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Abstract
This work aims to establish an analytical database of some painted plasters dating back to the 19th dynasty (1314-1304 BC) and recently discovered during the excavations of Cairo University at Saqqara area in Egypt. Appropriate representative samples were carefully chosen and collected from areas that had no aesthetic value or that were seriously damaged. In order to identify the stratigraphy, pigment particle size and texture of the paint layers, polished cross-sections of samples were analyzed by optical microscopy (OM). Scanning electron microscopy equipped with energy dispersive X-ray analysis system (SEM-EDS) showed the elemental microanalysis of the various materials used in construction of these paintings. The obtained results revealed the characterization of some inorganic pigments and plaster layers used in this period of the Egyptian history. Introduction Saqqara is an immense necropolis located about 30 km south of Cairo. The excavation campaigns of Cairo University started in 1984, in the south of the Unas’s pyramid causeway. There, many tombs dating back to the 19th dynasty were discovered. Some samples were collected from the painted plasters of the tomb of Mihew and tomb of hwi nfr. The stone blocks used in the construction of these tombs are not of high quality types of limestone, for this, they have been covered with stucco and white wash layers and painted with several scenes and inscriptions (figures 1 and 2). Pigments differ with respect to their chemical properties due to the fact that they are comprised of a wide variety of chemical compounds. The material’s color characteristics, such as hue and purity, rely not only on color absorption but also depend on the size, shape, and texture of the pigment particles. There are also other characteristics related to the shape and size of pigment particles such as, for example, that mineral pigments are often sharp and angular and traditionally have larger particle size [1]. The main objective of this work was to perform a preliminary analytical approach of the painting materials used for the construction of some painted plasters belonging to the excavations at Saqqara area in Egypt. The studies performed in the current work include application of optical microscopy (OM) and scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM-EDS). Experimental setup Methods for investigation and analysis Sampling Microscopic samples of blue, green and red-orange pigments were carefully collected from the wall paintings. Also, small fragments of coarse and fine plasters were collected and investigated. Optical microscopy Paint analysis usually begins with the visual investigation of the surface of the object, primarily with the purpose of locating intact and representative areas for further analysis. Taking even a tiny sample for a cross-section means removing and destroying a part of the artifact’s original structure. Thus, samples are best taken from areas with flaking paint, so that still intact paint layer will not be damaged. The size of the particle should be as large as necessary but as tiny as possible. Usually, a particle with a size of 1x2 mm is absolutely sufficient and attention should be given to the sampling procedure in order to collect all the paint layers [2]. The morphology of the pigment particle, including homogeneity, shape, size, surface character, and crystal form, are among the first in the sequence of observations that should be made in an investigation and that can help to determine the source of a pigment, and decipher subtle differences between natural and synthetic versions of a pigment. Optical microscopy can provide information such as: the sequence of paint layers, color and texture of those layers and layer thickness [3]. In order to analyze the stratigraphy of the mural paintings, some samples were embedded in Epoxy resin (EpoFix), cross-sectioned using variable speed silicon carbide papers and DP-lubricant blue for fine and cool polishing, and mounted on glass slides. The cross-sections were examined with a Zeiss Stemi DV4 stereomicroscope with a Sony DSC-S85 camera and under reflected light by a Leitz orthoplan (binocular polarized) microscope with a Nikon Cooplix 990 camera. Scanning electron microscopy (SEM-EDS) The scanning electron microscope is used to observe the pigment morphological features more accurately, and is most effective in the absence of organic binding media. When working with patinas and paint layers the backscattered electrons mode (BSE) usually provides more information concerning the elements distribution, due to the different atomic numbers of the elements present in the sample. This mode allowed us to distinguish the different layers with different elemental composition. Analyses in BSE on polished sections were used for elemental semi-quantitative chemical study of the painting layers. SEM analysis in the secondary electron mode (SE) on unpolished sections was used for microscopic observations of the layer’s microstructure and texture. The EDS mapping analysis offers a final piece of information needed for pigment identification, i.e., the elemental distribution within the different layers [4]. The pigment morphology was investigated using a JEOL JSM-840A scanning electron microscope and the microanalysis was carried out using an energy dispersive X-ray spectrometer (EDS) Oxford ISIS 300. Polished cross-sections were investigated by BSE for the purpose of pigment identification in each color layer. The elemental composition was determined using carbon coated cross-sections. Results Blue-green pigments The analysis of green pigment cross-section shows turquoise and green hues of coarse large particles embedded in glass-rich matrix. The thickness of the paint layer is slightly higher than others. Yellowish-brown spots were noticed scattered within the green particles (figure 3). Different bluish-green hues and some particles with brown color were also observed (figure 4). BSE analysis of the green pigment shows large crystals, probably of parawollastonite embedded in silica-rich amorphous phase (figure 5), and the EDS microanalysis shows the presence of silicon (31.12%), copper (5.66%) and calcium (9.34%), which is consistent with the possibility that a copper glass-rich bearing compound, such as the synthetic Egyptian green pigment, was used to produce the color. In the manufacture of Green Frit, a higher lime-to-copper ratio than for Egyptian Blue is required in order to stabilize the copper-bearing wollastonite as a liquidus phase. By comparison with Egyptian Blue, the chromatic phase in the Green Frit is wollastonite [(CaCu)SiO3] [5].
Figure 1. Painted inscriptions. Painted pastes with blue, bluish-green and red pigments filling pinkish and white plasters.
Figure 2. Painted inscriptions. Figures with red pigments and pink plasters. Figure 3. A close-up optical micrograph of green paint layer. Particles of green pigment are surrounded with yellowish-brown grains. Figure 4. Optical micrograph shows cross-section in bluish-green pigment and some particles with brown color are also noticed.
Figure 5. BSE micrograph of plaster layer and green pigment.
Egyptian green is a heterogeneous material like Egyptian blue and has a characteristic turquoise hue. Egyptian green is characterized by the presence of parawollastonite (CaSiO3) crystals, with a particle size less than 10 μm, and residual silica (quartz and/or tridymite or cristobalite), embedded in an amorphous, silica-rich glass phase. The copper ions in the octahedral environment of a silica-rich glass result in a turquoise color, which is affected when the temperature and the CuO concentration increase, but is not related to the flux concentration [6]. Also the Cu2+ ion is in an octahedral environment in the amorphous silica-rich matrix, which induces the green hue [6]. EDS microanalysis of yellowish spots mixed with the green crystals shows that the peak of iron is present which is consistent with the possibility that the ancient artist used on purpose a mixture of blue pigment (Egyptian blue) and yellowish-brown pigment (iron oxides based) to produce green hues, or that he mixed the Egyptian green with the yellow pigment to get special hues. Moreover, the mineralogical characterization to identify the crystalline phases in the samples is now in progress in order to obtain further information about the main components. Egyptian blue pigment appeared in Egypt during the 4th dynasty in the 3rd millennium BC. The use of the pigment spread from Egypt and the Near East to Minoan Crete and the Greek world, and then to the Roman world [7]. This pigment consists of cuprorivaite, calcium copper tetra silicate (CaCuSi4O10), blue tabular crystal about 15 μm to 30 μm in length, residual silica (quartz and/or tridymite) and an amorphous silica-rich phase. It was manufactured by mixing calcium salt (carbonate, sulphate or hydroxide), a copper compound (copper oxide or malachite), silica and alkali flux (sources of alkali could have been either natron from areas such as Wadi Natroun and El-Kab, or soda-rich plant ashes) [8]. This mixture was heated to a temperature between 850 and 1000° C to produce a colored glass or frit and later ground to powder for use. Red-orange pigment Ochres form a very wide class of natural inorganic pigments thanks to their extensive color range that can vary from deep red or brownish to orange and finally to bright yellow. Red ochre was used in Egypt from the 5th dynasty till the Roman times. There are three main factors that influence the color of ochres. Firstly, is the nature of the iron oxide chromophore. It is likely that the darker red ochre contains predominantly hematite, Fe2O3, while the paler yellow ochre is richer in the hydrated iron oxide, goethite, Fe2O3•H2O or FeOOH. Secondly, is the presence of other minerals, e.g. clay minerals or other metal oxides. Thirdly, is the particle size distribution within the material [9]. Hematite particles of about 1 μm have a distinct violet tint differing from the bright red colour of hematite with sub-micrometer particles, e.g. pedogenic [10]. The optical investigation of the painting layer with red-orange pigment shows that the pigment was applied over an unprepared underlying plaster layer rich in voids and gypsum and quartz particles as we can see that the painting layer shows irregular line with different thicknesses (figure 6). BSE investigation of red pigment shows massive granular aggregate particles (figure 7) while EDS microanalysis shows that the peak of iron (19.05%) is present, indicating the existence of hematite (Fe2O3) as the possible material producing the red color. Other elements of sulfur and calcium refer to the presence of calcium sulphates, as well as aluminum and silicon indicate possible existence of aluminosilicate material. The observation at high magnifications showed a difference in size between particles with gypsum (CaSO4•2H2O) and calcite (CaCO3) as the particles with gypsum are larger while the calcite ones are much smaller. The presence of titanium in the studied samples could be a result of the presence of ilmenite (FeTiO3) which is found in the Egyptian sand or possibly forming intergrowths with hematite [11]. Plaster layers From the optical analysis (figure 8) we can distinguish two main layers of the plaster used to overcome faults in the poor stones and to produce flat and smoothed surface for painting. The bottom coarse layer is known as arriccio and consists mainly of quartz grains, calcite and calcium sulphates, while the fine coat known as intonaco is mainly based on gypsum with variable amount of calcite (limestone powder). BSE analysis shows clearly the two layers of plaster (figure 9): the thick layer of coarse plaster and the irregular fine white wash. EDS microanalysis of the coarse plaster revealed high quantity of silicon associated with quartz, and calcium and sulfur associated with calcium sulphates (gypsum/anhydrite). The EDS microanalysis of white wash identified sulfur, calcium and magnesium as the major ions present, most probably due to the existence of calcium sulphates, calcite and some dolomite (CaMg(CO3)2). Remains of pink-colored plaster were noticed as paste filling the sunken areas in the walls, probably used to overcome imperfections in the wall. The EDS microanalysis of the pink plaster revealed that iron is present, thereby indicating the presence of iron oxides. A high quantity of calcium, most probably from calcium carbonates, was also detected. In addition, EDS microanalysis showed the presence of large amounts of aluminium, silicon and potassium suggesting the presence of clay minerals.
Figure 6. Optical micrograph shows cross-section of red paint layer.
Figure 7. BSE micrograph of plaster layer and red ochre pigment. Figure 8. Optical micrograph shows cross-section of plaster layers. The thin white wash layer lies on a thick coarse plaster. Figure 9. BSE micrograph of plaster layers. Table 1 illustrates the major ions present in the samples analysed by EDS microanalysis and the expected coloring material.
Conclusions The preliminary examination of pigments was performed indicating extensive usage of pigments commonly used in ancient Egyptian wall paintings. The pigments identified by optical observations, element analyses and morphological study had shown that: 1. The EDS detection of iron in yellowish-brown spots in green pigment samples indicates that the green-turquoise pigment was probably produced using Egyptian green and yellowish-brown pigment based on iron oxides, or the color was probably obtained by mixing Egyptian blue with yellowish-brown pigment based on iron oxides. 2. The results concerning the red pigment are in accordance with previous findings by Mahmoud et al. [12] in their studies of samples from painted limestone blocks from the same excavations, The red-orange pigment is mainly obtained from iron oxides (hematite, Fe2O3). The presence of aluminium and silicon detected by EDS analysis suggests the existence of aluminosilicate materials (clay minerals, etc.) normally found in ochre pigments. 3. Two layers of plaster were noticed: the coarse one with higher thickness, consisting mainly of quartz, calcite and gypsum; and the fine white wash thin irregular layer consisting mainly of gypsum and limestone powder. A pink paste was also observed covering some areas in the walls; iron oxides, calcite and gypsum in addition to clay minerals were used to produce this kind of plaster. Further investigation of additional samples is now in progress using different analytical methods (μ-XRF, XRD and μ-Raman spectroscopy) in order to provide a more detailed image of the chromatic palette and the composition of these murals. The results will be used in the conservation-restoration intervention of these paintings. Acknowledgments The chief of Cairo University excavations at Saqqara area is kindly acknowledged for the permission to collect the studied samples. References [1] R. J. Gettens and G. L. Stout, Painting Materials: A Short Encyclopedia, Dover Publications, New York, 1966, pp. 131-143 [2] W. Ullrich, “Cross-section Analysis of Paint layers–Materials, Methodology and Examples", Journal of Cultural Property Conservation 4, 2008, pp. 49–56 [3] C. L. Silva, A Technical Study of the Mural Paintings on the Interior Dome of the Capilla De La Virgen Del Rosario, Iglesia San José, San Juan, Puerto Rico, MSc. thesis, University of Pennsylvania, USA, 2006 [4] A. S. Škapin, P. Ropret and P. Bukovec, "Determination of pigments in colour layers on walls of some selected historical buildings using optical and scanning electron microscopy", Materials Characterization 58, 2007, pp. 1138–1147 [5] A. El Goresy, "Polychromatic Wall Painting Decorations in Monuments of Pharaonic Egypt: Compositions, Chronology and Painting Technique", in The Wall Paintings of Thera: Proceedings of the First International Symposium, Volume I, S. Sherratt (Ed.), Thera (Hellas, Greece), 30 August-4 September, 1997, pp. 49-70 [6] S. Pagés-Camagna and S. Colinart, "The Egyptian Green pigment: Its Manufacturing process and links to Egyptian blue", Archaeometry 45:4, 2003, pp. 637–658 [7] L. Mirtit, A. Appolonia, R. Casoli, P. Ferrari, E. A. Lurenti, C. Amisano and G. Chiari, "Spectrochemical and Structural Studies on a Roman Sample of Egyptian blue", Spectrochimica Acta 51A: 3, 1995, pp. 437-446 [8] G. H. Hatton, A. J. Shortland and M. S. Tite, "The production technology of Egyptian blue and green frits from second millennium BC Egypt and Mesopotamia", Journal of Archaeological Science 35: 6, 2008, pp. 1591–1604 [9]. J. L. Mortimore, L-J. R. Marshall, M.J. Almond, P. Hollins, W. Matthews, "Analysis of red and yellow ochre samples from Clearwell Caves and Çatalhöyük by vibrational spectroscopy and other techniques", Spectrochimica Acta Part A 60, 2004, pp. 1179–1188 [10]. D. Hradil, T. Grygar, J. Hradilova, P. Bezdička, "Clay and iron oxide pigments in the history of painting", Applied Clay Science 22, 2003, pp. 223–236 [11]. M. Berry, "A study of pigments from a Roman Egyptian shrine", AICCM Bulletin, December 1999, pp. 1-9 [12]. H. H. M. Mahmoud, M.F. Ali, N. Kantiranis, A. N. Anthemidis, J. A. Stratis, "Identification of some ancient Egyptian pigments in painted limestone block from Cairo University excavations at Saqqara area", The first conference of faculty of Archaeology, Cairo University (Giza through ages), Cairo, Egypt, March 3-6, 2008
About the author
Hussein Hassan M.H. Mahmoud
Conservator
Contact: marai79@hotmail.com
Hussein Hassan Mahmoud is a conservator of mural paintings. He is currently Assistant lecturer at the Conservation Department of the Faculty of Archaeology at the Cairo University, Egypt.
Mr. Mahmoud has a Bachelor’s degree in Conservation and Restoration of Monuments and Works of Art from the Cairo University and a Master’s degree in Conservation of Mural Paintings from the same university. At the moment he is working on his PhD thesis focusing on the degradation of ancient Egyptian pigments in mural paintings. In 2001 he participated in the conservation-restoration project of the decorated wooden ceilings of El-Ghuri mosque, Old Cairo. In 2002-2004 he also participated in the conservation-restoration project of the ancient mural paintings of two Pharaonic tombs (TT277, 278), Western Thebes, Upper Egypt, in collaboration with the Higher Supreme of Antiquities in Egypt. His main interests are the application of nanotechnology in conservation and the application of modern analytical techniques, namely micro-Raman and micro-FTIR spectroscopy, micro-XRF and SEM-EDS microanalysis, in the characterisation and diagnosis of mural paintings and objects of cultural heritage.
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