23 Apr 2003
A combination of infrared and ultraviolet light restores part of Athens' Acropolis in Greece to its former glory.
Fundamental and frequency-tripled light from an Nd:YAG laser is being used to clean one of the world's best-known ancient monuments.
Laser specialists on the Greek island of Crete developed the new method, which prevents the common "yellowing" effect seen when stonework is laser-cleaned.
Now, the two-wavelength method has been selected to restore the Pheidian frieze, which is regarded as one of the most precious and fragile parts of the ancient Acropolis in Athens.
Two out of sixteen marble blocks of the West part of the frieze have been cleaned using the two-wavelength combination so far. The remaining fourteen blocks will be treated by the end of 2004, according to Costas Fotakis, director of the Institute of Electronic Structure and Laser (IESL), which is part of the Foundation for Research and Technology-Hellas (FORTH) in Crete.
After centuries of exposure to the elements, during which time it experienced a number of earthquakes, fires, an explosion and - more recently - the severe pollution of the modern city, the frieze was removed from the Parthenon in 1992.
Since then, scientists have been working with the Committee for the Conservation of the Acropolis Monuments (ESMA) research team to select the best method to remove the black encrustations on the frieze surface.
Laser-cleaning was initially one of four methods shortlisted for the delicate job (forty methods were originally considered), but the established technique - using 1064 nm radiation from an Nd:YAG laser - was found to discolor the ancient marble, giving the stone a yellow appearance.
Fotakis and colleagues at FORTH-IESL discovered that during conventional laser removal, selective vaporization of the dark encrustations on the marble surface alters the absorption spectrum of the remaining encrustations. This causes the yellow discoloring.
"Such discoloration is avoided if ultraviolet radiation is used," said the FORTH-IESL team. "In this case, photomechanical phenomena dominate the cleaning process and the removal of unwanted layers takes place in discrete steps."
The FORTH-IESL team found that simultaneous use of both the fundamental and third-harmonic wavelengths from a pulsed Nd:YAG source, with the pulses overlapping eachother in space and time, cleaned the marble with no discoloration or surface damage. They have now applied for a patent covering both the method and the laser-cleaning system.
Michael Hatcher is technology editor on Opto & Laser Europe magazine.