Synchrotron radiation-based µ-XANES and µ-XRD for the characterization and degradation of chrome yellow pigments: a focus on paintings by Vincent van Gogh Letizia Monico CNR-ISTM (Perugia, Italy) University of Antwerp (Belgium) Darkening of chrome yellows in late 19th C paintings* Van Gogh was already aware of the instability of the chrome yellow pigments “[…] You were right to tell Tasset that the geranium lake should be included after all, he sent it, I’ve just checked — all the colours that Impressionism has made fashionable are unstable, all the more reason boldly to use them too raw, time will only soften them too much. So the whole order I made up, in other words the 3 chromes (the orange, the yellow, the lemon) the Prussian blue, the emerald, the madder lakes, the Veronese green, the orange lead, all of that is hardly found in the Dutch palette, Maris, Mauve and Israëls. […]” (letter n. 595, To Theo. Arles, 11 April 1888) Bank of the Seine (1887 V. van Gogh; Van Gogh Museum, Amsterdam, NL) Falling leaves (Les Alyscamps) (1888, V. van Gogh; Kröller-Müller Museum, Otterlo, NL) What is changing? Sunflowers (1889, V. van Gogh; What can be done? Van Gogh Museum, Amsterdam) * L. Monico et al., Anal. Chem. 83 (2011) 1224-1231; L. Monico et al., Anal. Chem. 86 (2014) 10804-10811. Properties of lead chromate-based pigments 2- [SO4 ]>40% chrome orange chrome yellows (1-x)PbCrO4∙xPbO PbCrO4 PbCr1-xSxO4 solubility Sulfates [BaSO4, CaSO4∙2H2O, KAl(SO4)2∙12H2O, PbSO4] Extenders Talc, kaolin, calcite (commercial formulation) Other chromate-based yellow pigments (CaCrO4 /BaCrO4) Darkening of chromate-based pigments: interest in the painting conservation field Evolution of the synthesis procedure Lightfastness controlled experimental conditions [pH, improvement temperature, presence of specific reagents (e.g., Until 1950 NH4HF2)] Keen interest of the darkening Coating methods (Sb-based compounds, of the chrome yellow pigments Al/Ti/Ce hydrous oxides, amorphous silica) in the industrial field. Replacement with the more stable lead molybdate compounds. Late 20th c – early 21th c Reconsideration of the problem of Analysis of several chromate samples taken the darkening of chromate-based from paintings and historical paint tubes. (a) yellow pigments (Pb-, Ba-, Sr-,Ca-, Zn/K-chromate) in the context of Darkening of zinc chromate-based yellows.(b) the conservation of paintings. (a) D. Bomford et al., in: “Art in the Making: Impressionism”, National Gallery Publications, London, 1990, p. 158; A. Burnstock et al., Z. Kunst technol. Konserv. 17 (2003) 74-84. (b)L. Zanella et al., J. Anal. Atom. Spectrom. 26 (2011) 1090- 1097; F. Casadio et al., Anal. Bioanal. Chem. 399 (2011) 2909-2920. Aims and analyzed materials 1 - What is the alteration mechanism of the chrome yellow pigments? 2 - What are the factors that induce the darkening of these compounds? 3 - How we can prevent/mitigate the degradation process on original paintings? 1) Photochemical aging of late-19th century oil paint tubes Flemish Fauvist Elsens (Bruxelles) Rik Wouters How the sulfate anions influence (1882-1913) the stability of chrome yellows? Vis light Vis - Degradation UVA S-rich paint S-rich areas Orthorhombic Monoclinic PbCr0.75S0.25O4 PbCr0.4S0.6O4 2) Study of a series of paintings by Vincent van Gogh and related micro- samples containing different types of chrome yellows Analytical methods “Conventional-source” methods Speciation/high spatial resolution methods XRD SR µ-XRD (P06 and L beamline; micro-Raman DESY/HASYLAB, Hamburg) FTIR (transmission, ATR, reflection mode) SR µ-XANES/µ-XRF at the Cr and S K-edges UV-visible (reflectance mode) and (ID21 beamline; ESRF, Grenoble) colorimetry Energy Electron Loss Spectroscopy (EELS) STEM-EDX Electron Paramagnetic Resonance (EPR) Capability to distinguish among different chrome yellow types (PbCr1-xSxO4, with 0≤x≤0.8) Information about the alteration products Information about the oxidation state and the distribution of a given element Additional analytical/morphological information at the nano-scale level Similar information by employing portable instrumentations for non-invasive in situ measurements Characterization and photochemical stability of different crystal forms of the chrome yellow pigment In-house synthesized and commercial pigments 1) Synthesis of PbCrO4 and PbCr1-xSxO4 (0.1≤ x ≤ 0.75) Pb(NO3 )2 + (1-x) K2CrO4 + xK2SO4 → PbCr1-xSxO4 ↓ + 2KNO3 2) Preparation of oil paint model samples commercial 2- 50% 75% [SO4 ] 0% 10% 25% S1mono S1ortho S3A S3B S3c S3D D1 D2 C PbCrO4 In house-synthesized PbCrO4 +PbSO4 PbCr1-xSxO4 (1:2) 3) Photochemical aging treatment (CIBA and BASF) SOLARBOX 1500e system Cermax Xenon lamp different wavelength bands UVA-visible light of the UV-visible light Characterization of different chrome yellow types* 2- [SO4 ] 0% 10% 25% 50% 75% SR µ-XRD (P06 –DESY) S K-edge XANES (ID21–ERSF) orthorhombic PbCrO 4 monoclinic PbCrO 2.481 S(VI) S 4 3A PbSO 4 S (111) monoclinic PbCr S O 3B monoclinic orthorhombic 1-x x 4 orthorhombic PbCr S O 1-x x 4 S 3c (201) monoclinic S orthorhombic 3D S (111) S1mono S S S S S3D 1ortho PbSO 1ortho 3A 3B 3c 4 (120) STEM-EDX D 1 S S (111) PbSO4 Cr Cr S Normalized fluorescence 3D Intensity 2.48 2.49 2.50 Energy (keV) S 3C PbCr0.6S0.4O4 With increasing Cr content S 3B Gradual disappearance of the 100 nm PbSO4 200 nm pre-edge signal at 2.481 keV. S 3A Several post-edge features Predominantly Predominantly S (111) (020) become less clearly defined. monoclinic Cr-rich orthorhombic S-rich 1mono 16.20 16.74 17.28 nanorods nanoparticles Q (nm-1) With increasing Cr content Sulfate groups are more isolated Possibility to distinguish different types of the Shift of the diffraction peaks chrome yellow pigments also by means of IR toward lower Q values. and Raman spectroscopies Increasing of lattice parameters. *L. Monico et al., Anal. Chem. 85 (2013) 851-859. A Artificially aged paint model samples* in-house synthesized commercial 2- B [SO4 ] 1 historical 0% 10% 25% 50% 75% 0% 50% 65% 0% 4 orthorhombic PbCr0.75S0.25O4 O B 0.6 S monoclinic 2 monoclinic Vis light Vis - 0.4 Vis light Vis - UVA PbCr UVA orthorhombic C S1mono S3A S3B S3c S3D S1ortho D1 D2 C monoclinic Monoclinic PbCrO +PbSO Cr(VI)→ Cr(III)? 4 4 PbCrO PbCr1-xSxO4 (1:2) 4 2- Orthorhombic phase and [SO4 ]≥50 wt % Spectroscopic measurements at high spatial resolution Thin alteration layer SR-based µ-XANES and µ-XRF analysis at the Cr and S K-edges (~3-4 µm thickness) (ID21 beamline; ESRF, Grenoble) * L. Monico et al., Anal. Chem. 83 (2011) 1214–1223; L. Monico et al., Anal. Chem. 85 (2013) 85 860-867. Cr reference compounds: Cr K-edge XANES spectra Intense pre-edge peak PbCrO 5.993 keV 4 1s →3d (dipole-forbidden) Cr O Cr(VI) compounds 2 3 non-centrosymmetric tetrahedral coordination. Cr(III) compounds centrosymmetric octahedral geometry. Pre-edge peak area proportional to the shift of the position of the amount of Cr(VI). absorption edge Shift towards higher energies: increasing in the valency of the absorbing atom and/or of pre-edge peaks of low intensity the electronegativity of the nearest neighbour 5.990 keV atoms. 1s→3d(t ) 2g 5.993 keV Normalized Fluorescence 1s→3d(eg) 6.00 6.03 6.06 Energy (keV) Historical sample and paint model S3D: XANES analysis unaged Sample A S3D (PbCr0.2S0.8O4) aged Reproduction of the same 5.993 alteration process as Only Before Cr(VI) observed on the historical Cr(III) sample A* Aged Cr(VI) After UVA-visible light Normalized Fluorescence 6.00 6.02 6.04 Energy (keV) 3 comp.: KCr(III) sulfate or Cross-sectioned samples Cr(III) acetyl-acetonate, 3 comp.: Cr(III) sulfate or acetate, Cr O ∙2H O and PbCrO XANES spectra: XANES spectra: 10.5 μm Cr O ∙2H O and PbCrO 2 3 2 4 8 μm 2 3 2 4 100 Brown area 100 90 Cr(III) 90 Cr(III) 2 comp.: Yellow area 80 80 Cr2O3∙2H2O 70 200 μm 40 μm 70 and PbCrO4 60 60 2 comp.: 50 50 Cr O ∙2H O All XANES spectra were fitted as a 2 3 2 40 40 and PbCrO4 30 linear combination of a limited set of 30 20 Cr-reference compound profiles 20 10 Cr(VI) Cr(VI) 10 Cr relative (%) abundance 0 Reduction of the original Cr(VI) Cr relative (%) abundance 0 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 9 10 Depth (m) ~60-65% of Cr(III)-species at the surface Depth (m) * L. Monico et al., Anal. Chem. 85 (2013) 85 860-867. Cr chemical state maps: historical sample A* 5.993 6.086 ID21 beamline Cr(VI) distribution 0 100 200 300 Lateral distance, micrometer al distance, micrometer VL 60-70% al distance, micrometer Map size (v×h): 42 ×300 μm2 30-40% 2 Cr(III) distribution pixel size (v×h): 0.25 × 1 μm dwell time: 100 ms/pixel 0 100 200 300 Lateral distance, micrometer In line with the linear combination fitting of the XANES spectra, Cr(III) species are localized in the upper 3-4 µm of the paintal distance, (up to micrometer 60-70% ). * L. Monico et al., Anal. Chem. 83 (2011) 1214–1223. A Aged paint models: general observations* [SO 2-] commercial 4 B S (m) 80 1 1mono 0% 10% 25% 50% 75% 50% 65% 0% D (m) 1 monoclinic orthorhombic S (m) 70 3B ] (%) ~ 20-25% Cr(III) Monoclinic total Before B PbCrO4 60 2 S (m+o) 3C After 50 ~ 45-60% Cr(III) S1mono S3A S3B S3c S3D D1 D2 C C A B1 B2 C [Cr(VI)]/[Cr 40 S (m+o) A A B B 1 B B 2 C C 3D 1 2 PbCrO4 +PbSO4 Monoclinic (1:2) y=(83.1±0.8)+(-0.54±0.02)×x, R=0.95 Before 30 PbCr1-xSxO4 0 18 36 54 72 [SO 2-](%) 4 After A B1 B2 2- No significant changes when SO4 <50% and when only the monoclinic phase is present.