Apr 2, 2013
An ancient Chinese purple colorant, often called Han purple, is one of the earliest known man-made pigments.
You can see Han purple on the famous Terracotta Army of Xi’an, now on view in the exhibition China’s Terracotta Warriors: The First Emperor’s Legacy, and on a few rare artifacts in the permanent collection. Here at the Asian Art Museum, we are hunting for Han purple and surveying our early Chinese ceramics to find evidence of this special color. The objects shown here are a lidded box and some beads.
Colors have always carried symbolic and emotional cultural significance, and purple is among the rarest and most desirable of colorants. Because it was so scarce, it was often restricted to the wealthy or powerful. The use of purple paint on these artifacts shows the importance and high status of these burial sites as well as the sophisticated aesthetic of the time. The decorated surfaces, using contrasting colors against a dark background, may have imitated fine lacquerwares of the period.
Han purple and Han blue, along with the closely related color Egyptian blue, have been making news in the world of art conservation. Scientists at the British Museum have recently published research describing the unique light-absorbing and emitting properties of these copper silicate compounds. The pigments emit powerful rays of light in the near-infrared range (around 900 nm) when exposed to a simple LED flashlight. Under the right conditions, the pigments show up with startling clarity under infrared sensors. Because Han purple reacts so strongly, even very faint traces of the color— invisible to the naked eye— can be seen with this enhancing technology.
Purple hues are most often made by mixing red and blue, but a few true violet colorants such as Han purple are found in nature. This bright lavender pigment and its near relative, Han blue, are misnamed, since both colors have been found well before the Han dynasty in China, perhaps as early as the Western Zhou dynasty (1045-771 BCE). The colors first appear on glass beads found among grave goods in wealthy burials. Later, the pigments were used in paint to decorate ceramics, wall paintings and metal wares. The popularity of Han purple peaked in the Qin and Han dynasties (221 BCE to 220 CE), and declined during the Tang dynasty (618-907 CE).
Very few pigments were chemically synthesized before the modern era– most are made by simply grinding up colored stones. Natural ultramarine (from ground lapis lazuli), appears in Afghanistan around the 6th century CE, and Maya blue (a cooked compound of indigo and white clay) appears in Mesoamerica around 800 CE. Only Egyptian blue is found earlier than Han purple: it appears about 3600 BCE on wall paintings in Egypt. At other museums, conservators have also detected traces of Egyptian blue on Greco-Roman marble sculptures and Assyrian reliefs.
Han purple is made by melting silica (sand) with copper and barium at very high temperatures – 850-1000 °C – to create barium copper silicate (BaCuSi2O6). The ingredients and manufacturing process are closely related to ceramic glaze technology, and the color appears early on in glazes and glass. Lead is also found in some formulations, and may have been added used to lower the melting point. Han purple can fade and decompose over time, particularly in contact with acids or heat. Han blue (BaCuSi4O10) and Egyptian blue (NaCuSi4O10) are more stable in chemical structure and fade less easily.
Pigments are made from inorganic materials such as powdered minerals, and are far more durable than dyes.Dyes are organic compounds, and are traditionally made from plants or animals. Tyrian purple and natural madder, two other precious purples used in ancient Greece and Rome from around 600 BCE, are organic dyes. Both tend to fade quickly from light exposure and deteriorate. In contrast, Han purple has survived for thousands of years in Chinese tombs and wall paintings.
Scientists have recently undertaken new experiments with Han purple, using samples taken from the Xi’an warriors. One group has studied the relationship of the pigment to glass and glass manufacture, and how it was used. Stanford physicists Liu Zhi, Apurva Mehta, et al. examined pigment samples from the Xi’an warriors and published their findings. See Liu, Z. et al, “Influence of Taoism on the invention of the purple pigment used on the Qin terracotta warriors,” Journal of Archaeological Science 34 (11):6 (2007). A summary article is available here. Other scientists have focussed on the physical properties of the pigment itself. Researchers at Stanford and elsewhere have found that when Han purple is exposed to extreme cold and a high magnetic field, the chemical structure of the pigment enters a new state that helps explain the physics of superconductors. This PBS video shows some of the physics experiments currently underway.
http://www.ldeo.columbia.edu/edu/eesj/gradpubs/GeneralMags/patel_chinesepurple_Archeology.pdf
http://en.wikipedia.org/wiki/Han_purple_and_Han_blue
http://en.wikipedia.org/wiki/Egyptian_blue
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