In this part, we describe the composition and microstructure of the inclusions within the orange–yellow glazes, first of Khorsabad and Babylon, then of Aššur that is a more complicated case due to the strong alteration of the glazes and to the coexistence of various yellow hues. The antimonates existing in these glazes are diverse, especially the lead antimonate of pyrochlore structure. Their investigation by EDX analysis and Raman spectroscopy is reported in the last paragraph.
3.2.1. The Orange–Yellow Colour of Khorsabad and Babylon
The microstructures of the orange–yellow glazes of Khorsabad and Babylon are very similar in their main features. A transparent, very slightly honey-coloured glass matrix contains a high number of large inclusions, typically 50 to 100 µm wide, of pigment with a variety of colours in the white–yellow–orange palette. These pigment inclusions all consist of assemblies of finely divided crystals, of which size is the order of 1 µm. They are unevenly distributed in the glass. Their microstructure and irregular distribution strongly suggest an ex situ technique for the fabrication of these glazes, consisting of the ex situ preparation of the pigments and their mixing with glass frit or with a glass-forming mixture. Similar to ancient Egypt and Rome [24
], lead antimonates from the ancient Near East were indeed probably mostly prepared ex situ [14
], although examples of possible in situ crystallization during the firing of the glaze are also reported [12
Khorsabad glazes bear antimonates and hematite (Fe2
) as major and minor pigment inclusions, respectively. It was possible to distinguish five types of colours and the chemistry of the antimonates, which are depicted in Figure 8
. At first, on the right-hand side of Figure 8
b, a big “bunch of grapes” inclusion of Ca-antimonate, white in colour, represents one minor type of antimonate. Then, Figure 8
a–c shows the three major types of antimonates, all are lead-antimonates with various colours and impurities. These types are named according to their optical colour: Pale Yellow (PY), Dark Yellow (DY), and Orange–Red (O–R). At last, Figure 8
d shows the fifth type of antimonate, of the Orange–Brown colour (O–B). Its grey contrast on the BSE image comparatively to the white lead-antimonates is due to the fact that it is not a lead-antimonate but an iron-antimonate.
In the orange–yellow glazes of Babylon (Figure 9
), three different types of lead-antimonates were also observed. They are named PY, DY, and O–R according to their Pale Yellow, Dark Yellow, and Orange–Red optical colour and similar to the above description for Khosabad glazes. DY antimonates are clearly the most numerous in the orange–yellow glazes of Babylon, and PY the least. In addition, the area covered by the optical microscopy image contains a small red hematite inclusion marked with R. Hematite inclusions are present in the Babylon orange–yellow, although not as abundant as in the Khorsabad orange–yellow. This may be related to the macroscopic observation that, concerning the orange–yellow colour, Babylon glazes are slightly more yellow while Khorsabad glazes are slightly more orange. In photographs, the b colour coordinate is systematically higher for Babylon than for Khorsabad. Thus, this colour will be referred to as yellow for Babylon and orange for Khorsabad in the following.
These three types of lead-antimonates have been characterized by EDX analyses and Raman spectroscopy, which confirmed their distinctness and their good correspondence between Khorsabad and Babylon. This will be reported in the last paragraph.
Note that contrary to Khorsabad, we did not observe the presence of Ca-antimonates, neither as cubic crystals nor as needles.
Moreover, the pigment inclusions generally show many signs of chemical reactivity with the glass phase during the firing and cooling of the glaze. This reactivity is out of the scope of this paper, although it promises a wealth of information on the thermochemical conditions of preparation of these materials. Figure 10
shows several typical cases of reactivity in a Khorsabad glaze, with the aim to mention it for further studies. Three zones are highlighted in the area covered by the optical microscopy image:
Zone (1) contains a hematite inclusion surrounded by a lead antimonate inclusion shaped as a croissant. In the magnified BSE image of the lead antimonate crystals, a chemical contrast can be observed with greyer crystals on the side of hematite. From EDX mapping of this area (not shown), this bright/grey contrast is due to lead/calcium substitution within the crystals;
Zone (2) corresponds to a large inclusion of calcium antimonate (grey “bunch of grapes” shaped inclusion, already shown in Figure 8
b) laid next to two smaller inclusions of lead antimonates (white on the BSE image). The magnified BSE image of these Ca-antimonates show that some crystals are crossed from side to side by white Pb-rich lines;
Zone (3) is very complex with iron enrichment in the left-centre of the zone, although no hematite crystal can be discerned. On this left-centre zone, a bow of lead antimonates surrounds long needle-shaped crystals of Ca-antimonates, the morphology of the latter suggesting that they have formed by precipitation and growth. The internal antimonate crystals of the bow are strongly Ca-enriched. On the right-centre zone, four small inclusions of lead antimonate are visible, displaying different shades of yellow on the optical microscopy image. Two inclusions at the bottom are dark yellow and Fe-enriched, while the larger inclusion on the right is pale yellow and Ca-enriched. This latter inclusion even bears white Ca-antimonate crystals on the top of it.
3.2.3. Compositional Analysis of the Lead Antimonate by EDX and Raman
As described in the previous paragraph, three types of lead antimonates have been distinguished according to their colour in optical microscopy images, namely Pale Yellow (PY), Dark Yellow (DY), and Orange–Red (O–R) antimonates. To understand the origin of their colour, these three groups have been distinctly characterized by EDX and Raman. Their cationic ratio (by EDX) and their Raman vibrational spectra allow assigning all three types to the pyrochlore structure of general stoichiometry A2B2O7, with A being mainly Pb2+ and B being Sb5+. It is important to emphasize that this fluorite-related crystalline structure is adopted by many compounds and solid solutions, as it can withstand numerous chemical substitutions. It belongs to the cubic system with space group Fdm. With respect to the fluorite structure, half of the cationic sites are SbO6 octahedra (Sb in 48f position), sharing three edges and forming SbO6 chains along the face diagonals of the pyrochlore cubic cell. The other half of cationic sites are distorted PbO8 polyhedra (Pb in 48f) bearing six equatorial oxygens (O1 in 48f) and two axial oxygens at a larger Pb-O distance (O2 in 8b). Contrary to the fluorite structure, the 8a site is vacant (no oxygen). Every PbO8 polyhedron shares one axial oxygen (O2) with three other PbO8, forming large Pb4O tetrahedra (the 8b position of the O2 has Td symmetry).
The EDX analyses of the lead-antimonates have been averaged according to their type or colour, for Khorsabad, Babylon, and tentatively for Aššur, in Table 2
. For the well-conserved glazes of Khorsabad and Babylon, the contribution of the glass matrix has been estimated by normalizing the average composition of the glass matrix of orange–yellow glazes (described later in this article) to the Si contamination value measured in the antimonates (about 2 at.% of Si). This contribution is necessary to assess the insertion of Ca and Fe in the pyrochlore structure because these two elements are also contained in the glass matrix.
First, it is clear from Table 2
, and the comparison with the glass matrix contribution that Na, Mg, Al, Ca, and Fe elements enter the pyrochlore structure. Among these elements, Ca and Fe enters in a minor but not negligible amount with respect to the main Pb and Sb cations. Mg and Al enter the pyrochlore as minor elements that will be neglected in the following. Concerning Na, its contribution is not negligible, especially in the PY antimonates for which the Na content is close to the Ca content.
To obtain the composition of the pyrochlore formula unit and the type of chemical substitutions, the at.% of the main cations, Pb2+
, have been normalized to a total of four as in A2
. The numbers are given in Table 3
. For Khorsabad, the orange–brown antimonates (O–B) are also included in the table, although they are actually iron-antimonates, with a Fe/Sb ratio close to 1. Table 3
also reports the (Pb + Ca)/(Sb + Fe) ratio and the Pb/Sb ratio in the last two columns. As expected according to the sizes and valency states of the cations, the (Pb + Ca)/(Sb + Fe) ratio is close to one (except for Khorsabad O–R), which indicates that Ca2+
ions substitute for Pb2+
in the pyrochlore structure. Note that the valency state of Fe is assumed to be +3 in the pyrochlore, considering the equilibrium with the +5 valency for Sb. The presence of hematite in the vicinity of the antimonates also points out an oxidized state for these glazes.
The PY inclusions may be a distinct case because of the high content in Na that is close to the Ca one. The formula unit of Babylon PY considering Na in addition to Ca and Fe is given in Table 4
. The formula unit for Khorsabad PY is similar. According to this formula unit composition, the type of cationic site hosting Na+
ion is not obvious, as the (Pb + Ca + Na)/(Sb + Fe) ratio is significantly higher than 1. Na+
ions may possibly enter other available sites in the pyrochlore structure, such as interstitial sites.
The three types, or colours, of the lead antimonates, correspond to different concentration ranges of chemical substitutions. The Pale Yellow (PY) is a lead antimonate particularly enriched in Ca and Na. The Dark Yellow (DY) is slightly poorer in Ca but richer in Fe, with a remarkably homogeneous Fe content (according to the s.d. value) of about 0.32–0.37 per formula unit. The Orange–Red (O–R) is more complicated to define because the content in Ca and the cationic ratio are more diverse, but in all three cases, the Fe content is the highest, from 0.36 to 0.55 per formula unit. As we will see later, the Raman spectra of the O–R antimonates bear the main features of the pyrochlore structure, except for an additional large band at about 630 cm−1
. By optical microscopy, O–R inclusions are always observed in red areas so that the inclusion and its red area produce an orange–red stain (this is well visible in the orange glaze cross-section of Khorsabad in Figure 8
and in the Aššur microscopy image of Figure 13
). Therefore, it is possible that the high Fe content stems from contamination of the analysis by the immediate environment of the antimonate that is red and very likely Fe-enriched. The 630 cm−1
could be a signature of this iron oxide-rich medium.
The Raman spectra are shown in Figure 16
, according to the colour and the corpus. In order to help their interpretation, powders of pyrochlore lead antimonates with Pb2
composition and Pb2
composition have been synthesized in the laboratory and used as references. Their Raman spectra are depicted in Figure 17
The Raman spectrum of the pure Pb2
of this study, in black in Figure 17
, is characterized by two equivalently intense features: a peak at 513 cm−1
and a double peak at 107 cm−1
and 117 cm−1
. Other well-defined features are two peaks at 198 cm−1
and 226 cm−1
, then low-intensity peaks at 300 cm−1
, 350 cm−1
, 400 cm−1,
and 423 cm−1
. The intense peak at 513 cm−1
bears a low-frequency shoulder at 475 cm−1
. In the Fe-substituted Pb2
compound, the intensity of the 513 cm−1
peak has considerably decreased with respect to the low-frequency intense peak, which has shifted to 140 cm−1
and has become unique (no doublet). The 200–480 cm−1
range has been globally modified and contains a large main peak at 338 cm−1
, with shoulders at 305 and 393 cm−1
, and another large, low-intensity peak at 455 cm−1
. Rosi et al. have published an analysis of the Raman spectra of Pb2
as pure form with different Pb/Sb ratio and of Pb2
with Sn and Zn substitution for Sb [27
]. The spectrum of our Pb2
synthesized reference is very similar to their spectra for pure Pb2
. In particular, the peak at 513 cm−1
, which is attributed to the symmetric stretching of SbO6
octahedra, tends to be more intense than the double peak at 107–120 cm−1
, attributed to the stretching of the Pb4
O tetrahedron. Substituting Sb5+
yields spectra very similar to the spectrum of our Pb2
reference, with the collapse of the 513 cm−1
peak, probably due to the disruption of the SbO6
network, and the high-frequency shift of the 107–120 cm−1
peak towards 140 cm−1
. This similarity of the Raman spectra of Pb-Sb-Zn and Pb-Sb-Fe antimonates has also been emphasized in the study of Cartechini et al. [39
The Raman spectra of the lead antimonates in the orange–yellow glazes of Khorsabad, Aššur, and Babylon all correspond to the modified pyrochlore structure bearing Fe3+
, and their grouping per colour puts in evidence common spectral alterations according to the colour. Among the three types of lead antimonates, the Raman spectra of the dark yellow (DY) are very close to the spectrum of the Pb2
reference. This is consistent with the composition of the DY antimonate that is enriched in Fe substituting Sb, with 0.32 to 0.37 of Fe per formula unit. In comparison with this now well-identified spectrum, the two other spectra, for pale yellow and orange–red colours, show characteristic differences. For the PY of Khorsabad and Aššur, the 513 cm−1
peak has a higher intensity with respect to the peak at 140 cm−1
, and this latter peak is divided into two peaks at about 127 and 140 cm−1
. Moreover, the intensities of the small peaks at 206 and 393 cm−1
are slightly higher. According to the EDX results, these spectral modifications can be associated with the enrichment of Ca substituting Pb in the pyrochlore structure. In the BSE image of zone (1) in Figure 10
, the white to grey contrast corresponds to a range of enrichment in Ca. By carrying out a Raman mapping of this zone, we have noticed that the more Ca in the pyrochlore, the more intense the 127 cm−1
contributions to the first peak (Raman and EDX mapping of zone 1, not shown). Note that for Babylon, as described before, PY antimonates appeared much less numerous than the O–R and DY ones in the yellow glazes from Babylon, and it was difficult to distinguish them optically. Therefore, a series of Raman spectra for DY/PY crystals are presented together (bottom of Figure 16
b). These spectra remarkably show the same intensity variations as for Khorsabad DY/PY: when the intensity of the 513 cm−1
peak is high, that of the 127 cm−1
shoulders and 393 cm−1
peak are also high (the tendency is less clear for the 206 cm−1
peak). For Aššur, the two spectra for DY and PY could be distinctly obtained, as shown in Figure 16
On the other hand, the Raman spectrum of the O–R colour bears similar modifications as the PY colour mentioned for Khorsabad and Aššur (more intense 513 cm−1
peak and the 140 cm−1
peak becomes a doublet); however, the 206 and 393 cm−1
peaks are generally weak, and the most characteristic feature is a wide peak at about 615–630 cm−1
. This type of lead antimonate is particularly enriched in iron, leading us to the suggestion that this feature may be a signature of some Fe-oxide-rich surrounding of the antimonate. A more detailed two-dimensional analysis with high spatial resolution, such as µ-XRF [24
], would be necessary to answer. In these O–R spectra, the double peak at 127–140 cm−1
and the intensity of the 206 cm−1
peaks are variable, which may be consistent with the variability of Ca content observed by EDX. The same O–R spectrum with the 630 cm−1
band has been encountered in all the Italian Renaissance Majoliques plates and tiles of the study of Rosi et al. [28
]. Among the various pigments used by these authors as internal references, this spectrum is close, but not identical, to the spectrum of the pigment PV-NY-34 synthesized with Pb3
and Pb:Sb = 3:2, without any flux. This pigment was heterogeneous, and its XRD pattern could not be clearly attributed to any Pb-Sb compound of the PDF database. In our case, the stoichiometry of this phase as obtained by EDX is close to that of the pyrochlore, and an association with an excess of Fe is certain.
Raman spectra of the other antimonate compounds found in the orange glazes of Khorsabad are depicted in Figure 18
. In Figure 18
a, the red spectrum has been measured on the big isotropic calcium antimonate crystals of the “bunch of grapes” group in zone (2) of Figure 10
. The blue spectrum has been taken on needle-shaped Ca-antimonate crystals as such present at the bottom of the bunch of grapes in zone (2) or in the middle of the Pb-antimonate circle in zone (3) of Figure 10
. Both spectra relate to the same CaSb2
phase, characterized by the sharp peak at 671 cm−1
. At last, Figure 18
b shows various spectra obtained for Fe-antimonate crystals in two different bricks. Fe-antimonates are not abundant in these glazes, and we did not study them closely. For the one group examined by SEM-EDX, the contrast on BSE images was homogeneous, and the Fe:Sb EDX ratio was very close to 1:1. However, the Raman spectra are diverse, with different positions for the most intense peaks, so there must be some chemical and structural heterogeneity within this composition.