3.1. Imaging Techniques
The imaging techniques employed provided information about the history of the painting from a conservation point of view and the artistic technique. Starting with the simplest one, the raking light showed a uniform craquelure through all the painted surface. Focusing on Judith’s face (
Figure 3), in the raking light photography (
b), the craquelure seemed to follow the border of the left side of the face.
In
Figure 4, the UV fluorescence photography and the X-radiography are shown. In the first image (
a), the organic materials are highlighted. Old varnish looks as a green opaque layer on the painted surface. It is evident from this picture that it was not uniform, but in some areas, it was absorbed by the underlying material, e.g., in the red dress of Judith, the darker areas were still covered by the varnish, while in the lighter ones, it seemed to be absorbed.
The peculiarity of X-radiography (
Figure 4b) is to give evidence of the presence of radiopaque pigments with high Z elements. These materials are present in all the lighter hues, in which it is possible to delineate single brushstrokes, the pictorial layer being thick.
Focusing on the detail of Judith’s face (
Figure 3), the information obtained with the two imaging techniques was complementary. In
Figure 3c, the UV light underlines the presence of darker purple areas, due to non-fluorescent materials. These materials are localized only on Judith’s face and refer to modern pigments due to restoration.
In
Figure 3d, the radiography showed the missing of the original radiopaque materials almost in the same areas of the restoration, highlighted by the UV fluorescence, as in the neck of Judith, due to the use of modern and light Z element pigments. Furthermore, in other areas of the face, the original materials were still present under the restoration ones. Focusing on the left side of the face, it is possible to recognize that the border of the face was shifted to the right, and the surface seemed to be scratched. This area corresponds to the craquelure, as seen in raking light detail (
Figure 3b).
The images obtained with UV fluorescence and X-radiography guided the choice of XRF points, recognizing the restored areas from the original parts.
3.2. Spot Analysis
The X-ray fluorescence analysis was performed on 74 selected points (
Figure 2), considering the colors and the presence of retouches due to restoration. In
Table 1, a summary of the results obtained for each color is reported.
The first observation is that for each color, the first element revealed was lead. Furthermore, calcium and iron were present in every point analyzed. These elements could indicate that the ground layer was composed of gypsum, lead white, and an earth, as was typical for a 17th Century painting.
Copper was also present in every spectrum, mainly as a trace element (except for the green areas). This could indicate the presence of a copper-based pigment or an earth rich in copper in the ground layer [
5]. For the white areas, the counts per second (cps) of lead were the highest, confirming the use of lead white (2PbCO
Pb(OH)
) in the artist’s palette [
6,
7].
In the yellow areas, the presence of lead, tin, and antimony could indicate the use of a specific yellow pigment (PbSnSbO
), the lead-tin-antimony yellow, used since the 17th Century [
8]. The determination of this pigment can be carried out by verifying the correlation between tin and antimony [
7]. Lead was not considered because it was present also in the preparation and as a white pigment, so the rate of cps was influenced [
7]. The absence of this correlation indicated that for the yellows, two pigments were most likely used: Naples yellow (Pb
(SbO
)
) and lead-tin yellow Type I (Pb
SnO
), due to the absence of Si, which characterizes lead-tin yellow Type II (Pb(Sn,Si)O
) [
6].
However, other yellow pigments could be used or mixed, such as yellow ocher (Fe) and organic yellows.
Concerning the blue, the presence of copper was not enough to hypothesize the use of a copper-based pigment. Despite this, the presence of silicon, together with potassium and other trace elements, suggested the use of lapis lazuli (3Na
O·3Al
O
6SiO
2NaS with CaCO
and FeS
) [
9]. An iron blue pigment was excluded due to the strong correlation between iron and manganese, suggesting the use of an umber earth [
7] to darken the color, as will be explained later.
In the green areas, instead, copper was one of the main elements, suggesting that the use of a copper-based pigment was the more plausible hypothesis. To identify clearly which one of the various copper green pigments available in the 17th Century (as malachite, verdigris, and copper resinate) [
9] was used, molecular spectroscopy techniques should be employed.
The main element present in red areas of the painting was mercury, so there was a clear indication for the use of cinnabar (HgS), the only ancient pigment composed of this element [
6]. However, the presence of lead could refer both to lead white, from the preparation layer or employed to lighten the hue, and to minium (Pb
O
) or another lead-based pigment, probably mixed with or underlying cinnabar. For the red as well, a good correlation between iron and manganese suggested the use of an umber earth for the darker areas.
Browns are characterized by iron and manganese between the main elements. As reported in the literature [
5,
7,
8,
10], red and yellow earth pigments are mainly colored by iron oxides and/or hydroxides and often contain other minerals such as quartz, clay, etc. Manganese oxides or hydroxides are present in brown earths and umbers. The ratio between iron and manganese determines the hue of the color.
We looked for the correlation between Fe and Mn counts in order to obtain an indication about the use of umber pigments. In
Figure 5, we report all the points in which Fe and Mn were revealed, where at least three different and statistically-significant (
) correlations were shown, pointing out three umber hues.
In Judith’s hair, the presence of mercury indicated that the umber was mixed with cinnabar to obtain a more reddish brown.
Considering the darker brown of the background, the detection of phosphorus could be related to the use of bone black, the carbon-based black obtained through the calcination of animal bones, thus containing calcium phosphate [
11]. The XRF technique working in air was blind to all the elements with Z lower than silicon, so carbon could not be detected [
7]. However, the presence of P revealed only for this dark hue was a clue for the use of bone black.
To characterize fully the presence of this carbon-based pigment, IR reflectance spectroscopy (a noninvasive and in situ technique) was applied with good results [
11].
The most interesting data concerned the flesh tones of the three characters, focusing on Judith’s head, which is the most debated area of the painting. For all of them, lead, mercury, iron, and manganese were detected, suggesting the mixture of lead white and potentially minium, cinnabar, and earths, including umber for the correlation of iron and manganese. The use of minium must be confirmed with other techniques, as XRF cannot discriminate between the lead pigments.
For Holofernes’s head, copper was revealed as well. In this case, the artist probably used a copper green to obtain a deathly pale skin.
Focusing on Judith’s head, the artistic palette seemed to be the same as the rest of the painting and, for this reason, we can infer the period of realization to be probably the same.
Thanks to UV fluorescence photography and X-radiography, it was possible to analyze the restoration materials. The analyzed points concerned Judith’s skin and parts of the restoration materials overlapping the original ones; hence, it was not surprising to find lead, iron, and the other elements already discussed. What is relevant, instead, was the detection of zinc and titanium as the main elements for all the restored areas, which could refer to zinc white (ZnO) and titanium white (TiO
), two modern pigments used since the second quarter of the the 19th Century [
12] and 1916, respectively [
13].
Furthermore, the restoration on the red lips of Judith revealed cadmium and selenium, possibly suggesting the use of cadmium red (Cd(S,Se)), another modern pigment employed since 1917 [
12].
The detection of these three modern pigments, zinc white, titanium white, and cadmium red, indicated that the restoration was performed during the 20th Century.
To conclude, a preliminary interpretation about the identification of the artistic materials was explained, even if there was no information about the use of organic materials, such as red lacquers and other organic pigments. The use of other techniques would help to characterize these materials and to identify clearly the presence of other pigments, such as minium and copper green pigments.