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What Is Used for Blue Paint in Buddhist Cave Paintings

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Historic context

The region associated with East Turkestan was an important site of Buddhism between the second and tenth centuries (Fischer et al., 1994). From the tenth century, Buddhism as the major religion lost its importance and was replaced by Islam. As a consequence, Buddhist buildings and monuments gradually disappeared due to desert sandstorms, excavations, and pillaging, all resulting in the destruction of monastic and temple complexes. Today, the mentioned territory is located in the province of Xinjiang Uyghur Autonomous Region, in the far west of the People's Republic of China.

By the early twentieth century expeditions hailing from countries like Japan, the United Kingdom, Russia, and Germany, then deemed to be the 'Great Powers' (Taniguchi, 2010), led to a series of research journeys following the northern Silk Road. During these expeditions, wall paintings were detached from their original settings, i.e. the interior walls and ceilings of Buddhist cave complexes (Russell-Smith, 2012). Geological and archeological reports of four Prussian expeditions formed the basis of the following research (Yaldiz & Zieme, 1987). Moreover, expedition diaries document the wall paintings in a very detailed manner in the form of descriptions, photographs, sketches, and tracings, and map the context and work in situ. They represent the primary sources of information on the expeditions today.

In 1906, during the third Prussian Turfan expedition, Buddhist cave sanctuaries were rediscovered in the valley of Sim-sim close to Kiriš in the district of Kucha (Fig. 1). Cave temples hewn into rock cliffs were found, for example cave no. 40, the so called Ritterhöhle (see Fig. 2A, exterior view and floor plan). The cave's name arises from the representation of a knight figure in a suit of armor located on the central wall paintings (Grünwedel, 1912). The interior walls, arches, and vaults of those Buddhist places of worship were entirely decorated with religiously influenced paintings (Fischer et al., 1994). Due to the style of their letter band, Pinault (1993–94) assigned the wall paintings to the first half of the seventh century.

Figure 1 Map of the northern Silk Road region in East Turkestan. The lines represent the four Prussian expedition routes (1902–1914). The blue line marks the third expedition (1905–1907). The inset in the lower right corner shows a detailed magnification of the region around Kucha, including the Buddhist cave sanctuary area in Sim-sim. Figure based on original maps owned by the Asian Art Museum.

Figure 2 (A) North entrance of cave no. 40 (photograph by T. Gabsch). The inset lower left shows the floor plan made by Grünwedel (in the collection of the Asian Art Museum); the annotations describe the dimensions (2M35c = 2.35 m). The red line on the left wall of the inset indicates the location where the wall painting fragments of this study were detached. (B) South side of the interior space (cella) of cave no. 40 (photograph by T. Gabsch).

The objects examined in this work are wall painting fragments that were detached in eight pieces (Fig. 3). Despite their heavily sooted surfaces, Grünwedel (1912) justified their removal by their high artistic quality. The paintings show three prophecy scenes. In his expedition diary, Grünwedel (1912) assigned these paintings to the side walls of the cella (Fig. 2B) of cave no. 40, marked red in the left part of the floor plan shown in Fig. 2A.

Figure 3 Photograph of the eight wall painting fragments; total dimensions 2.45 × 0.90 m.

Bartus participated in all four Prussian expeditions and developed a unique method for detaching the paintings. A detailed description of the method is given by von Le Coq (1926). First, the surfaces of the paintings were cut with a sharp knife into appropriate shape to have them fit into the packaging in transport crates. The cut was made through the whole painting and render. Later, a cut was made into the side of the wall with a hoe, allowing the use of a padsaw to detach the painting. The fragile fragments were transported overland to Europe, and in 1907 they were brought to the Indian Department of the Museum of Ethnology in Berlin. Because of the start of the First World War, a continuous scientific investigation became impossible.

Fig. 2B shows the condition of the interior space of cave no. 40 in 2011. It can be recognized that in large areas the wall paintings are widely removed and that heavy soot deposits are present in the dome (note that in this and other figures, the dimension refers to the full scale bar and not the individual sections).

Due to loss or damage incurred during transport and/or handling, many of these fragile objects were in a poor condition and not easy to handle after their arrival in Berlin. To preserve and prepare them for exhibition, each fragment was stabilized and placed in a bed of gypsum during a restoration campaign in the 1920s. The gypsum was reinforced with anchoring elements. During this treatment, parts of the wall painting were positioned erroneously, both regarding their surface level and their horizontal position in the pictorial composition.

Fig. 3 shows an overall photograph and Fig. 4 shows a stratigraphic schematic of the eight wall painting fragments at the Asian Art Museum. Each fragment is embedded in the 1920s gypsum support backing (labeled '0' in Fig. 4). On earthen render (1), which consists of layers of rough and fine render, a thin white ground layer (2) can be seen. On that, a single or multilayered painting (3) is applied (now partially covered with soot).

Figure 4 Schematic representation of the detached fragments. The numbered stratigraphy is explained in the text; total thickness of the fragments is approximately 6 cm.

More than one third of the wall paintings of the Turfan Collection are considered lost as a result of the First and Second World Wars. Other wall painting fragments and artefacts (especially manuscripts) were taken from Berlin to the Soviet Union in 1945; today fragments of the Ritterhöhle are also in the collection of the Hermitage in St Petersburg.

No written documents provide evidence for further interventions and treatments until 1973, when the paint layer surfaces were cleaned with ethanol and consolidated using a polyvinyl acetate dispersion with the trade name Caparol ® .

Studies on Central Asian wall paintings – a review

The following is a brief review of research into Central Asian painting technology, including a summary of analytical techniques used in the most relevant studies. Although not all objects investigated are from the same century and area as those investigated in this study, they still give useful comparative and contextual information on the polychromy of Central Asian wall paintings.

Painting materials

Painting materials of Central Asian wall paintings corresponding to the Late Antique and early middle Medieval periods were first investigated in a scientific manner at the beginning of the twentieth century (Gettens, 1938b).

In the 1970s, Riederer (1977) undertook scientific research on paintings from the Berlin collection. By employing analytical methods such as X-ray diffraction (XRD), infrared (IR) spectroscopy, emission spectroscopy, microscopy, and microchemical methods, he derived general conclusions concerning the use of pigments and colorants of the wall paintings from East Turkestan. The identified white pigments included gypsum, anhydrite, and, rarely, lead white. The blues were lapis lazuli and indigo. The green pigment was identified as atacamite. Concerning red pigments, Riederer mentioned mercuric sulfide in its mineral (cinnabar) or synthetic form (vermilion). He did not detect this pigment in wall paintings from the Kucha region, however, where mainly red iron oxide was detected. The presence of the pigments lead oxide and massicot is also mentioned by Riederer, who associated the brown-violet painting areas in fragments from Kizil with discoloration of a red lead oxide. Regarding organic colorants, in addition to indigo, Riederer (1977) suggested the possible use of gamboge, kermes, and red lake (rose madder).

Results obtained for binding media are consistent with scripts dating from the seventh century CE which report that animal skin glue was used in paintings (Riederer, 1977). Early studies of the wall paintings in Bamiyan (Gettens, 1938a) and Kizil (Gettens, 1938b) as well as Nagpall and Agrawall have indicated that glue was the binding medium in these cases, based on microchemical tests. Birstein (1975) also found evidence for animal glue (gelatin) as the organic component of paint and plaster layers in Central Asian paintings, as well as plant gums such as apricot or cherry, using chromatographic analysis of hydrolyzed samples.

Recent studies at the Hermitage on paintings from Kucha (it is not mentioned from which cave) used microchemical and spectroscopic methods to identify lapis lazuli, cinnabar, lead oxide, red arsenic sulfides, copper green, bone black, white lead, gypsum, and calcite (Kossolapov & Kalinina, 2007). A similar range of pigments was reported by Zuixiong (2010) for the Kizil Grottoes, located close to the Kucha region. XRD analysis confirmed the presence of vermilion, lead oxides, red ocher, lapis lazuli, copper hydroxychloride minerals such as atacamite, and gypsum. High-performance liquid chromatography indicated an animal glue binder, thought to be made from ox hide.

Between 2004 and 2009, studies of fragments of the giant Buddha statues in Bamiyan were performed with polarized light microscopy, XRD, XRF, and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and element mapping, allowing the detection of gypsum, red iron oxide, white lead, minium, ultramarine, and charcoal black (Blänsdorf et al., 2009).

Another recent study applied synchrotron-based micro-analytical techniques to the characterization of pigments and binders in Bamiyan Buddhist wall paintings. Combined XRF, X-ray absorption spectroscopy, XRD, and Fourier-transform IR spectroscopy carried out on cross section samples indicated the presence of protein and polysaccharide binders, as well as atacamite, minium, and goethite pigments (Cotte et al., 2008).

More detailed reviews of scientific research on Central Asian wall painting techniques can be found elsewhere (Yamauchi et al., 2007; Agnew, 2010).

Preparation techniques

For providing a better adhesion for the earthen render (made of local clay), rock faces were first roughened using a chisel. Multi-layer preparations were typically used. The first rough layer of render was prepared with chopped straw or animal hair for reinforcement. A second, thinner earthen layer generally contained less fibrous material, although sand was sometimes added. The materials used for the subsequent ground layer varied widely, depending on the region: gypsum, anhydrite, calcite, and lead-containing pigments have been detected. It can be assumed that organic binders (plant gums and/or animal glues) were added to extend the processing duration. Typically, the ground layer is thin (10–20 µm), although this may also vary depending on the region (Riederer, 1977).

Additional techniques for preparing the paintings include preliminary sketches, tracings, and the use of a compass. In this context, Riederer refers to von Le Coq who provided evidence for the use of pouncing (Riederer, 1977); this preparation technique was found in paintings from cave no. 123, which were restored and reconstructed in 2002 and are now part of the permanent exhibition at the Asian Art Museum in Berlin.

Methodology of technical analyses

Pigment analysis was the focus of the current study; binding agents have not yet been investigated. The methods used were optical microscopy (OM), polarized light microscopy (PLM), SEM with EDX, XRF, X-ray diffraction (XRD) analysis, and Raman spectroscopy. Table  summarizes the techniques used, the objectives of the analyses, and the areas/samples analyzed.

Table 1 Analytical methods used for investigation with sample numbers and locations

Figure 5 Optical micrograph detail of fragment V, with arrows and numbers marking the different layers: earthen render (1), white ground (2), red paint layer (3), and soot layer (4).

Figure 6 Cross sectional image (darkfield) of samples from a blue paint area (A) and a red paint area (B) of fragment IV.

Figure 7 Cross sectional image (darkfield) of sample from a red paint area of fragment I (A) with SEM detail of the rectangular area indicated (B), shown overlaid with EDX elemental maps; the individual element maps are shown in (C–J).

Figure 8 Detail of fragment I (A) with XRD spectrum (B) of a sample taken from the region indicated by a red rectangle. The asterisks indicate peaks for gypsum.

Figure 9 Detail of a decorative area of fragment V showing multilayered application of red paint (A). The blue and green marked areas indicate locations of XRF analyses (data shown in (B) and (C), respectively).

Figure 10 Detail of fragment I (A) along with a more highly magnified detail (B) and a cross sectional image of a sample from the green paint area (C).

Figure 11 Secondary electron (SEM) image of a sample from fragment V (20° angle) showing (i) soot and (ii) synthetic binder.

Optical (digital) microscopy was used first to study the appearance and condition of the surface of the eight wall painting fragments and to select appropriate sites for analysis and sampling. XRF was performed preferentially as a non-invasive method, since only very limited sampling of the original paint layers was possible.

In cases where sampling was feasible, original material was removed with a scalpel in the form of microscopic, multilayered flakes and prepared as cross sections for microscopic analyses. Additional samples were taken in the form of surface scrapings to investigate the constituents of individual layers.

In addition to the paint layers, the materials used for the ground and the earthen render layers, as well as the soot and synthetic binder that had been applied on the surface, were investigated. Altogether nine samples were taken, as listed in Table .

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Source: https://www.tandfonline.com/doi/full/10.1179/2047058414Y.0000000152

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