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How Lasers Can Preserve History

May 7, 2014

Central Park's 3500-year-old Egyptian obelisk has seen better days. Centuries of wear-and-tear have camouflaged the monument's ancient hieroglyphics, and now conservators are cleaning up with the help of high-powered lasers.


Cleopatra's Needle is an ancient Egyptian obelisk, but right now it looks like a construction project. Scaffolding snakes up the 69-foot spire as conservancy crews clear away 3500 years of soot and grime.


Originally built in honor of Pharaoh Thutmose III, the obelisk has black streaks that mark its time spent buried near the Nile, its travel across the Atlantic, and near 130 years facing the pollution-filled elements in New York City. Now it's getting a long overdue cleaning—using high-powered lasers.


The Central Park Conservancy, the organization tasked with the upkeep of all monuments and works of art in Central Park, has had the needle's cleaning on its to-do list since 2011. The Conservancy and personnel at the Metropolitan Museum of Art's photo studio began documenting the obelisk and assessing exactly what the monument needed. Soon conservators came in to test different types of cleaning techniques. "The typical ways of cleaning are using chemicals or micro-abrasive cleaning," says Marie Warsh, director of preservation planning for the Conservancy. "Those tests revealed that the lasers were the most sensitive."


The $500,000 conservation project was under way, but centuries of soot and an intricate maze of hieroglyphics presented a unique challenge for Bartosz Dajnowski and his team. "It's difficult to make out a lot of the hieroglyphs because the black streaks are like camouflage, like stripes running across it," says Dajnowski, vice director of Conservation of Sculpture and Objects Studio, the art conservancy group responsible for restoring the obelisk.


Nestled behind the Met, you can hear the steady hum of generators powering a collection of refrigerator-sized machines as Dajnowski describes the atomic chaos behind laser preservation to us.


"The key to it is discrimination," he says. "We have a discriminating factor between the layer we're removing and the layer we're trying to protect...We can choose parameters, such as wavelength, to absorb specific materials." Dajnowski uses the metaphor of two cars—one black, one white. The black car heats up quicker because its color absorbs light as the white car reflects. In terms of Cleopatra's Needle, the infrared wavelength of the laser is set to a specific distance, in this case 1064 nanometers, and can absorb into the carbon and soot deposits but not the obelisk's stone facade.


It's at the atomic level that things get interesting. "What happens is that layer absorbs the laser light," he says. "The atoms absorb the energy and they expand rapidly from the accumulation of energy, and by expanding so fast, they get blasted apart and they eject off the surface." Imagine a beach ball inflating at the speed of an airbag, says Dajnowski. In other words, explosive. In the swirling atomic aftermath, dirt is obliterated leaving only Egyptian artwork in its place.


The team has seven lasers, some on hand only for backup. The average laser is about the size of refrigerator, though Dajnowski developed his own laser, the GC-1 (the GC stands for "Game Changer"), that's much smaller and portable. The lasers can clean up to several square feet per hour with Dajnowski's own unit upping the efficiency to 10 square feet an hour.


However, with lots of intricate hieroglyphics, Cleopatra's Needle will be more of slow crawl. As the conservators move across the obelisk, they control energy density, pulse duration, scan pattern, and focal distance. Essentially a fiber laser is generated in the device, funneled through a fiber wire into what looks like a heavy-duty handheld scanner. Conservators then hold the scanner at the predetermined focal point in front of the monument to get the best results. The laser works most effectively when it's in focus and perpendicular to the stone surface, and with so many gouges and bumps in the stone face, whether made by the elements of the original Egyptian stone mason, cleaning the Needle will require a little more attention than the average project.


Depending on the weather, the team could be working away in Central Park for several weeks to almost two months. But Dajnowski doesn't mind. Standing at the foot of the monument, he mentions the poetry behind his work. "On this monument there's a lot of references to Ra, the god of the Sun," he says. "How poetic that we're conserving it with light."


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