Laser beams reduce 'street art' to powder. Robert Uhlig on a discovery that promises social and environmental benefits
PAINT-STRIPPING is turning to the high-tech tools of space in the war against graffiti.
A scientist at Lawrence Livermore National Laboratory in California has successfully completed experiments that use a laser to turn spray-painted graffiti into powder without damaging the underlying surface.
Dennis Matthews, who works in the laboratory's laser development department, stumbled on the idea when a neighbour complained that a piece of "street art" had appeared on a building near his home. He used one of the laboratory's lasers to test his theory - and it worked.
When the laser hits the surface layer of paint, the beam converts into a sound wave which passes through the paint, strikes the underlying surface and rebounds. The rebounded sound wave collides with the incoming beam and the paint disintegrates into powder.
"I was astounded," said Matthews. "I thought it would work in principle, but I couldn't believe the actual result. After we turned the laser on a piece of graffiti-covered wood, the wood looked like it was fresh from the timber yard."
All the methods now used to remove graffiti have shortcomings. Sandblasting is regarded as an environmental hazard by some authorities because it creates sand and paint particles so small they remain airborne and so become potentially harmful.
The preferred alternative to sand blasting is soda blasting. A quantity of sodium bicarbonate - to create abrasive friction - is mixed with water and applied in a high pressure spray. But soda blasting is not very efficient and pavements and sewage systems become filled with sodium bicarbonate which, while not a designated hazard, is not entirely benign.
Worst of all, soda blasting "crazes", or scars, the surface beneath the paint, and ruins highly finished surfaces, such as polished granite.
Matthews is aiming for a laser graffiti apparatus that would fit in a van. He believes a prototype van would probably cost $1 million ( £650,000), including development of a portable version of the laser he has in mind. Production models, he says, would drop to around £60,000, which compares well with the cost of soda blasting vans. Only two people would be required to operate a laser unit, rather than three for the soda blaster, the laser would operate some 100 times faster than a soda blaster, and the environmental hazard would be eliminated.
Matthews plans to equip his prototype with a vacuum mechanism to suck off the paint as it turns to powder.
"When the laser is finished doing its thing, there would be no particulates in the air, no chemicals splashed on the sidewalk and not even powdered paint lying on the ground," he said.
LASERS are very efficient at removing paint not only from hard surfaces like marble, but also from porous surfaces such as concrete and masonry - but they have trouble with painted metal street signs. The laser is unable to distinguish between the sign's paint and the graffiti paint, and removes both.
Safety considerations would include operator goggles and a shroud or cloak made of dark glass around the end of the laser arm to shield passers-by from the laser light, which can be harmful if shone directly into the eye. There is a danger of surface burns should a laser be turned on humans or animals, but Matthews says he would minimise this risk by designing the optics so the beam was focused for only a few inches from the end of the wand, beyond which it would scatter.
