RICERCA ITALIANA     

Evaluation of effects of laser Cleaning/Ablation tretment on materials of interest in cultural heritage

Università degli Studi di Napoli "Federico II"

Abstract

About one half of the whole world cultural heritage is located in Italy. This is therefore a fundamental resource requiring and implying a serious responsibility for its conservation and restoration. Physics is nowadays bringing a more and more significant contribution to the field of cultural heritage, even if with some delay with respect to other fields, such as Medicine, regarded as primary. It is depressing to observe that Italy, despite its natural inclination, is far from reaching the experience of other countries as Japan or United Kingdom. We believe that the time is ripe to begin fundamental scientific research oriented not only to the execution of actions with sophisticated instrumentation, but also to the development of new devices and to the optimization of their use.

Laser Cleaning/Ablation (LC/LA) technique is currently accepted, likewise other "traditional" (mechanical or chemical) ones, as a technique to remove extraneous materials from a solid substrate. The technique, born in microelectronic industry, in recent years has been successfully applied in the field of conservation and restoration of cultural heritage, with promising results.

Within the field of cultural heritage the evaluation of the effects of laser radiation interaction with the surface under treatment must be considered from a global point of view, taking into account artistical, historical, cultural aspects. Final result of a LC/LA operation must be obviously estimated by humanistic and historical experts taking into account elements having little significance in other applications: a slight discoloration probably does not affects the efficiency of an electronic circuit but may significantly reduce the quality of an artwork.

Results of restoration treatments have been analyzed by means of modern analysis and monitoring techniques, but the impossibility of wide trials has highly limited the full application of scientific method, especially in its forecasting ability. As a consequence, the choice of the technique to be applied is often leaven to individual feeling, due to lack of deep knowledge of laser radiation interaction with material of interest in cultural heritage. Long tradition and reasonable caution induce to prefer more invasive but well-known traditional techniques.

In order to contribute to the resolution of such problems we propose, within this research project, to use advanced surface analysis techniques to study the effects of Laser Cleaning/Ablation from two points of view: the quality of results obtained and the determination of the damage caused. Sample choice and overall results will be mainly carried out by humanistic and historical researcher in Lecce and Genova research units, sample treatment will be carried out at Lecce and Napoli research units, experienced in this field, and sample analysis will be carried out mainly at Brescia and Genova research units having large experience. <<<

Principal Investigator

Luciano Rosario Maria VICARI Università degli Studi di NAPOLI "Federico II"

Research Objectives

The use of Laser Cleaning/Ablation (LC/LA) techniques, i.e. the use of pulsed laser radiation to remove extraneous materials from a solid substrate, has found in recent years an increasing interest in several fields ranging from microelectronic industry to applications in conservation and restoration of cultural heritage. Within this field the most common application of LC/LA is the removal of undesired surface layers (pollution crusts, corrosion layers, surface treatments, fungi, bacteria, etc.) from artifacts of both organic (paper, parchment, textiles, wood, etc.) and inorganic (stones, metals, glass, terracotta, etc.) materials. Application of LC/LA techniques requires knowledge of effects of laser radiation interaction with both the substrate and the substance to be removed. Radiation, thermal and mechanical effects must be taken into account in order to determine the parameters (laser wavelength, laser beam fluence, pulse repetition rate, etc.) to be used in LC/LA operation. In order to achieve good results LC/LA operating parameters must be below "damage threshold" of substrate and above the "cleaning threshold" of the material to be removed.

Determination of such threshold values is non easy, especially in cultural heritage field since involved materials are usually very complex (substrate is often inhomogeneous and surface is often irregular) and the result of treatment must be mainly evaluated from point of view of the fruition of the artwork, even if on the basis of quantitative chemical and physical parameters.

Main objective of this research project is to determine what are the chemical/physical parameters to be monitored during or analyzed after LC/LA treatment and what are their relative weights in order to achieve an overall evaluation of the treatment carried out on objects of interest in cultural heritage field. We want to determine a set of quantitative evaluation rules arranged in a way to be meningful for both workers (technicians) and managers (Museum and Fine Arts Agency etc.) of restoration treatment. In order to pursue this aim several researchers from different fields (chemists, physicists, historians) will work together within this research project.

At the same time it will be possible to get a better knowledge of interaction of pulsed laser radiation with layered materials and a better classification of materials and techniques in order to supply a scientific and technical basis to be used by cultural heritage operators and commissioners.

The whole project is of great technical and scientific interest above all for its interdisciplinary. It will allow to implement the knowledges in laser-matter interaction field and in particular for materials of historical-cultural interest (such as pigments, stones, marbles, metals, paper...). The transfer of the knowledges and the competences through the training of staff for this specific program will be an ulterior added value of this project. The research results, systems and achievements will be communicated via the usual channels: scientific publications, contributions to national and international Conferences, input to Educational Courses, etc., also in order to guarantee the transfer of knowledge from the beginning of the project into new products and services. <<<

First Results

Results of first phase will be

* avaiability of sample sets

* definition of LC/LA treatment protocols

* purchasing of instrumentationResults of second phase will be

* preliminary chemical/physical analysis of reference samples

* sample sets treated following prevoously defined LC/LA treatment protocols

* set-up of purchased instrumentationAt the end of this phase the results of a large number of chemical and physical analysis will be available for selected sample sets.Results of this phase will be main results of the research project:

* definition of interventions suitable for each material

* individuation of the most relevant chemical/physical parameters to be monitored during or analyzed before/after LC/LA treatment

* determination of a set of quantitative evaluation rules arranged in a way to be meningful for both workers (technicians) and managers (Museum and Fine Arts Agency etc.) of restoration treatment <<<

Timescale

24 months

National and international background

Laser Cleaning (LC) birth-date can be placed at the beginning of the 1990s^1,2 even if the idea of using laser radiation to remove some substance from a substarate has been shown in 1965 by A. L. Schawlow³ to remove typewritten characters without damaging the underlying paper. In Dry LC (DLC) particle removal is due to sudden expansion of both substrate and particle as a consequence of pulsed laser irradiation, while in Stream LC (SLC), also called "wet" or "liquid assisted" LC^2,4,5, particle removal is due to vaporization of a thin liquid layer sprayed over surface to be cleaned. Besides these "direct" (laser radiation directly impinges on the surface to be treated) LC techniques some "indirect" (laser radiation does not impinge on the surface to be treated) techniques, such as "Shock-wave Laser Cleaning"⁶ or "Verso Laser Cleaning"⁷, have been proposed and used.

"Laser Ablation" (LA) refers to the interaction of a laser beam with a solid target producing vaporization of a small amout of irradiated material and has been extensively used and studied since high-power lasers have been available⁸. If the laser pulse is short enough, laser radiation interacts only with the irradiated zone, without involving the underlying or adjacent layers. Since both pulse repetition rate and number of pulses can be easily controlled, ablation can be carried out for sub-micrometric depths. The removal of a surface layer from a solid substrate can also be obtained by a before-evaporation ablation⁹ mechanism such as laser induced "buckling".

Laser Cleaning/Ablation (LC/LA), a term used in order to gather together all these techniques based on different physical mechanisms but all characterized by use of the energy from a pulsed laser beam to remove unwanted material from a substrate, is very important for modern high technology¹⁰ and is used extensively in the fabrication of printed circuit boards, in lithography and epitaxial growth and for the cleaning of micro-optical and micro-mechanical components.

The recovery of the Cultural Heritage often needs the cleaning of the surface of artefacts (frescoes, burlaps, coins, etc.) covered by surface layers (oxides, carbides, sulphates, fungi, bacteria, etc.) produced by chemical, physical, and biological phenomena which can act synergistically. Cleaning process should be carried out gradually and selectively, in order to avoid the removal of required layers (painting, protection layer, natural ageing layer, etc.), the alteration (discoloration, acceleration of deterioration, etc.) of artefact surface and the formation of harmful by-products. The authentication and the dating of an artefact require an accurate technical analysis of the materials after removal of extraneous surface layers.

Traditional restoration techniques used for the recovery of the manufactured objects can be divided in two categories: a) mechanical techniques (abrasion of the deposited layers) and b) chemical techniques (use of solvents and chemical reagents). However, such techniques present some drawbacks: the first one can damage the artworks (e. g., indentation of the marble or the metal, as an example), the second one can damage both the manufactured objects (e. g., chemical attack of the painting pigment) and the atmosphere (elimination of used potentially aggressive reagents).

Application of LC/LA technique to artworks has been proposed in the early 1970s by J. F. Asmus¹¹ but only in last decade (1990s) it has become more and more considered in the conservation field^12,13 and has been successfully used for both organic¹⁴ (paper, parchment, textiles, wood, etc.) and inorganic^15,16 (stones, metals, glass, terracotta, etc.) materials. At the same time the conservation institutions devoted more attention to this method and the number of professional restorers using it is increasing.

Even if there are still open problems, the interaction of pulsed laser radiation with matter has been extensively studied from a theoretical point of view: beside classical book by J. F. Ready¹⁷ it is possible to cite more recent books by several authors^18,19. However in applications concerning cultural heritage samples are, usually, very complex: base material is neither homogeneous nor well defined (e.g. there are a lot of types of marble), sample surface is often neither plane nor regular (e.g. the surface of a coin), surface sample is often covered by multiple layers (painting, oxides, etc.). As a consequence the usual procedure is to perform preliminary tests using both monitoring and analysis techniques before treatment of each artefact.

Some techniques, such as optical microscopy and scanning electron microscopy (SEM) were used to evaluate the results, examine the irradiated areas and determine the cleaning efficiency²⁰. More recently, material characterization before and after cleaning treatments has been carried out by means of structural and spectroscopic techniques. Infrared spectroscopy (IR), microraman²¹, X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) were employed in order to characterize the stone surface and to assess the effects induced by laser radiation. XRD technique²² is well-established and widely employed in order to determine the nature of crusts and their relationship with the substrate. Over the last years two new diffraction techniques have been developed: glancing incidence X-ray diffraction (GIXRD)^23-27 and microdiffraction with bidimensional detector (micro-XRD2)²⁸. Since GIXRD allows the structural characterization of few atomic layers on sample surface, it is sensitive to very small amounts of phase laying on the surface. For this reason, it is possible to obtain information, which can be hardly obtained otherwise, such as the presence of crystalline phases of pigments, from samples. Another emerging technique is micro-XRD2, which is characterized by a narrow beam and by a 2D detector. The beam dimension (about few tens of microns) allows to analyse samples with a spatial resolution. Moreover, it is possible to investigate samples with complex geometry, since planar surfaces are not required. By means of bidimensional detector it is possible to collect the whole diffraction pattern in only one exposure, so that the results can be easily compared to conventional diffractograms. From 2D diffraction pattern is it possible to discriminate phases characterized by a different microstructure, thus allowing the analysis of heterogeneous samples. It is also possible to identify and quantify preferred orientation of a phase and the presence of residual stresses. The great potential of these techniques applied to cultural heritage has been reported^29-31. Some Authors report the comparison among different complementary techniques. For example, spectroscopic diagnostics can be very useful to verify the cleaning procedure and guarantee non-destructive cleaning³². IR spectroscopy is also employed to verify the laser-cleaning performances in the removal of particles and organic films from surfaces³³. A systematic study of chemical and physical changes induced by exposure to UV excimer laser light of unvarnished tempera paint samples has been undertaken as a part of the research activities included in the European project "Advanced workstation for controlled laser cleaning of artworks". To this purpose, a broad range of techniques has been used, including optical and vibrational spectroscopy and analytical mass spectrometric techniques. Integration of the results obtained by these techniques allowed the investigation of the nature and degree of changes of irradiated paints³⁴.

The discoloration of pigments or substrate induced by laser radiation is an important issue in LC/LA applications to artworks. The problem in widely reported in literature^35,36 but it was shown that a useful LC/LA procedure can be achieved, after a systematic study so as to choose the proper wavelengths³⁷. Spectrophotometry, i.e. measurement of diffuse reflection spectrum of sample surface, allows to compute parameters (e.g. L*a*b* defined by CIE-Commission Internationale de l'Eclairage) able to quantitatively describe human eye perception of sample colour.

In addition, by putting laser ablation together with LIBS (Laser Induced Breakdown Spectroscopy), the composition of the evaporated material can be investigated through the analysis of the emission spectra recorded during ablation. Indeed, the laser pulse, focused on the sample surface determines the formation of a plasma emitting radiation (fluorescence coming from atoms and/or excited ions) that is characteristic of the elements present in the sample. Therefore, by monitoring the emission lines of pollution elements one can control the cleaning process; by comparing the emission spectra recorded before and after the treatment on an artwork one can evaluate its effectiveness. The LIBS technique, born and developed above all with the aim of studying the laser induced plasma during the thin film deposition by pulsed laser ablation³⁸, has widely been used in the Cultural Heritage field^39-46.

In conclusion LC/LA is currently well known in the conservation field and a growing number of people are interested in using this technology, but there is still resistance in accepting it as a safe tool⁴⁷ due to some open problems (e.g. yellowing⁴⁸, discoloration⁴⁹, etc.) and the absence of general criteria to quantitatively evaluate overall (taking into account both cultural heritage and chemio/physical point of view) result of treatment. Therefore there is the necessity of interdisciplinary experimental work in order to understand how to convert the quantitative results of several specific monitoring or analysis techniques into an overall evaluation of the effects of LC/LA treatment in the field of cultural heritage. <<<