The chemistry of ancient color
The plains of Bagan are dotted with over 2,200 temples and pagodas, most built between the 11th and 13th centuries. Inside many of these structures, murals depict religious narratives and royal life. For centuries, these paintings remained in near-total darkness, which preserved their original materials. Modern scientific analysis reveals the specific chemical compounds the Bagan artists used for their palette.
Researchers use a suite of non-invasive tools to analyze the pigments without damaging the artwork. Portable X-ray fluorescence (ED-XRF) and Raman spectroscopy identify the mineral-based pigments. The reds are predominantly hematite (Fe₂O₃) and vermillion (HgS), while blacks are carbon from lampblack. Arsenic-based compounds like orpiment were sometimes used for yellow tones. To bind these ground minerals to the plaster walls, artists mixed them with organic materials. Gas chromatography-mass spectrometry (GC-MS) has identified plant-derived gums, such as acacia, and proteinaceous glues as common binders. This painting method, applying pigments mixed with a binder to a dry plaster wall, is known as the a secco technique.
Fading with the lights on
The fragile components of Bagan's murals are the organic dyes, which were used alongside the more stable mineral pigments. These dyes, derived from plants, are susceptible to photo-oxidation, a chemical reaction triggered by light that breaks down the color-producing molecules. For the nearly 900 years that the temples were dark, unvisited interiors, these organic colors remained intact. The opening of the temples to tourism and the installation of artificial lighting introduced the energy needed to initiate this degradation, causing the colors to fade.
Scientific studies, for example at the Me-taw-ya temple, use techniques like Fourier-transform infrared spectroscopy (ATR-FTIR) to analyze the organic materials. These methods can distinguish between the original plant gums used as binders, such as those from the Neem or Acacia catechu tree, and modern polymers like polyvinyl acetate that were sometimes applied during past conservation efforts. This detailed chemical knowledge helps conservators understand the murals' original composition and the specific causes of their deterioration. The analysis shows that the artists used a multi-layer technique, applying the paint over one or more layers of plaster made from lime, sand, and sometimes mud.