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, environmental impact and safeguard of the so-called “age patina”.
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 alreadymentioned 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 couple of weeks ago the restoration of the North Door of the Baptistery of Florence came also to an end: the laser was here extensively employed for the removal of black encrustation over the gilded parts. Large bronze sculptures were also lately laser cleaned such as the Etruscan statue “Arringatore” or the sculptural group “Decollazione del Battista” by Vincenzo Danti from the Baptistery of Florence. Also some modern copper alloy outdoor sculptures underwent laser treatment such as the Monument to Lord Nelson in Liverpool and the Monument of Queen Victoria in Southport.
Laser cleaning of ceramics objects is still a challenging problem given the sensitivity of clay materials to laser radiation. Moreover ceramics often show a hard, thick and insoluble earthy encrustations that have a rather high ablation threshold in contract to the ceramic substrate: in this case the self-limiting phenomena does not apply and the cleaning has to be performed with extreme care.
[di Alessandro Zanini]