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ARCHAEOLOGISTS visiting the Mayan ruins at Chichén Itzá in
Yucatán, Mexico, have always been struck by the vivid blue paint
appearing repeatedly in murals and on pottery. Now a team of Mexican scientists
has found out why the colour survived centuries of heat and moisture without
fading. The research may even help in the today’s quest for brighter and more
durable paints.

Archaeologists have known for decades that the paint, called Maya blue, was
made of indigo and a white clay called palygorskite. But just like the colour in
blue jeans, indigo fades away with time and washings. The puzzle was how the
Mayans made their indigo last.

The clay is the key. By peering at late classical (AD 600-900) murals with an
electron microscope, an interdisciplinary team of scientists has found that
needles of palygorskite expand when mixed with indigo.

The enlarged, cage-like arrangement of atoms may shield the molecules of
indigo from the ravages of time, says Miguel José-Yacamán, a
materials scientist at the University of Mexico in Mexico City.

But sceptics remain. No direct proof exists that indigo chemically binds to
the lattice, says Patrick McGovern, a chemist and archaeologist at the
University of Pennsylvania.

In any case, indigo alone does not make a brilliant blue.
José-Yacamán and his colleagues discovered the secret ingredient:
tiny particles of metal, a million times smaller than the full stop at the end
of this sentence. These nanoparticles are mostly iron, with some manganese,
chromium and titanium (Science, vol 273, p 223).

While pigments made of iron usually look red, the extremely small size of
nanoparticles makes them look blue. Nanoparticles scatter light in a nonlinear,
quantum fashion. In Maya blue paint, the atoms in the metallic particles vibrate
near the wavelength of the re-emitted light. This resonance amplifies the light,
helping give the paint its brilliance.

Maya blue is unique to Mesoamerica and is unlike the blue of ancient or
medieval paintings from Europe. “As far as I know, no paint in Europe had
nanoparticles,” says José-Yacamán. “You can’t produce them by
crushing.” And only the Mayans made paint from palygorskite, he adds.

The Mayans produced the paint with sophisticated techniques, says
José-Yacamán. He found an almost identical composition in the
eight paint samples, even though they came from sites dozens of kilometres
apart. “It’s consistent with the highest quality control of modern paints,” he
says.

To find the source of the nanoparticles, the researchers tried making their
own Maya blue paint. They took palygorskite from an ancient Mayan mine near
Mérida, the capital of Yucatán. When boiled with indigo from the
xiuquilit plant—the local name for species of Indigofera
—the clay turned to Maya blue. The microstructure exactly matched the
ancient paint. The critical nanoparticles probably came from the plant, which is
typically iron-rich in the Yucatán, José-Yacamán says.

The team plans to analyse other paints and murals from Mayan sites.
José-Yacamán says Mayan techniques and ingredients could produce
Maya blue in commercial quantities. If so, modern designers and artists could
soon be benefiting from the brilliance of ancient technology.