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This article is the first of a series devoted to the application of lasers to art conservation, mainly focused on the cleaning process, and it will cover notions from the basics of lasers to its application to paintings and other works of art.
Introduction
The laser just turned 50 years-old. It has, since long, been part of our daily life and it made possible many technologies that many of us couldn’t live without, from dvd readers to telecommunications. But the laser has also been an important development for art conservation. During the development of laser technology the interaction between the laser and the irradiated material has been of the utmost interest in several fields of knowledge. Among its many applications, in art lasers can be used mainly for analysis, when integrated in analytical devices such as spectroscopy, and for cleaning. Laser technology should be always considered as an alternative tool to solve specific problems, when traditional methods are not a viable option. Lasers in Conservation 50 years after the first laser was built there are more than 10,000 laser wavelengths known. Laser technology has a great potential in the development of alternative procedures for conservation mainly due to particular properties of the light beam but also due to its controllable and reproducible technique. Its application in the conservation field includes a wide spectrum of uses, namely surface cleaning, removal of overpaintings and other layers, and analysis of art materials. Laser has always captured much attention due to its potential over traditional cleaning techniques, either chemical or mechanical ones. It became the first tool that did not interact physically with the artwork, minimizing thus the contact with the surface and the stresses or damages that can be inflected during normal procedures. This minimum contact respects the “minimum intervention” principle that every conservator aspires to follow. Research in laser cleaning is focused on the development and optimization of the controllable removal of surface selected layers. Although in the beginning laser technology could have been seen by some as a new fast technique to clean any material, soon it was obvious that the technique required much research before being applied to works of art. Nowadays its potential is to complement the established traditional techniques by overcoming their disadvantages and limitations. Although conservation is traditionally a conservative field, scientific research is very active in both the material characterization and the development of new technology to the cultural heritage field. Traditional cleaning techniques employed by painting conservators are either of mechanical or chemical nature. The use of solvents presents several disadvantages such as the difficulty to control their degree of penetration into the paint layers, interfering thus with the chemical stability of the original, and their toxic nature. It was attempted to overcome the former disadvantage by the development of gels instead of the liquid form, although this technique is also not exempt from disadvantages. An important factor is that some layers to be removed are partial or completely insoluble to strong solvents that can endanger the paint layer and the integrity of the painting. Mechanical cleaning is performed most of the times with a scalpel, which is probably the most characteristic tool used by conservators, but which presents the associated hazard of the direct physical action over the paint layer that can damage the surface. The choice and control of these techniques is directly dependant on the individual skills of the conservator, as the borderline between optimum cleaning and over cleaning is often very thin. Laser technology, and more particularly, laser cleaning provides advantages such as selectivity (it is possible to remove unwanted layers without removing the original material with appropriate parameters), non mechanical contact (enabling the treatment of highly sensitive surfaces), environmental friendliness (avoiding the use of hazardous chemicals or solvents), precise action and reliability.
Historic background
Lasers are based on the principle of stimulated emission which was first deduced by Albert Einstein in 1917. This principle is simply the process by which electromagnetic waves of a certain frequency can induce (or stimulate) an excited atom or molecule to decay from a high to a low energy level, emitting thus more electromagnetic waves. In the early 50’s, Columbia University researcher Charles Townes thought that he could induce molecules to emit at certain wavelenghts. In 1954 Townes and his colleagues built the first device based on that principle. The device was named ‘maser’, which stands for Microwave Amplification by Stimulated Emission of Radiation and it was able to emit at a very precise unique wavelength in the microwave region. This breakthrough was then reproduced by others, originating several modifications. In 1958, Townes and Schawlow [1] proposed that the emission could also be done in the infrared and visible region of the electromagnetic spectrum. Two years later, in 1960, the first light-emitting maser was constructed by Maiman [2] using a flash-pumped rod of ruby. The name of this light-emitting maser was changed to Light Amplification by Stimulated Emission of Radiation, or laser. After that, lasers development was soon directed for industrial applications. In short, lasers are devices that produce and amplify an intense beam of highly coherent and highly directional radiation by stimulated emission in specific frequencies of the electromagnetic spectrum. The first application of lasers to art conservation happened in the early 70’s by a team leaded by John Asmus [3, 4]. He was invited by geophysicist Walter Munk to produce high-resolution holographic records from marble sculptures for archival purposes using laser technology in Venice, Italy before further degradation. For the job, the team used a ruby holographic laser, the most powerful in existence at the time. In 1972, and after having produced over 50 holograms, Asmus was introduced to the difficulties of cleaning crumbling marble sculpture by restorer Giulia Musumeci. Asmus had previous experience with laser ablation due to past research on using lasers for space exploration and came with the idea of using the laser to ablate the black crusts from the marble with minimum damage [5]. The initial holography program turned then into a laser cleaning project of stone statues, the first of a new area of research. However, before starting to clean sculptures, it was required to assess the safety of the use of lasers for this purpose. Further research funded by the Samuel Kress Foundation and the Smithsonian Institution between 1972 and 1974 found that laser could be applied to clean countless materials used in works of art such as “marble, limestone, oolite, sandstone, stucco, concrete, terra cotta, most metals, leather, velum, paper, cotton, wool, silk, moleskin, and wood” [5]. However, Ruby and Nd:YAG lasers had limitations back then, among which the low pulse repetition laser, low reliability, high costs, etc., that prevent them from being extensively employed [6, 7]. Although promising, there was few research developed in the field in the 70’s and 80’s. The 80’s was still an incipient period, although lasers were produced with increasing technological advancements. Their use involved a very high cost when compared with the traditional cleaning practices and their precise short- and long-term effects on the works of art needed further research. The fact that lasers were normally located in industrial environments was not adequate to the cleaning of works of art. During that period this new technology faced scepticism from the conservation community and it was mainly thanks to the work of Asmus and colleagues that research continued [8, 9]. It was only in the 90’s, with the progressive development of laser technology, that this slow paced research field met a new interest from European researchers. More studies begun. mainly backed up by European Programs funding, and several research groups arose [10-14]. In this period, stone-based materials were the focus of research and the success cases contributed to the debute of research in other areas to start, mainly in paper, textiles, glass, metals and paintings. Due to the increasing interest in laser applications to conservation, an international workshop was organised in 1995 gathering the main researchers of the area. The meeting was so successful that it turned into a biannual conference with the name LACONA (Lasers in the Conservation of Artworks), and has become one of the most relevant international meetings in this field of research. Since then, laser technology has continue to evolve and is now much better known by conservators and other professionals. Its research and use has been consolidated by a permanent interest from several research groups, mainly located in Europe.
1. Laser scanning device. Lasers can be used to scan 3D objects of any size, from coins to building facades.
Laser Fundamentals
Lasers are devices that can produce and amplify coherent radiation by stimulated emission in specific frequencies of the electromagnetic spectrum. Nowadays lasers can emit in a large range of the electromagnetic spectrum, namely at wavelengths from the long infrared to X-ray regions [15]. Principles of Laser Radiation The theoretical basis of lasers was presented in 1917 by Einstein [16] when he described fundamental concepts of emission and absorption of light by matter: stimulated absorption, spontaneous emission and stimulated emission of radiation. Spontaneous emission is the process when excited particles transit to a stable state of lower energy resulting in the spontaneous emission of a photon. Stimulated emission occurs when energy of the same frequency of the spontaneously emitted radiation is incident on the material forcing the particle to undergo a level transition emitting radiation. In this particular case, however, the photons emitted by stimulated emission have the same phase, same frequency and direction of propagation as the incident radiation. The resulting radiation beam is therefore considered coherent, monochromatic and highly directional. It is the most low-divergent and monochromatic light source that is known to man. Lasers emit radiation at several wavelengths covering a broad range of the electromagnetic spectrum, from the microwave to the soft X-ray region. Each laser can only operate at a specific wavelength, except for free electron lasers that have the potential to operate at all wavelengths. The most important regions for most applications are the infrared, the visible and the ultraviolet regions.
Basic Structure
A basic standard laser requires the same 3 basic components: a power source, an active medium and a resonance cavity. The active medium must have a metastable state in which the electrons can be trapped. After excitation of the active medium by energy pumped from the power source, the active medium particles tend to achieve an excited metastable state with a consequent inversion of the population. The inversion of the population occurs when the majority of the particles (atoms, ions or molecules) are in an excited state rather than in a low-energy state. This condition is critical as the radiation is emitted when the particles decay from energy level. The active medium is contained in the resonance cavity, the main mechanism of the laser, where the light is amplified. The cavity has two mirrors at its extremities, opposite to each other. While one is totally reflective, the other is partially transmitant making possible the exit of the light beam from the cavity. After the spontaneous emission of light is produced in all directions, the photons that travel in the parallel direction of the resonance cavity axis can start the emission of other photons. The light amplification is achieved by successive reflections in the mirrors on the referred axis. When the amplification exceeds the loss of the cavity, a coherent beam of light is produced. Operational Modes Lasers operate in two fundamental modes: continuous or pulsed (either normal or Q-switched). As self-explained, continuous mode is when transmission from the resonance cavity emission is continuous in time and pulsed mode is when transmission is made intermittently. The pulsed mode can be originated by the device set-up or it can be induced mechanically by means of a switch (called Q-switch). Q-switching is a technique used to obtain strong pulses. In the normal laser configuration the energy drains out of the population inversion as fast as is pumped in. However, if the feedback of the light to the mirrors is blocked, the energy is stored until a certain level is reached. When the feedback is unblocked, the energy is released in a single and very high peak pulse. This technique makes possible to produce laser beams with different lengths (τ) depending on the switching frequency. Lasers are complex devices and their explanation can sometimes be hard to comprehend. However, it is required to understand the principles by which they function in order to understand how they can operate and how they can be later used for conservation purposes. The next article of this series will continue introducing the application of lasers in conservation. It will contain, in detail, the interaction of the laser radiation with matter, exploring why it is so convenient for cleaning procedures, and the most used laser types in conservation.
[1] A.L. Schawlow, C.H. Towens, “Infrared and optical masers”, Physical Review 112, 1958, pp. 1940-1949 (PDF)
[2] T.H. Maiman, “Stimulated optical radiation in ruby”, Nature 187, 1960, pp. 493 [3] J. F. Asmus, G. Guattari, L. Lazzarini, G. Musumeci, R. F. Wuerker, “Holography in the conservation of statuary”, Studies in Conservation 18, 1973, pp. 49-63, URL [4] J. F. Asmus, S. G. Murphy, W. H. Munk, “Studies on the interaction of laser radiation with art artifacts”, in R.F. Weurker (ed.), Developments in laser Technology II, Proc. SPIE 41, 1973, pp. 19-30 [5] R. Bordalo, “John Asmus, from Lasers to Art Conservation”, e-conservation magazine 3, 2008, pp. 12-19, http://www.e-conservationline.com/content/view/598/ [6] A. Martini, “Utilità del laser nel restauro della pietra e del marmo”, Quaderni della Soprinten¬denza ai Beni Artistici di Venezia, Venezia, 1978, pp. 151-152 [7] L. Lazzarini, La pulitura dei materiali lapidei da costruzione e da scultura, Cedam, Padova, Italy, 1981 [8] J. F. Asmus, “More light for art conservation”, IEEE Circuits and Devices Magazine, March Issue, 1986, pp. 6-14 [9] J. F. Asmus, “Lasers in conservation”, Conservation News 34, 1987, pp. 9-10 [10] M. Cooper, Laser cleaning in conservation: an introduction, Butterworth Heineman, Oxford, 1998 [11] A. C. Tam, W. P. Leung, W. Zapka, W. Ziemlich, "Laser cleaning techniques for removal of surface particulates", Journal of Applied Physics 71, 3515, 1992, doi:10.1063/1.350906 [12] S. Georgiou, V. Zafiropulos, D. anglos, C. Balas, V. Tornari, C. Fotakis, “Excimer laser restoration of painted artworks: procedures, mechanisms and effects”, Applied Surface Science 127-129, 738, 1998, doi:10.1016/S0169-4332(97)00734-4 [13] R. Oltra, O. Yavas, F. Cruz, J. P. Boquillon, C. Sartori, “Modelling and diagnostic of pulsed laser cleaning of oxidized metallic surfaces”, Applied Surface Science 96-98, 484, 1996,  doi:10.1016/0169-4332(95)00500-5 [14] C. Fotakis, Optics and Photonics News 6 (5), 30, 1995, URL [15] Marvin J. Weber, Handbook of Lasers, CRC Press, 2001 [16] A. Einstein, “Zur Quantentheorie der Strahlung”, Physikalische Zeitschrift 18, 1917, pp. 121-128
About the author
Rui Bordalo
Conservator-restorerContact: rmbordalo@gmail.com Rui Bordalo is a conservator-restorer specialised in easel paintings. He has a particular interest in the study of art materials and in the application of new technologies to conservation. This interest led him to pursue a PhD at the Courtauld Institute of Art in the application of laser technology in the cleaning of paintings. He currently teaches several disciplines of the conservation course at Portucalense University, Porto. He is a board member of the Portuguese Association of Conservator-Restorers (ARP) and a Committee member of the European Confederation of Conservator-Restorers' Organisations (ECCO) since 2005. He is also one of the founders of e-conservation magazine, where he is currently the editor-in-chief.
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