This 2000-Year-Old Pigment Can Eliminate The Third Dimension

by Esther Inglis-Arkell

Han purple is an ancient pigment that wasn't reconstructed by modern chemists until 1992. After the chemists got done with it, it was the physicists' turn. Han purple, they found, eliminates an entire dimension. It makes waves go two-dimensional!

The Chemistry of Han Purple

You'll see Han purple on the famous terracotta warriors surrounding the tomb of the first emperor of China, or on ancient pottery and other works of art. Where you won't see it is on anything made between 220 A.D. and 1992, because after the pigment disappeared it took 1700 years to re-discover it. Elisabeth FitzHugh, a conservator at the Smithsonian, pinned down the chemical composition of the pigment and announced it was a barium copper silicate. (The paper describing the discovery is a fun read. It starts by pointing out the inferiority of other ancient purple pigments, which tended to be closer to red than purple. It also stresses that Tyrian purple, made from sea snails, was a textile dye, not a pigment, and that it could range anywhere from "reddish-blue to purplish-violet." Take that, Phoenicians!)

Exactly how some inventor stumbled on a way to make the pigment is still a matter of debate. An early theory, not believed by many, is that the Chinese learned how to make purple pigment from the Egyptians. Egyptian purple pigment seems to be similar, but the chemical formulas don't add up — Egyptians used calcium instead of barium. It's also not an easy process to pass from one culture to another. To get the elements to melt together, they have to be heated to about 850-1000 °C.

Most researchers think that because it contains both silicon and barium Han purple was a by-product of the glass-making process. Barium makes glass shinier and cloudy, which means this pigment could be the work of early alchemists trying to synthesize white jade.

Han Purple and the Third Dimension

Barium copper-silicate doesn't just have archaeologists and chemists intrigued. At normal temperatures, it's an insulator and is nonmagnetic. Along with its many fine properties - prettiness, historical importance, a hint of aristocratic style — barium copper-silicat has many electrons, some of which spin up and some of which are spin down.

Something unusual happens as the temperature drops and as a magnetic field is applied, although the temperature has to drop pretty far, going down to between one and three degrees Kelvin, and the magnetic field has to be about 800,000 times the strength of Earth's magnetic field. The results are worth it — the electrons seem to merge, taking on one spin, and acting as one electron.

That sounds like an ordinary superconductor, you say. Then you're as foolish as a Phoenician in sub-par purple! Han purple still has a trick up its sleeve. Drop the temperature some more and something happens to the magnetic wave traveling through the substance. 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.

Scientists think that this has something to do with the structure of barium copper silicate. It's components are arranged like layers of tiles, so they don't stack up neatly. Each layers' tiles are slightly out of sync with the layer below them. This may frustrate the wave and force it to go two dimensional.

Anyone wonder if ancient physicists discovered this? And if the secret to making Han purple was lost because they waved themselves into two dimensions?