Cr(III) Cr(VI)
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.
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