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. Laser cleaning gained in the last years a prominent position among the cleaning techniques for the conservation of Cultural Heritage. The use of laser for the restoration of artworks started in the 70s but the technique actually began its rise at the end of the 90s when scientific studies validated the efficacy of the use of a laser beam to clean masterpieces. The emission parameters of the lasers have been also optimized in order to guarantee a safe and efficient cleaning of different substrates. Laser cleaning was initially applied only for the removal of black crusts from white marbles but, thanks to the technological innovation and to the background of scientific studies, its use has been extended to other materials such as metals, gilded bronzes, wood, ceramics and wall paintings. Laser ablation provides indeed many advantages with respect to mechanical and chemical methods in terms of gradualness, self-termination, selectivity, environmental impact and safeguard of the so-called “age patina”.
Laser cleaning has been widely applied for the cleaning of frescoes and wall paintings also in very particular and extreme environments such as catacombs. One beautiful case study regarding underground locations is the Catacomb of Domitilla (figure 1), in Rome, in particular the “bakers’ niche” which is located on the first floor of the catacombs. Its walls are mainly frescoed, often with dry overpainting. The microclimate inside the hypogean structures of the catacombs is usually quite stable, featuring high relative humidity between 96% and 100%, and temperatures around 14-17°C all through the year. One of the most common decay problems concerns the precipitation and crystallization of calcium carbonate that covers the frescoes almost entirely. An instance of such decay is the typical dark film covering the vaults and the upper walls of the rooms that may range from thin films to very thick layers.
Nevertheless, the results obtained with traditional conservation methods were unsatisfactory as they did not result in the complete cleaning of the surface. Therefore, after preliminary tests, two laser systems with optimized pulse duration have been used for the removal of the black film. The cleaning has been extremely satisfactory and brought back to light the beautiful colors of the original paintings. (figures 2 and 3).
A similar conservation problem was faced in the first century A.D. Roman complex of the Underground Basilica of St. Maria Maggiore in Rome. This is a beautiful and fascinating hypogean temple decorated with fine stuccos and a fresco. Here, too, the laser had to intervene for removing thick layers of mineralized carbonation that obscured and covered the precious depictions. (figure 4).
The application of laser cleaning on metals concretely started with the case study of the gilded bronze panels of the “Gates of Paradise” by Lorenzo Ghiberti of the Baptistery in Florence (figure 5): a careful optimization of laser parameters was performed and led to the introduction a Long Q-switching laser system with pulse duration of 100 ns. The effectiveness and safety of the laser for the cleaning of amalgam gilding, gold laminas, silver and bronze were proved during the years thanks to interesting conservation treatments such as the restoration of the bronzes statues of David by Verrocchio, David and Attys by Donatello, the Etruscan statue “Arringatore (figure 6) and the cleaning of the other two doors of the Baptistery: the North door by Lorenzo Ghiberti (figure 7) whose cleaning ended up in 2015 and the South door, the oldest of the three, by Andrea Pisano that is currently under restoration at the Laboratories of the Opificio delle Pietre Dure in Florence.
In the last years the laser cleaning technology overcame the boundaries of Europe to land also in Asia. Two stone portals of the Royal Palace of Patan in Nepal (figure 8), 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 the laser which has proven the most effective in the removal of the bitumen from the stone of the portals.
[by Laura Bartoli, Alessandro Zanini]