Scientific examination of the sculptural polychromy of cave 6, Yungang, China
The present study of the sculptural polychromy of Cave 6, Yungang, forms part of an overall programme of investigation of the site by the Getty Conservation Institute (GCI). The research was undertaken in partial fulfilment of the requirements for an MSc in painting conservation at the Courtauld Institute of Art, with the analytical work carried out at the GCI and under its direct supervision.
Yungang is situated in the Shanxi province, about 300 km west of Beijing. Cave 6 is the most historically significant and artistically important of the 53 principal caves at Yungang. Commissioned by a Northern Wei emperor at the end of the 5th century, it is one of the last of the Buddhist cave-temples to be cut at Yungang. It is square in plan, with a central stupa pillar rising three stories to the ceiling. It contains several hundred devotional figures and narrative subjects carved from the sandstone and fully polychromed. The cave has had a long history of neglect, destruction, deterioration, and intermittent repair, and the present condition is extremely variable.
The primary goals of the research were to determine the nature of the original polychromy and its present condition. Historical research, site examination, sampling and analysis were carried out to ascertain the stratigraphy, the components of the various layers, and their condition. It is anticipated that the information obtained will be relevant for conservation of the cave.
Analytical techniques used included optical and electron microscopy, microchemical tests, infrared spectros-copy, energy-dispersive X-ray microanalysis, methods widely applied for the study of paint layers.
The polychromy of the original, 5th-century painting scheme was found to be composed in almost all cases of two layers: a clay-based preparation applied directly to the stone, and a single pigment layer. The pigments were: ultramarine, vermilion, red lead, and red and yellow earth. Wherever it occurred, red lead was found to have converted to plattnerite (PbO2). By contrast, the palette of the first scheme of repainting was found to be extraordinarily limited, including only red and green, raising questions of survival or of selective repainting. The pigments were: vermilion, red lead, atacamite, malachite, and possibly green earth. Atacamite (a copper chloride green) occurs widely in this painting phase as well as in subsequent ones.
Although a large number of later repainting phases is evident (as many as 12), due to the discontinuity of the painting phases it was not possible from the stratigraphic evidence of the samples to make secure correlations. Several new pigments were introduced in subsequent decorative schemes, most notably gold.
Gold occurs widely, typically for flesh painting in imitation of gilt bronze, and generally in those areas which are both most conspicuous and accessible to intervention. Other pigments include: Prussian blue, azurite, artificial ultramarine, artificial malachite, orpiment, massicot (?), and carbon black.
Comparison with technical studies of other Central Asian and Chinese polychromy indicates that the techniques and materials used in Cave 6 are essentially similar. The most conspicuous feature of all of this painting is the use of clay-based grounds; this occurs regardless of support (whether stone or earth) and medium (whether painting or sculpture). Several aspects were highlighted as meriting further research, including the alteration of lead-containing pigments, the synthesis and use of atacamite, the nature and use of clays, the nature and use of organic binding media, and the absence of green in the palette of the original scheme.
The study showed that the surviving polychromy is extremely fragile, resulting both from its original technique and past deterioration. Failure of adhesion occurs at the various interfaces: between support and ground, ground and paint layer, and between subsequent phases of repainting. This has significant implications for its preservation, since it is highly susceptible to mechanical damage, and it is therefore likely that the current regimen of regular dusting puts the painting at risk.