In situ Raman microscopy applied to large Central Asian paintings

摘要

Procedures and versatile Raman instruments are described for the non-destructive in situ analysis of pigments in large paintings. A commercial Raman microscope is mounted on a gantry for scanning paintings with dimensions exceeding 1 m(2). Design principles and the physical implementation of the set-up are outlined. Advantages/disadvantages and performance of the gantry-based instrument are compared with those of a mobile Raman probe, attached to the same Raman microscope. The two set-ups are applied to Central Asian thangka paintings. The utility of the gantry-mounted Raman microscope is demonstrated on a 19th century Buddhist painting from Buriatia, South Siberia. Surprisingly, three arsenic-based pigments, i.e. orpiment, realgar, and pararealgar, are found all in the same painting. Pararealgar is used for painting the numerous yellow areas. Realgar is admixed with red lead for adjusting its orange tint. Finally, orpiment is blended with Prussian blue for producing green. Traditional malachite is used in addition as a non-adulterated green pigment. The mobile Raman probe was employed for examining a Tibetan painting of the 18th century from Derge monastery in the Kham area of Sichuan. The highly unique painting could be dated well and its origin accurately located. In fact, the painter's workshop, where the thangka has been executed, is shown in great detail on the painting itself. The painter's palette of this thangka matches the canonical set of pigments used in Tibet for more than 10 centuries.