AsianScientist (Aug. 5, 2014) – Scientists have identified the binding material used to apply pigments to the two thousand year old terracotta soldiers buried with Chinese emperor Qin Shihuang. The research has been published in the journal Chinese Science Bulletin.
Efforts at preserving or restoring the terracotta army have been hampered by a lack of understanding of the materials used to create and lock in colors. Archaeological excavations and research conducted since the discovery of the First Emperor’s polychrome army have revealed that “the surfaces of the terracotta warriors were initially covered with one or two layers of an East Asian lacquer… obtained from lacquer trees,” according to authors Drs. Yan Hongtao and An Jingjing, scientists at the College of Chemistry and Materials Science, Northwest University, in the Chinese city of Xi’an.
Prior research has shown that the polychrome layers applied to the terracotta soldiers were composed of natural inorganic pigments and binding media. These pigments have been identified as including cinnabar [HgS], apatite [Ca5(PO4)3OH], azurite [Cu3(CO3)2(OH)2] and malachite [Cu2CO3(OH)2], etc., but the precise composition of binding media used in the painting process had long eluded scientists.
Studying the binding material has been challenging because only extremely low levels of the proteinaceous binding media have survived after being submerged in almost six meters of water-saturated loess for more than two millennia.
“Following almost 22 centuries of storage under these conditions, the remaining pieces of original polychromy that have survived on the sculptures contain extremely small amounts of the binding media,” the researchers wrote in an article.
“A large amount of the polychromy has already been lost through pillaging as well as damage resulting from fires and the long-lasting effect of water,” they explained.
To solve the more than 2000-year-old enigma, these researchers used matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) to identify the binding material. MALDI-TOF-MS offers high levels of sensitivity, requires only a minimal sample pretreatment process and can be used to reliably identify different types of proteinaceous material.
The researchers prepared “artificially aged” model samples by mixing different pigments with either animal glue or an adhesive concocted from free-range chicken eggs. To replicate the processes involved in the degradation of the pigments and binding media of the actual terracotta warriors, the model samples were buried in loess soil at a depth of one meter for a year.
Historical samples of the polychrome terracotta army were obtained from the Museum of Emperor Qin Shihuang’s Terracotta Army in Xi’an to facilitate a comparative analysis.
The peptide mass fingerprints of the historical samples were found to be very similar to those of the animal glue model samples, having most peak masses in common. This indicates that the binding material used was based on animal glue, even though this proteinaceous material underwent significant changes in terms of protein content during the two millennia the terracotta army was deployed underground.
“To the best of our knowledge,” wrote the researchers, “this work represents the first account of the proteinaceous binding media from a 2200-year-old historical sample in China being identified by MALDI-TOF-MS.”
Source: Science China Press; Photo: Fabio Achili/Flickr/CC.
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