Comparative study between IR and UV laser radiation applied to the removal of graffitis on urban buildings
Introduction
Over the last years, the possibility of the use of laser ablation as an alternative method to the conventional techniques of mechanical cleaning (sandblasting) or chemical removal [1], [2], [3], [4], [5], [6], [7] has been considered. The laser cleaning technique allows some benefits: the different and characteristic absorption at the laser wavelength exhibited by each substance allows a selective removal of the material without damaging the substrate, which allows controlling, with high accuracy, the depth of the removed layer. This method is intrinsically fast and clean, and circumvents the use of toxic and polluting solvents, required with the more conventional cleaning procedures, thus preventing also damage to environment. The laser methods make possible the treatment of surfaces that with other techniques cannot be cleaned or where the conventional cleaning process is much more expensive. In addition, combination of the laser cleaning process with the use of analytical techniques, allows us to identify the substrate, carry out a layer analysis of the surface and control the cleaning process in real time [8], [9]. The tendency is to extend the field of application of lasers from the cleaning of individual artworks to the cleaning of larger areas such as entire building façades. The high power of the lasers used in this application, the fast cleaning rates and diversification of substrates could result in some health hazards, as surface ablation can generate micro powder hazard and toxic substances [10], [11]. To ensure safe laser cleaning conditions, critical requirements are the use of laser goggles, fume extraction equipment to avoid inhalation and adequate operator training [12], [13], [14], [15].
The mechanism responsible for the laser cleaning process depends on the wavelength, duration and energy of the laser pulse as well as on material properties. In the particular case of laser cleaning of artwork, the sample's fragility, its unique nature and cultural value, make necessary to characterize the object extensively to guide the restoration process. For this task, the methods of analysis based on laser ablation are particularly suitable, although it is necessary to assess carefully their possibilities and limitations, the optimal conditions for their use, and the response of the materials [8].
The fundamental wavelength of a Q-switched Nd:YAG laser (λ = 1064 nm) is nowadays the most utilized wavelength to clean stone (including marble) surfaces [1], [16], [17], [18]. The major advantage of using this wavelength is commonly described as self-limiting-phenomenon, since the gap between the ablation thresholds of incrustation and marble/stone reaches a maximum at this wavelength [18]. Nd:YAG lasers have also demonstrated their effectiveness in the cleaning of a great variety of substrate materials: paper, wood, metal, leather, parchment and carpets [19], [20], [21]. However, the efficient removal of thin layers of aged varnish in polichromies and canvas required pulsed excimer laser, due to the high absorption of polymers in the UV region, in a process which leaves intact the inner layers and reveals the original colors [9], [22]. At the moment, these methods are in development, and numerous questions about their safe and efficient use have still to be answered. In addition, there is still some controversy about the efficiency and mechanism responsible for the ablation process, since photochemical and photothermal mechanisms, as well as a combination of both have been proposed [23], [24], [25]. In a photochemical process, the absorption of one or more photons results in an electronic excitation followed by decomposition of the compound caused by direct bond breaking in the solid, in competition with various relaxation processes. In a thermal mechanism, the initial electronic excitation produced by the absorption of the laser radiation is converted into vibrational excitation via fast relaxation processes, which causes thermal decomposition of the compound. Probably, in the ablation process both mechanisms are involved, and depending on the properties of the irradiated materials and the irradiation conditions, such as wavelength, one of them may be dominant.
In this study, we carry out a comparison between the performance of the fundamental radiation of the Nd:YAG laser (λ = 1064 nm) and the radiation of a XeCl excimer laser (λ = 308nm) in the removal of graffitis from construction materials. The goal of the present work has been to evaluate and compare in a systematic way the efficiency and quality of the painting removal by IR irradiation at 1064 nm, which acts essentially via a photothermal mechanism, and at 308 nm, where the UV photons are expected to induce photochemical processes. This comparative study can be of importance for both technical applications and fundamental understanding of the ablation behavior. When dealing with artworks, a careful control of the cleaning procedure is needed in order not to damage the original surface and preserve the artistic value of the work. Although in the case of removal of vandalic painting from public buildings such rigorous control is not needed, nevertheless it is important to avoid damage on the construction materials, mainly in historical buildings and monuments. Laser-induced breakdown spectroscopy (LIBS) technique was applied for spectroscopic analysis of the emission from the plasma generated when the laser radiation interacts with the material's surface [5], [8], [26]. In addition to the elemental composition, LIBS, if combined with traditional techniques such optical microscopy (OM) and scanning electron microscopy (SEM), provides information on structural composition facilitating in this way the choice of the appropriate parameters for the cleaning process. Finally, the monitorization of the acoustic signal generated by the material removal during the laser ablation [27], as registered with a microphone, allows the determination of the ablation threshold fluence for the analyzed paints.
Section snippets
Samples
The study was carried out using the following samples of commercial sprays: white, pink, black, golden, silvery, yellow, green all of them [Trans-color] and blue [Felton]. The main component of these sprays is generally an acrylic or styrenic copolymer, being the main difference between them the specific mixture of pigments and dyes used as well as the presence of small amounts of other additives. Details of the particular paint formulations are proprietary of the manufacturer and not available.
Results and discussion
The LIBS technique allows the elemental analysis of the ablated material from both paint and substrate, and thus an on-line control of the ablation process. First, the substrates with no paint on them were irradiated and the subsequent plasma emission detected and analyzed. Characteristic emission lines of each one of the studied substrates under irradiation at 308 nm as well as their elemental assignation [28] are collected in Table 1. Fig. 2 depicts as an example the LIBS spectra of glass and
Conclusions
The ablation process of spray-paintings under UV and IR laser irradiation reveals that, from a practical point of view, both types of lasers (XeCl excimer at 308 nm and IR Nd:YAG at 1064 nm) have potential to be used in the cleaning of graffitis since they induce removal processes highly efficient and suitable for on-line control by LIBS technique, with minimum requirements in the alignment of the detection system. The ablation rate studies and the evaluation of the threshold fluences by
Acknowledgements
This work was supported by Project PROFIT-030000-2000-79, CESPA being the company backing it. The authors wish to thank the Ministerio de Educación y Ciencia of Spain for financial support through a Ramon y Cajal to C. Gómez.
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