and -XRD for the characterization and degradation of chrome yellow - PowerPoint PPT Presentation

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 19 th 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- ]>40% [SO 4 chrome orange chrome yellows (1-x)PbCrO 4 ∙xPbO PbCrO 4 PbCr 1-x S x O 4 solubility Sulfates [BaSO 4 , CaSO 4 ∙2H 2 O, KAl(SO 4 ) 2 ∙12H 2 O, PbSO 4 ] Talc, kaolin, calcite Extenders Other chromate-based yellow pigments (CaCrO 4 /BaCrO 4 ) (commercial formulation)

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 NH 4 HF 2 )]  Coating methods (Sb-based compounds, Keen interest of the darkening 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 20 th c – early 21 th 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-19 th century oil paint tubes Flemish Fauvist Elsens (Bruxelles) Rik Wouters How the sulfate anions influence (1882-1913) the stability of chrome yellows? UVA-Vis light Degradation  S-rich paint  S-rich areas Orthorhombic Monoclinic PbCr 0.75 S 0.25 O 4 PbCr 0.4 S 0.6 O 4 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  SR  XRD µ-XRD (P06 and L beamline; DESY/HASYLAB, Hamburg)  micro-Raman  FTIR (transmission, ATR, reflection mode)  SR µ-XANES/µ-XRF at the Cr and S K-edges  UV-visible (ID21 beamline; ESRF, Grenoble) (reflectance mode) and colorimetry  Energy Electron Loss Spectroscopy (EELS)  STEM-EDX  Electron Paramagnetic Resonance (EPR)  Capability to distinguish among different chrome yellow types (PbCr 1-x S x O 4 , 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 PbCrO 4 and PbCr 1-x S x O 4 (0.1≤ x ≤ 0.75) Pb(NO 3 ) 2 + (1-x) K 2 CrO 4 + xK 2 SO 4 → PbCr 1-x S x O 4 ↓ + 2KNO 3 2) Preparation of oil paint model samples commercial 25% 50% 75% 0% 10% 2- ] [SO 4 S 1mono S 1ortho S 3A S 3B S 3c S 3D D 1 D 2 C PbCrO 4 In house-synthesized PbCrO 4 +PbSO 4 PbCr 1-x S x O 4 (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- ] 0% [SO 4 10% 25% 50% 75% SR µ-XRD (P06 – DESY) S K-edge XANES (ID21 – ERSF) orthorhombic orthorhombic PbCrO 4 S 3A S(VI) monoclinic 2.481 Normalized fluorescence monoclinic PbCrO 4 PbSO 4 S 3B (111) monoclinic orthorhombic monoclinic PbCr 1-x S x O 4 orthorhombic PbCr 1-x S x O 4 S 3c (201) S 3D S 1ortho S 1mono S 1ortho S 3A S 3B S 3c S 3D (111) PbSO 4 (120) STEM-EDX D 1 Intensity S S (111) PbSO 4 Cr Cr S 3D 2.48 2.49 2.50 Energy (keV) S 3C PbCr 0.6 S 0.4 O 4 With increasing Cr content S 3B  Gradual disappearance of the PbSO 4 pre-edge signal at 2.481 keV. 200 nm 100 nm S 3A  Several post-edge features Predominantly Predominantly S 1mono become less clearly defined. (111) (020) orthorhombic S-rich monoclinic Cr-rich 16.20 16.74 17.28 nanorods nanoparticles -1 ) Q (nm Sulfate groups are more isolated With increasing Cr content  Shift of the diffraction peaks Possibility to distinguish different types of the toward lower Q values. chrome yellow pigments also by means of IR and Raman spectroscopies  Increasing of lattice parameters. *L. Monico et al., Anal. Chem. 85 (2013) 851-859.

Artificially aged paint model samples* in-house synthesized commercial 2- ] [SO 4 historical 0% 0% 10% 25% 50% 75% 0% 50% 65% orthorhombic PbCr 0.4 S 0.6 O 4 PbCr 0.75 S 0.25 O 4 UVA-Vis light orthorhombic UVA-Vis light monoclinic monoclinic S 1mono S 3A S 3B S 3c S 3D S 1ortho D 1 D 2 C Cr(VI)→ Cr(III)? monoclinic Monoclinic PbCrO 4 +PbSO 4 PbCrO 4 PbCr 1-x S x O 4 (1:2) 2- ]≥50 wt % Orthorhombic phase and [SO 4 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 4 Normalized Fluorescence 5.993 keV 1s →3d (dipole -forbidden)  Cr(VI) compounds Cr 2 O 3 non-centrosymmetric tetrahedral coordination.  Cr(III) compounds centrosymmetric octahedral geometry.  Pre-edge peak area proportional to the amount of Cr(VI). shift of the position of the  Shift towards higher energies: increasing in absorption edge 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 1s→3d( e g ) 6.00 6.03 6.06 Energy (keV)

Historical sample and paint model S 3D : XANES analysis unaged Normalized Fluorescence 5.993 S 3D (PbCr 0.2 S 0.8 O 4 ) Sample A aged Reproduction of the same Only alteration process as Cr(VI) Before observed on the historical Cr(III) sample A* Cr(VI) Aged After UVA-visible light 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, XANES spectra: 10.5 μm XANES spectra: Cr 2 O 3 ∙2H 2 O and PbCrO 4 Cr 2 O 3 ∙2H 2 O and PbCrO 4 8 μm Brown area 100 100 Cr relative abundance (%) Cr(III) Cr relative abundance (%) Cr(III) 90 90 2 comp.: Yellow area 80 80 Cr 2 O 3 ∙2H 2 O 40 μm 70 70 200 μm and PbCrO 4 2 comp.: 60 60 Cr 2 O 3 ∙2H 2 O 50 50 All XANES spectra were fitted as a and PbCrO 4 40 40 linear combination of a limited set of 30 30 Cr-reference compound profiles 20 20 Cr(VI) 10 10 Cr(VI) Reduction of the original Cr(VI) 0 0 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 9 10 ~ 60-65% of Cr(III)-species at the surface Depth (  m) 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 Later al di s tanc e, m i c rom eter 60-70% VL 30-40% Map size (v×h): 42 × 300 μm 2 Cr(III) distribution pixel size (v×h): 0.25 × 1 μm 2 dwell time: 100 ms/pixel 0 100 200 300 Later al di stanc e, m ic romete r In line with the linear combination fitting of the XANES spectra, Cr(III) species are localized in the upper 3-4 µm of the paint (up to 60-70%). * L. Monico et al., Anal. Chem. 83 (2011) 1214 – 1223.