Conventional imaging

Portrait of a Woman by Edgar Degas (Fig. 1a) has historically been known to have a concealed figure, and the work has been criticised since at least 1922 for the gradually increasing outline of the underpainting22. Degas painted directly on the underlying portrait with no intermediate ground paint layer using exceptionally thin paint layers, thus little pigment is present to provide hiding power. The hiding power of paint layers often decreases as oil paintings age. The index of refraction of the natural oil medium has a tendency to increase over time, thus the difference between the pigment’s and the oil’s indices of refraction become smaller, leading to less light scattering at the oil-pigment interface and therefore yielding lower opacity23. The gradual increase in transparency of pigments such as emerald green (Cu(C 2 H 3 O 2 ) 2 ·3Cu(AsO 2 ) 2 )24 and lead-based pigments25 has been observed and studied, with metal soap formation considered to play a major role in the process24,26.

The identity of the woman in the black dress and bonnet is currently unknown. In the visible light image (Fig. 1a), it can be observed that the underlying portrait runs in the opposite orientation to the upper composition. The shoulders of the hidden portrait are the source of the diagonal lines radiating from the present sitter’s bonnet to the top corners of the painting. The sitter’s face appears discoloured as the impression of the hidden composition shows through.

An X-radiographic image (Fig. 1b) and a reflected infrared image (Fig. 1c) of the painting represent the limits of conventional practice for imaging the work27. The X-radiographic image indicates that the underlying portrait is a young woman in three-quarter view. The main source of contrast is provided by the face and ear. Infrared imaging is sensitive to carbon-based pigments and is often used to reveal underdrawings which may consist of graphite or charcoal for example28,29. In the reflected infrared image, it is apparent that the black paint used in the garment of the upper painting has high opacity to infrared radiation (Fig. 1c). This is indicative of a carbon-based pigment such as carbon black, and its extensive use in the present portrait provides limited views to the underlying work. Portions of the underlying figure’s face are observed where it is overlapped by the present sitter’s face. Overall the underpainting cannot be resolved as more than a faintly outlined female figure by conventional techniques, and it has long been considered to be indecipherable, to the disappointment of academics studying the artwork22.

High-definition XRF mapping

The use of the Maia 384 detector array30 at the X-ray Fluorescence Microscopy (XFM) beamline31 has enabled the rapid collection of high-definition synchrotron XRF data over areas spanning tens of dm2 (Fig. 2). The 31.6 megapixel scanning XRF elemental maps obtained from Portrait of a Woman are presented in Fig. 3. Under the experimental conditions, relevant detectable elements range from K-edge excitations of Z = 20–33 (Ca to As) and L-edge excitations of Z = 50–80 (Sn to Hg).

Figure 2: Simplified schematic of the synchrotron-based scanning X-ray fluorescence microscope. A monochromatic undulator-based X-ray source is focused by Kirkpatrick-Baez mirrors. The focused beam passes through an aperture in the Maia detector onto the raster-scanned sample. The X-ray photon events are detected in a backscatter geometry and analysed to produce elemental and scatter maps. (Edgar Degas, French, 1834–1917, Portrait of a Woman (Portrait de Femme), c. 1876–80, oil on canvas, 46.3 × 38.2 cm, National Gallery of Victoria, Melbourne, Felton Bequest, 1937). Full size image

Figure 3: High-definition 31.6 megapixel X-ray fluorescence elemental maps of Portrait of a Woman. (a) Eleven elemental maps providing an overview of the construction of the painting (426 × 267 mm2 scan). The maps have been downsized by averaging over 4×4 pixels and displayed as the square root of the elemental counts with the threshold value displayed in square brackets (e.g., Zn has a maximum display threshold of 100 counts, corresponding to 104 photons per pixel). (b) Detail of zinc map, also in square-root counts. The fine brush work of the hidden sitter is clearly revealed. Image size shown is approximately 118×66 mm2 (~2.2 Mpixel). (Edgar Degas, French, 1834–1917, Portrait of a Woman (Portrait de femme) c. 1876–80, oil on canvas, 46.3 × 38.2 cm, National Gallery of Victoria, Melbourne, Felton Bequest, 1937). Full size image

XRF imaging can be used to deduce pigment use based on the elements observed within the context of the painting. However it cannot be used to unequivocally identify pigments. Pigment identification can be further supported when different elements are highly co-located. Co-located elements could indicate a pigment intrinsically containing different metals or a mixture of pigments used by the artist to achieve the desired colour. For instance, Fe and Mn are co-located in the hidden sitter’s hair (Fig. 3a), strongly suggesting the use of the brown pigment umber, a fine-grained rock consisting of manganese oxides and hydroxides (5–20% composition) with iron oxyhydroxides (~45–70%)32. The Fe:Mn atomic ratio of 6:1 determined from the XRF data is consistent with the composition of umber and its use in the hair.

The As and Cu maps suggest a possible headdress or adornment was attempted on the hidden sitter (Fig. 3a). Cu and As are often associated with green pigments belonging to the copper arsenite group32. A historic pigment commonly associated with the copper arsenites is Scheele’s green, which is considered to be a mixture of several components of varying composition, (e.g., copper diarsenite (2CuO·As 2 O 3 ·H 2 O), copper metaarsenite (CuO·As 2 O 3 ), copper arsenate (Cu(AsO 2 ) 2 ), etc.)32. In regions with high correlation of As and Cu, we find the atomic ratio of As:Cu as 2:1, consistent with the use of copper arsenite. Arsenic is also present in the hair and bonnet of the upper composition; however these areas are relatively free of Cu, barring a region of repair, suggesting the presence of another arsenic pigment. Potential arsenic pigments are likely arsenic-sulphur based such as realgar (As 2 S 2 , orange-red), pararealgar (As 4 S 4 , red-orange) or orpiment (As 2 S 3 , yellow to greenish-yellow), however the relatively low energy sulphur fluorescence (~2.3 keV) is below the low energy limit of the fluorescence detector to aid this line of reasoning.

The hidden sitter’s face consists of several elements. The Zn map provides the best overview of the face and showcases Degas’ brushwork (Fig. 3b). Here Zn would most likely be in the form of zinc white pigment (ZnO), which came into widespread use after 184532. Zinc appears to be the most thickly applied element detected on the face, and it also present in the ear and hair of the hidden sitter. Similar to the Zn map, Co defines the hidden sitter’s face and ear, and it is also present in the hidden sitter’s garment. Cobalt is probably present as a blue pigment, which is useful in defining flesh tones, with examples being cobalt blue (CoAl 2 O 4 ) or smalt (Co-doped alkali glass). Mercury is predominant in the facial area and would most likely correspond to the red pigment vermilion (HgS), which would contribute to a pink flesh tone. It is primarily used on the lips, face and ears of the hidden portrait. Cobalt is also present on the lips of the present portrait. Iron is also present in areas of the face that are free of Mn, suggesting another pigment besides umber as was postulated for the hair. Hematite (Fe 2 O 3 ) and goethite (FeO(OH)) are plausible pigments for use in the creation of flesh tones as they have the ability to generate red and yellow hues.

The background of the painting is defined by chromium, which is also present in some features of the hidden sitter’s face such as the eyes. Based on the background colour, several possibilities exist such as chrome yellow (PbCrO 4 , PbCrO 4 ·xPbSO 4 or PbCrO 4 ·xPbO). Chrome yellow has a tendency to darken with time and its degradation process with respect to the works of Van Gogh has been studied in detail33,34,35,36. The Zn:Cr ratio varies over the background, ranging from approximately 1:1 above the hidden sitter’s head to 5:1 on the left and right sides of the painting. Based on the 1:1 Zn:Cr ratio, another possibility is the use of zinc yellow (K 2 O·4ZnCrO 4 ·3H 2 O)32 or the green pigment viridian (Cr 2 O 3 ·2H 2 O) augmented with a Zn-containing pigment. Zinc yellow is also sensitive to degradation, with George Seurat’s masterpiece A Sunday on La Grande Jatte displaying the effect37,38.

The overall counts observed for Ni is low, suggesting Ni is present in low concentrations or in lower paint layers where its fluorescence would be more attenuated. At the cost of lower spatial resolution, an improved signal to noise was achieved by averaging the map over 4 × 4 pixels, which is an advantage of high definition mapping when counting statistics are low. The Ni map shows a general Ni distribution throughout the painting, and it appears that Ni is predominant in areas of the hidden sitter’s face. The rather uniform distribution throughout the remainder of the painting may suggest that Ni is present in the ground layer. Based on the rather low counts observed for Ni, it is unlikely a Ni-based pigment. Nickel is often present as an impurity in many pigments, with lead white or cobalt-based pigments being common examples39.

The hidden sitter’s garment forms an outline showing through the upper composition in the visible light image of the painting (Fig. 1a), and it is found to consist primarily of Mn, Co and Hg. These elements do not appear to cover the whole of the hidden garment, thus it may also consist of, for example, low Z inorganic pigments, carbon-based black or other organic dye-based pigments which are undetectable by XRF.

The black painted areas of the upper artwork have low concentrations of Ca (Fig. 3a). Thus the black pigment used is unlikely to belong to the cokes family of black pigments, such as bone black32. Bone black contains approximately 84 wt% Ca 3 (PO 4 ) 2 40, thus Ca fluorescence would be readily observed from the painting surface if bone black had been used. The upper portrait’s pigment composition is more likely to belong to the flame carbons family of blacks, such as lamp black, with the carbon source possibly from a hydrocarbon precursor32.

The Ba distribution image is particularly useful in identifying the location of the upper portrait relative to the earlier composition. Barium is routinely observed in art as barium sulphate (BaSO 4 ) and was in common use for preparation of commercial canvas grounds and used as a low-cost filler material or hue alteration in commercial pigment mixtures, including zinc white and viridian32.

X-ray scatter maps

The X-ray scatter maps (Fig. 4) provide further complementary information to the elemental maps. The inelastic (Compton) scatter is sensitive to the lighter elements and thus enables imaging of dense organic components such as the canvas (Fig. 4a)10. In contrast, the elastic (Rayleigh) scatter is more sensitive to the heavier elements such as lead and mercury (Fig. 4b). Here the elastic scatter provides an image of the ground layer and for example, the lead white paint brush stroke running across the hidden sitter’s forehead, and is complementary to the inelastic scatter map. Earlier damage and restoration is clearly identifiable in the scatter maps and is highlighted in Fig. 4. A negative image of the hidden sitter’s face is observed in both scatter maps. We attribute this primarily to the heavy application of zinc-based paint in this area (Fig. 3), which would attenuate the incident X-ray beam and then further attenuate (self-absorb) the scattered X-rays from the underlying ground layer and canvas. Overall these attenuation effects would yield lower sensitivity to materials below the zinc layer.

Figure 4: X-ray scatter maps. (a) Inelastic and (b) elastic scatter. The boxed areas are shown inset for greater detail, and reveal an approximate 21 × 13 mm2 puncture damage, previously restored by filling and overpainting, and not readily evident by visible light examination. Maps displayed in a linear count scale. Full size image

Pb Raman Imaging

X-ray Raman scattering is an inelastic scattering of X-rays from core electrons. It is normally a weak process, but can become considerably stronger through a resonance effect if the incident photon energy is immediately below the absorption edge of a matrix material41. The incident beam excitation energy, 12.6 keV, was chosen ~0.4 keV below the Pb L 3 edge to minimize the Raman scattering signal from the Pb-rich pigments and ground layer. Another consideration in the choice of energy was to remain above the Hg L 3 absorption edge at 12.284 keV and keep the incident energy high enough to minimise the inelastic scatter tail from interfering with fluorescence lines and thereby limiting sensitivity. We have previously used the 12.6 keV incident beam energy to successfully image a painting wholly covered in lead white paint10. The Maia 384A detector’s low energy sensitivity cutoff is approximately 4 keV, so (surface) Pb detection via the low energy Pb M fluorescence lines (~2.3 keV) was not possible. In this work we found that Raman scattering could be detected and used for imaging Pb.

To demonstrate the Raman scattering effect below the Pb L 3 edge (13.035 keV), Fig. 5a shows logarithmic plots of three spectra of a paint sample containing lead white (basic lead carbonate, 2PbCO 3 ·Pb(OH) 2 ) obtained at 12.6, 12.8 and 13.0 keV. Zn fluorescence, originating from the canvas preparation, is also indicated in the plot. The ratio of the integrated intensity of the most intense Raman scattering band to the elastic (Rayleigh) scatter I Raman /I elastic = 0.55 at 13.0 keV, and I Raman /I elastic = 0.034 at 12.6 keV, illustrating the rapid intensity drop of the Raman signal as a function of energy below the Pb L 3 edge. The change in elastic scatter intensity was negligible over this incident energy range.

Figure 5: Pb Raman imaging. (a) X-ray spectra obtained from a lead white-containing paint sample demonstrating Raman scattering for three excitation energies below the Pb L 3 absorption edge (13.035 keV). The spectra have been offset by factors of 10 for clarity. (b) Pb Raman scatter map (detail) indicates areas of lead-based paint application to the painting. Map displayed in a linear count scale. (c) Detail of Portrait of a Woman shows that the white brush strokes yield the strongest Pb Raman signal. The low intensity of the Raman scatter at 12.6 keV excitation energy renders surface Pb most sensitive to detection, as scatter from below the surface would be attenuated by the overlying high density paint layers. (Edgar Degas, French, 1834–1917, Portrait of a Woman (Portrait de Femme), (c). 1876–80, oil on canvas, 46.3 × 38.2 cm, National Gallery of Victoria, Melbourne, Felton Bequest, 1937). Full size image

Due to its relatively low intensity at 12.6 keV excitation energy, the Pb Raman scattering was best detected in areas of the portrait containing surface applications of Pb-based paint, in particular the white brush strokes below and to the right of the sitter’s face (Fig. 5b,c). A relatively low Pb Raman signal was observed for the Cr-containing background of the painting, which may support that a non-Pb based Cr-containing pigment such as zinc yellow or viridian was used rather than Pb-based chrome yellow (vide supra).

The Raman signal is likely not practical to use as a reliable imaging method for paintings given their highly variable nature. However it does highlight that the choice of excitation energy is an important experimental consideration when working immediately below an absorption edge of any painting component.

Colour Reconstruction

Elemental maps enable false colour reconstruction of concealed artworks, which provide insight to the colour palette of the artist. Previous researchers2,42 have attributed a false colour to their elemental maps, and were able to create a plausible colour representation of the underpainting. For instance, with the Van Gogh painting Patch of Grass the false colour effect was achieved by manually overlaying two elemental maps with manually assigned colour and transparency2.

A false colour image of the underlying painting from Portrait of a Woman was made using a methodology for layering multiple elemental maps. It was generated using custom-written software capable of merging the high resolution, high dynamic range elemental images manually assigned with colours most likely associated with each element (e.g., red for Hg, blue for Co). The colour is chosen based on published examples of the typical colour of a pigment. Pigment colour is not a standardized value, and varies considerably in natural pigments43. The resulting false colour image (Fig. 6) is a plausible representation of the artist’s work from the period44,45, and we have presented it to emphasise the underlying image.

Figure 6: The hidden portrait of Emma Dobigny. False colour reconstruction of Degas’ hidden portrait (detail). The image was created from the X-ray fluorescence microscopy elemental maps. (Edgar Degas, French, 1834–1917, Portrait of a Woman (Portrait de femme) c. 1876–80, oil on canvas, 46.3 × 38.2 cm, National Gallery of Victoria, Melbourne, Felton Bequest, 1937). Full size image

The Hidden Portrait

Based on the observed XRF elemental maps, we propose that the revealed underpainting is a previously unknown portrait of the model Emma Dobigny. Dobigny, whose real name was Marie Emma Thuilleux, modelled for Degas between 1869–1870 and is reported as a favourite model of Degas and other French artists of the period44. We observe strong resemblance between the revealed underpainting and several of Degas’ portraits of Emma Dobigny. Literature suggests that Degas had a special fondness for Emma Dobigny44 which may account for the otherwise unfinished or unsatisfactory painting being retained by Degas.