Influence of Taoism on the invention of the purple pigment used on the Qin terracotta warriors

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Abstract

Until the 19th century, most pigments were based on naturally occurring colored minerals and dyes, with three significant exceptions: Egyptian Blue, Chinese Blue/Purple and Maya Blue. The former two are alkaline-earth copper silicates, and because of this similarity it has been proposed that the Chinese pigments were derived from Egyptian Blue. Herein, we analyzed clumps of pigment from the Qin warriors and discovered that in spite of the structural similarity to Egyptian Blue, the micro-structural morphology of Chinese Purple is very different. Therefore, we believe that the synthesis technology for the Chinese pigments was a by-product of high-refractive index glasses (artificial jades) produced by Taoist monks. Further, the disappearance of these pigments from Chinese art and monuments concurrently with the decline of Taoism not only substantiates the link between the two, but also gives a striking example of how cultural changes in the society affected the scientific developments in ancient China.

Introduction

In March 1974, Chinese farmers made a remarkable archaeological find: during the sinking of wells for farmland irrigation construction near Xi'an (Shaanxi province, China) they discovered an army consisting of more than 8000 life-size terracotta figures of warriors and horses dating from the First Emperor of the Qin dynasty, Shi Huang Di (reigned ca 221 BC–ca 210 BC). The figures, facing east and ready for battle, were individually modelled with their own personal characteristics, and were accompanied by their weapons, real chariots, and objects of jade and bone. How, more than 2000 years ago, the ancient Chinese constructed these large and heavy statues and what technologies they used to finish such a large project are questions which are still only partially answered by modern archaeologists.

The discovery that BaCuSi2O6 (FitzHugh and Zycherman, 1983, FitzHugh and Zycherman, 1992), also known as “Chinese Purple”, was the main constituent of the purple pigment used in the paint covering the warriors constitutes an enigma in itself. This pigment was also used later in the Han dynasty in pottery (hence its other common name of “Han Purple”) and for trading. BaCuSi2O6 is a mineral that has never been found in nature, which implies that the makers of the warriors must have been able to synthesize it. The process to synthesize BaCuSi2O6 is now known to be highly complex (Berke and Wiedemann, 2000, Berke, 2002) and how the early Chinese chemists managed to synthesize barium copper silicates in an almost pure form, even preceding the invention of paper and the compass, is a mystery. Interestingly, these same materials are now being studied to gain insights into the mechanisms of high temperature superconductivity (Jaime et al., 2004, Sebastian et al., 2006).

In a detailed study, Berke (Berke and Wiedemann, 2000, Berke, 2002) showed that the manufacture of Chinese Purple was a very complicated process and that barium (BaSO4 or BaCO3), copper and lead compounds as well as quartz were used in the preparation. He pointed out that lead oxide played a very important role as a catalyst in transforming barite (BaSO4) into barium oxide (BaO). At 900–1100C: BaO + CuO + 2SiO2 = BaCuSi2O6; since BaSO4 decomposes at a much higher temperature (1560 C), PbO catalyze a dismutase reaction leading to the in situ decomposition of BaSO4 (PbO + BaSiO4  BaO + PbSO4). Berke discussed the striking similarities of the Chinese Purple and Chinese Blue (BaCuSi4O10) with the Egyptian Blue pigment (CaCuSi4O10) (Riederer, 1997). He conjectured a connection between the manufacture of the two pigments in the form of technology transfer from the makers of Egyptian Blue to the makers of Chinese Purple, and proposed that the Chinese Purple was in fact derived from the Egyptian Blue (Berke and Wiedemann, 2000, Berke, 2002). This would have been the earliest technology transfer between these two ancient civilizations. This supposition, however, leaves many unanswered questions. First, it is unlikely that the Chinese chemists could have acquired the technology (not just the pigment) from Egypt well before the official “silk road” (125 BC). Some earliest Chinese Purple samples date back to the “Warring States” period (479–221 BC). Considering the time needed to develop Barium based pigments, this technology transfer, if there was one, must have happened well before the “Warring States” period. But even if there existed a connection between China and Egypt, it doesn't explain why the Chinese decided to substitute Ba for Ca (Kerr and Wood, 2004) and face the challenges related to the consequent elevation of the synthesis temperature. Egyptian Blue forms at ∼800 °C–900 °C (Berke, 2002, Riederer, 1997), whereas Chinese Purple starts to form between 900–1100 °C and Chinese Blue at temperatures in excess of 1100 °C (Berke and Wiedemann, 2000, Berke, 2002). An additional problem with the Egyptian-Chinese connection theory is that, to our knowledge, no Ca-bearing Egyptian Blue has been found in China.

In order to address these questions, we re-examined the chemistry and the morphology of purple pigments found on one of the Qin Terracotta warriors (Fig. 1). By combining our findings of the technology used in the synthesis of Chinese Purple with existing archaeological evidence, we conclude that Taoist alchemists invented this pigment as well as the related pigment Chinese Blue independently from any Egyptian influence.

Section snippets

Experimental methods

Our investigation was based on a two pronged approach. We used a small fraction of our specimen, ground it into fine powder and used synchrotron radiation high-resolution powder X-ray diffraction (XRD) analysis to identify the crystallographic phases present. Then based on this inventory, we used spatially resolved X-ray and electron micro-beam techniques, such as micro X-ray diffraction (μXRD), micro X-ray fluorescence (μXRF) and Scanning Electron Microscopy (SEM) based Energy Dispersive X-ray

Results

Fig. 2 shows the powder XRD pattern of the purple pigments, alongside a diffraction spectrum of BaCuSi2O6 obtained from the International Centre for Diffraction Data (ICDD, No. 00-043-0300). The majority of the diffraction peaks found in the purple pigments belong to BaCuSi2O6. Some peak intensities of the sample do not follow the intensities of BaCuSi2O6 standard exactly. It is know that the silicate minerals are very prone to preferred orientation. For such a textured material, the

Discussion

Our results show that the process and the technology for making Chinese Purple are quite different than that used for Egyptian Blue. Use of lead fluxes plays a crucial role in lowering the synthesis temperature and stabilizing Chinese Purple over Chinese Blue and forms the foundation of this pigment synthesis technology. Furthermore, the combination of lead and barium compounds in the synthesis of the pigment suggests a plausible identity of the inventors of this technology as will be discussed

Conclusion

In summary, we argue that Chinese Purple was invented by Taoist alchemists as a by-product of the technology originally developed for synthesizing barium-containing Chinese glasses, which, in turn, were originally developed for the purpose of imitating jade. The barium compounds were added to increase the refractive index of the glass, thus giving the glass a similar appearance as jade. The development of this process also benefited from two well-developed technologies in ancient china: the

Acknowledgement

The research was partially funded by the France-Stanford Centre for Interdisciplinary Studies. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences; and the Advance Light Source which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Dept of Energy.

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