A Lost Purple Pigment, Where Quantum Physics and the Terracotta Warriors Collide

The Terracotta Warriors in China (photo by Kevin Poh/Flickr)

The connection between contemporary quantum physics and China’s ancient Terracotta Warriors is a lost pigment called Han purple. The vibrant hue appeared in the Zhou dynasty and faded out sometime near 220 AD; art didn’t see a purple as vivid until 19th-century manufacturing.

Han purple has strange properties, particularly at low temperature points. Back in 2006, researchers at Stanford, Los Alamos National Laboratory, and the Institute for Solid State Physics described this phenomenon as a “Flatland.” When exposed to extremely low temperatures, magnetic waves going through the pigment lose their third dimension. Recently Esther Inglis-Arkell at io9 returned to this research, explaining:

At higher temperatures, it propagates like a regular wave, traveling in three dimensions. Get under one degree Kelvin, and it no longer has a vertical component. It propagates in two dimensions only.

This fluctuating state of matter, likely caused by the pigment’s diversely layered barium copper silicate structure, isn’t seen often. We’ve examined obsolete pigments at Hyperallergic before, whether a brown made of actual mummies or poisonous arsenic greens. Han purple is one of the first known synthetic pigments, and its rarity made it a powerful color.

This is where the Terracotta Warriors come in, as the Qin dynasty funerary army retains traces of the color (although its hues largely oxidized after exhumation). Some have speculated that the purple came via the Silk Road, with information from Egypt and its famous blue traveling the distance; however, at Symmetry, a particle physics online magazine, Lori Ann White writes: “Researchers discovered that Chinese pigment-makers used lead to lower the melting point of the barium in Han Purple, a step not taken in the production of Egyptian Blue.” She goes on to say that glass makers “in ancient China may have stumbled on Han Purple while trying to develop a jade-like glass, a process that also involved lead.”

The peculiar Han purple, sourced from a byproduct and with its two-dimensional properties, may also have a brand new technological purpose: some scientists are looking to the research on it to help inform the process of building quantum computers.