Select NAAR images and XRF maps that reveal the most information about the hidden painting are presented below. In general, the NAAR images or MA-XRF element maps that provided information about the distribution of lead (Pb), mercury (Hg), copper (Cu) and iron (Fe)/manganese (Mn), corresponding to the pigments lead white, vermilion, copper blues/greens and iron earths, respectively, were found to be particularly informative of the underlying figure.

Lead

Lead has a very small cross section for activation by thermal neutrons [5], and thus the NAAR images are unable to provide information about the distribution of this important element. However, MA-XRF easily detects Pb from both the L and M emission lines, and the difference in emission energies can be exploited to provide information about the distribution of this element at different depths. The Pb-M α map (collected at 2.3 keV, Fig. 2a) measures only the Pb present in the uppermost portion of the painting, while the Pb-L α distribution map (collected at 10.4 keV, Fig. 2b) reflects the presence of Pb both at and below the surface. Indeed, the lower image is clearly revealed by the Pb-L α map: The face of the lower figure is visible just to the right of the upper head (and inverted), with the noses of the upper and lower figures lying adjacent to one another. In the area to the right and above the surface figure a cloak-like garment belonging to the figure located beneath is also visible. As with the X-radiograph, the underlying figure is more easily seen when the Pb-L α map is rotated 180° (Fig. 2c); to facilitate discussion of the lower figure, all subsequent images will be shown in the rotated orientation.

Fig. 2 a MA-XRF Pb-M α element map; b Pb-L α element map; c Pb-L α element map, rotated 180° to better show face and cloak of underlying man; d enlargement of area in c of face of underlying figure, with arrows pointing to the two eyes Full size image

Due to the central role of lead white in Rembrandt’s technique [21], notably as a key component in the paint used for flesh tones as well as its general use as a base for modifying the color tone of many other pigments, it is perhaps not surprising that the Pb-L α map, similar to the X-radiograph, reveals an almost complete image of the underlying figure and in particular the man’s face. Notably however, the Pb-L α map clearly shows a significant change that is also present, but much more difficult to see, in the X-radiograph. There are two proper right eyes, located one over the other (see Fig. 2d); Rembrandt revised the initial position of the eye in favor of a slightly lower placement.

Mercury

The face of the underlying figure is most clearly revealed by the Hg-L α MA-XRF map (shown in Fig. 3a), indicative of the presence of the pigment vermilion (HgS), which was used to tint the flesh tones of the underlying figure, and, to a lesser extent, the upper figure. Because the Hg-L α map does not have a significant contribution from the surface image it reveals some features of the underlying figure even more clearly than the Pb-L α map, such as the proper left eye. Along with revealing the main features of the face—including the lips, right cheek, ear, nose, jaw and chin—the Hg-L α map, similar to the Pb-L α map, notably shows that the eyes of the underlying head were lowered slightly after the initial execution. No indication that the position of the rest of the head of the underlying figure was adjusted is evident.

Fig. 3 a MA-XRF Hg-L α element map; b NAAR Image Plate Set 9 (composite of six imaging plates, wait time 8 days 9 h, exposure time 2 days 12 h) with gray scale inverted to facilitate comparison with MA-XRF element map; c NAAR Image Plate Set 9 (as acquired) Full size image

The radionuclide 203Hg has a half-life of 46.6 days and is strongly activated by thermal neutrons [5]. In the NAAR images, the face of the underlying figure first becomes visible approximately 2 days following the activation, but is obscured by elements present in the upper painting. The face becomes clearer in images captured between 6 and 11 days following activation, and remains visible, with diminishing intensity, in all subsequent exposures (including the final exposure taken 78 days post-activation). The NAAR image that most clearly shows the underlying face was captured by Image Plate Set 9 (shown in Fig. 3b, c). To facilitate comparison with the MA-XRF scans, in Fig. 3b, the gray scale is inverted—with lighter values corresponding to greater exposure/nuclide concentration (all subsequent NAAR images will be similarly presented with inverted gray scales). In Fig. 3c, the NAAR image is shown as traditionally presented—as a blackened film, with darker regions corresponding to higher exposures/greater nuclide concentration. The NAAR image is dominated by Hg and thus, like the Hg-L α map, reveals the face of the underlying figure. However, this NAAR image also contains contributions from other elements (most notably in the area of the feather of the upper figure visible in the bottom center of Fig. 3b, c, and the background in the upper right corner), hampering straightforward interpretation of the distribution of pigments.

Copper

The MA-XRF map showing the distribution of Cu-K α is presented in Fig. 4a. This map clearly shows that the clothing of the underlying figure and the underside of the feather in the upper figure contain significant amounts of a copper-containing pigment. A Cu-containing pigment also seems to have been used to delineate the shadows near, and edges of, the soft cap worn by the figure in the upper painting. As shown in Fig. 4b, adjusting the dynamic range of the Cu-K α map to highlight the lower-intensity values (oversaturating the areas of higher Cu density) reveals that Cu is also present in the cloak of the upper figure. Figure 4c contains XRF spectra averaged over comparable areas of the cloak in the upper and lower figures (areas are indicated in Fig. 4b); from these, it is clear that Cu is indeed present in both cloaks, with a greater proportion present in the cloak of the lower figure than the upper. Comparing the peak areas of the Cu-K α emission line from each spectrum gives an estimate of at least 3× greater Cu in the area of the cloak of the lower figure than the upper, although of course that ratio will vary depending on the areas selected. It should be noted that this abundance ratio represents a lower limit, due to absorption of Cu fluorescence originating from the lower cloak by the layers of paint used to obscure the hidden image and create the background of the upper painting.

Fig. 4 a MA-XRF Cu-K α element map; b map shown in a with contrast adjusted to highlight lower-intensity regions; c area-averaged XRF spectra from area of cloak in lower figure (green trace) and upper figure (black trace). Areas over which average spectra were acquired are indicated in b (outlined in green) Full size image

Unfortunately, the identity of the copper-containing pigment cannot be determined from XRF data alone. Due to the pristine nature of the paint surface, opportunities for sampling are extremely limited and restricted to the very edges of the painting; none of the few samples taken to date captured the Cu-containing layer in the lower cloak. That azurite may be present throughout this area as coarsely ground particles is suggested by X-ray diffraction analysis (portable, noninvasive XRF/XRD spectrometer, InXitu Duetto [22, 23], Cu X-ray source (20° ≤ 2θ ≤ 50°, 0.3° resolution for XRD, 3–15 keV, 250–300 eV resolution for XRF)), which produced X-ray diffraction rings with only a few bright spots at the spacing corresponding to the main peak of azurite. However, this does not preclude the possibility that other Cu-containing pigments, such as copper resinate, might also be present. While it is known that Rembrandt used azurite in his early works to enliven brown tones or as a drier [21], given the circumstantial nature of this evidence, we cannot draw any conclusions from these data; additional analysis is planned to more directly probe Rembrandt’s use of Cu-containing pigments in both the upper and lower compositions.

Multiple NAAR images reveal the shape of the cloak of the underlying figure: It is visible in the first exposure, taken 15 min after activation (Film 1, see Fig. 5a) and then again in all the exposures taken between 24 and 58 h after activation (appearing most strongly and clearly in Film 8, exposed at 26 h post-activation, shown in Fig. 5b). These two time ranges, during which the shape of the copper-containing cloak is best visualized, correspond to the maximum emission from 66Cu and 64Cu, with half-lives of 5.12 m and 12.7 h, respectively. The image in Film 8 most closely matches the Cu-K α MA-XRF map, including the weaker level of emission from the cloak of the upper figure. The image in Film 1 shows a greater relative emission intensity in the region of the cloak of the upper figure, which is due to contributions from 66Cu together with other elements with similarly short decay half-lives. More significantly, Film 1 contains an additional feature—the proper left edge of the face of the underlying figure. This feature is not attributable to Cu, since it does not also appear in Film 8, nor in the Cu-K α MA-XRF map. This feature therefore must be due to the presence of other elements with short decay half-lives (discussed below) that have contributed to Film 1.

Fig. 5 a NAAR Film 1 (wait time 15 m, exposure time 5 m), red arrows indicate edge of contour defining proper left edge of face of underlying figure; b NAAR Film 8 (wait time 26 h, exposure time 24 h) (both films are presented with grey scales inverted) Full size image

Iron/Manganese

The proper left edge of the face of the underlying figure is visible in the NAAR image collected immediately after activation (Film 1, Fig. 5a, discussed above), is strongest in the exposures collected at 3 and 6 h after activation (Film 5, collected 3 h post-activation, is shown in Fig. 6a, red arrows point to outline of face), and then fades in subsequent exposures. The final image in which it can be seen, very faintly, is the film collected 24 h after activation. There are a number of radionuclides with half-lives in this time frame, including 56Mn (2.579 h), 31Si (2.62 h), 42K (12.36 h) and 24Na (14.96 h). These elements could reasonably be present in the iron earth pigments often employed by Rembrandt. Unfortunately, no Fe distribution can be inferred from the NAAR images; during neutron activation, its most abundant isotope (56Fe) is converted into another stable isotope (57Fe).

Fig. 6 a NAAR Film 5 (wait time 3 h, exposure time 3 h), red arrow indicates edge of contour defining proper left edge of face of underlying figure; b MA-XRF Fe-K α element map; c MA-XRF Fe-K α element map adjusted to better show gorget/collar of hidden man; d area-averaged XRF spectra from area of gorget/collar in lower figure (red trace) and upper figure (black trace). Areas over which average spectra were acquired are indicated in c (outlined in red) Full size image

While MA-XRF mapping can detect Fe, Mn, and, to a lesser extent, K, in the air-path configuration employed here, it cannot detect Si and Na. The Fe-K α MA-XRF map is presented in Fig. 6b (the Mn-K α map, not shown, shows the same features as the Fe map, but with lower intensities). A slight indication of the proper left edge of the underlying face can be seen in the Fe-K α map (this feature is more visible with adjustment of the brightness/contrast of the image, shown in Fig. 6c), but not as clearly as in the NAAR image (Fig. 6a). That the left side of the face of the underlying figure is not readily apparent in any of the XRF maps collected suggests it may be due to an element with limited or reduced fluorescence intensity—either something that is effectively absorbed by an overlying layer, or consisting of lower Z elements. The strong correlation between the Fe-K α and Mn-K α distribution maps suggests Rembrandt used iron earth pigments in the form of umbers (alumina-silicate earth pigments colored by iron oxides Fe 2 O 3 and FeOOH, with umbers also containing MnO 2 ) [24, 25]. Taken together, the complementary information provided by the NAAR and MA-XRF data suggests that this portion of the face of the underlying figure was executed in an iron earth pigment—specifically, an umber—for which the Fe and Mn content is visualized in the MA-XRF maps, and the Si content is visualized in the NAAR image.