Micro-sample analysis of the blue headscarf
Paint samples from shadow areas: stratigraphy and material composition
The study of paint cross-sections helped to determine the layer stratigraphy of the blue headscarf, and to characterise the ultramarine pigment and other materials that Vermeer used to paint it. Two sample locations were carefully selected from the right (shadow) side of the blue headscarf based on the preliminary macro-imaging results: a bright blue brushstroke that has some relief—applied in the final stages of the painting process—and a dark blue tone from an adjacent area, applied thinly (Fig. 1b, c). Both areas exhibited a superficial whitish surface haze, but the haze was more noticeable in the darkest blue. Characterisation of both samples was achieved using LM, SEM–EDX, and µ-FTIR-ATR (Table 1).
In the cross-section from the bright blue brushstroke (sample 41), LM and SEM–EDX identified a thick (up to 70 µm) layer of blue paint (Fig. 2, layer 3) applied on top of a black underlayer (Fig. 2b–d, layer 2). The grey ground, identified in other cross-sections [7] and sample 42 (below), is missing in this sample. The underlayer contains predominantly black particles. The splintery shape of the particles, together with the detection of C using SEM–EDX, suggests a charcoal black. In the blue layer, bright blue particles and some fine white particles in a translucent matrix are observed in the LM image (Fig. 2b–d, layer 3). Using SEM–EDX, Na, Al, Si, and S were co-detected (Fig. 2f, g, h, i, m) in the particles observed as bright blue in the LM image (Fig. 2b, c): this indicates the presence of lazurite, the blue mineral of ultramarine pigment. Surrounding these particles, an abundance of Ca was also detected (Fig. 2k, n) with EDX. Only in very few spots, Ca is co-localised with S, suggesting its main presence as chalk (CaCO3), with some particles as calcium sulphate. The proportion of lazurite to chalk is estimated to be about 50:50, based on the EDX maps from Na, Al, Si and Ca (Fig. 2f, g, h, k). Further in the surrounding matrix, Pb was detected in the fine white particles observed in the LM image, suggesting the presence of lead white pigment. Of the remaining particles, co-locations of Si, Al, and K (without detection of S) as well as Si, Ca, and Mg (in one particle) were detected with EDX. These elements suggest the presence of accessory minerals related to lazurite in lapis lazuli: respectively, a feldspar mineral (KAlSi3O8) and diopside (MgCaSi2O6). The presence of only a few particles of accessory minerals relative to the large number of lazurite particles implies that a high-quality ultramarine was used.
In agreement with the SEM–EDX results, µ-FTIR-ATR analysis of sample 41 identified lazurite and chalk in the blue paint layer (Fig. 2o–q). The distribution of lazurite is mapped using a characteristic absorption band at c. 978 cm−1 from silicate (Si–O stretch) (Fig. 2o, p) [27]; chalk has a sharp absorption band at c. 870 cm−1 from carbonate (C–O out-of-plane bending), which was used to map its distribution (Fig. 2o, q) [27]. In addition to lazurite and chalk, µ-FTIR-ATR detected some particles of gypsum (CaSO4∙2H2O) distributed throughout the blue layer. Figure 2r shows the distribution of the strong sulphate band (S–O stretch) at c. 1110 cm−1, characteristic of gypsum (Fig. 2o, r) [27].
The cross-section from the dark blue shadow (sample 42) shows a thick layer of medium-grey ground (Fig. 3b–d, layer 1, up to 50 µm thick), followed by a thin black underlayer (Fig. 3b–d, layer 2, c. 10 µm thick), and a thin blue surface paint layer (Fig. 3b–d, layer 3, c. 10 µm thick). The grey ground consists of chalk, lead white, some earth pigments, and fine carbon black [7]. The black underlayer was found to contain some chalk, gypsum, and a few aluminium (Al)-containing particles, in addition to the many splintery particles of charcoal black. Its composition is comparable to that of the black underlayer in the background [9]. The backscatter image also showed a few light grey amorphous clusters within the black underlayer, within which mainly Pb, K and S (data not shown) were detected. This points to the presence of palmierite (K2Pb(SO4)2): a common degradation product found in historical oil paintings [15, 28,29,30].
Similar to the blue paint layer in sample 41, SEM–EDX and µ-FTIR-ATR detected characteristic bands showing that the thin blue surface layer is composed of mainly ultramarine (lazurite) and chalk, with minor additions of a fine lead white, and gypsum (Fig. 3e–m). The layer also contains very few particles of a translucent red pigment that shows a pink UV-induced fluorescence (Fig. 3c). Using EDX, Al, Ca and S were co-detected; these are associated with a lake substrate, implying the use of a red organic lake on an Al substrate. The red lake was present in such a small quantity that it presumably had little effect on the eventual colour of the paint; other components (especially chalk) seemed to have a much larger impact on both optical and handling properties of the paint.
Macroscopic X-ray powder diffraction imaging (MA-XRPD) in reflection mode detected calcium oxalates and imaged their distribution in the shadow zone of the blue headscarf [29]. Calcium oxalates, like palmierite, are common degradation products in oil paintings, and here seem related to the presence of chalk [31, 32]. They may be held responsible for the whitish and patchy appearance of the shadow zone of the headscarf. The µ-FTIR-ATR analyses of samples 41 and 42, however, were not able to detect any calcium oxalates at the surface or within the blue paint layers. Although µ-FTIR-ATR is an effective method to identify calcium oxalates on paint fragments, the spatial resolution achieved here is limited to c. 4 to 6 µm. If the formation of calcium oxalates is a surface phenomenon, the layer of oxalates would be only c. 1 µm thick in cross-section, and this is below the spatial resolution of the present µ-FTIR-ATR instrument. It is also possible that the calcium oxalates formed in such low quantities that they could not be detected with µ-FTIR-ATR.
Samples 41 and 42 are from neighbouring regions and both show similar compositions, yet each has a different build-up with respect to blue: one thick and the other thin. From the SEM–EDX and µ-FTIR-ATR, it can be concluded from the relative abundance of bright blue particles of lazurite that Vermeer used a high-quality ultramarine.
Synchrotron µ-XANES of ultramarine pigment
Using µ-XANES, further information about the preparation of ultramarine pigment in samples 41 and 42 was determined: specifically, whether the lazurite was extracted from a heat-treated lapis lazuli rock. The location of lazurite particles in each sample was determined from µ-XRF maps of Si, Al, and S (Fig. 4a), and several µ-XANES spectra were then acquired across each lazurite particle. Figure 4b presents a selection of averaged µ-XANES spectra from lazurite particles within both samples (solid curves); the uppercase letters refer to the particles circled in Fig. 4a from which the spectral averages were taken. The general pattern of each matches that found in the literature for lazurite with a relatively low-intensity peak at 2469.0 eV, an ‘envelope’ of peaks between 2470.0 and 2475.6 eV, and a relatively high-intensity peak at 2482.2 eV [33,34,35,36]. With the exception of the peak at 2482.2 eV, which is attributed to sulphate, the peaks are reported as combined contributions from neutral, radical, and/or dianion sulphur species [33,34,35,36,37].
More closely, the relative ratio of peaks within the ‘envelope’ region—namely those at 2471.2, 2472.5, and 2473.8 eV—indicate the influence of heat-treatment [12]. It was recently found that for lazurite extracted from lapis lazuli heated at 600 °C the relative intensity of the peak at 2471.2 eV was greater than that at 2472.5 and 2473.8 eV In contrast, for lazurite from lapis lazuli left at room temperature or heated only at 415 °C, a local maximum was observed at 2472.5 eV. With further heating to 750 °C, a clear minimum at 2472.5 eV was also visible [12]. Using these criteria, conclusions may be drawn about the approximate temperature of treatment for the lapis lazuli rock from which the ultramarine in the headscarf of the Girl was extracted. Specifically, the ‘envelope’ region of spectra F, G, and H in Fig. 4b each shows a higher intensity at 2471.2 eV relative to that at 2472.5 and 2473.8 eV, yet no distinct minimum at 2472.5 eV. Such spectra correlate well with reference spectra of lazurite from lapis lazuli heated at 600 °C [12]. The remaining spectra of Fig. 4b are intermediate between those from low temperature treatments and those of treatments at 600 °C, indicative of the heterogeneity that arises with heat treatments [12]. Whether the heterogeneity is a result from mixing different batches of ultramarine pigments or from the partial conversion of species with heating cannot be determined. The similarity of the spectral shapes found in the two samples implies Vermeer used the same type of ultramarine for both applications; however, many more lazurite particles would need to be measured to determine this with certainty.
Finally, µ-XANES also confirmed the presence of gypsum particles as revealed by µ-FTIR-ATR. In sample 41, µ-XRF detected particles containing S without a corresponding detection of Al and Si (Fig. 4a, upper, blue false-coloured particles). The µ-XANES spectra from such particles—exemplified in Fig. 4b (uppermost curve, dotted blue)—present the characteristic spectral shape between c. 2482 and 2500 eV for gypsum [38].
Admixture of chalk or an organic lake pigment?
The origin and effect of the large amount of chalk in samples from the shadow of the headscarf in Girl with a Pearl Earring remained partly unresolved. Some calcite is typically associated with the lazurite mineral. However, the amount of chalk present in the blue layers of samples 41 and 42 (c. 50 vol%) can be considered too high to be only associated with ultramarine. It must come from another source. It could be an adulterant to bulk up the costly ultramarine, but the high quality of the pigment makes this unlikely. Another possibility is that extra chalk was added deliberately to the ultramarine glaze paint, to adjust its handling properties. A more likely possibility is that the chalk is present as a substrate of a—now faded—organic lake pigment. The use of yellow lakes on a mainly chalk substrate has been reported in many genre paintings by Vermeer and his contemporaries, also in combination with ultramarine [3, 5]. In Vermeer’s View of Delft (c. 1660–1661, Mauritshuis), the foliage containing ultramarine has a bluish appearance as a result of the fading of the yellow component, most likely a yellow lake on a chalk substrate [39].
In a cross-section, a gradient within a paint layer can be an indication that a yellow lake has faded: the upper part of the layer looks very pale, while the bottom part still exhibits some colour [40]. In Girl with a Pearl Earring, the chalk matrix surrounding the ultramarine particles in the ultra-thin blue layer of sample 42 is completely colourless (Fig. 3b, c). If it originally contained a lake, the dyestuff has now completely faded. The UV-induced fluorescence image of sample 41 showed a slightly stronger yellowish fluorescence at the bottom of the blue layer, but this was not convincing enough to conclude that a (yellow) lake was present (Fig. 2b, c). Unfortunately, there was not enough material from either of these samples to confirm the presence of an organic dyestuff using UHPLC analysis.
In an attempt to find more indication for the use of a lake, the use of lakes in other areas of the painting—the background and the Girl’s jacket—was investigated as part of the current study. Sample 23 from the edge of the headscarf, adjacent to the dark background, clearly shows a gradient within the surface paint layer; the bottom part exhibits a yellowish UV-induced fluorescence, while the top part is much paler (Fig. 5b, c, layer 3. Table 1). It contains a relatively thick layer (c. 30 µm) with ultramarine and high amounts of chalk; the proportion of ultramarine to chalk estimated to be about 25:75 on the basis of the LM and SEM images (not shown). It appears that some glaze from the background of the painting (see below) was blended into this paint, showing fine blue particles of indigo at the top of the layer (instead of ultramarine). Analysis with UHPLC-PDA-FLR of two tiny unmounted fragments from the same location detected indigotin as the main organic colourant, as well as several unidentified yellowish components (spectra not shown). In the 1990s, Groen et al. [6] determined that the glaze in the background of the painting contained indigo and a yellow lake (weld) on a chalk substrate with minor traces of alum. The yellow component has completely faded over the entire background, except in a few restricted areas that had been covered with old retouchings, removed during the 1994 treatment [8]. As part of the Girl in the Spotlight research, two small paint fragments from the background glaze were analysed using UHPLC-PDA-FLR, which confirmed the presence of luteolin (from weld) and indigotin (from indigo) in the glaze [9].
Although the Girl’s jacket appears yellow where it faces the light, the paint also contains a small amount of ultramarine, mixed with lead white, yellow ochre and a tiny amount of red lake. The shadow of the jacket contains ultramarine, yellow ochre and a red lake pigment; hardly any chalk was detected [7]. Analysis with UHPLC-PDA-FLR of a dislodged paint fragment from the jacket (sample 14a) detected carminic acid, and some other red fluorescent colourants,—such as dcIV, dcVII, kermesic acid and dc equivalents—implying the use of American cochineal (Dactylopius coccus Costa.) (spectra not shown). The co-detection of Al, K and S in the red lake particles with EDX indicated that the red dyestuff has an alum substrate (spectra not shown).
In all occurrences of a red lake in Girl with a Pearl Earring, the substrate was alum [7, 15]. In contrast, the yellow lake (weld) identified in the background glaze was precipitated or adsorbed on chalk with only minor traces of alum [6, 9]. This further supports the hypothesis that the chalk found in the blue paint layers of the headscarf is the substrate of a—now faded—yellow lake, and not from a red lake. This is also in agreement with lake recipes from contemporary sources. The recipes for yellow lakes (schiet-yellow) typically contain more chalk than alum [41, 42]. Reconstructions of yellow lake recipes by Hermens and Wallert showed that the bulk of the material consisted of calcium carbonate white, with some calcium sulphate and some aluminium hydroxide [41]. Thus the presence of some calcium sulphate (gypsum) (identified with SEM–EDX and µ-FTIR-ATR, Figs. 2, 3), evenly distributed within the blue paint layers of the Girl’s headscarf, may also originate from a lake pigment. Paint reconstructions by Saunders and Kirby demonstrated that yellow lakes made with calcium-based substrates are most vulnerable to light-induced fading [43].
As part of the current study, schematic reconstructions were made to further investigate the visual and rheological effect of adding chalk or yellow lake to an ultramarine paint (Fig. 6c–j). An ultramarine paint was mixed with varying quantities (0, 25, 50 and 75 volume%) of either chalk, or weld (Reseda luteola) precipitated onto a chalk/alum substrate (weight proportions of chalk:alum 4:1). The methodology is described in Additional file 1. Pure ultramarine in oil was difficult to apply evenly, and making a long fine brushstroke required reloading the brush with paint (Fig. 6c). Adding chalk improved the handling properties significantly, and only a small amount was required (Fig. 6e); there was no noted improvement when a larger proportion (50–75%) was added (Fig. 6g, i). Adding too much chalk negatively affected the appearance of the paint, making the glaze more opaque and ‘cloudy’.
Reconstructions showed that adding yellow lake also improved the handling properties of an ultramarine paint. It was observed that a small amount (25%) of yellow lake did not change the colour of the blue paint significantly (Fig. 6f); The 50:50 ratio of yellow lake:ultramarine produced a blue-green (Fig. 6h). Only the 75:25 yellow lake:ultramarine created a distinctively green colour (Fig. 6j). The effect of the addition of a yellow lake on the original appearance of the headscarf is discussed at the end of the next section.
Painting process of the blue headscarf
The results of complementary analyses using MS-IRR, MA-XRF, RIS—combined with micro-sample analysis and microscopic examination of the paint surface and high-resolution 3D digital microscopy—revealed the steps that Vermeer took to paint the different zones of the headscarf. The headscarf is visually divided into three distinct zones (Fig. 7a): the strongly lit part on the left (Fig. 7a, zone 1), a middle tone that is a slightly brighter blue (Fig. 7a, zone 2), and the shadow on the right (Fig. 7a, zone 3).
In the initial stages of the painting process, Vermeer prepared different parts of the composition with monochrome underlayers: in shades of cream, brown and black [7, 9, 15]. A black underlayer was restricted to the right side of the headscarf, which would eventually become the shadow (Fig. 7a, zone 3). MS-IRR (1900–2500 nm) showed that it was applied with broad horizontal brushstrokes above her ear, and continues towards the crown of the Girl’s head (Fig. 7b) [7]. This underlayer corresponds to layer 2 in the cross-sections (Figs. 2, 3).
In the left (light) zone and middle zone (Fig. 7a, zones 1 and 2), microscopic examination of the paint surface showed that the light blue paint appears to have been applied directly onto the grey ground layer. The carbon black in the ground can cause a decrease in slope for NIR wavelengths as was observed with reflectance imaging [8]. 3D microphotographs reveal the grey colour of the ground beneath where the paint is thin, slightly translucent, and/or abraded (Fig. 1d).
MA-XRF maps for Pb (Fig. 7c–e) showed that the paint in the left (light) and middle zones contain a significant amount of lead white. RIS identified mixtures of lead white and ultramarine in different proportions, showing most lead white in the left (light) zone [8]. In the false-colour RIS image obtained at 750, 550 and 450 nm (Fig. 7h), this zone appears light blue. Here the light blue paint was left exposed at the surface to create the highlight on the left side (Fig. 7a, zone 1).
After the shadow and light zones had been established, Vermeer applied a thin blue layer—with little or no modelling—over the shadow, and partially extending over the middle section of the headscarf (this is layer 3 in sample 42, see Fig. 3). MA-XRF maps for Pb show that the paint in the shadow zone contains relatively low quantities of lead white (Fig. 7d, e). The thin blue paint application in the shadow zone was also visualised in the Ca and K maps (Fig. 7f, g). This is in agreement with sample 42 (Fig. 3, layer 3), in which ultramarine and chalk (presumably from a lake substrate) were detected in the thin blue paint. For the construction of the darkest blue shadows, Vermeer made deliberate use of the black underlayer to play a role in the final colour: he applied the final blue paint very thinly with respect to other layers, leaving the black underlayer partially exposed. This effective layer build-up is typical of Vermeer’s painting technique [44].
In the next step, Vermeer defined the middle section (Fig. 7a, zone 2) of the headscarf with a blue paint: more intense in colour than the highlight on the left, but also containing lead white and (more) ultramarine. The colour of the paint is not entirely homogeneous: some brighter strokes were applied wet-in-wet to enliven the middle zone. Vermeer applied the blue paint rather thickly on top of the light blue layer, using curved brushstrokes to create ‘undulating’ folds between the highlight and midtone (Fig. 1e), and between the midtone and shadow. He dragged the paint in fine lines towards the back of the headscarf to depict linear folds. The distribution of this layer is most clear in the false-colour RIS, where it appears light pink (Fig. 7h).
In the final stages, Vermeer applied an intensely coloured blue glaze to delineate the deepest folds in the scarf (this is layer 3 in sample 41, see Fig. 2). The glaze consists of mostly ultramarine and chalk (possibly from an organic lake, which has since faded). In the false-colour RIS image, the glaze appears as dark magenta (Fig. 7h). It shows that the glaze is mostly concentrated on the right side of the headscarf, but some brushstrokes extend slightly over the middle section. One wide brushstroke of blue glaze that overlaps the lighter blue layer counterbalances the undulating folds in the layer beneath (Fig. 1f). A thick glaze applied with a narrow brush created folds that extend towards the back of the Girl’s head. While this intensely coloured glaze would have given depth to the shadow of the headscarf when it was first painted, it has since degraded (see sections above). For instance, a fold that is clearly visible as a magenta line above the Girl’s ear in the false-colour RIS image is barely visible to the naked eye (Fig. 7a, h).
The blue paint layers in the headscarf were painted rather late in relation to the other colour areas within the painting. When the contours where different colours overlap with each other are examined under the stereomicroscope, it is clear that the blue paint layers of the headscarf were painted after (and partially on top of): the yellow part of the scarf, the (underlayer of the) background, and the skin tones.
The last touches that Vermeer applied are small dots on the surface of all three zones of the headscarf, which can be seen in the 3D digital micrographs [45]. Similar dots are found throughout the painting, including on her yellow jacket, and on the border of the yellow ‘tail’ of her headscarf. These dots create the illusion of texture without specifying a particular type of fabric; however, the subtle undulating way that Vermeer painted the folds in the fabric suggests a silky cloth.
Vermeer’s sophisticated layering within the headscarf creates the illusion of a folded cloth strongly lit from one side. Unfortunately, the illusion has been somewhat compromised by degradation and fading. Although we cannot know with certainty what the function of the chalk was in dark blue areas in the Girl’s headscarf, the reconstructions were helpful in developing a hypothesis (Fig. 6c–j). The headscarf of Girl with a Pearl Earring may originally have had a much richer variety of blue shades, ranging from a vivid light blue in the lit part (zone 1) to a dark greenish blue in the shadow (zone 3). Vermeer mixed ultramarine into different proportions of lead white for the light blues of the lit and middle zone of the headscarf (zones 1 and 2). He created the deepest blue-green shadows in the right (shadow) zone, making sophisticated use of the black underlayer; this effect is less visible now because of the whitish surface haze. Alternating wide and narrow brushstrokes of a greenish blue—now a patchy bright blue due to loss of the yellow component—were applied on top of the deep blue-green shadow zone to achieve modelling. The back contour of the headscarf above the Girl’s neck was probably green.
The ‘tail’ at the back of the Girl’s headscarf currently has a stark contrast between a light yellow (left) and a bluish shadow (right), with a yellow-green border at the bottom (Fig. 6a). RIS and MA-XRF showed that the highlights in the border contain both ultramarine and lead–tin yellow, and do not appear discoloured [8]. The XRF map of Ca (Fig. 6b) suggests that Vermeer also mixed a yellow lake—now completely faded—into the blue (shadow) zone of the ‘tail’, which implies that it was once blue-green or green. The application of a deep blue-green or green glaze for the shadow area would also have better matched the yellow-green highlights at the border of the yellow headscarf.
