Multiple temporal regime laser for art conservation
The idea of using laser ablation to remove undesired material layers—such as environmental deposits and patinations applied in the past—from the surface of artifacts of cultural interest dates back several decades.1 Nevertheless, only in recent years has this idea gradually undergone a methodological revolution. That is, thorough studies of specific ablative processes and the development of dedicated laser systems have facilitated rapid progress in the field of cultural heritage preservation.2, 3
Several crucial issues are involved in the conceptually simple work of laser-aided removal of a stratification from the surface of a given artwork, usually referred to as ‘laser cleaning.’ A number of conservation problems can be encountered in which the laser irradiation aims at the complete or partial removal of, for example, black crusts (comprising gypsum and carbon particles from urban pollution), deteriorated coats applied during previous restoration works,4,5 corrosion products, calcareous growths,3 aged varnishes,6, 7 overpainting, and biodeteriogens (algae, fungi, bacteria, and lichens),8 from a valuable layer (of the artwork), which is often fragile and photosensitive. Such a variety of materials and microstructures, along with the need for high selectivity, makes the optimization of laser ablation difficult. Furthermore, practical aspects related to the restoration yard (such as reliable beam delivery and safety), as well as to the overall aesthetic result of the treatment, must be taken carefully into account.
During the past decade, we have shown that fiber-coupled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser systems (emitting at 1064nm) with pulse durations between several tens of nanoseconds—long Q-switching (LQS)—and tens of microseconds—short free running (SFR)—offer the most variety in terms of operating characteristics.3 The free running regime corresponds to a basic laser setup (the Nd:YAG active medium, a pumping lamp, and a cavity comprising back and front mirrors). The lasing occurs in a free oscillating regime, without modifying the optical characteristics of the cavity. By contrast, the Q-switching regime (in which Q is the quality factor of the laser cavity) corresponds to a configuration whereby the emission is controlled by an optical switch, which enables one to inhibit the emission during the pumping and deliver the light in a very short time (1–10ns). Usually, the ablation thresholds of the LQS and SFR temporal regimes can be drastically reduced and side effects (such as photothermal and photomechanical damage) are prevented by spraying a small amount of water on the surface under treatment. In this way, LQS and SFR laser regimes can exploit ablative processes that range from cold spallation to massive vaporization, minimizing the energy dose dissipated into the layer being uncovered and reducing the direct and indirect photomechanical stresses to it.
After demonstrating the effectiveness of such a technological approach through phenomenological and physical investigations, as well as a number of successful practical applications, we recently introduced—in collaboration with an industrial partner—a novel Nd:YAG (1064nm) laser system. The schematic setup combines passive Q-switching and SFR regimes in the same laser source: see Figure 1(a). This combination enables the selection of pulse durations, from about 10ns (Q-switching) to 120ns (LQS), or 40–120μs (SFR). Furthermore, in the LQS configuration, single-, double-, and triple-peak pulses can be produced by suitably increasing the flash-lamp pumping: see Figure 1(b).
The availability of these various temporal regimes in a single system considerably increases the potential for successful laser ablation treatments (see Figure 2). Although general ‘recipes’ cannot be determined because of the variability of materials and stratigraphies in art preservation, our experiments can provide rough indications for using the device. For example, soft spallation of deposits and coatings from wall paintings, fire gilding, and silver surfaces can often be safely achieved using single-peak LQS pulses (LQS1),4 whereas SFR pulses—which exploit slow vaporization dynamics—are usually suitable for uncovering oil gilding2 and stone surfaces. For wall paintings, in many cases, the combination of these two regimes can extend the set of treatable paint layers, minimize undesired side effects, and carefully control the final result. Furthermore, multiple LQS peaks (LQS2, 3) might also remove coherent layers, such as those formed by calcareous growths.6 Finally, the combination of Q-switching, LQS, and SFR for stone conservation treatments can enable the optimization of the efficiency and selectivity of the ablation process and control the final chromatic appearance of the uncovered surface.5, 6
We assessed these principles using model studies2–5,9and successfully applied the techniques in a number of restoration projects of unique masterpieces. For example, we restored several marble and bronze sculptures: Donatello's Profet Abacuc, Amore Attis, and David; Nanni di Banco's Quattro Santi Coronati and Assunta; Giambologna's Ratto delle Sabine; Verrocchio's David; Ghiberti's San Matteo and Porta del Paradiso; and the Etruscan masterpiece Arringatore from the Trasimene Lake. We have also extended combined LQS and SFR laser treatments to wall paintings, and have achieved excellent results on, for example, the painting of the Santa Maria della Scala museum complex in Siena, Castle of Quart (Aosta, Italy), catacombs of Rome, and Santa Croce Cathedral in Florence, Italy.5, 6 Currently, the system is being used in several important restoration projects, including the Florence Baptistery's North Door, a gilded-brass masterpiece by Lorenzo Ghiberti. The testing phase on this project has already begun: see Figure 3(a). Recently, we were able to remove an overpainting from a 20th century easel painting—see Figure 3(b)—which will be the subject of forthcoming publications.
In summary, our novel multiple temporal regime laser system represents a significant step ahead, consolidating the use of laser ablation in the conservation of artifacts of cultural heritage. In future works, we will extend its application to other conservation problems concerning, in particular, polychrome stones (granite, ammonitic red, etc.) and painted surfaces. We will also continue to promote the dissemination of the novel technology in the field of art restoration.
The laser development was supported by the project TEMART (Tuscany Region, POR-CReO/FESR 2007–2013). The application studies are ongoing within the framework of the European Project CHARISMA (FP7 Capacities, Research Infrastructures, grant agreement no. 228330).
Salvatore Siano is a physicist, senior researcher at the Istituto di Fisica Applicata ‘Nello Carrara’–Consiglio Nazionale delle Ricerche (IFAC-CNR). His activity is focused on laser and optoelectronic techniques for material knowledge and the conservation of cultural heritage. He has coordinated the activities of several projects in this arena.