are formed as several superimposed layers (with an overall thickness between 0.1 to 0.5 mm). While the intact stone has a color similar to white marble, the crusts color range between dark orange to bright ochres (Figure 5). The SEMEDX analyses showed that these encrustations are composed of Ca, Si, Al, with minor variations.
It was seen in the maps (Figure 2) that a kind of dark coloration is developing as small to medium size patches on the reliefs, especially on the previously encrusted surfaces. This feature is present as a very thin layer, rich in Sulphur, which is formed on the external surface of the overall encrustation. While the older encrustations are basically close to the rock composition with small amounts of aluminum-silicates contamination, the newer phenomenon is connected with a composition of gypsum and bassanite highlighted as minor phases in XRD analyses.
The appearance of crusts in these cases does not change so much (Figure 6). However, the progressive formation of sulphate layers can lead to black crust formation, as an impact of high atmospheric pollution (Fronteau et al., 2010). We still need to complete our studies on the air pollution data for this area.
Figure 3: Comparison between an old photo shot no later than 1939 (Schmidt, 1970) and recent condition of the reliefs, showing the progress of weathering and material loss.
Figure 4: XRD patterns of rock samples of Xerxes’s tomb. Main phases: calcite, minor phases: magnesian calcite.
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Figure 5: Encrustation on the reliefs, (a) Sampling location in the pathway of moisture with crusts of different colors, (b) the polished cross section of crusts, (c) and (d) SEM elemental maps of Si and S for the same sample.
Another secondary decay phenomenon related to the formation of crusts regards the presence of microrganisms. They form a dark film, which is also accompanied by pitting effects (Sohrabi et al., 2017). Microscopic observations showed that this is originally the same encrustation layer which is then contaminated with biological growth. The presence of micro-organisms in the pores of the encrustation layer caused a greyish or darker color. In places near the cracks and fissures conducting the rainwater, it went deeper and caused the detachment of stone pieces from the rock. Therefore, cross sections showed the presence of biologic material not only on the surface layer but also on the opposite side of the sample (Figure 6).
Finally, a very interesting feature was identified in a few samples under the encrustation. SEM/EDX analysis shows a considerable amount of phosphorous in this layer (Figure 7). It is found that this feature is linked to the technique, which was used for polychrome decoration. The first finding of such feature was given nearly two centuries ago by the French archaeologist who discovered traces of blue paint on Darius tomb, under a thick layer of what was described as a calcareous cover, but currently we may classify as a crust layer (Dieulafoy, 1885: 227). There was no report about the polychromy on other tombs of Naqshe-Rustam, especially the Xerxes’s. However, during the conservation activities in the site a few traces of red paint were identified on the rock reliefs, which was given to the authors for analytical studies. We found the same layer with a high amount of P, interpreted as a ground under the paint in those samples.
Similar phosphorous containing material, probably as a product of burning bones, was found in Persepolis, as a ground layer for painting, in constructions attributed to Xerxes era (Ridolfi et al., 2018).
We need to continue the mapping and also to 76compare analytically our findings on Xerxes tomb with the earlier polychromy on the tomb of Darius. Moreover, this discovery enabled us to distinguish the traces of P-rich ground layer of the original polychromy from other superficial depositions on the rock reliefs. This provides an important measure for future conservation works in order to avoid errors such as overcleaning.
Figure 6: Black Biofilm on the reliefs, (a) sampling location, (b) the cross-section of the stone surface with biofilm, (c) and (d) SEM graphs of the surface of same sample showing the biological growth in the porous structure of the crusted surface.
Figure 7: White layer with traces of polychromy on the surface of reliefs, (a) sampling location, (b) cross-section of sample showing traces of red color on the surface with a ground layer between paint and stone, (c) SEM elemental map for Phosphorous.
Conclusions
In this study, it was used a combined methodology including historical archival study and laboratory analysis, in order to identify the decay patterns and evaluate their progress in the course of time, especially during the last 100 years. Moreover, the results of this study uncovered new features about the polychrome layers on the rock reliefs, which can be used in future conservation planning in order to save as much as possible of the historical materials. Analysis of these materials together with recording of their traces and relevant materials on the maps, were carried out for the first time on the tomb of Xerxes. The results showed interesting features about the origins of the phosphorus-containing layer which is in agreement with other evidence of polychromy during the second half of the Achaemenian period.
Acknowledgements
The authors want to thank Dr. H. Fadaei, the head of Persepolis-Pasargadae scientific foundation for supporting this study.
References
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