Non-invasive analysis and large scale imaging of murals at the UNESCO World Heritage Site of Mogao caves in Dunhuang, China
The UNESCO World Heritage Site of the Mogao caves, along the ancient Silk Road, consists of 492 richly painted Buddhist cave temples dating from the 4th - 14th century. The 45,000 square metres of wall paintings in nearly 500 caves are an immense resource for the study of the history of art, architecture, religion, technology, politics and cultural exchange.
Exterior of Mogoa caves
In situ PRISMS scanning in 2013
Exterior of Mogoa caves
Because the wall paintings of this historical site are vulnerable, any inspection needs to be non-invasive and non-contact.
The aim of this project was to develop a methodology for the non-invasive and in situ analysis of the large scale murals, even when they are located in unreachable areas (e.g. ceilings). Our research was mainly focused on the analysis of murals in cave 465. Cave 465, located at the northern end of the site is unique in its Indo-Tibetan tantric Buddhist style, and like many other caves, the date of its construction is still under debate.
Addressing the Challenge
The collaboration with the Dunhuang Academy was initiated in 2011, and since then the ISAAC mobile laboratory has visited the site of the caves several times, applying a range of non-invasive imaging and spectroscopic techniques. More specifically, remote standoff spectral imaging has been applied on large mural surfaces using the PRISMS system in the telescope configuration.
This enabled the imaging of painted areas located also on the ceiling of the caves. Remote sensing hyperspectral imaging has been also applied on areas of special interest, using the high spectral resolution version of the PRISMS system in the telescope configuration.
The ISAAC mobile laboratory in cave 465, Mogao caves
OCT (Optical Coherence Tomography) scanning enabled the high resolution, depth resolved examination of the paint layers structure and sequence. Moreover, a range of spectroscopic techniques, from FORS to Raman and XRF spectroscopies, were used to provide complementary information, including detailed identification of the paint materials used.
(a) Colour image of the eastern panel of the southern wall of the main hall of Cave 465 showing Mahāmāyā and his consort Buddhaḍākinī in union; (b,d) Two zoomed-in areas from the top and bottom of the wall along with their corresponding spectral imaging cluster maps showing areas which share the same spectral information derived from a mosaic of PRISMS spectral imaging data (c,e) with a unique false colour assigned to each cluster; (f) XRF spectrum and (g) Raman spectrum of a region on the bottom figure (marked by a yellow circle) in one of the clusters represent by the dark green false colour in the cluster maps.
Making a Difference
In the context of this project, a methodology for the automatic analysis of large scale spectral imaging data was developed. Moreover, for the first time, a total non-invasive and in situ examination of various degraded paint layers was performed, revealing their original composition. The scientific examination of the murals provided very important information about the history of cave 465, placing its dating in the period between late 12th to the 13th century.
Kogou, S., Shahtahmassebi, G., Lucian, A., Liang. H, Shui, B., Zhang, W., Su, B. and van Schaik, S. 2020. From remote sensing and machine learning to the history of the Silk Road: large scale material identification on wall paintings. Sci Rep 10, 19312 (2020). https://doi.org/10.1038/s41598-020-76457-9
Liang, H., Lucian, A., Lange, R., Cheung, C.S. and Su, B., 2014. Remote Spectral Imaging with Simultaneous Extraction of 3d Topography for Historical Wall Paintings. ISPRS Journal Of Photogrammetry And Remote Sensing, 95, pp. 13-22.
Lange, R., Zhang, Q. and Liang, H., 2011. Remote Multispectral Imaging with Prisms and XRF Analysis of Tang Tomb Paintings. Proceedings of SPIE, 8084, 80840y
Liang, H., Keita, K., Vajzovic, T., and Zhang, Q., 2008. PRISMS: Remote High Resolution In Situ Multispectral Imaging of Wall Paintings. In: International Council of Museums, Committee for Conservation (ICOM-CC) Triennial Conference, New Delhi, 2008, New Delhi.
Liang, H., Keita, K. and Vajzovic, T., 2007. PRISMS: A Portable Multispectral Imaging System for Remote In Situ Examination of Wall Paintings. Proceedings of SPIE, 6618, 661815
Professor Su Bomin (Dunhuang Academy)
Dr Sammy Cheung (Nottingham Trent University)
Shui Biwen (Dunhuang Academy)
Zhang Wenyuan (Dunhuang Academy)
Yu Zongren (Dunhuang Academy)
Rebecca Lange (Nottingham Trent University)
Andrei Lucian (Nottingham Trent University)
Sotiria Kogou (PhD, Nottingham Trent University)
Alex Hogg (MSci, Nottingham Trent University)
Stuart Christian (BSc, Nottingham Trent University)