Search Engines: Serendipity A longer lasting blue

IN THE Seventies, a Mexican archaeologist by the name of Louis Torres became interested in the staggering Mayan ruins at Chichen-Itza. He was fascinated by the intricate murals which adorned the Temple of Warriors, and was particularly struck by a vivid blue paint, known as Maya blue, which was still brilliant, despite 1,000 years of weathering.

Torres sent a sample of Maya blue to Professor Miguel Jose-Yacaman, an amateur archaeologist and a physicist at the Nuclear Institute of Mexico. He examined the flakes of paint under his electron microscope, but he could see nothing that explained the paint's peculiar shade of blue or its longevity. In the end, he lost interest, put the samples in a drawer, and forgot about them.

Twenty years later, Yacaman was invited to speak at a conference on the Mayan civilisation. In desperation, he dug out the Maya blue samples, dusted them off, put them under his spanking new high-power electron microscope, and turned it up to full blast. Suddenly, the secret of Maya blue was revealed.

Maya blue is a natural blue dye, from a local variety of indigo. Generally, the problem with indigo is that it is easily washed away, which is why jeans fade so easily. However, the main constituent of Maya blue is palygorskite, a clay which acts as a cage for trapping molecules of indigo and prevents them leaching out.

Yacaman identified another ingredient, in a discovery that had repercussions for his main research project. The paint contains microscopic grains of metal which make the blue much more vivid. These grains act in a similar way to dust particles in the atmosphere. Atmospheric dust particles scatter sunlight, intensifying red light to the detriment of other colours. This is why sunsets over polluted cities are often deep red. The size of the particles determines which colour is enhanced. The 19th-century physicist Michael Faraday demonstrated the effect with gold particles suspended in water. Depending on the size of the gold particles, the water varies in colour from yellow to blue. In the case of Maya blue, the metallic grains are less than 10 millionths of a millimetre in diameter, just the right size to preferentially intensify blue light.

Inspired by Maya blue, scientists are beginning to introduce the use of microscopic particles in an attempt to enhance the colours of a range of optical devices, such as light-emitting diodes (LEDs) and miniature lasers, which is what Yacaman is working on at the moment.

For physicists, one of the main technical problems is how to make particles of just the right size, typically just a few dozen molecules in diameter. For archaeologists, the problem is trying to figure out how the Mayans succeeded in making such particles more than 1,000 years ago.

Simon Singh is the author of `Fermat's Last Theorem' (Fourth Estate, pounds 6.99)