Irises
Over a ground preparation composed principally of lead white, Van Gogh built up the flowers with cobalt blue, zinc white, and eosin red-containing paints, and with Prussian blue and eosin red in the outlines, as reflected in the Pb, Co, Zn, and Br distribution maps presented in Fig. 1b–e, and in the Fe distribution map shown in the Additional file 1: Figure S1. The identification of these pigments was corroborated by Raman spectroscopy and SEM–EDS analysis on paint cross-sections, some of which are shown below, and on sample scrapings. Guided by the XRF distribution maps, a limited number of samples for analysis by Raman spectroscopy, SEM–EDS, SERS, and/or HPLC were removed from this painting. The locations of the samples relevant to this article are indicated in Fig. 1a.
Examination of the paint cross-section removed from a flower revealed that the blue and red lake-containing paints were applied wet on wet (Fig. 2a, b). Normal Raman spectra acquired in the red lake pigment layers in this cross-section present bands at ca. 642, 712, 1174, 1280, 1345, 1501 and 1620 cm−1, that are consistent with the presence of an eosin lake [25], and a strong feature at 978 cm−1 due to PbSO4 [26]. In Fig. 3a, the spectrum recorded in the red paint layers in this sample (spectrum 1) is compared to a spectrum acquired in an eosin red lake reference sample, with barite as a substrate, from a 1900 catalogue of early synthetic dyes [27] (spectrum 2). SEM–EDS analysis in the red layers in this sample confirmed the presence of lead sulfate and showed Br due to eosin red. In the red layer at the bottom of the stratigraphy of the cross-section, Ca and Ba, most likely originating in calcite and barium white, were detected by SEM–EDS. An SEM image of this sample is presented in Fig. 2c.
Geldof et al. in their study on the use of eosin red lakes in 34 paintings by Van Gogh reported that in all cases in which the inorganic substrate of the lake had been analyzed, Al has been identified [14]. In two of these paintings, Garden with Butterflies and Portrait of Dr. Gachet, both dating to 1890, these authors identified Pb and S most likely in the form of PbSO4 in the eosin lake-containing paint. They proposed that the compound is formed as a product of the reaction of the dye and aluminum sulfate in the presence of lead acetate, which accords with a 19th century recipe for making the lake, but that it is also possible that PbSO4 was added as an extender.
In this sample, SEM–EDS analysis confirmed the presence of Co and showed that Zn is also present in the blue paint. When the cross-section is examined under UV illumination, relatively large pigment particles that fluoresce bright yellow are visible in the blue paint layers (Fig. 2b); in normal illumination they appear an indistinct brownish color. SEM–EDS analysis of these particles showed that they are mainly organic, and that they contain mainly Zn and Br, in addition to C (the corresponding spectrum is shown in the Additional file 1: Figure S2. Since all the sample cross-sections were ion-milled, smearing from other paint layers is ruled out as the source of these elements. The normal Raman spectrum acquired in the particles that fluoresce yellow suggests the presence of a lake pigment (Fig. 3a, spectrum 3), but this lake could not be firmly identified. The presence of Br in our sample indicates that the lake might be a compound related to eosin red. Eosin equivalents, with similar absorption spectra but different retention times from eosin and probably formed during the production of the dye, and eosin related components with different absorption spectra than eosin that are either degradation or side products, have been reported in paintings by Van Gogh based on HPLC analyses [14]. According to F.H. Jennison’s 1900 catalogue on the manufacture of lake pigments, ZnSO4·7H2O was used to precipitate eosin lake pigments, ‘but not often’ [27]; however, no S was identified in our sample by SEM–EDS. Other possible sources of Zn are ZnO, which has also been reported as a substrate in lakes [28], and zinc chloride, that was used as a condensing agent in the manufacture of fluorescein, a starting material for the synthesis of eosin [29]. To our knowledge, Zn has not been previously identified in substrates of eosin lakes used by Van Gogh, as mentioned above, but, in some cases, this may be explained by the fact that Van Gogh frequently used eosin lake pigments in mixtures with zinc white [14], and that ion milling of sample cross-sections is necessary to rule out its interference. To identify the lake pigment, or mixture, in this sample with more certainty, a larger sample would need to be removed for HPLC analysis. If in fact the yellow fluorescent particles contain a red lake, the paint used in the cross-section shown in Fig. 2a, would have been purple.
In the background of the painting, some faint pink strokes are visible with the naked eye, but no clear indication of the presence of Br was obtained by XRF mapping (Fig. 1e). Raman spectroscopy and SEM–EDS analyses of a cross-section removed from the left edge of the painting (Fig. 2d–f) showed the wet on wet application of eosin red and lead white paints, that the eosin red lake was precipitated on an Al-containing substrate, and that the small dark pink particles present in this sample contain eosin red on a CaCO3 substrate. It is well known that the inorganic substrates in lake pigments determine in part the stability of the colorants towards the exposure to light [11, 13]. To date, no inorganic substrates other than Al-based have been reported for eosin paints in works by Van Gogh [14], and no studies have been published on the permanence of eosin red as precipitated on CaCO3.
SERS (Fig. 3b) and HPLC measurements (Additional file 1: Figure S3) on a microsample removed from the edge of the background confirmed that the only lake pigment used by Van Gogh in this passage is eosin red. SEM–EDS analyses showed that the white ‘layer’ on top of the stratigraphy is relatively more porous than the lower portion of the sample (Fig. 2f) and that Br- and Al-containing particles are present together with lead white. These results indicate that this is not a separate layer but part of the pink layer beneath and that it presently looks white due to fading of the eosin red lake.
Van Gogh made multiple orders for eosin paint under the commercial name of laque geranium, or geranium lake, as soon as he arrived in Arles and continued to use it until the end of his life [15].
The possible fading of a red mixed with the light green paint in the vase was also investigated by SEM–EDS and Raman analysis of a paint cross-section and it was found that it was intended to be light green, as it is painted mainly with zinc white with some emerald green and lead white, and no lake pigment particles are present.
Color-calibrated microscopy was used to measure L*a*b* values in the layers composed of eosin red lake in the cross-sections removed from the flower and from the background of the painting (Fig. 2a, d, respectively). L*a*b* values were also obtained from reflectance spectra acquired in lake paint reconstructions of similar composition prepared by van den Berg et al. following a ninenteenth century recipe [11, 30]. These color coordinates and the ones obtained by reflectance spectroscopy in the flowers and in areas of the edge that had been protected by an aged brown paper were used for the virtual re-colorization of the painting [1]. Reference was also made to the X-radiograph of the painting and to the Pb and Zn distribution maps for the color simulations.
Roses
Color reproductions of Roses dating from 1928 to 1957 [1, 31] were useful for evaluating the relative values within the painting but only as subsidiary information to XRF mapping and other imaging techniques for assessing the distribution of the red lakes. The Br distribution map matches the location of the darkest reds in the flowers but the lightest pink passages did not register (Fig. 4a, c). The pink in the blooms were achieved mainly by applying eosin red and zinc white paints wet on wet and also by mixing these two paints. This complex layering and mixing interfered with and complicated the evaluation of the distribution of the eosin red lake by XRF mapping alone, but the chemical analysis of samples helped to clarify some of these issues and also to evaluate the distribution the brazilwood lake in the tabletop and in the stems.
Results of RIS analysis are presented in Fig. 5 for a detail area in Roses, similar to the detail area shown in Fig. 4b–e. In this area, three red spectral ‘endmembers’ were found that well describe the dull red observed in the painting. The red paints mapped to portions of the ‘white’ roses and the roses’ buds (Fig. 5a). The red spectral endmembers in the 400 to 580 nm spectral range show an absorption from ~400 to 550 nm with decreasing absorption at longer wavelengths (Fig. 5b). At wavelengths longer than ~580 nm, the red layer becomes increasingly transparent and the underlying paint dominates the reflectance spectra. Two of the red paint endmembers are on a white paint layer and one (bottom spectrum in Fig. 5b) is on a green paint layer of the bud which results in the absorption at wavelengths beyond ~580 nm. In short, all three spectra suggest that a similar red pigment was used in these areas. These spectra have shapes indicative of a red lake pigment [20] rather than a semi-conductor, which would have a sharp transition (i.e. vermilion, cadmium reds or red lead) and they do not match iron oxide-based pigments such as Mars red [20]. The combined map shown in Fig. 5a overlays with the deep red and light reds in the color image and, interestingly, with the Br distribution map in many areas. This also suggests the red pigment used is an eosin-based red lake. Also, the map of the red pigment/s aligns quite well with the strong red features seen in the book-plate image detail in Fig. 4b.
The RIS maps give information in the visible at or near the paint surface as the optimal depth depends on the absorption and scattering properties of the outer paint layer whereas the XRF distribution maps can provide information at layers below the surface paints (noting that the matrix effect modulates this). This likely explains the few differences observed between the Br distribution map obtained by XRF mapping and the RIS maps for the reds. This is apparent in the comparison of the XRF distribution map for Pb and the RIS map due to lead white (Figs. 4e, 5c).
For mapping the presence of lead white (basic lead carbonate) the narrow feature at 1446 nm (Fig. 5d), which originates from the hydroxyl first overtone, was used [32]. The lead white used by Van Gogh is most likely a commercial product, expected to have a larger proportion of the basic lead carbonate over lead carbonate. In the RIS map, the 1446 nm feature is localized to one flower (top right) and a portion of the lower right flower (Fig. 5c). The white pigment for the other flowers is likely zinc white given the XRF distribution map for Zn (Fig. 4d) and the finding of strong absorption from 350 to ~370 nm in the site specific fiber optics reflectance spectra.
SERS and HPLC analyses of a microsample removed from the edge of the pink tabletop that had been protected from light exposure by the frame rebate (Fig. 4a) showed that Van Gogh used brazilwood, and that no cochineal or any other organic colorant is present. Brazilwood was also identified by SERS and HPLC in a microsample removed for the dark red paint used for some of the stems, in which the lake—used without the admixture of white—does not appear to have faded to the extent observed in the flowers. The corresponding SERS spectra are shown in Fig. 6 together with the spectrum of a brazilwood reference, and the HPLC data is included in Additional file 1: Figure S4. Brazilwood and eosin lakes are the most fugitive red lake pigments among others used by Van Gogh such as cochineal lake [11]. Therefore, carminic acid, the main component of cochineal and a more stable colorant, would have been detected by HPLC analysis if it was present in the sample. Burnstock et al. have reported that the most severe fading tends to occurs where the lakes are mixed with lead white or zinc white [11], as observed in the blooms.
Geldof et al. in their study of Van Gogh’s palette in the last period of his life (1888–1890) found that, starting in the summer of 1889, Van Gogh used a paint containing a redwood colorant and cochineal precipitated on an Al/Sn-containing substrate, with a small amount of CaCO3, and state that he likely bought this as a ready-made paint mixture since they did not identify redwood separately [15]. Redwood alone had been previously identified in Garden of the Asylum [12] by thin layer chromatography; both lakes were identified separately in paintings done while Van Gogh was in Paris [13].
Two microsamples were removed from the tabletop and mounted as cross-sections for Raman and SEM–EDS measurements. The first sample was taken from an area where the pink paint is protected by the blue horizon line, on the right edge of the painting, and the second one from a spot where the red lake pigment has visibly faded (Fig. 4a). In the first sample, the pink layer is applied over the white ground preparation containing lead white and CaCO3 (Fig. 7a, b). Raman analysis of the pink layer showed that the brazilwood lake is mixed with zinc white, while SEM–EDS revealed that the lake is precipitated onto a substrate composed of Sn and Al, and that there are a few CaCO3 particles in this layer. It should be noted that, in the pink layer, the red colorant has faded in the spot where the Prussian blue paint layer on top of the stratigraphy has a gap; this spot is indicated by an arrow in the photomicrograph presented in Fig. 7a.
The second sample cross-section removed from the tabletop is missing the ground preparation layer (Fig. 7c, d). In the dark pink layer at the bottom of the stratigraphy, the brazilwood lake is precipitated onto a substrate containing Sn and Al and it is mixed with zinc white and few CaSO4 particles. In the white layer at the top of the sample, relatively bigger CaSO4 particles, CaCO3 particles, relatively smaller amounts of particles containing Sn and Al, also smaller in size, and larger amounts of zinc white were detected by SEM–EDS when compared to the dark pink layer beneath. The presence of particles containing Sn and Al in this white layer indicates that it must have been colored by a brazilwood lake similar to the one in the darker pink layer beneath and that this colorant has now faded. The relatively smaller amount of particles containing Sn and Al and the larger zinc white content in this white layer indicates that, originally, it must have been of a paler pink.
In a sample removed from a rose in the right side of the painting (Fig. 4a), eosin red and zinc white-containing paints, applied wet on wet, were identified by Raman spectroscopy and PbSO4 was found to be present in the red paint by Raman and SEM–EDS (Fig. 7e, f). The composition of the red paint in this sample shown from Roses is similar to the one observed in the sample taken from a flower in Irises and shown in Fig. 2a–c.
For the virtual color simulation of the tabletop, L*a*b* values were obtained by calibrated microscopy in the cross-section shown Fig. 7a, where the eosin red lake is protected by the Prussian blue layer, and by calibrated photography in the right edge of the painting, in an area that had been protected from exposure to light. For the color simulations of the flowers, the L*a*b* values used were those measured in the cross-section shown in Fig. 7e and those obtained in multiple spots in the roses by reflectance spectroscopy [1]. Reference was also made to the X-radiograph of the painting and to the Pb and Zn distribution maps for the color simulations.