Arts & Culture: Going the Extra Tile
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The Temple of the Emerald Buddha in Bangkok is considered the most sacred temple in Thailand, and it ranks in the city’s top ten tourist attractions. Tens of thousands of people flock there daily to view the ornately decorated building (Fig. 1) and the 26-inch, gold-clothed jade Buddha it houses. But the modern world’s pollution is taking a toll on the temple. Physicists are now involved in a restoration project aimed at bringing the temple back to its full glory.
The outside walls and pillars of the hall containing the Emerald Buddha sparkle with mirrored glass mosaics. These red, blue, green, yellow, and colorless decorative tiles were added to the temple during renovations in the 1830s. But over the last fifty years or so, air pollution from factories and cars has degraded the natural latex adhesive used to hold the tiles in place. Look closely at the mosaics and you will notice that many tiles are missing, having fallen off and ended up in landfill. Conservators at the temple have attempted to restore the mosaics using modern glass. However, today’s glass is aesthetically different from the nearly two-hundred-year-old glass that has been lost (Fig. 2). In an effort to make new tiles that more closely resemble the old, conservators drafted a team of physicists led by Wantana Klysubun at the Synchrotron Light Research Institute in Thailand.
The colors of the modern glass tiles currently in use at the temple are far brighter, almost garishly so, than the antique tiles. “I think the newer glass mosaics are rather ugly, while the antique glass mosaics are really beautiful,” said Bruce Ravel, a scientist at the National Institute of Standards and Technology in Maryland, who presented the project earlier this month at a meeting of the New York State Section of the American Physical Society on physics and the arts. Ravel has been working with Klysubun’s team to uncover the coloration mechanisms and chemical composition of the original mosaic tiles, so that conservators can accurately recreate them.
The researchers’ analysis of the tiles, performed using x-ray spectroscopy, shows that the base material for the tiles is leaded glass, made up of 55% lead oxide and 45% silica. This kind of glass is still used today and is found in expensive crystal wine goblets. Its color comes from impurities present in the glass. The yellow, blue, green, and colorless glasses all have slightly different amounts of iron, manganese, and cobalt; for example, the yellow glass has more iron and the blue glass more cobalt. But none of these impurities can turn the glass red, presenting a mystery that the researchers have been trying to unravel.
“[The red glass] is really different from the other four,” said Ravel. Analysis of the red glass tiles shows that, in addition to iron, cobalt, and manganese, these tiles also contain gold. The amount of gold present is minute, around 50 parts per million, but because the element is dispersed within the glass as nanometer-sized particles, it’s enough to change how the glass absorbs, reflects, and transmits light. “The Thai glass craftsmen of the 19th century were nanoengineers,” said Ravel. The nanoparticles absorb light in the green and yellow part of the spectrum but let the red light through, causing the tiles to appear red to the human eye (the blue is absorbed by other impurities present).
Now that the researchers know exactly what elements go into making each colored glass tile, Klysubun’s group has started to make tiles in their lab and has successfully reproduced mosaic patterns. But the glass-making process has opened a host of questions regarding how the craftsmen actually made the glass. It turns out that you can’t just chop up gold and toss it into melted glass and magically get the nanoparticles needed to turn the glass red. “You have to do chemistry,” said Ravel. The team has been using gold chloride, which, when mixed with tin oxide, reduces to metallic gold. But making the gold chloride in the first place is tricky and dangerous. The process requires pure chlorine gas, which would have been hard to create and handle with 1830s technologies. The gas is also corrosive, highly toxic, and might explode if impurities do find their way in. “That’s clearly not something they did in the 1830s,” said Ravel. Unfortunately the craftsmen never recorded their glass recipes; this part of the glassy mystery may have to stay unanswered.
–Katherine Wright
Katherine Wright is a Contributing Editor for Physics.