Reading pastor by Ferdinand Hodler (private collection)
A sample (which separated into three subsamples during sampling) was collected from an area affected by the degradation phenomena in the lower left edge of the painting illustrated in Figure 2. Details of the analytical findings are given in Table 1.
Synchrotron radiation X-ray tomographic microscopy (SRXTM)
The first subsample (234 × 167 μm2) was studied using SRXTM. The study of paint samples using SRXTM brings information on the three dimensional distribution of phases of interest in an unmanipulated paint sample [1]. The reconstructed data provided a cube of 8-bit grey scale images with a pixel size of 370 nm. The grey level of each phase depends on its absorption and scattering interaction with the X-ray beam and therefore on composition. Segmentation based on grey scale level allows the visualization of the distribution in three dimensions of each phase in the context of the entire sample. This approach has proved extremely useful in the study of reactivity in paintings [2] and has uniquely allowed the study of porosity properties in 19th century ground layers [3]. In the present case we have focused on the three phases of interest: the metal particles, the paint and the green agglomerates. In Figure 5, a light microscopic image of the sample (Figure 5a) and the reconstruction of the distribution of the phases of interest are shown (Figure 5b, c). On the sample surface four distinct green agglomerates can be identified. At the core of the green agglomerates, the metallic pigment particles can be observed. The thin, elongated and folded morphology of the particles suggests these originate from a metallic foil. The fragments have a thickness varying from 1–4 μm. The study of the virtual cross sections (Figure 6a, b) extracted from the tomographic dataset provide further information. A first observation is that the sample is composed of two paint layers of different X-ray linear attenuation coefficient. Above the paint surface and only there, not mixed with the paint, are elongated, flat and highly attenuating particles, as expected from metal foil fragments. These metal fragments are embedded in lower absorbing masses contained in the varnish layer. Ferdinand Hodler applied the metal not embedded in the paint, but mixed within the varnish layer.
Analytical microscopic study of a sample in cross section
A cross section of a second microsample collected from the same location as the one studied by SRXTM was prepared.
LM and SEM–EDX analysis of the cross section surface confirmed that the metallic particles were brass with a copper to zinc ratio of approximately 85:15. Element mapping using SEM–EDX detected chlorine associated with the metal particles in particular but also with the surrounding environment and the paint in general. Detailed backscattered electron images (SEM–BSE) of the brass foil suggested surface pitting of the alloy (Figure 7a). Clear interpretation of the origin of the metal foil surface structure requires higher magnification than the current 10000×. Higher magnification images of the metal surface could not be obtained possibly due to insufficient surface quality as a result of micromesh dry polishing.
The brass of the composition found in the painting by Hodler is described as malleable and ductile [4] but is not the highest quality brass as used as gilding metal which has a higher copper content (95:5 Cu:Zn) [5]. Element analysis using SEM–EDX detected chlorine associated with the metal particles which is compatible with the use of sodium chloride as an aid in the grinding process of the metal foil. The use of salt as grinding agent has been mentioned in literature dealing with Rococo and Baroque techniques [4]. The same source mentions that this method is more suited to the preparation of gold and silver pigment. In the case of copper containing alloys and, in order to avoid corrosion, grinding without salt is recommended, indicating that the corrosion acceleration in the presence of chlorine had been known.
ATR–FTIR–FPA imaging of the cross sections confirms that the green agglomerates are composed of copper and zinc soaps and shows that the distribution of the two types of soaps is not identical. Although the signal was relatively noisy, it could be concluded, based on spectral comparison in different regions, that areas of the soap agglomerate on the upper side of the metal leaf are richer in zinc carboxylates as suggested by the mapping of the 1540 cm−1 (zinc carboxylates) and 1585 cm−1 (copper carboxylate) peaks (Figure 8). Interestingly, an enrichment in lead carboxylates of the lower area of the aggregate can be seen in the 1525 cm−1 peak intensity distribution (Figure 8c). This component originates, most likely, from the reaction of the lipids in the paint/varnish matrix and the lead white in the paint layer [6].
Bulk analysis (FTIR and GCMS)
The single point analysis of the varnish layer in the painting by Ferdinand Hodler in an area where the metal pigment had been applied, showed evidence of the presence of lipids suggesting that the pigment would have been dispersed in oil prior to application.
The green agglomerates were composed of a combination of copper and zinc carboxylates (single point FTIR spectra compared with reference compounds and published spectral library [7]) of saturated fatty acids with a minor content on diacids (GC–MS), indicating that the green mixture of copper/zinc soaps resulted from the reaction between saturated fatty acids of the binding medium the metal pigment was applied with or, from the paint layer below and the copper and zinc of the brass powder originally used by Hodler.
In the case of Reading pastor two factors play a role in the reactivity of the brass pigment. On the one hand the presence of chlorine and on the other the high content in zinc. The presence of highly mobile anions such as chloride are known to accelerate the corrosion process [8, 9]. Brasses with substantial amounts of zinc (above 15%) become destabilized and lose zinc by dezincification [10] with a preferential initial formation of zinc corrosion products. The heterogeneous distribution of zinc and copper soaps in the agglomerates and the structure of the metal foil surface could be suggestive of dezincification of the brass alloy prior to the formation of soaps.
Portrait of a young girl by Filippo Franzoni
This painting (Figure 3) was heavily retouched and had been wax resin relined. The sampling strategy and analysis was limited to the characterization of metallic pigment application and characterization of the agglomerated composition. One sample was collected and a cross-section prepared (Figure 9a). Detailed chemical profiling of the binding medium was not carried out as it would have been severely compromised due to the presence of the lining material.
The analytical results that allow the characterization of the build-up are given in detail in Table 2. In the painting by Franzoni, the metal pigment (characterized as a Cu:Zn 90:10 brass by SEM–EDX) was applied mixed with the beige oil paint of the background. The brass pigment and the reaction phases are well embedded in the paint and not on the surface as observed before in the painting by Ferdinand Hodler. The brass fragments are typically 1–2 μm thick and up to 40 μm long. No chlorine was detected, suggesting a different preparation method for the pigment.
The distribution of zinc and copper soaps is also heterogeneous with increased signal from zinc carboxylates in the outer rim of the agglomerate and from copper carboxylate in the core region (Figure 9d). The presence of zinc in the paint matrix, even if at a very low level, hinders the further meaningful interpretation of the metal soap distribution data. Detailed BSE images of the metal foil suggest progressing pitting corrosion (Figure 7b). The copper content of the brass pigment used by Franzoni was higher than that found in the painting by Ferdinand Hodler but neither are of gilding metal quality (95:5). The technique used by Filippo Franzoni of mixing the brass pigment in oil paint might have promoted the formation of the zinc and copper soaps.