Laser ablation is one of the most important irradiation effect which can be induced on optically absorbing materials. Laser cleaning is a particular case of laser ablation where a specific substrate is uncovered through the removal of undesired layers.
The application of laser cleaning in the conservation of artworks was proposed by J.F. Asmus and L. Lazzerini since the beginning of seventies through a set of practical tests carried out in Venice on encrusted stone artefacts: the novel approach, though, did not overcome the experimental stage for several years mainly because of the technological limits of the pulsed laser sources available at that time. During the eighties the technological level of the laser devices increased significantly but the costs were still out of scale for the specific field of application.
Since the second half of the 1980s laser cleaning was widely applied in stone artifacts restoration in Italy, France, England, Portugal, Austria and other countries, mainly for the removal of black crusts produced by environmental pollution but also for the removal of intentional dark layers applied in the past. Laser cleaning was applied massively on restoration of stone reliefs, historical façades, ancient archaeological artworks such as the West Frieze of the Parthenon and famous Renaissance masterpieces such as Profeta Abacuc, San Marco and Pulpito by Donatello, panels of the Giotto’s tower of the Florence Cathedral by Andrea Pisano, San FIlippo and Santi Quattro Coronati by Nanni di Banco, Fonte Gaia by Jacopo della Quercia and Loggia della Mercanzia in Siena, the capitals of the leaning Tower and of the Cathedral in Pisa, some parts of the S.John’s Baptistery in Florence and many others. This extensive application of laser cleaning was accompanied by basic studies on the phenomenological characterization of irradiation effects, diagnostic of the material removal and physical modelling which allowed the definition of operative fluence ranges ensuring discrimination between encrustation to be removed and the substrate to be preserved.
Laser ablation provides indeed many advantages with respect to mechanical and chemical methods in terms of gradualness, selftermination, selectivity, environmental impact and safeguard of the so-called “age patina”.
In the last years the laser cleaning technology overcame the boundaries of Europe to land also in Asia. The cleaning of the portals of the Royal Palace in Patan, Nepal is a clear ex- ample of the power of this technique. Two stone portals of the Royal Palace of Patan, built in local sandstone, were covered with a thick bituminous layer difficult to be removed with traditional cleaning techniques. During a previous intervention the use of solvents (acetone and white spirit) followed by mechanical action by scalpel had been able to remove this thick black layer only partially. The use of the sandblaster has been discarded due to the fragility of the stone and the consequent possible loss of material as well as for reasons related to the humid climate in Nepal that would have affected the emission of sand from the nozzles.
After some preliminary laser cleaning tests it was decided to perform the conservation of the portals only using this method which has proven the most effective in the removal of the bitumen from the stone of the portals.
The laser is currently used also for the cleaning of the beautiful white marble temples in Agra, India. The laser is able to remove pollution crusts from the white surface leaving the original marble bright and clean.
The Venus temple in Baalbek, Lebanon, has been also subjected to laser cleaning on the frieze. The black crust has been removed by using a fast Q-switched laser and, also in this case, the “age-patina” has been fully preserved.
Laser manufacturers are working on the next generation of devices which will be even faster in the cleaning and ideal also for big and flat stone surfaces. Technology is moving forward ... and laser cleaning is following this trend!
[by Laura Bartoli, Alessandro Zanini]