulse ablated a thin layer of the ink; the second 193 nm laser pulse induced multi analytes in the desorbed ink to fluoresce. Based on the fluorescence spectra, we could tell all the inks apart by partialleast-square discriminant analysis, including the two challenging ink samples. Meanwhile, the sampled area was not visibly damaged even under 64× magnification. We plan to extend the analysis to more Chinese ink types from different manufacturers and different eras in order to establish a comprehensive spectral library. At the same time, we will investigate the chromatographic effects of xuan paper by measuring the chemical depth profiles of the ink-on-xuan samples using PLIF and LIBS. Although the present study focused on the analysis of Chinese inks on xuan paper, the method can be applied elsewhere. In essence, we demonstrated that multi elements in a sample plume, present even at trace level, can all be induced to fluoresce simultaneously to allow multivariate sample sorting while the specimen is not visibly damaged. Chemical mapping at tens of µm resolution or better should also be possible. The method is therefore well suited for the analysis of aged manuscripts and questioned documents, as well as other rare and precious samples. 14 ACS Paragon Plus Environment

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ASSOCIATED CONTENTS The Supporting Information is available free of charge on the ACS Publications website at DOI: Transmission FTIR and attenuated total reflection FTIR spectra of ink-on-xuan samples

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +852 3411 5813. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Prof. Jin Yu of Lyon 1 University for useful discussions, Dr. Jeffery Huang for the use of the ATR-FTIR, and Mr Sunny Verma for the spectral preprocessing. This work was supported by the Research Grant Council of Hong Kong under grant number HKBU200610, HKBU200513 and F-HKBU202/13, the Science and Technology Development Fund of Macau SAR under grant number 045/2014/A1, the Faculty Research Grant of Hong Kong Baptist University, and the Multi-year Research Grant of the University of Macau under grant number MYRG2015-00042-FST.



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REFERENCES

(1) Christie’s September 2015 Asian Art Week auction brought in US$ 54.9 million in just four days, as reported in: http://www.christies.com/auctions/Asian-Art-Week-New-YorkSeptember-2015?sc_lang=en#auctions-section. Last accessed Nov 16, 2015. (2) Fawcett, H. "Demand grows for Chinese fake art," Al Jazeera English, Youtube, 29 Aug 2010. http://www.youtube.com/watch?v=0xyZ63s43uM. Last accessed Nov 17, 2015. (3) Richardson, L. Jr., A Catalog of Identifiable Figure Painters of Ancient Pompeii, Herculaneum, and Stabiae, Johns Hopkins University Press: Baltimore, 1999. (4) Salisbury, L. and Sujo, A., Provenance: How a con man and a forger rewrote the history of modern art, Penguin Press: New York, 2010. (5) Gilberto, E. and Spoto, G. (Eds.), Modern analytical methods in art and archaeology, Wiley-Interscience: New York, 2000. (6) Tang, Y.; Smith, G.J. J. Cult. Herit. 2013, 14, 464-470. (7) Chen, G.; Katsumata, K.S.; Inaba, M. Restaurator, 2003, 24, 135-144. (8) Mullock, H. The Paper Conservator 1995, 19, 23-30. (9) Internet article on xuan paper posted by Books LLC, http://www.booksllc.net/sw2.cfm?q=Xuan_paper. Last accessed Nov 17, 2015. (10) Hagiopol, C.; and Johnston, J.W. Chemistry of Modern Papermaking, 1st Ed., CRC Press: Boca Raton, 2011. (11) Swider, J.R.; Hackley, V.A.; and Winter, J. J. Cult. Herit. 2003, 4, 175-180. (12) Richardin, P.; Cuisance, F.; Buisson, N.; Asensi-Amoros, V.; and Lavier, C. J. Cult. Herit. 2010, 11, 398-403. 16 ACS Paragon Plus Environment

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(13) Wei, S.; Fang, X.; Cao, X.; and Schreiner, M. J. Anal. Appl. Pyrolysis, 2011, 9, 147153. (14) Wei, S.; Fang, X.; Yang, J.; Cao, X.; Pintus, V.; Schreiner, M.; and Song, G. J. Cult. Herit. 2012, 13, 448-452. (15) Yamasaki, A. Fullerene Sci. Technol. 1995, 3, 529-543. (16) Gu, A.; and Shen, W. Sciences of Conservation and Archaeology 2013, 25, 59-64. In Chinese with English abstract. (17) Tanase, I.G.; Udristioiu, E. G.; Bunaciu, A. A.; Aboul-Enein, H.Y. Instrum. Sci. Technol. 2009, 37, 30−39. (18) Williamson, R.; Raeva, A.; and Almirall, J.R. J. Forensic Sci., 2016, 61, 706-714. (19) Cheng, H.; He, W.; Yao, H.; Tang, J.; Yang, F.; Ma, C.; Shan, G.; Zhong, Y.; Wang, W. Sciences of Conservation and Archaeology, 1997, 9, 16–19. In Chinese with English abstract. (20) Zhang, W.; Liu, H.; and Guo, S. Sciences of Conservation and Archaeology 1995, 7, 2127. In Chinese with English abstract. (21) Hahn, D.W. and Omenetto, N. Appl. Spectrosc., 2012, 4, 347-419. (22) Becker J.S. Inorganic Mass Spectrometry – Principles and Applications, Wiley: West Sussex, 2007. (23) Chu, P.C.; Cai, B.Y.; Tsoi, Y.K.; Yuen, R.; Leung, K.S.Y.; and Cheung, N.H. Anal. Chem., 2013, 85, 4311-4315. (24) Cai, Y.; Chu, P.C.; Ho, S.K.; and Cheung, N.H. Frontiers of Phys., 2012, 7, 670-678. (25) Wang, X.; Huang, Z.; Chu, P.C.; Cai, Y.; Leung, K.S.Y.; Lum, T.S.; and Cheung, N.H. The mechanism of ArF laser induced fluorescence of dense plume matter, J. Anal. At. Spectrom. 2016, DOI: 10.1039/C6JA00290K. (26) Saitzyk, S.L. Art Hardware, Watson-Guptill, 1987. 17 ACS Paragon Plus Environment


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(27) Ho, S.K.; Cheung, N.H. Anal. Chem. 2005, 77, 193-199. (28) Cheung, N.H. Appl. Spectrosc. Rev. 2007, 42, 235-250. (29) Chu, P.C.; Yip, W.L.; Cai, Y. Cheung, N.H. J. Anal. At. Spectrom. 2011, 26, 12101216. (30) Cai, Y. Minimally destructive multi-element analysis of colorants for forensic applications and artwork authentication, PhD Thesis, Hong Kong Baptist University, 2013. (31) Motto-Ros, V.; Negre, E.; Pelascini, F.; Panczer, G., and Yu, J. Spectrochim. Acta B, 2014, 92, 60-69. (32) Zlotnick, J.A. and Smith, F.P. J. Chromatogr. B, 1999, 733, 265-272.


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FIGURE CAPTION

Figure 1. (a) Schematics of the PLIF setup. The first laser pulse at 355 nm was expanded by lenses L1 and L2, reflected downwards by dielectric mirror M and focused onto the sample by lens L3. Focusing was adjusted by motorized translation of L1. After 11 µs, a second laser pulse at 193 nm intercepted the plume. The 193 nm beam was focused by lens L4 to a point 8 mm in front of the plume and 145 µm above the sample surface. The plume fluorescence was collected by lens L6 and imaged by lens L7 onto an optical fiber bundle, the other end of which was connected to a spectrograph. At the same time, the sample surface was monitored by a CCD camera C via lens L3, beam splitter S and lens L5. (b) A typical CCD image of the surface of Chinese black ink on xuan. The overlap of the red cross hair and the green reference circle indicates that the target surface is at the correct height z. The scale bar represents 600 µm. (c) The working principle of the z alignment. When the target surface is at the correct height (position 2), the red cross-hair centers on the reference circle. When the surface is too high (position 1), the red cross-hair is displaced to the right. When it is too low (position 3), the red cross-hair is displaced to the left.

Figure 2. [Ca] and [S] in the two-layer stacked samples as measured by the portable XRF probe. Shown is the net analyte concentration [X] (in %) in the ink-xuan sample when [X]b of the blank paper is subtracted. The associated uncertainties are shown as error bars.

Figure 3. Ink L on xuan paper. (a) Front side of ripe xuan sample. (b) Back side of ripe xuan sample. (c) Front side of raw xuan sample. (d) Back side of raw xuan sample.



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Figure 4. PLIF spectra of five different inks on ripe xuan. The five inks are coded L (green), O (red), R (gray), R’ (blue) and C (black). Each trace is the average of 100 single-shot spectra. Nine 1024-pixel spectral regions each covering about 40 nm are stitched to form one panoramic spectrum with gaps marking the boundaries. The summation of intensities over each 1024-pixel region is normalized to 5,000 to remove shot-to-shot fluctuations. The five traces are offset vertically for clarity. The leading and trailing pixels are zeroed to indicate the baseline. Intensities of some spectral regions are scaled for graphical clarity. The scale factors are shown at the bottom. The standard deviation of 100 single-shot events is indicated by two-sigma-full-height error bars drawn against three spectral features: the Cu 324.754 nm line of ink O (red), the Fe I 344.061 nm line of ink R (gray) and the Ca I 527.027 nm line of ink L (green). These and other prominent features are labeled by arrows at the top.

Figure 5. PLIF spectra of five different inks on raw xuan. Display format identical to that of Figure 4 except error bars are drawn against the Cu I 324.754 nm line of ink O (red), the Fe I 344.061 nm line of ink R (gray) and the Ca I 422.673 nm line of ink L (green).

Figure 6. PLS-DA coefficient spectra of the five inks on ripe xuan. The five traces are offset vertically for clarity and with the leading and trailing pixels zeroed to indicate the baseline. The gap between the top two traces is 0.02. The corresponding spectral features are labeled by arrows at the top.

Figure 7. PLS-DA coefficient spectra of the five inks on raw xuan. The display format is identical to that of Figure 6 except the gap between the top two traces is 0.01.


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Figure 8. Micrographs of sampled spots of ink R’ on ripe xuan. Top row, ablation by 355 nm laser pulse at fluence of 180 mJ/cm2. Shown are craters after (a) zero shot, (b) one shot, and (c) two shots. Bottom row, 355 nm laser fluence increased to 3.1 J/cm2, with craters formed after (d) zero shot, (e) one shot, and (f) two shots. Scale bar is 300 µm.



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Table 1. The inks and papers used in this study. Ink L O R R’ C

Brand name Qing mo Tang mo Jianshan wandei Huangshan song yan Qi ye jinlan

Manufacturer Hu Kaiwen, De Ji branch Hu Kaiwen, Tun Xi branch Hu Kaiwen, Jing De branch Hu Kaiwen, Cheng Wen Tang branch Caosugong

Made from Lacquer soot Tung oil soot Pine soot Pine soot Carbon black

xuan raw ripe

Brand name Yu xing jing pi Yu xing fan xuan

Manufacturer Anhui Sheng Jing Xian Yuquan Xuan Zhi Anhui Sheng Jing Xian Yuquan Xuan Zhi

Density (g m−2) 24.1 28.6


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Table 2. Concentration (µg/g) of Ca, Fe, Cu and Al in Chinese inksticks and xuan papers as measured by atomic absorption (AA) and inductively-coupled plasma mass spectrometry (ICPMS).

µg/g

raw xuan ripe xuan ink L ink O ink R ink R’ ink C

AA Ca 2,280 1,020 143,000 600 1,030 706 906

Fe 146 136 424 429 2,640 400 400

ICPMS Cu 1.71 3.31 8.33 612 80.1 13.4 10.1

Al 115 324 217 5.56 124 142 137



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Table 3. Confusion matrix of the sorting of inks on ripe xuan.

Test class L O R R’ C

# test obser’ns 100 100 100 100 100

L 100 0 0 0 0

O 0 99 0 0 0

Model class R R’ 0 0 1 0 96 4 2 98 0 2


C 0 0 0 0 98 avg

Correct 100% 99% 96% 98% 98% 98.2%

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Table 4. Confusion matrix of the sorting of inks on raw xuan.

Test class L O R R’ C

# test obser’ns 100 100 100 100 100

L 96 2 0 4 0

O 1 98 1 1 4

Model class R R’ 3 0 0 0 83 1 1 94 1 7


C 0 0 15 0 88 avg

Correct 96% 98% 83% 94% 88% 91.8%


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TOC

− ink #1 − ink #2

307


327

347

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Figure 1. (a) Schematics of the PLIF setup. The first laser pulse at 355 nm was expanded by lenses L1 and L2, reflected downwards by dielectric mirror M and focused onto the sample by lens L3. Focusing was adjusted by motorized translation of L1. After 11 s, a second laser pulse at 193 nm intercepted the plume. The 193 nm beam was focused by lens L4 to a point 8 mm in front of the plume and 145 m above the sample surface. The plume fluorescence was collected by lens L6 and imaged by lens L7 onto an optical fiber bundle, the other end of which was connected to a spectrograph. At the same time, the sample surface was monitored by a CCD camera C via lens L3, beam splitter S and lens L5. (b) A typical CCD image of the surface of Chinese black ink on xuan. The overlap of the red cross hair and the green reference circle indicates that the target surface is at the correct height z. The scale bar represents 600 m. (c) The working principle of the z alignment. When the target surface is at the correct height (position 2), the red cross-hair centers on the reference circle. When the surface is too high (position 1), the red cross-hair is displaced to the right. When it is too low (position 3), the red cross-hair is displaced to the left.

F-1 ACS Paragon Plus Environment


50 L-raw

net [X] (%) measured by XRF

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L-ripe

40

O-raw O-ripe

R-raw

30

R-ripe R'-raw

20

R'-ripe C-raw

10

C-ripe

0 Ca

S

Figure 2. [Ca] and [S] in the two-layer stacked samples as measured by the portable XRF probe. Shown is the net analyte concentration [X] (in %) in the ink-xuan sample when [X]b of the blank paper is subtracted. The associated uncertainties are shown as error bars.


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a

b

c

d

Figure 3. Ink L on xuan paper. (a) Front side of ripe xuan sample. (b) Back side of ripe xuan sample. (c) Front side of raw xuan sample. (d) Back side of raw xuan sample.



Al I

L

normalized intensity

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O R

R’

C

2x 290

0.3 x 340

390

3x 440

490

540

0.3 x 590

640

wavelength (nm) Figure 4. PLIF spectra of five different inks on ripe xuan. The five inks are coded L (green), O (red), R (gray), R’ (blue) and C (black). Each trace is the average of 100 single-shot spectra. Nine 1024-pixel spectral regions each covering about 40 nm are stitched to form one panoramic spectrum with gaps marking the boundaries. The summation of intensities over each 1024-pixel region is normalized to 5,000 to remove shot-toshot fluctuations. The five traces are offset vertically for clarity. The leading and trailing pixels are zeroed to indicate the baseline. Intensities of some spectral regions are scaled for graphical clarity. The scale factors are shown at the bottom. The standard deviation of 100 single-shot events is indicated by two-sigma-full-height error bars drawn against three spectral features: the Cu 324.754 nm line of ink O (red), the Fe I 344.061 nm line of ink R (gray) and the Ca I 527.027 nm line of ink L (green). These and other prominent features are labeled by arrows at the top. F-4 ACS Paragon Plus Environment

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normalized intensity

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3x 290

2x 340

390

440

490

540

0.6 x 590

640

wavelength (nm) Figure 5. PLIF spectra of five different inks on raw xuan. Display format identical to that of Figure 4 except error bars are drawn against the Cu I 324.754 nm line of ink O (red), the Fe I 344.061 nm line of ink R (gray) and the Ca I 422.673 nm line of ink L (green).


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relative weight


290

340

390

440

490

540

590

640

wavelength (nm) Figure 6. PLS-DA coefficient spectra of the five inks on ripe xuan. The five traces are offset vertically for clarity and with the leading and trailing pixels zeroed to indicate the baseline. The gap between the top two traces is 0.02. The corresponding spectral features are labeled by arrows at the top.


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relative weight

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290

340

390

440

490

540

590

640

wavelength (nm) Figure 7. PLS-DA coefficient spectra of the five inks on raw xuan. The display format is identical to that of Figure 6 except the gap between the top two traces is 0.01.



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Figure 8. Micrographs of sampled spots of ink R’ on ripe xuan. Top row, ablation by 355 nm laser pulse at fluence of 180 mJ/cm2. Shown are craters after (a) zero shot, (b) one shot, and (c) two shots. Bottom row, 355 nm laser fluence increased to 3.1 J/cm2, with craters formed after (d) zero shot, (e) one shot, and (f) two shots. Scale bar is 300 m.


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Elemental Analysis of Chinese Black Inks on Xuan Paper by ArF Laser

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