Figueiredo, M. Ondina

Silva, Teresa

Veiga, JP

Silva, Teresa Pereira

Veiga, João Pedro

Laboratório Nacional de Energia e Geologia Repository

Ancient Chinese blue-and-white porcelain glazes : can we measure the oxidation state of cobalt through X-ray absorption spectroscopy?

Repositório do LNEG

1º Encontro Nacional dos Utilizadores da Radiação de Sincrotrão  1st. Meeting of Synchrotron Radiation Users from Portugal  1º ENURS / 1st. MSRUP 1 Ancient Chinese blue-and-white porcelain glazes: can we measure the oxidation state of cobalt through X-ray absorption spectroscopy?  M.O. Figueiredo1,2 (*), T.P. Silva2,1, J.P. Veiga1 1  CENIMAT / I3N, Fac. Sci. Technology, New Univ. Lisbon, 2829-516 Caparica,  2  National Laboratory of Energy & Geology (LNEG), Apartado 7586, 2720-866 Alfragide, Portugal.  (*)Email: ondina.figueiredo@lneg.pt   Key-words: Cobalt speciation, XANES, blue glazes, Chinese porcelains   Abstract Archaeological findings of ancient Chinese porcelains can contribute to set up dating processes once the composition of cobalt blue pigments is known to have changed between the 14th and the 17th century. A study of blue-and-white porcelain shards recovered during recent excavations was carried out by non-destructive laboratory X-ray fluorescence spectrometry for chemical characterization, combined with X-ray absorption spectroscopy using synchrotron radiation to ascertain the coordination of pigmenting cobalt ions and infer the corresponding formal valence. A detailed analysis of the near-edge region of Co 1s X-ray absorption spectra (XANES) was carried out on the blue-and-white glazes from those porcelain fragments. Pre-edge features and edge details are discussed in comparison with spectra obtained from model compounds with known crystal structure − Co3 O4, Co Al2 O4 and Co2  Si  O4,  plus a cobalt-based blue pigment (cerulean, Co2-x Sn O4).  Chemical data allowed to validate the manufacture period of studied Chinese porcelains advanced by Art Historians on the single basis of stylistic features (late 16th and medium 17th cent.) and the spectroscopic results confirm a coordination environment of pigmenting Co2+ ions close to tetrahedral, substantiating the dual role of cobalt as network former plus modifier in the glaze of ancient Chinese porcelains. Furthermore, the present study confirms the questionable validity of inferring the oxidation state of 3d transition metal ions wholly from X-ray absorption spectroscopy data.  Introduction Ancient blue glazes owe its colour mainly to copper and cobalt, either employed separately or added together to the siliceous matrix in order to attain the desired tonality. Once the speciation sate of the chromophore elements controls the final colouring effect, a particularly suitable technique to interpret pigmenting performance through non-invasive assays is X-ray absorption spectroscopy (XAFS) using synchrotron radiation. Indeed, the analysis of both near-edge features (XANES) and extended fine structure (EXAFS) has been extensively and                  Monte de Caparica, 16/01/2012 Organização: Francisco M. Braz Fernandes, Maria João Romão, Rui M.S. Martins 2successfully used to study the speciation state of chromophore ions and modifier elements in ancient glassy materials.  The trade of Chinese blue-&-white porcelains to Europe became intense after the maritime contact established by the Portuguese navigators by the end of the 16th century. European museums and traders face nowadays an increased need for establishing the authenticity of such art objects through non-destructive assays. As a contribution, a set of Chinese porcelain fragments recovered during archaeological excavations recently conducted in the Monastery of Santa Clara-a-Velha at Coimbra and in Lisbon Old-city were studied by combining a bulk chemical characterization using X-ray fluorescence spectrometry and XAFS to ascertain cobalt coordination & speciation state(s). Following first approaches [1,2], the theme is developed with emphasis on the questionable formal valence(s) of cobalt ions.   Dating of studied archaeological Chinese porcelain shards Ancient Chinese porcelains were lately the object of compositional studies applying analytical techniques based on X-rays and neutron beams to find out production periods and sites [3-5]. A set of Chinese shards supposedly manufactured in late 16th to early 17th century was selected for the present study by visual inspection. A shift from imported As-rich Persian cobalt pigments towards Mn-rich native Chinese blue pigmenting ores – asbolane, a cobalt-rich manganese wad with general formulation (Co, Ni)1-y (MnO2)2-x (OH)2-2y+2x – is known to have occurred during the early Ming Dynasty (AD 1368-1644) [6]. Accordingly, arsenic content combined with Fe/Co & Mn/Co ratios configure a suitable dating criterion [7] that supported the manufacture period suggested by Art historians on the basis of a stylistic analysis [1].   Experimental and materials Co 1s X-ray absorption spectra were collected at the ESRF (Grenoble/France) using the instrumental set-up of beamline BM-29 (recently succeeded by the renewed line BM-23), by applying a Si (111) double-crystal monochromator with detuning for harmonic rejection, thus assuring an energy resolution around 0.8 eV close to the cobalt absorption edge. The surface of glazed debris was directly irradiated to detect the fluorescence yield using a 13-element germanium detector. Pressed pellets of slightly grinded model compounds mixed with boron nitride were prepared in a proportion adequate to optimize the absorption jump while minimizing the total absorption, in this way improving the display of pre-edge details and edge features of spectra collected in transmission mode. The energy calibration was assured by irradiating a cobalt metal foil and assigning the first inflection point to 7709 eV.  To model cobalt coordination plus speciation state various compounds were irradiated: the blue oxide CoAl2O4 (with spinel-type structure and Co2+ ions in regular tetrahedral coordination); Co2 Si O4 (with olivine-type structure, Co2+ with slightly distorted octahedral coordination); Co3 O4, also a spinel where the metal occurs in two formal oxidation states (one Co2+ ion in tetrahedral coordination and two Co3+ ions in octahedral coordination per formula unit); and a synthetic blue pigment (cerulean, ideally Co2 Sn O4, a spinel with  Co2+ ions in tetrahedral and octahedral coordination).    1º Encontro Nacional dos Utilizadores da Radiação de Sincrotrão  1st. Meeting of Synchrotron Radiation Users from Portugal  1º ENURS / 1st. MSRUP 3Results and discussion Co K-edge XANES spectra are very similar for the studied porcelain samples. As expected from distinct oxygen environments – tetrahedral in Co Al2 O4, octahedral in Co2 Si O4 – the spectra collected from these double oxides containing exclusively Co2+ ions show similar details of steeply rising edges but display dissimilar edge-crest plus post-edge and pre-edge regions. Comparing the spectra collected from all model compounds, the influence of coordination geometry irrespective the formal valence of cobalt ions becomes apparent.  The first derivatives of normalized XANES spectra highlight the comparative variation of edge features, emphasizing the dominant effect of coordination environment of cobalt ion over its formal valence in the chemical formula of the compound.  Differences in pre-edge details obtained by deconvoluting the spectra with FITYK program [8] may be explained by the coordination geometry and local symmetry of cobalt ions, tetrahedral vs. octahedral, quite independently from the formal valence. Indeed, the absorption edge of 1s XANES spectra from 3d transition metals is due to electronic transitions from the 1s core state to the 4p conduction band, while pre-edge details arise from quadrupole transitions to 3d empty states; in case the inherent inversion symmetry is broken (as for Co2+, a high spin 3d 7 ion with filled eg and t2g orbitals), local mixing of 3d-4p wave functions allows for dipole transitions to occur, intensifying the pre-edge feature [9].  Comments and conclusions Actual results are most timely because the relevance of interpreting the electronic behavior of cobalt largely overpasses the problematic of ancient blue glazes. Under the form of nano-crystalline thin films which magnetic behavior has been extensively studied, cobalt oxide-based materials have nowadays a great diversity of technological applications, ranging from heterogeneous catalysts to optical sensors and semiconductors.  Particularly, the present work clearly supports previously published considerations [10] on the questionable reliability of inferring a formal oxidation state for 3d transition metal ions solely from XAFS data [9]. The fragility of such inference is clearly illustrated by a recent study [11] claiming the occurrence of cobalt ions in two formal valence states within the glaze of blue-and-white ancient Chinese porcelains: the poorly-defined “cobaltic oxide” Co2 O3 [12,13] was used as model compound and the mean coordination number deduced by EXAFS analysis for Co3+ ions was lower than three – a value hardly explainable in view of the recognized structural array of silica-rich glazes. Indeed, the formal valence of a metal ion is strictly connected with the chemical formula of the carrier compound, while X-ray absorption data reflect the electron binding situation of the element, even if a specific chemical formulation cannot be stated  (as is the case of amorphous phases like ceramic glazes).   Acknowledgements Work developed under Project PTDC/HAH/69506/2006 financed by the Portuguese Foundation for Science and Technology (FCT-MCTES).  The financial of EU to perform                  Monte de Caparica, 16/01/2012 Organização: Francisco M. Braz Fernandes, Maria João Romão, Rui M.S. Martins 4XAFS experiments at the ESRF through the action Access to Research Infra-structures is acknowledged.  References [1] M.O. FIGUEIREDO, T.P. SILVA, J.P. VEIGA, M.I. PRUDÊNCIO, M.I. DIAS, M.A. MATOS, A.M. PAIS (2010) Blue pigments in XVI-XVII century glazes: a comparative study between Portuguese faiences and Chinese porcelains, in J.L. Ruvalcaba Sil, J.R. Trujeque, J.A. Alatorre, A.V. Castro (Eds.), 2nd Latin-Amer. Symp. Phys. Chem. Methods in Archaeol., Art and Cultural Heritage Conservation (LASMAC & IRMC 2009), Univ. Nac. Autonoma de México, (2010) 81-85 [ISBN 978-607-02-2017-3]. [2] M.O. FIGUEIREDO T.P. SILVA, J.P. VEIGA (2011) A XANES study of cobalt speciation state in blue-and-white glazes from 16th-17th cent. Chinese porcelains. Submitted to J. Electron  Spectroscopy and Related Phenomena. [3] P.L. LEUNG, H. LUO (2000) A study of provenance and dating of ancient Chinese porcelain by X-ray fluorescence spectrometry. X-Ray Spectrom. 29 (2000) 34-38. [4] H.S. CHENG, B. ZHANG, H.N. XIA, J.C. JIANG, F.J. YANG (2002) Non-destructive analysis and appraisal of ancient Chinese porcelain by PIXE.  Nuclear Instr. & Methods B 190 488-491. [5] R. WEN, C.S. WANG, Z.W. MAO, Y.Y. HUANG, A.M. POLLARD (2007) The chemical composition of blue pigment on Chinese blue-and-white porcelain of the Yuan and Ming Dynasties (A.D.1271-1644). Archaeometry 49 101-115. [6] H. GARNER (1956) The use of imported and native cobalt in Chinese blue-&-white.  Oriental Art 2 48-51. [7] H.S. CHENG, B. ZHANG, D. ZHU, F.J. YANG, X.M. SUN, M.S. GUO (2005) Some new results of PIXE study on Chinese ancient porcelain. Nuclear Instr. & Methods Phys. Res. B 240 527-531. [8]   http://www.unipress.waw.pl/fityk [9] P. GLATZEL, G. SMOLENTSEV, G. BUNKER (2009) The electronic structure in 3d transition metal complexes: can we measure oxidation states? J. Phys.: Conference Series 190 (11 pp)  [10] F. de GROOT, G. VANKÓ, P. GLATZEL (2009) The 1s X-ray absorption pre-edge structures in transition metal oxides. J. Phys. Condensed Matter 21 (7pp)  [11] L. WANG, C. WANG (2011) Co speciation in blue decorations of blue-and-white porcelains from Jingdzhen kiln by using XAFS spectroscopy. J. Anal. Atomic Spectrom. 26 1796-1801. [12] J.-F. HUANG,  et al. (2002) Geneses of cobaltic oxide. J. Chinese Chem. Soc. 49 819-824. [13] J. WANG (2005) Low energy electron diffraction studies of transition metal oxide surfaces and films. Ph.D. Thesis, Univ. Halle, Wittenberg, Germany (108 pp).     

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Ancient Chinese blue-and-white porcelain glazes : can we measure the oxidation state of cobalt through X-ray absorption spectroscopy?

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