Anal. Chem. 2009, 81, 2600–2610
Characterization of a Degraded Cadmium Yellow (CdS) Pigment in an Oil Painting by Means of Synchrotron Radiation Based X-ray Techniques Geert Van der Snickt,† Joris Dik,‡ Marine Cotte,§,| Koen Janssens,*,† Jakub Jaroszewicz,† Wout De Nolf,† Jasper Groenewegen,‡ and Luuk Van der Loeff⊥ Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium, Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, NL-2628CD Delft, The Netherlands, Laboratoire du Centre de Recherche et de Restauration des Muse´es de France (LC2RMF), CNRS UMR 171, Palais du Louvre, Porte des Lions, 14, Quai Franc¸ois Mitterand, F-75001 Paris, France, European Synchrotron Radiation Facility, Polygone Scientifique Louis Ne´el, 6, rue Jules Horowitz, F-38000 Grenoble, France, and Conservation Department, Kro¨ller-Mu¨ller Museum, Houtkampweg 6, NL-6731AW Otterlo, The Netherlands On several paintings of James Ensor (1860-1949), a gradual fading of originally bright yellow areas, painted with the pigment cadmium yellow (CdS), is observed. Additionally, in some areas exposed to light, the formation of small white-colored globules on top of the original paint surface is observed. In this paper the chemical transformation leading to the color change and to the formation of the globules is elucidated. Microscopic X-ray absorption near-edge spectroscopy (µ-XANES) experiments show that sulfur, originally present in sulfidic form (S2-), is oxidized during the transformation to the sulfate form (S6+). Upon formation (at or immediately below the surface), the highly soluble cadmium sulfate is assumed to be transported to the surface in solution and reprecipitates there, forming the whitish globules. The presence of cadmium sulfate (CdSO4 · 2H2O) and ammonium cadmium sulfate [(NH4)2Cd(SO4)2] at the surface is confirmed by microscopic X-ray diffraction measurements, where the latter salt is suspected to result from a secondary reaction of cadmium sulfate with ammonia. Measurements performed on cross sections reveal that the oxidation front has penetrated into the yellow paint down to ca. 1-2 µm. The morphology and elemental distribution of the paint and degradation product were examined by means of scanning electron microscopy equipped with an energy-dispersive spectrometer (SEM-EDS) and synchrotron radiation based micro-X-ray fluorescence spectrometry (SR µ-XRF). In addition, ultraviolet-induced visible fluorescence photography (UIVFP) revealed itself to be a straightforward technique for documenting the occurrence of this specific kind of degradation on a macroscale by painting conservators. From an analytical chemistry and conservation science point of view, the paintings of the 19th century and the beginning of the 20th century form an interesting field of research as numerous new pigments were developed by the emerging chemical industry at that time. Synthetic inorganic and organic molecules were * To whom correspondence should be addressed. Phone: +32 3 820 2373. Fax: +32 3 820 2376. E-mail:
[email protected]. † University of Antwerp. ‡ Delft University of Technology. § CNRS UMR 171. | European Synchrotron Radiation Facility. ⊥ Kro ¨ller-Mu ¨ ller Museum.
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introduced as alternatives for the well-established pigments that were employed by painters in earlier periods. Examples of pigments launched in that period are emerald green (Cu(C2H3O2)2 · 3Cu(AsO2)2, chromium yellow (PbCrO4), cerulean blue (CoO · nSnO2), etc. These new painting materials often outclassed the traditional pigments with regards to coloring intensity, purity, cost, covering power, etc. However, not all of these modern pigments proved to be chemically stable on the long term. Such appears to be the case for cadmium yellow, a cadmium sulfide compound developed/invented by Stromey in 1818 and commercialized toward the middle of the 19th century.1 Cadmium sulfide exists in two crystalline and one amorphous form. The hexagonal form (R-CdS) is found in nature as the mineral greenockite, the cubic form (β-CdS) as hawleyite, while the amorphous form is only known as a chemically synthesized product. As stated by Eastaugh et al.,2 these three forms can coexist at room temperature. According to Fiedler and Bayard, the pigment cadmium sulfide was historically produced by means of either a dry or a wet process. For the dry process, either (i) metallic cadmium, (ii) cadmium oxide, or (iii) cadmium carbonate was employed as starting material. This was mixed either stoichiometrically or with an excess of sulfur and heated to 300-500 °C in the absence of air.1 After the completion of any of the three reactions below, the resulting material was subsequently washed and ground. Cd + S f CdS
(i)
2CdO + 3S f 2CdS + SO2
(ii)
2CdCO3 + 3S f 2CdS + SO2 + 2CO2
(iii)
During the wet process, CdS was obtained as a precipitate by adding a soluble sulfide (such as hydrogen sulfide, sodium sulfide, or barium sulfide) to a soluble cadmium salt (such as cadmium (1) Fiedler, I.; Bayard, M. A. Cadmium Yellows, Oranges and Reds. Artists’ Pigments. A Handbook of their History and Characteristics; Cambridge University Press: Cambridge, 1986; Vol. 1, pp 65-108. (2) Eastaugh, N.; Walsh, V.; Chaplin, T.; Siddall, R. The Pigment Compendium [CD-ROM]; Elsevier, 2004. 10.1021/ac802518z CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
chloride, cadmium nitrate, cadmium sulfate, or cadmium iodide), for example:
light. The absorption of photons with energy equal to the band gap results in the formation of electron/hole pairs:7,8
CdCl2 + H2S f CdS + 2HCl
CdS + hν f e- + h+
CdSO4 + Na2S f CdS + Na2SO4
Subsequently, the cadmium compound itself decays as the positive holes (h+) oxidize the solid according to the reaction9
CdI2 + BaS f CdS + BaI2
CdS + 2h+ f Cd2+ + S(s)
Cadmium yellow (CdS) is highly insoluble in water (Ks ) 3.6 × 10-29), alkalis, and weak organic acids. It is soluble in concentrated hydrochloric and nitric acids and in boiling diluted sulfuric acid.1 The compound also dissolves in ammonium chloride and hydroiodic acid.3 Manufacturers started to make intense use of this pigment from the moment that cadmium became commercially available as a base material (ca. 1840s). This popularity was mainly due to the pigment’s high tinting and covering power, bright yellow color, wide applicability (artists’ paint, metallurgy, ceramics, medical use, etc.), and suitability for mass production. In addition, CdS was thought to be highly stable in oil paint and water colors, and this in contradiction with chrome yellow (PbCrO4), the only bright yellow alternative available for painting at that time.1 By consequence, prominent 19th to 20th century painters such as Claude Monet,4 Vincent Van Gogh,1 and Pablo Picasso5 frequently employed CdS, as was amply documented by earlier analytical research. However, in spite of its excellent reputation with regards to permanency, fading of the yellow color of CdS and loss of adhesion of the oil paint has been reported by several authors.5,6 Also in the Royal Museum of Fine Arts of Antwerp (Belgium) and in the Kro¨ller-Mu¨ller Museum (Otterlo, The Netherlands), degraded cadmium yellow was found on several unvarnished paintings of the Belgian avant-garde artist James Ensor (1860-1949). In the painting: “Still Life with Cabbage” (Inv. nr. KM 105.303, dated ca. 1921), the painting conservator of the Kro¨ller-Mu¨ller Museum noticed a local variation in color intensity after removal of the frame of the painting. As Figure 1 demonstrates, the yellow paint along the border of the painting seemed to have largely preserved its original vibrant color, whereas the yellow paint in the rest of the painting exhibited a rather dull shade in comparison. This suggested that the observed alteration was induced by environmental influences of either chemical or physical nature or both. Thus, the frame may have functioned as a protective shield, locally screening off the underlying paint from incident light (UV), fluctuations in relative humidity, airborne particles, etc. The most striking feature of this degradation was the presence of whitish, semitransparent globules that were observed on top of the surface during examination by means of an optical microscope (OM). Especially the interaction with incident daylight is expected to play an important part in this degradation process as the semiconductor CdS can act a photocatalyst when excited by visible
After further oxidation of the sulfur (S0) to the sulfate stage by atmospheric oxygen, the overall reaction becomes10
(3) Curtis, P. J.; Wright, R. B. J. Oil Colour Chem.’ Assoc. , 37, 26–43. (4) Roy, A. National Gallery Technical Bulletin 2007, 28, 58–68. (5) Leone, B.; Burnstock, A.; Jones, C.; Hallebeek, P.; Boon, J.; Keune, K. The Deterioration of Cadmium Sulphide Yellow Artists’ Pigments. In Preprints of The 14th Triennial Meeting of ICOM International Committee for Conservation; 2005; Vol. 2, pp 803-813. (6) Van Asperen de Boer, J. R. J. kM (Sticht. kunstenaarsmateriaal) 1994, 12, 20.
CdS + 2O2 f Cd2+ + SO42In addition, the conservator noticed that the degraded area could be clearly distinguished from the nondegraded zone under the frame by means of ultraviolet-induced visible fluorescence photography (UIVFP). The latter is a relatively straightforward manner of imaging that is widely employed in conservation studios to discern local variations in the condition of painted and/or varnished surfaces. In general it is used to distinguish more recent paint from the (older) original paint; such retouching areas may contain deviant pigments and/or liants, fungi, former conservation products such as wax or resin coatings, etc.11 which feature a different fluorescent response versus wavelength than the original surrounding material. As demonstrated by Figure 1, in the Ensor painting studied here, the nondegraded areas of CdS clearly produce an orange-colored fluorescence upon irradiation with UV, whereas the degraded paint fluoresces a distinctive brownish color. This color variation is most pronounced in impasto areas, e.g., areas where the artist locally applied a thick stroke or dot of paint. The greenish color in the overall UV photo (see Figure 1B) is attributed to zinc white (ZnO), a pigment which is known to produce this color upon UV radiation. The light blue color along the border of the painting is associated with the preparation layer containing lead white (2PbCO3 · Pb(OH)2). It is of great interest to conservators to chemically characterize the white globules and to gain a better understanding of the overall degradation process of the CdS in order to ensure the optimal conservation of this work of art for subsequent generations. Preliminary investigations by means of conventional laboratorybased methods such as scanning electron microscopy coupled to energy-dispersive X-ray analysis (SEM-EDS), microscopic X-ray fluorescence (µ-XRF), and X-ray diffraction (XRD) indicated the need for high spatial resolution and species-selective analytical techniques to characterize the degradation products and the material immediately adjacent to it. As the thickness of the altered layer appeared to be in the micrometer range and the presence of several cadmium compounds (in the original paint) was suspected, it was decided to examine the samples by means of synchrotron radiation based methods. The option to use this family of nondestructive methods was also inspired by the fact that the white degradation products appeared unstable under laser beams (7) Aslani, I. Who is afraid of discoloured Yellow? MSc. Thesis, Delft University of Technology, 2006, pp 20-23. (8) Davis, A. P.; Huang, C. P. Water Res. 1991, 10, 1273–1278. (9) Williams, R. J. Chem. Phys. 1960, 32, 1505–1514. (10) Henglein, A. Ber. Bunsen.-Ges. Phys. Chem. 1982, 86, 301–305. (11) de la Rie, E. R. Stud. Conserv. 1982, 27, 1–7.
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Figure 1. (A) Optical photograph of the oil painting “Still Life with Cabbage” by James Ensor (ca.1921, KM 105.303). (B) Detail of the exposed, yellow paint surface (×40) showing white globules. (C) Detail of the right-lower corner of the painting: the yellow paint covered by the frame displays a more vivid yellow color, whereas the paint in the exposed areas has become dull. (D) UV fluorescence photograph of the entire painting. (E) Detail of the same area as shown in panel C in UV fluorescence: the yellow paint under the frame fluoresces in brown, whereas the exposed yellow paint mostly produces an orange color.
used for Raman spectroscopy and under electron beams used for transmission electron microscopy. Additionally, the potential of synchrotron radiation based techniques for the study of degradation processes in polychromed layers has been abundantly established in the past few years by several authors.12-17 In what (12) Cotte, M.; Checroun, E.; Susini, J.; Walter, Ph. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 841–848. (13) Lluveras, A.; Boularand, S.; Roque´, J.; Cotte, M.; Giraldez, P.; Vendrell-Saz, M. Appl. Phys. A: Mater. Sci. Process. 2008, 90, 23–33. (14) Cotte, M.; Susini, J.; Metrich, N.; Moscato, A.; Gratziu, C.; Bertagnini, A.; Pagano, M. Anal. Chem. 2006, 78, 7484–7492. (15) Krug, K.; Dik, J.; den Leeuw, M.; Whitson, A.; Tortora, J.; Coan, P.; Nemoz, C.; Bravin, A. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 247–251. (16) Cotte, M.; Welcomme, E.; Sole´, V. A.; Salome´, M.; Menu, M.; Walter, Ph.; Susini, J. Anal. Chem. 2007, 79, 6988–6994. (17) Cotte, M.; Susini, J.; Sole´, V. A.; Taniguchi, Y.; Chillida, J.; Checroun, E.; Walter, Ph. J. Anal. At. Spectrom. 2008, 23, 820–828.
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follows the chemical transformation leading to the color change and to the formation of the globules is elucidated. EXPERIMENTAL SECTION The conservator of the Kro¨ller-Mu¨ller Museum supplied two types of samples for analysis from the painting “Still Life with Cabbage” (inv. nr. KM 105.303, see Figure 1). Prior to the actual sampling, suitable spots were carefully selected under the binocular microscope within the discolored yellow area. This area was located on the bottom right edge (spectator viewpoint) of the painting (see Figure 1, parts A and C). Type 1 samples were obtained by cautiously removing a minute piece (
