In 1974, while sinking irrigation wells in the Chinese province of Shaanxi, a
group of farmers made an astonishing archeological discovery. Buried beneath
their feet stood a contingent of 8,000 life-sized terra cotta warriors and
their horses, facing east, ready for battle. The figures, later found to be
more than 2,000 years old, were accompanied by weapons, real chariots, and
objects of jade and bone. It was later determined that this army was built to
protect the tomb of the first emperor of the Qin dynasty, Shi Huang Di.
Today many puzzles still surround these statues. One of the most intriguing of
these mysteries is the origin of a synthetic purple pigment, often called
"Chinese purple" or "Han purple," that their ancient creators used to adorn the
soldiers and their accessories.
Until the 19th century, most pigments were made from naturally occurring
minerals or organic dyes. One well-known exception is "Egyptian blue," which is
chemically very similar to the Chinese purple. Because the Egyptian and Chinese
pigments are so chemically close in composition, some have proposed that the
technology to create Chinese purple was in fact derived from the Egyptians. If
that were so, Chinese purple would represent one of the earliest known cases of
cross-cultural technology transfer, dating to a period before the Silk Road
opened China to the west, even before the invention of paper or the compass.
To test this hypothesis, SSRL researchers Zhi Liu, Apurva Mehta and colleagues
from Stanford University, the Advanced Light Source and China used a series of
synchrotron-based x-ray techniques, in particular micro-x-ray diffraction and
micro-x-ray fluorescence, to characterize the exact chemical make-up
of the pigments in Chinese purple. Liu found that deep within the structure of
Egyptian blue, calcium plays a key role in the pigments' molecular identity,
whereas in Chinese purple, that same slot is filled by the element barium. Liu
concluded that, based on this difference, and despite the two pigments'
structural similarity, Chinese artisans invented Chinese purple independently
of Egyptian influence.
"Stable purple is rare in nature," Liu said. "People already knew what the
pigment was, but by making a chemical map and combining it with archeological
evidence, we were able to solve the puzzle of where it came from."
Scholars believe that Chinese alchemists 2,200 years ago were most interested
in synthesizing jade, a mineral regarded as sacred in Taoist culture. Liu's
analysis found that, based on the combination of compounds used to make the
pigment and the technologies available to Chinese artisans at the time, Chinese
purple was most likely first developed by Taoist glassmakers attempting to
create artificial jade.
To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/research/highlights_archive/chinesepurple.html
Z. Liu, A. Mehta, N. Tamura, D. Pickard, B. Rong, T. Zhou, P. Pianetta,
"Influence of Taoism on the invention of the purple pigment used on the Qin
terracotta warriors", J. Archaeol. Sci. (2007), doi:10.1016/j.jas.2007.01.005
