laser conservation of art

Cultural heritage materials — art and artefacts — can be sensitive, ephemeral and unique, much like the multitudes of added layers that are found obscuring their

surfaces. Previous cleaning and restoration attempts, and general storage, use and misuse lead to the build-up of a myriad of types of dirt and other materials on top of the original work. In order to preserve our past and avoid causing further damage to artefacts during cleaning, an awareness of the original material is required during restoration, as well as a careful and informed approach to conservation. Such understanding requires knowledge of materials science as well as a framework for treatment.

Artists themselves have, of course, always been aware of the importance of the properties of the materials they use for achieving specifi c eff ects and durability. In an early fi ft eenth century treatise, describing the pigment vermillion (HgS), the renaissance painter Cennino Cennini warned that:

“Th is colour calls for various temperas, according to the situations in which you have to use it…But bear in mind that it is not its nature to be exposed to the air, but it stands up better on panel than on the wall; because, in the course of time, from exposure to the air, it turns black when it is used and laid on the wall”1.

As technology improves, so does our scientifi c appreciation of the material complexity of works of art — for instance, a recent discovery showed that the iridescence of ceramic glazes from the Italian Renaissance results from the formation of nanoclusters of silver and

copper in the lustre2. Th e enormous range of materials encountered in art — such as pigments, dyes, organic binding media, stone, metals, organic fi bres and tissues — and the complex way in which they have been used means that their analysis and an explanation of the mechanisms of their ageing constitutes a multifaceted problem, spanning many subfi elds of materials science and chemistry. Th e modern characterization, analysis and conservation of artworks therefore requires a concerted

multidisciplinary approach. What makes conservation a particularly demanding materials science problem, however, is that treatment and restoration of art, especially when we consider irreversible interventions like cleaning, cannot rely on trial and error and require a thorough understanding of the original materials, as well as the eff ects of the treatment applied.

Th e use of lasers for conservation and cleaning of works of art seems a risky suggestion, given the complexity of the

Laser conservation of artAUSTIN NEVIN1,2, PARASKEVI POULI1, SAVAS GEORGIOU1 AND COSTAS FOTAKIS1,3

are in 1the Institute for Electronic Structure and Laser, Foundation for Research and Technology Hellas (IESL-FORTH), PO Box 1385, Heraklion, 71110, Greece, 2the Courtauld Institute of Art, University of London, Somerset House, Strand, WC2R 0RN, London, UK and 3the Department of Physics, University of Crete, PO Box 2208, Haraklion 71003, Greece.e-mail: [email protected]; [email protected]

Focusing a laser on the dirt covering a precious work of art may seem like a dangerous thing to do, but this unexpected technique has found a variety of cleaning applications. Analogies from other fi elds of materials science can provide guidance for its use, and model experiments ensure it doesn’t all end in disaster.

Figure 1 The Acropolis, Athens, Greece. The inset shows detail of a fi gure from the West Frieze of the Parthenon (block no. 3 (ΔΖ ΙΙΙ)). The left half of the fi gure is dark from the build-up of pollution, and the right half of the fi gure has been cleaned with combined ultraviolet and infrared laser irradiation. Inset reprinted with permission from ref. 8.

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physiochemical risks associated with the ablation of photosensitive substrates, the limited opportunities for testing on real cases before treatment is carried out and the irreversibility of the potential damage that could be caused. Surprisingly then, lasers have provided the basis for highly eff ective and versatile techniques, addressing many aspects of conservation, and have inspired new approaches, methodologies and investigations. Th is has been facilitated by the application of previous knowledge from simpler systems to the complex problems of cleaning dirt-encrusted artefacts.

Figure 1 shows the Parthenon — the temple of Athena, built in the fi ft h century bc on the Acropolis in Athens, Greece. Th e marble frieze that runs along the top of the temple depicts the festive procession of the ‘Panathenaia’ in honour of the city-goddess Athena. Pollution, caused primarily by the central location of the Acropolis in Athens — a city of around four million inhabitants and almost half as many cars — had led to the build-up of encrustations of thick layers of dirt, smog and other unwanted materials on the exposed surfaces of the 2,500-year-old frieze. Th e inset in Fig. 1 shows the result of cleaning one of the blocks from the Parthenon West Frieze using laser ablation, which did not compromise the remains of the original monochromatic surface layers.

Th e basis for the use of lasers in the cleaning of the Parthenon frieze is that in many cases on stone — for instance white marble — dark surface pollution contaminations are removed by ablation at specifi c wavelengths at much lower fl uences than those at which substrate damage occurs. Laser ablation has been used in the past to clean dirt from stone, but following extensive tests on a series of marble samples exhibiting pollution accumulations in diff erent thicknesses and morphologies, it was realized that the most controllable and selective laser cleaning called for a more elaborate irradiation mechanism than that brought about by exposure to a single wavelength. Instead, simultaneous irradiation at two wavelengths (1,064 nm and 355 nm) were used. Th e reasoning behind this development was that the use of two wavelengths in diff erent spectral ranges (infrared and ultraviolet) promotes contributions of diff erent material-removal mechanisms3. Importantly, in comparison with alternative techniques used for removing encrustations from stone (such as sand blasting and chemical solvents), laser ablation was able to clean dark pollutants selectively without aff ecting the underlying substrate.

A fundamental issue is addressed by considering the evolution of laser technologies into delicate cleaning techniques — that of introducing a new methodology into real-life applications with inherent risk. For the acceptance of lasers for the cleaning of art, several problems had to be overcome regarding potential side eff ects recognized by conservators, as well as scientifi c concerns regarding the fundamental aspects of the photophysics that drives ablation. Th e applications of laser cleaning have relied on analogies and expertise from less-complex fi elds. In many cases, studies on models have guided successful extensions of laser cleaning to new problems.

In contrast, limited understanding of laser-ablation mechanisms has restricted progress in other areas where extensive trial and error is similarly impossible, such as complex biological systems. Laser ablation has had only limited success for intravenous surgery; on the other hand photorefractive keratectomy (eye surgery), which also relies on laser ablation, has developed into a very successful technique4. Th e reasons behind the failures and successes of applications of laser ablation can be explained, but this is only because we now know more about the fundamental aspects of laser–materials interactions. Having learnt the hard way, from the use of laser ablation with little understanding of the operating mechanisms and potential consequences, scientists now realize that modelling and proper justifi cation of analogies with fundamental studies is crucial.

When considering the cleaning of paintings, extremely complex layered systems must be taken into account. Figure 2 shows the number of potential layers that may build up over the lifetime of a painting, including several layers applied intentionally by the artist, several protective and restoration layers, and the ubiquitous dirt additions on top and between layers. Th e high chemical complexity and ill-defi ned properties of paintings — with not only each layer but also each work diff ering in composition, condition and treatment — make both modelling and analysis especially tricky. Of course, it is not worth restoring a painting if the side eff ects accelerate degradation and deterioration, ultimately compromising the painting’s integrity. Th erefore, extensive studies have been performed using spectroscopic and analytical techniques to assess structural and chemical changes in paintings following laser processing. However, a case-by-case examination of plausible eff ects in realistic samples clearly becomes tedious and even impractical. Too many aspects and details must be analysed. Moreover, even if a detailed examination were possible, given the variety of systems involved, this would not specify general principles underlying applications as is necessary for systematic optimization. Th e questions must therefore always be raised: what are the criteria for specifying an appropriate model and how valid are the extrapolations made?

In an eff ort to model laser-ablation cleaning for paintings, we have considered systems consisting of simple photolabile dopants dispersed within polymers, aiming to emulate a photosensitive varnish or binding medium. We use aryl iodides as dopants, whose photosensitivity and reactivity is, in many respects, typical of the organic chromophores encountered in paintings. On excitation, the dopants dissociate into reactive aryl radicals, which may abstract a hydrogen atom from the polymer to form stable aryl compounds (for example, naphthalene), and/or react via diff usion-limited processes with each other to form the biaryl equivalent (binaphthalene). By doping these compounds in polymers and varnishes, we can probe the extent and type of chemical modifi cations induced in the substrate to give us insight into the potential modifi cations that could be induced in the paint and varnish being cleaned. Detailed studies have been performed on these models as a function of laser-irradiation parameters such as wavelength, fl uence and laser pulse width, and substrate properties such as absorptivity and chemical composition5.

Whereas laser parameters can be easily varied and controlled, the material

Figure 2 Stratigraphy of an idealized canvas painting. From the bottom up, the support is covered by a preparation layer, followed by brown under-drawing. This is then covered by a white preparation paint layer, a red pigment layer, a blue layer containing an organic photosensitive dye, two original varnishes and three restoration varnishes. Finally a layer of black ‘dirt’ covers the surface. Each paint layer is of the order of tens of micrometres.


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properties of varnishes and dirt on the painting to be cleaned cannot. An example of a material property that is of great importance in determining the eff ects of laser ablation is molecular weight6. Molecular weight does not generally aff ect the reactivity of radicals within the bulk material, aside from the limited infl uence on radical diff usion. However, models demonstrate that the effi ciency of ablation strongly depends on polymer molecular weight — depending on laser irradiation parameters, ablation is usually easier for molecules of lower molecular weight. Th e fl uences to remove higher molecular weight varnishes must therefore be increased, and the amount of unwanted by-products that are produced and accumulate in the substrate may also be greater as a result. Studies of processes on doped polymer model systems show that this dependency on molecular weight is not unique to varnishes, but rather a general characteristic of polymer ablation. Until now, this important parameter has not been considered in the practice of laser cleaning of art; rather the focus has been on the use of diff erent laser systems with diff ering wavelengths, pulse durations and repetition rates. Insights such as these have allowed more systematic refi nement of previous methodology, which now takes into account materials properties as well as laser parameters.

Analogies from other fi elds of materials science, including the extensive previous experience gathered in conventional laser material processing, may also serve as a guide for further developments in the use of lasers for conservation, but of course they also require validation before being used. For example, femtosecond-pulse laser technology may lead to radically new processing schemes. Multiphoton polymerization using femtosecond pulses has been used to create structures with details of sizes below the diff raction limit and for fabricating complex three-dimensional structures on the 100 nm scale7. Precise control over morphology and structuring are specifi c advantages of processing with femtosecond pulses. Following studies on model varnishes, we have found that ultraviolet femtosecond ablation also results in greater control of induced chemical modifi cations in specifi c fl uence ranges. Th e mechanisms of this specifi city seem to be highly novel. Recent applications of femtosecond laser structuring of biological tissues are probably successful owing to the absence of chemical alterations to ablated tissue. Th is is equally encouraging for the cleaning of polymeric coatings on paintings. From model studies, we foresee that extensions of such capabilities to conservation may eventually revolutionize the approach to cleaning.

Models have clearly become necessary for the understanding of fundamental photophysics associated with ablation. Th ey have also been crucial for the fi ne-tuning of treatment and the explanation for observations in the processing of materials, as found in the highly specifi c but related process of cleaning of art. Studies on models have indicated specifi c trends and advantages associated with diff erent laser ablation schemes, and have ultimately enabled the refi nement of laser cleaning approaches to restore, conserve and preserve art and a variety of heritage materials for future generations.References1. Cennini, C. Il Libro dell’Arte: Th e Book of Art Ch. XL (Dover,

New York, 1933).2. Padovani, S. et al. Appl. Phys. A 79, 229–233 (2004).3. Pouli, P., Frantzikinaki, K., Papakonstantinou, P., Zafi ropulos, V.

& Fotakis, C. in Proc. 5th Int. Conf. Lasers in the Conservation of Artworks (LACONA V) (Dickmann, K., Fotakis, C. & Asmus, J.) 333 (Springer Proceedings in Physics 100, New York, 2005).

4. Vogel, A. & Venugopalan, V. Chem. Rev. 103, 577–644 (2003).5. Bounos, G., Kolloch, A., Stergiannakos, T., Varatsikou, E. &

Georgiou, S. J. Appl. Phys. 98, 084317 (2005).6. Rebollar, E. et al. J. Appl. Phys. 101, 033106 (2007).7. He, G. S., Markowicz, P. P., Lin, T. C. & Prasad, P. N. Nature

415, 767–770 (2002).8. Papakonstantinou, E. et al. in Proc. 10th Int. Congress

Deterioration and Conservation of Stone (ICMOS, Sweden, 2007).

AcknowledgementsWe would like to acknowledge the contributions from our colleagues at IESL-FORTH and EU Research Infrastructure Laserlab-Europe (R113-CT-2003-506350). Th e conservation of the Acropolis West Frieze is a collaborative project between IESL-FORTH, ESMA (Committee for the Preservation of the Acropolis Monuments) and YSMA (Acropolis Restoration Service).

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laser conservation of art

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