LASER CLEANING OF CULTURAL HERITAGE

 

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 in Siena by Jacopo della Quercia, the capitals of the leaning Tower and of the Cathedral in Pisa 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, self-termination, selectivity and environmental impact.
Laser cleaning of stone is the main application of the method: the black crust is easily removed by the stone substrate without any damage. The laser, thanks to its gradualness and selectivity, is also able to preserve the “age patina” that can be found on most of ancient stone artworks. A clear example is the laser cleaning of the Diocletian Palace in Split (Croatia). The Diocletian Palace was built by the Roman Emperor Diocletian at the turn of the fourth century AD.  The perystile is located in the central part of the palace and has been built using the calcareous stone extracted from Brazza Island nearby. The main part of the calcareous stones constituting the peristyle were covered by a quite thick black crust (from 0.5 to 10 mm) made of gypsum and carbon. A SFR laser (Short Free Running) with pulse duration of 50-100us was used for the cleaning. For thin crusts of 0.5 mm a fluence of about 13 J/cm2 was sufficient for  the removal, while with crusts of 2 mm it was necessary to increase the fluence up to about 22 J/cm2. For thicker crusts a mechanical removal was used to reduce the thickness of the  encrustation: the finishing of the cleaning was performed always by laser because of its capability to safeguard the patina on top of the stone (figures 1 and 2).
An interesting example of laser cleaning of archaeological artworks is represented by the statues of the Dioscuri of Locri ,a splendid group belonging to a Doric temple of 450-425 BC, executed in Parian marble, probably in Magna Graecia workshop. Very thick incrustations of different nature and morphology, present on the front side of the dioscuro named A, needed to be removed. These carbonate deposits partially incorporated earthy material coming from the archaeological sediment and showed an irregular dark ochre hue. With the use of a short pulse laser (LQS) it was possible to get a very good removal of the concretion, lightening the chromatic tone of the complex, without affecting the original patina of the marble (figures 3 and 4).
The Temple of Hadrian erected in 145 AC is a temple to the deified Hadrian on the Campus Martius in Rome, Italy. The Temple retains eleven of the thirteen Corinthian columns of the north side, part of the entablature and the cella wall. On the marble two layers of film had been identified. The oldest, lighter and thinner, is organic and made from casein of goat milk and egg white. The second, much thicker and of brown-black color is a true dyeing, wherein a mixed animal glue of rabbit and pig origin was identified. The laser was able to remove the dark patina without attacking the light, oldest patina lying beneath, and without damaging the marble. Despite these good results, it was decided to keep both oxalate films for consistency with the intervention performed in the 80s on the free columns of the arcades. The laser was applied  to give continuity to the architrave, which is  a particularly damaged architectural element. The most recent patina, which interrupted the flat elements with alternating black stripes, and almost completely flattened ovoid shape of the reliefs has been removed or reduced (figure 5 and 6).

The application of laser cleaning on wall paintings concretely started with the case study of Santa Maria della Scala frescoes in Siena. 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 covered with layers of lime and paint whitewashing. When the traditional chemical and manual techniques proved unsuccessful, restorers thought about the groundbreaking use of lasers. Preliminary tests were carried out with a Short Free Running system (Eos 1000) and the Long Q-switching one (Eos 1000 LQS). These two intermediate-impulse systems proved extremely effective and safe compared to short-pulse Q switch lasers; used together or one by one, they resulted in the successful removal of the whitewashing, revealing the frescoes resting underneath.
After the successful cleaning in Siena, the laser started to be used also on extreme environments such as catacombs. One interesting example is the cleaning of the wall paintings in the “Baker’s niche” in the Domitilla’s catacombs in Rome. The bakers’ niche is located on the first floor of the catacombs of Domitilla. 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. Thanks to the laser the dark film was completely removed from all the shades of colors (white, green, ochre, etc) and the wall paintings  were completely uncovered by the black layer that concealed them (figure 7).
Another example of successful laser cleaning in hypogean environment is the cleaning of the decorated stuccoes of the vault of the Pythagorean Basilica of Porta Maggiore in Rome. The Basilica was built underground between 14 and 54 AD and was used for meetings of Neopythagoreans. The ground plan shows a basilica with three naves and an apse similarly to early Christian basilicas that appeared only much later in the 4th century. The vaults are decorated with white stuccoes symbolizing Neopythagorean beliefs but whose exact meaning remains a subject of debate. The stuccoes in the vault were covered by a thick and strong layer of calcium carbonate whereas the decorations on the walls were covered by earthy residues. A powerful QS laser was used to safely remove the encrustations bringing back to light the delicate decorative motifs (figure 8).
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: 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 related alloys were proved during the years thanks to interesting conservation treatments such as, in addition to the already-mentioned Gates of Paradise, the restoration of the bronzes statues of David by Verrocchio, David and Attys by Donatello, and the successful cleaning of a Roman Hoard composed by 300 silver alloy coins.
A complex project has been the intervention on one of the most well-known among the great Etruscan bronzes, the so-called “Arringatore” (the Orator), a votive statue that was found in XVI century near the Lake Trasimeno in Middle Italy and became part of the collection of Cosimo I de’ Medici in 1566. The statue, 1,79 m tall, cast by lost-wax technique, is composed of seven distinct parts welded together. Supposedly, the statue underwent several undocumented restoration and maintenance works along the centuries. A SFR laser treatment was carried out for removing a brown-black patination applied in the past. The unwanted patination was an organic-matrix layer of some hundreds microns including silicates, calcite, gypsum and a pigment load of carbon black and ochres, along with sporadic presence of Ca- and Cu-oxalates deriving from the mineralization of the binder. Water assisted SFR Nd:YAG laser irradiation at 2J/cm2 fluence was used to partially ablate and thermally disaggregate the organic-matrix patination: the cleaning was then mechanically finished using scalpel and brush. The recovery of the surface readability was evident as well as the tenorite black layer intimately bond to the metal surface. The laser was used also to remove undesired incoherent copper minerals after mechanical cleaning: the SFR laser irradiation at low fluences allowed a deeper degree of cleaning with respect to mechanical finishing and then allowed one to control the final chromatic hue of the surface uncovered (figures 9 and 10).

[by Alessandro Zanini, Laura Bartoli]

 

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