Stratigraphy of the wall paintings
Through the results of field investigations and sample analysis, the Jokhang temple wall painting is basically composed of five parts (from inside to outside): the basic support (flagstone wall), the ground layer, a white preparation layer, paint layer and a surface protective layer or varnish. The schematic diagram is shown in Fig. 2. It is noted that today traditional Tibetan wall paintings are made as follows: firstly, a clay mud (known locally as Ba-ga soil) is applied on a flagstone wall to smooth the wall and left to dry completely; a fine mud layer with loess is then applied; this is followed by a white preparation layer with Yang-ga soil; finally, after the white base dries, painters will brush a layer of wet glue on the white layer first, and then paint over the base. A varnish is usually applied to the final painting.
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1.
The flagstone wall.
The wall in the Jokhang temple is made of flagstones and is the basic support of the wall painting. Because the buildings are old, some walls have deformed and have shrunk under the influence of external environment. Thus, structural problems in the walls are one of the main factors leading to the loss of wall paintings.
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2.
The ground layer.
The ground is divided into three layers. The bottom is the Ba-ga soil, with coarse particles and mostly sandy loam, with a sand content not more than 12%. The middle layer is made of thin loess, in which some flexible materials (such as straw and gaunt) are generally added to reinforce and increase the flexibility of the layer. The last layer is Ang-ba soil, which has a fine particle size. However, only the highest grade wall paintings have so many ground layers, and may do not contain the Ang-ba layer. Since the stones on the flagstone wall are uneven, the ground layer varies in thickness from about 5 to 1 cm.
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3.
The preparation layer.
According to the local monks, the preparation layer is made of a local Yang-ga soil and is a very thin gray/white layer. Analytical results identify the main component of the white powder layer of the samples as magnesite (magnesium carbonate).
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4.
The paint layers.
The color of the Jokhang temple mural is very rich. Blue, red, yellow, green and gold are mostly used and constitute the main thematic colors. The paintings are executed with meticulous painting with exact delineation (see Fig. 3a). In addition to the main painting scheme, there are two specific styles in the wall paintings: paint on a black background with gold lines (Fig. 3b), and paint on the gold-silver powder background in black (Fig. 3c). Traditionally, when painting, mineral pigments are ground into powder and a glue solution is added to make a liquid paint. After the color has been adjusted, the painter will use the paint to draw. The process of Tibetan wall painting is very strict and is described in detail elsewhere [10, 34, 35].
Technical study of three fragments
Three larger fragments (Fig. 4) were sampled from the outside Gallery wall. Their surfaces are beautiful and lustrous coated with pigments, gold and a layer of protective material. Three layers (ground layer, white layer and painting layer) can be seen clearly. The ground layers are thick and some flexible materials like wheat straw are mixed in the earthen plaster.
Examination under the stereo microscope (Fig. 5) shows that the surfaces of the fragments show signs of the accumulation of dust, discoloration, cracks, scaling, cracking, and pigment loss. The original color of the apparent black lines is actually blue when observed under the microscope, which is the result of long-term smoke accumulation. Gold powder can be observed over the pigment layer. A protective layer is evident on the surface and many micro bubbles and fine scratches are seen in this layer likely due to the brushing process, or the cracking and flaking of the varnish due to natural aging.
The thickness of the pigment layer is about 30–80 μm, and the white layer is about 50–120 μm (Fig. 6). Gold powder can be seen over the paint layer and its thickness is about 20 μm (Fig. 6d). It is noted that in some samples there is evidence of overpainting (Fig. 6b) where a thick (about 50 µm) white layer has been applied over the red layer, and is also covered in what could be varnish. This suggests the application of another ground or preparation layer for new paint.
The ground layer is thick and is usually divided into two parts: next to the white layer is a thin layer of loess with fine clay particles tightly connected and distribute uniformly; below the fine layer is a coarse mud layer with larger clay particles mixed with different sizes of sand and small stones. The main components of the coarse ground layer were identified by XRD to be quartz, albite and microcline, and the main components of the fine layer are quartz, albite, microcline and calcite. The major components of the coarse and fine layers are almost the same, thus it is likely that the same kind of clay was used. The local Ba-ga soil, Ang-ba soil and loess soil were sampled and tested by XRD which revealed the main components of these soils are all quartz, calcite and albite, and are basically the same as those of coarse layer, fine layer and Ang-ba layers of the wall painting samples.
In some samples the blue pigment is found mixed with the green pigment (Fig. 6e), and the orange pigment contains red and yellow particles (Fig. 6f). As noted by Dambar [36], traditional Tibetan wall paintings mainly use white, yellow, red, blue and green pigments either on their own or mixed to create different tones as observed in the samples studied as part of this work.
Under UV illumination, the thick varnish layer is apparent in cross-sections as a strongly luminescent material (Fig. 7). In some cross-sections two varnish layers are observed, and it is possible that they are of different composition. In addition, it is noted that the varnish penetrates into the sometimes abraded surface of the paint, and this is an indication that varnish was likely applied at a date later than the completion of the paintings.
The chemical composition of the pigments was determined by Raman spectroscopy and EDX. The test location for each analysis is showed in Additional file 1. Pigments were identified on the basis of a comparison between Raman bands and published databases of mineral and synthetic pigments [37, 38]. The results are shown in Figs. 8 and 9 and indicate that the orange pigment is a mixture of red lead (Pb3O4) and goethite (FeO(OH)), sometimes containing orpiment (As2S3); the yellow pigment is orpiment (As2S3); the red pigment is cinnabar (HgS); the green pigments are either malachite (CuCO3·Cu(OH)2) or Emerald green [Paris green, Cu(C2H3O2)·3Cu(AsO2)2)]; the blues are azurite (2CuCO3·Cu(OH)2) or ultramarine blue (Na3CaAl3Si3O12S); the white is magnesite (MgCO3); and the black is carbon (C). In order to identify the nature of the ultramarine blue, dispersions were prepared for the blue pigment and reveal fine rounded particles typical of artificial ultramarine (Fig. 10), a nineteenth century pigment.
Under SEM (Fig. 11), four layers are seen in samples: the sealing layer or varnish, the paint layer, the white preparation layer and the ground (Indications of paint layer, ground layer, and preparation layer for each cross-section are showed in Additional file 2). In some samples gold is present beneath the varnish. It is noted that in sample a and e, the particles of the blue pigment are uniform and relatively round, which suggests that they are artificial and not natural ultramarine, as confirmed by dispersions of the pigment (Fig. 10). The particle sizes of the white preparation layer are small and have a uniform distribution. The EDX spectrum analysis shows that the main element is Mg. In combination with XRD and Raman, it is possible to identify the main component of the white layer as magnesite (MgCO3). The thickness of the ground layers is different and varies in each sample. In some samples there are two layers, while in others the preparation cannot be clearly distinguished. Clay used in the ground layers contains both quartz and feldspar particles of different sizes.
THM-Py-GC/MS analysis
Binding media and varnish in the samples were determined by THM-Py-GC/MS. The results (see Additional file 3) showed that fatty acid (especially high content of azelaic acid), oxidation products of tung oil after heat treatment, pyrolysis products of rosin resin and maker compounds of protein, were all detected in the samples. Therefore, it is likely that the varnish contains a mixture of drying oil (maybe tung oil) and rosin resin, while the binding media is a kind of animal glue which is likely used as the binding medium for the paint and preparatory layers. According to the literature and field interviews with local painters, today Tibetan painters generally use bovine glue as binding medium, which is similar to the medium identified by THM-Py-GC/MS. Thus, the animal glue used in the wall paintings of Jokhang temple is very likely to be bovine (or yak) glue.
There is no surface protective material on the wall paintings of Tubo Period (sixth century C.E.). However, after the fifth Dalai Lama, materials were used to protect the surface of the wall paintings. The materials can increase the gloss of the surface and can prevent the loss of pigment caused by repeated touching from thousands of believers. The traditional Tibetan protective materials (such as egg white, oils, etc.) are occasionally mentioned in local treatises. However, the recipes have not been written in detail and have been lost. Since 13th Dalai Lama Period, Tibetan painters have begun to use natural protective varnishes with imported resins, which were replaced by alkyd resin varnishes during the 1970s according to accounts of local painters. Tung oil and rosin resin were detected in samples of the varnish, both of which can be dissolved in turpentine, but it is not possible to date the application of the varnish due to the use of the same materials by painters today.
Implications for dating
The pigments used on Jokhang temple wall paintings are mainly natural mineral pigments, such as ochre, red lead, cinnabar, malachite, azurite, orpiment and magnesite. These pigments are all traditional pigments which are often used by Tibetan artists [21,22,23,24]. It is noted that the same mineral can show a variety of colors by different degree of grinding or different processing methods, and that malachite and azurite are often found together, as observed in cross-sections.
In addition to traditional mineral pigments, synthetic pigments, such as Paris green and artificial ultramarine are also detected. These two synthetic pigments were produced in Europe in early nineteenth century. Subsequently, they began to enter China and were identified on many paintings, architectural painting and temple murals of the late Qing Dynasty. For example, Paris green has been found in the pigments of a late Qing Dynasty Taoist portraits [39], the architectural painting of Zhendu gate in the Imperial Palace [40] and the Qing Dynasty wall painting in the Potala Palace and Drepung Monastery [41, 42]. The use of artificial ultramarine is more extensive than Paris green. In China, many paintings from the late Qing Dynasty were executed in artificial ultramarine. For example, most murals and painted sculptures of Qing Dynasty in Dun Huang grotto used artificial ultramarine [43]. Because Paris green and artificial ultramarine were also found in the samples of Jokhang temple, it is concluded that the wall paintings in the outside galley of Jokhang temple have been repaired and repainted in late Qing Dynasty, thus during the mid-nineteenth century or later.