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. One of the first case studies in the use of laser cleaning on wall paintings is dated back to the beginning of 2000 and took place in Siena, Italy. The Old Sacristy and the Chapel of the Mantle are two painted halls within the complex of Santa Maria della Scala in Siena, one of the oldest European hospitals opened about 1000 years ago and functioning until 70 years ago, Santa Maria della Scala has been gradually turned into a museum. The walls and the vaults of the Old Sacristy were painted by Lorenzo Vecchietta between 1446 and 1449 with scenes from the Old and New Testaments. The paintings were coated with layers of whitewashing applied in the past. Used as first-aid room, the Chapel of the Mantle shows three spans divided into groin vaults painted by Cristoforo di Bindoccio and Meo di Pero in 1370. Again, in the past the paintings had been almost completely whitewashed. Traditional chemical and manual techniques proved unsuccessful in the removal of the whitewash because it was strongly attached to the pigments underneath. For this reason, restorers thought about the groundbreaking use of lasers. Preliminary tests were carried out using two intermediate-pulse systems: used together or one by one, they resulted in the successful removal of the whitewashing, revealing the frescoes underneath.
After this first successful result, 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, 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.
During the last twenty years, the removal of incrustations has been carried out mostly manually, trying to remove the most of the concretion, at the same time protecting the original painting. Nevertheless, the results obtained with this method were unsatisfactory as they did not result in the complete cleaning of the surface. Therefore, after cleaning 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. The laser has been able to safeguard all the different shades of colors, from red to ochre, from white to green.
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, a three-nave hall destroyed and buried few decades after its completion. Here, too, the laser had to intervene for removing thick layers of mineralized carbonation that obscured and covered the precious depictions.
Thanks to the experience gained in the first laser cleaning interventions on wall paintings the technique has been lately applied on many other important wall paintings such as in the Villa of the Mysteries in Pompeii, in the Main Chapel of the Santa Croce Basilica in Florence, in the Santa Tecla’s Catacombs in Rome.
Armed with this experiences, this advanced cleaning technology for decorated surfaces has been applied successfully in other difficult interventions outside Italy such as on the murals of the temple of Mut in Jebel Barkal in Sudan, on the funerary paintings of the tombs of Xi`an and on the walls of the monastery of Quqa (Xinjiang) in China.
[di Laura Bartoli e Alessandro Zanini]