Early globalized industrial chain revealed by residualsubmicron pigment particles in Chinese imperialblue-and-white porcelainsXiaochenyang Jianga,1, Yanjun Wengb,1, Xiaohong Wu (吴小红)a, Jianfeng Cui (崔剑锋)a,2, Hongshu Lyua, Jianxin Jiangb,Guodong Songc, Hetian Jind, Dashu Qina,2, and Changsui Wange

aSchool of Archaeology and Museology, Peking University, 100871 Beijing, China; bDivision of Field Archaeology, Jingdezhen Institute of CeramicArchaeology, 333001 Jingdezhen, China; cDivision of Historic Archaeology, Inner Mongolia Institute of Cultural Relics and Archaeology, 010010 Hohhot,China; dLaboratory of Archaeological Science, Beijing Institute of Cultural Heritage, 100009 Beijing, China; and eDepartment of Archaeology andAnthropology, University of Chinese Academy of Sciences, 100049 Beijing, China

Edited by A. Mark Pollard, Oxford University, Oxford, United Kingdom, and accepted by Editorial Board Member Elsa M. Redmond January 27, 2020 (receivedfor review September 25, 2019)

The success of early Chinese blue-and-white porcelains reliedheavily on imported cobalt pigment from the West. In contrastto art-historical concept, which contains both typological evidenceand literature records, it is assumed that imported Sumali blue wascompletely superseded by domestic Chinese asbolane ore basedon the analytical results of the Fe/Mn ratio in imperial productionsfrom the Xuande reign (1426 to 1435 CE) onward. Using a focusedion beam transmission electron microscopy technique to reassessthis hotly debated question, we have identified two classes ofresidual submicron pigment particles in the blue glaze with diag-nostic differences in morphology, chemical composition, and dis-tribution behavior. Compared with the microstructural features ofthe blue-and-white porcelains of the Yuan and Qing dynasties, weshow that a mixture of imported and domestic cobalt pigmentswas used for aesthetic reasons, indicating that the overseas supplychain of imported pigment remained consistent and adequateeven though the authorities had terminated official trade andtributary activities after the death of Admiral Zheng He. This dis-covery further suggests that the globalized trading network andcross-regional industrial chain had been extensively established inthe 15th century. Moreover, we provide analytical evidenceagainst the fundamental assumption of the current Fe/Mn prove-nancing criteria, implying that the failures of previous chemicalanalyses can be attributed to elemental differentiation betweenthe silicate glaze and the arsenic pigment. We propose an innova-tive method for directly assessing original mineralogic informationfrom submicron residual pigment particles that provides a morereliable way to trace cobalt circulation and holds great promise forprovenance studies.

blue-and-white porcelains | residual pigment particles | globalized tradingnetwork | ceramic archaeology

The blue-and-white porcelains of the Yuan, Ming, and Qingdynasties, made in Jingdezhen, China, represent the highest

level of the ancient porcelain making handicraft industry interms of both quality and production volume and represent oneof the most influential aspects of human cultural heritage (1–4).Moreover, as the raw materials involve cross-regional circulationand trade, blue-and-white porcelain is an optimal subject fordiscussing the integration of materials, technology, and culturaltransmission in ancient Eurasia (3–6). Thus, related studies, es-pecially on the provenance of cobalt blue pigments, have con-sistently been a popular topic in archaeological research andhave drawn the interest of scholars across a broad range of dis-ciplines. As early as 1956, chemical analysis was conducted onthe cobalt pigments of selected porcelains at the ResearchLaboratory for Archaeology and the History of Art, University ofOxford (7). Since then, scientists and archaeologists haveobtained fruitful results through close collaboration, and blue-

and-white porcelain research has become an exemplary case ofinterdisciplinary cooperation (4–11). However, researchers haveconflicting interpretations regarding the use of pigments for theblue-and-white porcelains produced in the imperial kilns of theXuande reign of the Ming dynasty, fueling fervent academicdiscussion.Specifically, on the basis of the typological evidence (e.g., blue

hue, bleeding effect, iron spots) of the cobalt pigments andtextual records, archaeologists have declared that the importedSumali blue from the Middle East was used throughout theXuande Period, as it had been in the Yuan and Early Mingdynasties (12–14). In contrast, scientists argue that the im-ported Islamic pigment was abruptly superseded by Chinesedomestic asbolane ores in the Xuande reign given the high Mn/Fevalue of blue pigments, which is currently considered an effectiveempirical criterion to use in tracing chronological changes inthe blue pigments (1, 15–19). As this contradiction involves an

Significance

Our knowledge of the pigment provenance of blue-and-whiteporcelain is based primarily on the Fe/Mn criteria, which con-tradicts the archaeological concept (including typological evi-dence like blue hue, bleeding effect, iron spots) and historicalrecords. Here we propose a reliable method for tracing cobaltsources through the microstructural analysis of submicronresidual pigment particles to avoid influence from elementalfractionation. An application of the method to the Xuandeimperial productions demonstrates that an intentional ap-proach of mixing imported and domestic cobalt ores wasadopted, which could rationalize all of the apparent contra-dictions between compositional data and conventional au-thentication. Our findings further suggest that the overseaspigments continued to play an irreplaceable role from 15thcentury onward as the globalized trading network began tobreak free of regional political affairs.

Author contributions: J.C. and D.Q. designed research; X.J. performed research; Y.W., J.J.,G.S., and H.J. contributed the samples and the excavation information; X.J., X.W., J.C.,H.L., and C.W. analyzed data; and X.J., Y.W., and J.C. wrote the paper.

The authors declare no competing interest.

This article is a PNAS Direct Submission. A.M.P. is a guest editor invited by theEditorial Board.

Published under the PNAS license.1X.J. and Y.W. contributed equally to this work.2To whom correspondence may be addressed. Email: cuijianfeng@pku.edu.cn orqindashu@pku.edu.cn.

This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1916630117/-/DCSupplemental.

First published March 2, 2020.

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important and sensitive node in history—when and how do-mestic blue pigments replaced imported blue pigments—thisquestion has attracted special attention in the academiccommunity.There is a key assumption underlying the methodology of the

abovementioned scientific research—namely, that the under-glaze cobalt pigment would be completely or proportionallydissolved in the glaze layer so that the characteristic elementalratios in the blue area, which can be calculated by X-ray fluo-rescence spectroscopy, laser ablation inductively coupled plasmaatomic emission spectroscopy/mass spectroscopy results, shouldbe identical to the ratios of the original cobalt ores (20). In otherwords, the different ratios of impurities (such as Fe/Mn) in blueglaze are expected to reflect the different mineralogical originsof ore deposits. However, given that cobalt in the Earth’s crustoften exists in the form of sulphide and arsenide, and thattransition-metal elements, which include the characteristic ele-ments of blue-and-white porcelains, have significant differencesin affinities to oxygen, sulfur, and arsenic, in theory, strong ele-mental differentiation would inevitably occur during pigmentdissolution. In this scenario, oxophilic elements such as ironand manganese enter the silicate glaze, while sulphophilicelements such as copper and nickel enter the matte phase. If thisis the case, then it is necessary to reconsider whether the char-acteristic elemental ratios detected from the blue glaze can serveas an accurate indicator of the primary composition of cobaltores. We assume that this may be the reason why the existingprovenancing studies are only able to distinguish domestic andimported sources in general but fail to further determine thespecific mineral species and origins.Therefore, identifying the inherent cause of this contradiction

and providing a reasonable explanation represent key problemsin archaeology. In this study, we selected scientifically unearthedXuande imperial blue-and-white porcelain sherds with clear in-scriptions (大明宣德年制, made in the Xuande reign of the Mingdynasty) for the direct analysis of submicron residual pigment

particles based on the advanced focused ion beam transmissionelectron microscopy (FIB-TEM) technique. In addition, blue-and-white porcelain sherds of the Yuan dynasty (generally con-sidered a typical period during which imported pigments wereused) and blue-and-white porcelain samples of the Qing dynasty(generally considered a typical stage during which domesticpigments were used) were selected as reference materials toperform comparative studies. Moreover, we examined the inherentcauses of the Xuande contradiction and accordingly created apromising paradigm with which to accurately lift out the residualpigments and precisely characterize these particles. Our proposedmethod shows clear advantages over routine glaze compositionalanalysis in the capability to distinguish the specific ore deposits andhas great potential applicability in provenance studies and tracingof resource circulation.

ResultsRepresentative Imperial Blue-and-White Porcelain Sherds from VariousArchaeological Sites. To highlight the novelty of our approach, wereport the results obtained on a sufficient number of repre-sentative pieces (n = 275) of imperial blue-and-white porcelainspanning from the Yuan dynasty to the Qing dynasty in Chinaand Mongolia. These sherds were selected from four key ar-chaeological sites with definite information (SI Appendix, Fig.S1). Chronologically, these samples cover all the typical stagesof pigment usage, including the Yuan and Qing dynasties, whichadopted only imported and domestic cobalt pigments, respec-tively, and the Xuande reign, the most controversial transitionnode in the use of blue pigments. Spatially, these samples alsocover the important units in the operation chain of blue-and-whiteporcelain, including the exclusive official kiln on the pro-duction end—the Imperial Kiln Site at Jingdezhen—and threerepresentative sites on the consumption end: the ruins ofKarakorum (Ka; the early-stage capital and political center of theMongolian Empire), the Maojiawan porcelain pit in Beijing (theancient capital of the Yuan, Ming, and Qing dynasties), and the

Fig. 1. Microstructure of the residual pigment particles revealed in the Xuande imperial blue-and-white porcelain. Images demonstrate a typical morphologyof sample MX-JY-01 in backscattering mode. (A) Overview of the blue region. (B–F) Individual close-up views of areas of interest containing the two types ofrepresentative particles.

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Old Summer Palace (the ruins of the Royal Garden of the Qingdynasty). The selected Xuande imperial blue-and-white porce-lain sherds are not only painted with official inscriptions (SIAppendix, Fig. S2), which greatly aid their differentiation, but alsohave authenticating features of Early Ming Period production,such as a dark-blue hue, bleeding effect, and iron spots, indicatingthe use of imported cobalt pigments and the expected consistencybetween archaeological typology and literary records. However,the nondestructive results (SI Appendix, Table S1) of the in situportable X-ray fluorescence (pXRF) analysis demonstrate thepresence of an exceptionally high manganese content, which is asignature of Chinese native deposits and is consistent with previousstudies on Xuande blue-and-white porcelains (1, 4, 11, 15–19).

Two Types of Residual Pigment Particles with Different Properties AreDiscovered in the Xuande Imperial Blue-and-White Porcelains. Thediagnostic microstructural morphologies of the Xuande sherdsare illustrated in Fig. 1 and SI Appendix, Fig. S3. Generally, thecross-section of the blue region can be divided into three parts:the body region with massive irregular pores, unmelted quartzparticles, and interstitial glass phase; the glaze layer containingabundant glaze bubbles and few unmelted quartz particles; andan interaction layer with dense, dark-gray lath-like crystal clus-ters and bright particles observable in backscatter mode. High-magnification images clearly show that these particles can besubdivided into two classes on the basis of their contrast anddistribution behaviors. The high-contrast particles mostly havestandard spherical shapes and appear only among interstitialspaces of dark-gray needle-like crystals, whereas the irregu-larly shaped low-contrast particles are tightly enwrapped bywell-developed plate-like crystals, forming a core-shell struc-ture. Macroscopically, there are no clear clustering tendencies ofthe particles; rather, they are thoroughly mixed. In addition, animmiscible liquid-liquid phase separation structure formed bythe associated precipitation appears in the periphery of the crystalclusters.The corresponding compositional analysis (SI Appendix, Table

S2) and elemental mapping (Fig. 2) demonstrate that althoughthe two aforementioned particles are both rich in Co, theirspecific compositions are very different: the high-contrast parti-cles consist of heavy elements such as As and Ni and minoramounts of Cu, whereas the low-brightness particles contain lowatomic number elements such as O, Al, and Mn along with minoramounts of Ni. Furthermore, concentrated areas of Ca and Allargely overlap with the dark-gray crystals; in combination withstoichiometric calculation and the needle-like morphology, theresults clearly identify this class of crystals as anorthite.

FIB-TEM Analysis Reveals the Fine Structure of Residual Pigments. Toobtain accurate information regarding the chemical compositionand phase structure of the residual pigment particles and avoidmatrix effects arising from the overly large excitation volume ofscanning electron microscopy energy-dispersive X-ray spectros-copy (SEM-EDS), we used FIB to directly lift out several typicalpigment-rich domains and then used TEM to precisely explorethe fine structural information of the residual pigments. Thehigh-angle annular dark-field image (Fig. 3B) shows four regionswith different morphologies in the FIBed slice. Specifically, thelargest gray section in the field of view overlapping with Al- andCa-rich areas could be identified as anorthite (CaAl2Si2O8)according to the stoichiometric ratios of the elements and theelectron diffraction patterns. The high-brightness spherical parti-cle in the upper right part of the slice is ∼500 nm in diameter andlocated in the interstitial spaces within the anorthite crystals.The chemical analysis shows that the particle is rich in As, Co,

Ni, and Cu and contains almost no O, Si, Al, Mn, or Ca (Table1). However, the corresponding selected area electron diffrac-tion pattern and high-resolution lattice measurement cannot

Fig. 2. Elemental mapping demonstrates the elemental differences in re-sidual pigment particles in Fig. 1F. The distributions of the Al and Ca areconsistent with expectations in the dark-gray crystalline phase, confirmingthe existence of anorthite. The Fe-, Mn-, Si-, and K-rich regions are accordantwith the intercrystalline glass phase. In terms of residual pigments, thesebright particles coincide with the Co distribution as colorants overall. Thedistribution of the high-contrast particles is consistent not with Fe but in-stead with As, Ni, and Cu, whereas the low-contrast particles are positivelycorrelated with Al-, Mn-, and Fe-rich areas.

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match any references in the dataset, indicating the complexity ofthe structure, which may be an intermetallic compound such ascobalt-bearing speiss. The relatively low-brightness, irregularparticles on the left side of the slice have been completelyenwrapped by anorthite crystals and coincide with enrichment oflight elements, such as O, Al, Co, Fe, and Mn. Since the corre-sponding phase identification indicates a characteristic spinel struc-ture, we infer that these particles are cobalt aluminates (CoAl2O4)with impurity elements of Fe and Mn as a kind of solid solutionand replacement (11, 21). The large numbers of isolated smallliquid droplets are enriched in Fe and Mn and depleted in Siand K. The corresponding fast Fourier transform and selectedarea electron diffraction analysis confirm the existence of anamorphous liquid-liquid phase separation structure. In addi-tion, these small droplets are mostly smaller than 100 nm, andas the light-scattering source, they are prone to influence theblue hue and final appearance of the porcelain. Notably, theclose agreement between the SEM and TEM results excludesthe interpretation of TEM observations as isolated cases.

Comparative Analysis of the Yuan Blue-and-White Porcelains Revealsthe Microstructural Features of Imported Cobalt Pigments. It isgenerally acknowledged that the pigments used in early-stageblue-and-white porcelains are completely imported cobalt ores(22–26). Thus, four pieces of Yuan blue-and-white porcelainsherds were selected for comparison analysis in this study. Thebackscatter images (Fig. 4) clearly demonstrate massive high-brightness residual pigment particles in the blue regions, andthe morphology and distribution of these particles present cer-tain patterns. Most of the particles appear as normative spheres;

the residual particles located at the glaze–bubble interface aregenerally larger (>5 μm) than the residual particles at the body–glaze interface (submicron scale). The particles are principallydistributed at the glaze–bubble interface and its periphery (ac-counting for the majority of the particles), along the dark-grayanorthite crystals and in the periphery of the iron spots (a mod-erate proportion) and the body–glaze interface (the lowest pro-portion). Of note, no residual particles are wrapped by theanorthite crystals, and even in the iron spot region where anorthiteprecipitates are well developed, the pigment particles still directlycontact the anorthite crystals only rarely. The EDS analysis sug-gests that these high-brightness particles generally have high Cocontent; are enriched in As, Ni, and Cu; and are depleted in Feand O (SI Appendix, Table S3). These findings are not only similarto the published data (27) on the residual pigments of the Yuanblue-and-white porcelains excavated from the Luomaqiao kiln sitein Jingdezhen, but are also consistent with the morphology andchemical composition of the high-brightness particles in theaforementioned sherds from the Xuande imperial kiln, indicat-ing that the same cobalt source was successively used in theXuande reign.

Comparative Analysis of the Qing Blue-and-White Porcelains Revealsthe Microstructural Features of Domestic Cobalt Pigments. It isgenerally acknowledged that the cobalt pigments used in theQing dynasty are completely Chinese native asbolane ores (1, 4,6, 17). Therefore, three pieces of the Qing blue-and-white por-celain sherds were selected for comparison analysis in this study.The backscatter images (Fig. 5) show large numbers of brightpigment particles and surrounding thickset anorthite shells along

Fig. 3. TEM images of the FIBed slice of a blue region. (A) Overview of the slice. (B) High-angle annular dark field image of the slice, in which a high-contrastspherical particle can be observed in the upper right, low-contrast irregular particles are observed in the lower left, and intercrystalline liquid-liquid separateddroplets are seen in the lower right. (C–E) Close-up views of bright field images and the corresponding selected area electron diffraction (Upper Left) andhigh-resolution images (Lower Left) of the three aforementioned phases.

Table 1. Average compositions of typical phases in the FIBed slice (wt%)

O Na Mg Al Si K Ca V Mn Fe Co Ni Cu As S

High-contrast particles 0.95 0.83 0.04 0.46 0.29 1.67 0.21 b.d. b.d. 1.13 34.24 9.16 3.37 47.65 b.d.Low-contrast particles 49.23 0.42 2.89 24.56 3.81 0.11 0.54 0.66 5.09 3.63 8.22 0.84 b.d. b.d. b.d.Glaze 47.77 1.44 0.35 8.19 31.3 3.5 2.38 b.d. 2.45 1.33 1.21 0.03 b.d. 0.05 b.d.Gray acicular crystals 49.26 1.16 0.25 16.16 20.5 0.27 11.78 b.d. 0.22 0.19 0.21 b.d. b.d. b.d. b.d.

b.d., below detection.

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the body–glaze interface. Contrary to the morphology in theYuan blue-and-white porcelain, the residual particles of the Qingsherds appear as irregular shapes with diameters of several mi-crometers. Furthermore, all of the pigment particles in the Qingsherds are tightly enwrapped by abnormally developed plate-likeanorthite crystals, presenting a classic core-shell structure, and todate, none of them have been isolated in the glass phase. TheEDS analysis shows that the pigment particles have exceptionallyhigh levels of Al, Mn and Fe (SI Appendix, Table S4), which notonly are identical to the microstructure of the exported blue-and-white porcelains of late Ming dynasty produced in folk kilns, asreported by Jiang (11), but also coincide with the aforementionedresults of the low-contrast particles in Xuande imperial blue-and-white porcelain regarding their morphology, composition anddistribution, confirming that the same kind of cobalt ore appearsto have been initially used in the Xuande reign.

DiscussionThe discovery of the residual pigment particles in blue-and-whiteporcelain provides extraordinary evidence to answer the mostintriguing questions pertaining to the pigments used in theXuande reign. The results reveal two thoroughly mixed classes ofblue pigments with obviously different properties, includingmorphology, chemical composition, and distribution behavior.Specifically, microstructural results of Xuande imperial blue-and-white porcelain and those from comparative experimentalanalyses suggest that one pigment is spherical Co-Ni-(Cu)-Asintermetallic compounds associated with minor sulphophilic el-ements and appearing only among interstitial spaces of theneedle-like anorthite crystals, which is the same as the importedpigment used on the Yuan blue-and-white porcelains, while theother pigment is irregular cobalt aluminate spinels associated withoxophilic elements (such as Fe and Mn) and all closely enwrappedby plate-like anorthite crystals as the core-shell structure, which isthe same as the domestic pigment used on the Qing blue-and-white porcelains. It should be noted that these residue cobaltparticles can have one of three origins: (1) chemically unchangedresidual (primary) cobalt ore; (2) partially dissolved (refractory)particles that no longer have the original primary compositionand may have recrystallized; or (3) secondary particles that haveeither nucleated and grown as new phases or have been parti-tioned by phase-separation mechanisms. In this study, thespherical Co-Ni-As grains could be attributed to the secondarytype, which have compositions controlled by multiple solubilitycoefficients that partition some elements into phase-separateddomains, while the irregular Al-Mn-rich grains could be attrib-uted to the refractory/recrystallized type, which have undergonepartial fractionation. Although neither of these grains are iden-tical to the primary composition of the original ore, the di-agnostic elements (such as Ni-Cu-As and Mn-Al-O, respectively)are retained and can be used to point to former cobalt mineralsources as indicators.From the perspective of geochemistry, the parent minerals of

the imported particles observed in the Yuan dynasty and Xuandeblue-and-white porcelains, according to the element association,should be Co-Ni-(Cu)-As deposits such as smaltite [(Co,Ni)As2],skutterudite [(Co,Ni)As3], cobaltite [Co,Ni)AsS], or secondaryerythrine [(Co,Ni)3(AsO4)2·8H2O], which developed in the oxi-dation zones of the deposits. Likewise, based on the scenario ofMn and Al enrichment, the primary minerals of the native pig-ment particles observed in the Qing dynasty and Xuande blue-and-white porcelains could have been sourced from a secondaryweathering asbolane deposit (28, 29). Cobalt-bearing arsenideand asbolane ores present distinctive and inherent differencesin metallogenesis and deposit formation mode. Specifically, thearsenide deposit is formed principally by endogenic mineraliza-tion processes, including magma penetration and hydrothermalcirculation, and usually requires a low-oxygen fugacity environ-ment, whereas the asbolane deposit is formed mainly by epigenicprocesses, such as the weathering sedimentary, and requires highoxygen partial pressure. Furthermore, the formation of arsenidedeposits tends to occur in fault tectonic zones and orogenic belts,while the formation of weathering and leaching deposits requiresstable geological structures, such as a craton, and specific ther-modynamic conditions to decompose clay, along with sufficientrainfall to carry away the silicates. For example, the Lehuamanganese ore deposit in Jingdezhen, one of the largest asbolanemines in Asia and an important source of blue pigment recor-ded in ancient literature, is located in a subtropical monsoonregion with high average annual temperature and seasonaltemperature differences, abundant rainfall, and thick vegetation.In addition, as for element association, the asbolane deposit isoften composed of siderophile elements, while the arsenide de-posit is associated with chalcophile elements, which may reflect

Fig. 4. Microstructure of the residual pigment particles revealed in the im-perial Yuan blue-and-white porcelain. (A) Typical morphology of sample Y-BM-01, where massive residue pigment particles exist at the interface of glazebubbles. (B) Close-up view of A. (C) Typical morphology of sample Y-Ka-01, inwhich residual particles exist at the interface of bubbles and the body–glazeinterface. (D) Close-up view of the body–glaze interface in C. (E and F) Typicalmorphology of sample Y-Ka-02, in which pigment particles exist around “ironspot” zones. (G) Typical morphology of sample Y-Ka-03, in which residualpigment particles exist at the interaction layer between body and glaze.

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the different mineralogical origins of cobalt mines. In fact, al-though it is not possible to fully exclude other possibilities, basedon the acquired data from geological surveys to date, the specificcompositional features of high-contrast residue particles—nota-bly, the low content of Fe and Cu/Ni+Co— appear to have beenattributed only to Anarak deposit (30–32) on the Iran Plateau,which is located on the Uroumieh-Dokhtar magmatic belt andfamous for its classic five-element association of Ni-Co-As-Cu(U) (31). The signature we observed is also very similar to that ofthe blue pigment used in early Islamic glazed ceramics (33, 34),suggesting that the cobalt ores from the Anarak mining district,rather than those originating from Qamsar, Kashan as previouslythought (5, 35), might have served as one of the most likelysources of the imported pigment used for Chinese imperial blue-and-white porcelains.Given the distinctive and inherent differences in geochemistry,

these results strongly indicate that the two mixed classes of residueore particles cannot be naturally paragenetic assemblages but in-stead must be the result of a deliberate action aiming to concoctpigment materials for desired aesthetic appearance. Accordingly,this intentional mixing treatment could successfully rationalize allof the apparent paradoxes between compositional data and con-ventional authentication. The arsenic ore particles would result inearly-stage authenticating signatures, including dark hue, bleedingeffect, and iron spots, while the asbolane particles with othermanganiferous impurities would dissolve in blue design regionsand consequently decrease the Fe/Mn value to a great extent.From an archaeological perspective, it is speculated that this

ingenious mixing procedure derived from a technological inno-vation initiative to improve artistic performance initiatively ratherthan representing a passive compromise due to a shortage in thesupply of imported pigments. Specifically, it has been claimed thatthe emergence of native Chinese colorants might be related to thecollapse of extravagant tributary trades and officially sponsoredvoyages, which were considered the most reliable sources of im-ported blue pigments. Thus, when the pigment inventory was se-verely depleted, potters had to seek alternative but inferior local

colorants and adopt a mixing strategy to save valuable importedpigments while trying to maintain the original aesthetic appear-ance of blue-and-white porcelains (4, 5, 11). However, the largequantities of residual arsenic particles that we discovered inXuande imperial blue-and-white porcelains affirm the adequacyand continuity of the overseas supply chain of raw materials ofcobalt ores, even though the authorities had terminated officialtrade and tributary activities after the death of Admiral Zheng He.In fact, the contemporaneous 江西省大志 (Chorography ofJiangxi Province, Ceramics Volume, compiled by the governmentofficials) had recorded the methods of pigment usage (36)—“Pureimported pigments give rise to bleeding effect; and when nativepigments are added in excess, the hue becomes gloomy”—whichalso confirms that the ancient potters had already recognized theproperties of different cobalt pigments and, accordingly, were ableto perfectly complement imported and domestic materials bymixing them in certain proportions depending on the requiredaesthetic design. Thus, it appears likely that the extensive long-distance exchange network across Eurasia had been well estab-lished from the 15th century onward, and that the globalizationprocess could not be suspended by any regional political affairs.From a methodological perspective, the microchemical re-

sults (Table 1 and SI Appendix, Tables S2–S4) show that thecompositions of the imported residual pigment particles arecompletely different from those of solutes in blue glaze; namely,the contents of As, Ni, and Cu account for a dominant portion(>60 wt%) of imported pigment particles, whereas they are al-most below the detection limit of EDS in the surrounding glassymatrix, which provides convincing evidence to suggest that thecobalt-bearing arsenic pigments undergo significant elementaldifferentiation during high-temperature sintering, as we havespeculated. In fact, this differential scenario seems highly similarto the speiss smelting process in pyrometallurgy, where the me-tallic elements (e.g., Ni, Cu, Co) have considerable differences inchemical affinity to oxygen and arsenic and so are only slightlysoluble in silicates. Thus, the characteristic ratios of impuritiescalculated from bulk glaze analysis cannot reflect the primary

Fig. 5. Microstructure of the residual pigment particles revealed in the imperial Qing blue-and-white porcelain. (A) Typical morphology of residual particle-enriched areas of sample QK-JY-01. (B and C) Close-up views of core-shell structure in A. (D) Typical morphology of the interaction layer of sample Q-BY-01.(E) Close-up views of A. (F) Typical morphology of particle-enriched areas of sample Q-BY-02.

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compositional ratios of cobalt ores when the pigments are notproportionally dissolved in the glaze as previously expected,leading to the partial failure of conventional analysis with respectto determining the specific provenance of cobalt pigments. Inother words, the concern expressed by Pollard regarding howrepresentative ratios are calculated from XRF through the glazehas been confirmed, necessitating a reassessment of the funda-mental assumption of current provenancing methodology. In thepresent study, we add another layer of knowledge to this par-ticular subject and provide a potential paradigm for directlyassessing original mineralogical information from residual pig-ment particles based on the FIB-TEM technique, making itpossible to establish an intrinsic linkage between cobalt-bearingantiquities and specific ore deposits. Our practical protocols alsohave important implications in the field of archaeological dating,artwork authentication, and historical vicissitude.

Materials and MethodspXRF measurements were performed in-situ using a Bruker Tracer 5i in-strument under vacuummode via scanning at 40 kV, 25 μA, and 90 s. SEMwas

done with a Tescan Vega3 coupled to an EDAX Element EDS system(30 mm2 silicon drift detector) at the Archaeometry Laboratory of PekingUniversity. For this analysis, the specimens were prepared in cross-section bycutting, mounting in epoxy resin, grinding, and polishing. FIB was carried outfor specimen preparation with an FEI DualBeam Helios NanoLab 460HP, fol-lowing the method described in ref. 11. TEM was done on a FEI Talos F200Xequipped with a SuperX EDS with four silicon drift detectors, operated at200 kV in this study.

Data and Materials Availability. The data that support the findings of thisstudy are available from the corresponding author upon reasonable request.

ACKNOWLEDGMENTS. We thank Yimin Yang, Qinglin Ma, and Yaowu Hufor their consultation on this study; Yan Ge, Xiaoshuang Li, and Zheyu Wangfor sample selection; and especially Kunlong Chen, Siran Liu, and ChenyuanLi for their brilliant advice on manuscript revision. We also thank Yuanqiu Lifor her technical assistance with the experimental work, Nian Liu for layoutwork, and the anonymous reviewers for their constructive and inspiringcomments. This study was supported by the National Social Science Fund ofChina (18BKG017, 19CKG030, 15ZDB057, 16ZDA145 and 16ZDA144) and theChina Postdoctoral Science Foundation (2019M650009).

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