Ruth A. Mendonsa
The French, rightfully proud of their architectural heritage, spend about $8.6 million a year on high-technology restoration work, and 20 percent of that money is used to clean historic buildings using environmentally friendly techniques. Cleaning stone with lasers is an expensive undertaking and usually involves transporting heavy, cumbersome laboratory equipment to the site. Thanks to a European research project called Brite Euram, a laser system has been developed that is compact and lightweight, and saves time and money.
The system, manufactured by Quantel, is called Lama for a French acronym meaning portable laser for cleaning facades and historic buildings. It comprises two modules, a self-contained cooling system and a power supply unit with an Nd:YAG laser source that operates in Q-switched mode at 1064 nm. A 36-foot-long cord protects four optical fibers that feed the laser light to a handpiece that weighs less than 3 pounds and is the size of a camcorder.
Unlike the traditional bulky lasers used to restore architecture, the Lama laser delivers its light via fiber optics to a 3-lb handpiece. On a stone portico (above), the right-hand section was cleaned by microsanding, while the left and central sections were cleaned with the Lama laser, preserving the stone's yellow patina.
Cheaper, more efficient
The laser's mean output power is 20 W compared with 3 to 15 W for other available systems. The company says that because the speed of cleaning is proportional to the mean output power, Lama is seven to nine times more efficient and the cleaned square foot is much cheaper than with previous systems.
The laser system's optical fibers are key to its success. Not only do the fibers provide greater maneuverability than lab lasers outfitted with an articulated arm, but the beam delivered through the fiber achieves a spatially flat, rather than Gaussian, profile. With a Gaussian profile the difference in intensity between the center and the edge of the beam can lead to a damaged substrate. With a flat profile the cleaning action is completely even and all of the available energy is used.
The dark-colored stains absorb the 350-mJ maximum light energy. They are then vaporized into a rapidly expanding ionized gas. The resulting shock wave removes the remainder of the stain by mechanical action. The 10-ns pulses prevent the heat from diffusing into the substrate.
This technique has a double threshold: Below the lower level of energy intensity the laser light has no effect on the stains; above the upper intensity level it can deteriorate the substrate. In the working window between these two levels, the effect of the laser is self-limiting. Once the stain is removed, the continuing laser pulses do not damage the material's surface, and the stone retains its patina, which forms a protective coating.
Although the cost of laser cleaning is still about $30 per square foot, the project managers believe widespread use of the Lama system could bring the price down to $8 per square foot.