Capital ÆIN 1045
The VIL images show quite convincing patterns of photoluminescence emission consistent with the presence of Egyptian blue (Fig. 1b). The blue-painted areas observed on the base as well as the areas between the palm leaves at the top of the capital display very bright, coherent luminescence indicating a high concentration of Egyptian blue. Slightly less intense luminescence is observed on the palm leaves in the form of veins radiating from the midribs. The slight reduction in intensity suggests a smaller, yet considerable amount of Egyptian blue. This is in accordance with pigment mixtures containing a high concentration of the said blue pigment forming hues such as the dark, bluish-green colour used for the veins on the palm leaves.
Optical microscopic examination of the cross-sections attests to the presence of two decoration schemes on the palm leaves as well as the base of the capital. Both schemes comprise a white preparatory layer sequenced by one or more paint layers or gold leaf. Invariably, the second decoration scheme echoes the colours of the first (Fig. 3) suggesting that the capital was repainted in order to freshen up its original expression rather than to alter it. Furthermore, a greyish, waxy coating is observed covering a substantial part of the painted surface. According to the tentative pigment identification performed as part of the FTIR analysis in the binding media investigation, the green paint layer would appear to be based on atacamite [1].
ÆIN 1048
VIL imaging strongly indicates the use of Egyptian blue in various concentrations (Fig. 2e). The blue-painted areas comprising the vases and their stand as well as the half loaf of bread all display very bright, coherent luminescence in the images attesting to a high concentration of Egyptian blue. The green-painted areas on the falcon below show a distinct, yet considerably less intense luminescence suggesting that the green paint layer contains some Egyptian blue. In addition, luminescing particles form part of the white background colour as well as other depicted details including the green feather and the animal skin. In these cases, however, the concentration is quite low.
The results of the µ-XRF analyses also indicate unequivocally that the blue colour consists of Egyptian blue. As is shown in Fig. 4, Cu, Si, and Ca are all present in high concentrations in the blue-coloured areas, and no Cl is present (Fig. 4d). The green paint layer on the depicted feather contains high Cu and Cl concentrations (Fig. 5a, c) which points to the presence of atacamite (Cu2Cl(OH)3). Slight amounts of Mn seem to follow the distribution of Cu and Cl (Fig. 5d). The concentration of Si in the green paint layer analysed (Fig. 5b) is too low to determine whether a minute amount of Egyptian blue (CaCuSi4O10) forms part of the painted decoration in this area, as suggested by VIL imaging. The distribution of Ca and S shown in Fig. 5f, g indicates that the white layer underneath the green paint contains gypsum.
ÆIN 1058
The VIL images show luminescent particles forming part of almost the entire polychromy (Fig. 2f). On closer inspection, however, the distribution of the luminescent particles is evidently consistent with the white layer serving as the background colour as well as a preparatory ground layer for the rest of the painted decoration. Only the black-painted areas appear completely dark, attesting to the absence of Egyptian blue. The yellow, red, and green nuances all exhibit glowing particles whose exact relation to the painted decoration is uncertain in that they may form part of the paint layers or/and of the white preparatory layer underneath.
Figure 6 shows an optical image of the area which has been analysed with µ-XRF scanning. The same area has been mapped with µ-XRF depicted in Fig. 7. In the green areas, the µ-XRF elemental distribution graphs mainly reveal the presence of Cu and Cl (Fig. 7a, b). Based on the VIL images and the very low amounts of Ca and Si detected, Egyptian blue is unlikely to be present in the green-painted areas (Fig. 7c, d). Like the case of ÆIN 1048, the high amount of Cu and the presence of Cl is pointing to the presence of atacamite. The yellow paint layer in the scanned area shown in Fig. 6 is characterised by high amounts of As (Fig. 7e), which is suggestive of orpiment (As2S3), as well as Pb which indicates the presence of one or more lead-based components such as lead–antimonate (Pb2Sb2O7) and lead–tin yellow (Pb2SnO4).
The µ-XRF scanning also showed high concentrations of iron delineating the cracks and dents in the paint layers (Fig. 7f) pointing towards the presence of minute particles of soil deposited in the cracks. A point analysis in a white area showed almost pure calcium carbonate with traces of gypsum.
ÆIN 1059
The VIL image shows bright, coherent luminescence in the blue areas of the painted decoration consistent with a high concentration of Egyptian blue (Fig. 2g). In addition, luminescing particles are observed forming part of the white background colour. The concentration and distribution indicate a very small amount of Egyptian blue, presumably added for its cool, tinting effect. A similar pattern of luminescence is seen in the yellow-painted area. It is unclear whether the photoluminescent particles form part of the yellow paint or the white layer underneath it.
The presence of Egyptian blue in the blue-painted areas is confirmed by the high concentrations of Cu, Si, and Ca detected with µ-XRF point analysis and scans. A green-painted area has been analysed using the same approach (Fig. 8). The results of the calibrated semi-quantitative concentrations are listed in Table 1. Besides the elements listed in the table, the Sn L-alpha line clearly visible at 3.44 keV shows unequivocally that Sn is present in the green pigment. Chlorine is also present in Fig. 8. However, Cl cannot be quantified using the NIST-610 standard. The presence of Cu and Cl and the absence of Si are pointing to the use of atacamite. The detection of high amounts of Ca is likely caused by the X-ray beam going through the thin green paint layer and reaching the underlying Ca-rich preparation layer.
Table 1 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis of the green pigment in ÆIN 1059
A section of ÆIN 1059 comprising part of the yellow feather and the background was scanned with µ-XRF (Fig. 9a). The pigment of the yellow paint contains large amounts of Pb and As. No S was seen in the white background (not shown in Fig. 9), and the white background is high in Ca and Sr (Fig. 9b, c), which makes it likely to be calcite. The red line is high in Fe (Fig. 9d) which makes it likely that the pigment is haematite (Fe2O3). It is interesting to note that the area of high Ca (Fig. 9b) ends further towards the lower part of the field of view than the area of high Sr (Fig. 9c) so that a high concentration of Sr is also detected where it is covered by the red line and some part of the yellow paint. A likely explanation is that the K-alpha-line of Sr is so high in energy (14.2 keV) that the photons escape even when covered by the red paint layer and some of the yellow paint layer, whereas the K-alpha-line of Ca has a relatively low energy (3.7 keV), which causes an almost complete absorption of the Ca K-alpha photons on the way out from underneath the red and yellow paint. The fact that the yellow paint layer is only penetrable near the outline, indicates that the paint has been applied in a thicker layer in the rest of the yellow-painted area.
A single point analysis of the yellow pigments is shown in Fig. 10, and the results of the semi-quantitative concentrations are listed in Table 2. The main constituents are Pb and As. The L-alpha line of Pb (10.551 keV) and the K-alpha line of As (10.543 keV) overlap almost completely, but the L-beta line of Pb (12.61 keV) and the K-beta line of As (11.73 keV) are clearly separated and distinguishable in the spectrum, and it is therefore clear that both Pb and As are present in the yellow pigment. Calcium and Sr are probably from the underlying ground layer. Even though there is a large Ca-signal (K-alpha, 4.0 keV) and K-signal (K-alpha, 3.6 keV) the Sn L-alpha line at 3.4 keV is visible as a shoulder. The Sn K-alpha lines at 25.0 and 25.3 keV are also visible (although not depicted in Fig. 10). So, Sn is indeed identified by energy dispersive X-ray spectrum in appreciable amounts in the yellow paint. The K-lines for Sb is at 26.1 and 26.4 keV are visible in the spectrum, but the L-alpha line of Sb situated at 3.6 keV are swamped by the huge Ca system starting with K-alpha 1 at 3.69 keV and extending to the K-beta line. From the µ-XRF analyses it is therefore only possible to say that there is a small amount of Sb present based on the K-alpha line.
Table 2 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis (A3, ÆIN 1059) of the yellow paint
ÆIN 1060
Apart from the thin black outlining, VIL imaging reveals varying concentrations of luminescent particles on the entire surface (Fig. 2h). The luminescence patterns observed are consistent with limited amounts of Egyptian blue. The highest concentration is observed on the white shaft, the second highest on the white background and the yellow paint layers. The lowest concentration is found on the green area and the two widest black lines delineating the shaft. It is possible that Egyptian blue only forms part of the white nuances and that the luminescent particles observed on the other thinly applied paint layers stem from the white layer underneath. Considering the absence of luminescent particles on the finer lines delineating the motif, it would seem that the black paint used is not penetrable by VIL from underlying layers. It is probable that the particles observed on the black areas are to be found on top of rather than within the paint layers. Particles of Egyptian blue are known to travel to neighbouring areas, cf. the luminescent particles found on the broken surfaces of the painted relief.
The green paint was analysed by µ-XRF, the spectrum is shown in Fig. 11. The semi-quantitative results are listed in Table 3. A high concentration of Cu with some Cl has been detected in the green paint layer, making it likely that the pigment is atacamite. The low concentration of Si renders chrysocolla, and cupro-wollastonite improbable matches. The concentrations of Ca, Sr and small amounts of S are probably ascribable to the presence of calcite and gypsum in the white layer underneath. Manganese, Fe, Zn, and As are also detected in the green-painted area.
Table 3 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis ÆIN 1060 of the green paint
A sample of the green layer in ÆIN 1060 exhibited a FTIR spectrum with the typical bands of calcium oxalate (CaC2O4) (Fig. 12). The bands are located at 1620 cm−1, 1318 cm−1, 1092 cm−1, and 780 cm−1 and can be attributed to asymmetric C=O stretch band, symmetric C=O stretch band, asymmetric C–O stretch band, and O–C=O stretch band in calcium oxalate, respectively. The other bands detected in this spectrum fit well with data published for atacamite (Cu2Cl(OH)3) [56]. The CuCl and CuO vibrations were detected at 415 cm−1, 448 cm−1, 511 cm−1, and 598 cm−1. Deformation bands of hydroxyl are identified at 870 cm−1, 913 cm−1, and 997 cm−1 as well as two hydroxyl stretching bands at 3316 cm−1 and 3440 cm−1. The results are in agreement with the µ-XRF measurements and the FTIR results published for the sample ÆIN 1045 from the Palace of Apries [1].
Some very small intensity peaks that could belong to malachite (Cu2CO3(OH)2) can be seen in the spectrogram at 1490 cm−1, 1384 cm−1, 1043 cm−1, and 570 cm−1 (Fig. 12). However, other expected malachite peaks of strong and medium intensities are not seen in the spectrum, namely those located at 804 cm−1, 821 cm−1, 750 cm−1, and 523 cm−1 [56, 57]. It is therefore most likely that malachite is not present in ÆIN 1060.
The yellow paint layer has also been analysed with µ-XRF (Fig. 13) and the semi-quantitative results are listed in Table 4. The concentrations of As and S indicate that the yellow pigment is orpiment.
Table 4 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis ÆIN 1060 of the yellow paint
A micrograph of the white paint at the tip of the shaft is shown in Fig. 14 along with the energy dispersive spectrum of a single point analysis (Table 5). The paint layer is clearly white. However, the Ca determination is far from the expected 40 wt% (corresponding to 100% calcium carbonate)—only 19.4 wt% (see Table 6). There are also appreciable amounts of As and S present pointing towards the presence of some orpiment (Table 5). A likely interpretation is that the white paint, which cannot be securely identified by µ-XRF, contains some orpiment. There is nothing arguing against the white paint being mainly calcium carbonate with a minor amount of gypsum or anhydrite, but the issue cannot be resolved by µ-XRF alone, wherefore a small sample was procured for FTIR.
Table 5 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis ÆIN 1060 of the white paint
Table 6 DCCR calibrated results of the energy dispersive X-ray spectrum of a point analysis ÆIN 1060 of the white paint
The FTIR spectrum of this sample from the white layer in ÆIN 1060 exhibited three bands belonging to calcium carbonate (CaCO3) (Fig. 15). The bands located at 1420 cm−1, 873 cm−1 and 713 cm−1 can be assigned to the v3-asymmetric CO3 stretching band, the v2-asymmetric CO3 deformation band and the v1-symmetric CO3 deformation band, respectively. Their location suggests a micritic texture of the calcium carbonate more than a sparitic one [58]. The bands located at 600 cm−1, 1000 cm−1, 1150 cm−1 and the wide one centred around 3378 cm−1 could be due to the presence of gypsum. However, the shapes of the bands are not entirely typical for gypsum since the one at 600 cm−1 is quite sharp, while it usually forms a doublet or a triplet [59]. Bands from SO42− and H2O vibrations, which are usually seen in FTIR spectra of gypsum, are missing here. The identification of gypsum in the white layer can therefore only be tentative. The presence of an organic compound is seen with two bands at 2919 cm−1 and 2850 cm−1, both attributed to the C–H stretching vibrations and a weak band detected at 1740 cm−1, which could belong to carbonyl groups. The broad band around 3400 cm−1 could be attributed to the NH-group. Taking the relatively low intensity of these bands into consideration, the identification of the organic compound(s) is uncertain. However, these observations are not contradicting the conclusions drawn from GC–MS-analysis performed on this sample, which points to the use of a binder mixed of plant gum and proteinaceous material [1].
Further analyses of the yellow paint layers
It would be interesting to know to which extent the yellow paint layers contain Sb or Sn, as this would indicate the possible presence of lead–antimonate yellow or lead–tin yellow. To resolve this, a sample of the paint layer from ÆIN 1059 was analysed using LA-ICP-MS. An optical image from the microscope is shown in Fig. 16a. The opposite side of this cut is shown in Fig. 16b, now seen through the optics of the laser ablator, after the laser ablation analysis was performed.
Very small count rates were seen for the isotopes Na23, Mg24, Al27, Si29, Ba137, and Au197, indicating that the concentrations of these elements were very low indeed. The detection limit of the ICP-MS is generally at the ppb to ppt level. Consequently, it is unlikely that Na, Mg, Al, Si, Ba and Au are present in concentrations over the ppb-level. The isotopes present in appreciable amounts are plotted in Fig. 17. Four groups of co-varying elements are discernible. The first group of four elements, at the top in Fig. 17 marked in red, Pb, Sb, Cu, and Ag are present throughout the yellow paint layer with the highest concentrations in the lower part of the yellow paint layer. The next group consisting of Sn and As (curves marked in orange) features lower concentrations in the upper part and higher concentrations in the lower part of the yellow paint layer. Consequently, the first and the second groups are mainly distinguishable by Sn and As occurring in somewhat lower concentrations in the upper part of the yellow layer. The third group, Fe (marked in brown), is also present in the yellow paint layer, but with its highest concentration in the outer layer indicating that it is present in a separate mineral and has been applied last. Finally, there is the group consisting of Ca and Sr (marked in blue). They occur mostly in the white ground layer.
The LA-ICP-MS data shows the presence of both Pb and As in the yellow pigment in accordance with the µ-XRF data. The LA-ICP-MS data also irrefutably show the presence of both Sn and Sb in substantial amounts. In the yellow paint layer, the isotope Sn118 has an average count rate of 14,300 counts s−1 (Sn118 has a natural abundance of 24.2%), while Sb121 has a count rate of 380,000 counts s−1 (this isotope has a natural abundance of 57.4%). Judging from the low count rates it is likely that Ag (645 counts s−1) and Cu (567 counts s−1) are only present in trace amounts. As described in the methodology section, the LA-ICP-MS measurements were calibrated to yield semi-quantitative concentrations (Table 7). The semi-quantitative calibration was done using the count rate for Ca in the white ground layer and assuming that the concentration of Ca here was 40 wt%, consistent with almost pure calcite. Using this assumption, we arrive at a concentration of S and Sr in the white ground layer of 0.8 and 1.6 wt% respectively. This indicates that the calibration is fairly robust; in particular it is clear that the amount of gypsum is relatively low in the ground layer, c. 4.1 wt%, or even less if other sulphur-bearing minerals are present.
Table 7 Semi-quantitative concentrations of the average composition of the yellow paint layer in ÆIN 1059
To ascertain if these minerals are actually present in the yellow pigment and if they are the only ones, a small sample was extracted from the yellow-painted area on of ÆIN 1060 (KLR-12084) and analysed using µ-XRPD. Due to the thinness of the layers it was impossible to procure a sample of pure yellow pigment; the sample also contained visible fragments of the preparation layer and translucent wax together with yellow pigment.
The sample was crushed in an agate mortar and poured into a Pyrex capillary sample holder. The diffraction pattern (Fig. 18) showed the presence of two calcium related phases. Calcite was identified with many peaks fitting with the experimental pattern (PDF card 01-072-1652). Among them, the three peaks with the highest intensity were seen at 29.45° (2Θ), 48.58° (2Θ), and 39.42° (2Θ), which correspond to the [104], [116], and [113] peaks respectively. The gypsum (CaSO4·2H2O) pattern was also thoroughly consistent with the experimental diffractogram (PDF card 18-01-2012). The four peaks with the highest intensities are located at 20.72° (2Θ), 11.63° (2Θ), 29.11° (2Θ) and 31.16° (2Θ). They correspond to the [− 121], [020], [− 141] and [121] peaks respectively. The low intensity of the peaks belonging to the gypsum phase confirm the observation made in the LA-ICP-MS analysis on ÆIN 1059 about the small quantity of this mineral in that sample. A small displacement (< 0.20°) for the second and the third peaks of the gypsum can be seen. Reflections from wax were also present in the diffractogram, which agrees with the previous finding in the analyses of the binders [1]. The two main peaks were located at 21.48° (2Θ) and 23.84° (2Θ). Their high intensity reflects the relatively large amount of wax in the sample analysed.
According to the LA-ICP-MS results in ÆIN 1059, the yellow pigments could theoretically, i.e. stoichiometrically, be related to three specific phases in the sample: orpiment, lead–antimonate oxide and lead–tin oxide. The identification of orpiment (PDF card 01-071-2435) in the µ-XRPD pattern of ÆIN 1060 is mainly based on the presence of the [020] peak with the highest intensity located at 18.51° (2Θ). This peak is clearly visible despite the low intensity and it is not overlapped by any other phase. The peak with the second highest intensity is located at 32.99° (2Θ) and corresponds to the [311] plane. This peak can be seen in the experimental pattern accepting a small shift (≈ 0.3°), however, the intensity is very low. The third and fourth peaks with higher intensities, if present in the diffractogram, are overlapped by gypsum and wax peaks. None of the other peaks with a lower intensity can be seen. This lack of smaller peaks and the slight shift for the second peak probably reflects a small number of orpiment grains in the sample.
Lead and antimony were found under different forms in the µ-XRPD pattern of ÆIN 1060. The first oxide identified was the bindheimite (Pb2Sb2O6), which is the natural mineral form of lead–antimony oxide (PDF card 00-018-0687). However, this identification is mainly based on the peak with the highest intensity located at 29.71° (2Θ) originating from the [222] plane. Despite being close to a calcite peak [104], this peak is clearly visible even if the intensity is relatively low. It should be noted that there is also a very low intensity wax peak at 29.95° (2Θ). The other wax peaks with very low intensities are not visible in the pattern, thus this peak is very likely linked to the lead–antimony oxide phase. The peak with the second highest intensity is located at 49.50° (2Θ) with a [440] plane. This peak was also identified in the experimental pattern, but its intensity is very low. This can be due to the difference between the relative intensities of the first and the second peak in the theoretical pattern of bindheimite (the I.R. of [440] peak decreases to 30%). None of the other peaks with a lower intensity from this phase can be seen in the diffractogram. Another lead–antimony-based phase can tentatively be identified in the pattern as an iron–lead–antimony sulphide (Pb4FeSb6S13) (PDF card 00-041-1401). This phase is identified as parajamesonite in the PDF database. However, Papp et al. [60] have discredited this mineral identification by proving that the reference samples were mixtures of jamesonite and other mineral phases. Thus, this phase will only be named “iron–lead–antimony sulphate” according to its chemical composition. The two peaks with the highest intensity are located at 21.07° (2Θ) and 23.49° (2Θ). They are overlapped by the wax peaks. The third highest intensity peak is located at 19.09° (2Θ). This peak can be seen in the pattern and is not overlapped by any other phase. The fourth highest intensity peak is located at 40.38° (2Θ). The intensity of this line might be increased by the low intensity gypsum [− 152] peak which is close. For this phase, the Miller index was not specified on the PDF card.
The lead–tin oxide was the third compound suggested by the LA-ICP-MS results of ÆIN 1059. However, a better match can be found in the µ-XRPD pattern of ÆIN 1060 with a lead–tin-antimony oxide phase (PDF card 00-039-0928). The first peak with a [222] plane is located at 29.22° (2Θ) but it is overlapped by the calcite [104] peak. The peaks with the second and the third highest intensity are also overlapped by the calcite [116] and calcite [122] peaks, respectively. The peaks with lower intensities cannot be seen in the XRD pattern. The presence of a goethite phase (COD card 96-900-2156) cannot be identified in the XRD pattern. The first peak with the highest intensity with a [101] plane is located at 21.35° (2Θ) but it would be overlapped by a wax peak. The other peaks with a lower intensity cannot be seen in the diffractogram.
In the µ-XRPD pattern, wax, calcite, and gypsum are the most abundant and visible phases. Their signals somewhat conceal relevant parts of the other mineralogical phases, the presence of which was suggested by the LA-ICP-MS results on ÆIN 1059. Despite this drawback, it is possible to confirm the use of orpiment in the yellow pigment in ÆIN 1060. The use of Pb–Sb related minerals was also confirmed, but more precise mineralogical identification cannot be achieved by way of the µ-pXRD in this small sample. It seems possible that these phases are the result of a mixture of different compounds. The presence of sulphide and oxide could suggest that the Pb–Sb oxide is, at least partially, the oxidation product of Pb–Sb sulphide, or alternatively originates from the orpiment. The Sn and Fe related phases cannot be confirmed by µ-XRPD, maybe because of overlap. However, it seems very likely that these elements would have been embedded in the Pb–Sb oxide. Iron oxides usually occur along with other terrigenous minerals, such as clay, quartz or micas, and none of these are seen in the diffractogram. This particularity, together with the likely possibility of a mixture between different Pb–Sb–Sn bearing phases support the idea, that the preparation steps for the yellow pigment were very thorough (e.g. burning, sieving, water-washing) in order for clay and quartz to be completely removed from the pigment.
From a methodological perspective, it should be noted that a lack of accuracy in the peak positions and missing peaks are well-known problems when using µ-XRPD. This can be caused by at least four possible errors: small amounts of single crystals in the sample; the thinness of the cross section analysed; the particle statistic (smaller number of larger grains) and a possibly inhomogeneous distribution of grains [61].
Therefore, taking the results in their entirety, the most likely yellow pigment containing Sb is lead–antimonate yellow (Pb2Sb2O7), and the only viable yellow pigment containing Sn is lead–tin yellow (Pb2SnO4). These two pigments account for the presence of Pb, Sb, and Sn, and probably small amounts of Fe and S also. It must be stated, that although these two identifications are not only possible, but also likely candidates, there is still room for other possible mineral identifications. The third mineral containing As is orpiment. In Table 7 are listed calculations of the amounts of the three mineral phases, made on the assumption that all Sn is present as lead–tin yellow, all Sb as lead antimonate yellow, and all As as orpiment. Thus, the results of the LA-ICP-MS on the yellow paint layer point to a mixture of lead–tin yellow (Pb2SnO4, c. 9.9 wt%), lead–antimonate yellow (Pb2Sb2O7, c. 1.4 wt%) with traces of Ag and Cu, and orpiment (As2S3, c. 0.95 wt%).These concentration calculations are consistent with most elements, but leaves c. 10 wt% of Pb unaccounted for. This extra Pb could be speculated to be present as degradation products of lead–tin yellow and lead–antimonate yellow.