Research program
Evaluation of effects of laser Cleaning/Ablation tretment on materials of interest in cultural heritage
University Co-ordinator
Università degli Studi di LECCE -
FISICA - LECCE(LE)
Research Unit Leader
Maria Luisa DE GIORGI
Description
An international research activity in the field of the laser ablation has been consolidated in the past years in the Physics Department of the University of Lecce. This scientific activity is essentially based on thin film and nanostructure deposition by laser ablation, on material processing by lasers and on morphological, structural and optical characterizations of the produced films. Moreover, an activity of diagnostic of the emission of the plasma plume during the ablation and reactive ablation processes has been developed. In particular, by time- and space-resolved optical emission spectroscopy (LIBS) and imaging, the study of the plasma composition, of its spatial (at different distances from the target) and temporal (at different delay times from the arrival of laser hit onto the target surface) evolution has been carried out in order to let to infer information about the kinetic of the chemical species present in the plasma and about the mechanisms of the compound formation.LIBS has been recently applied also to artwork diagnostic in collaboration with the Provincial Museum "S. Castromediano" in Lecce. The studies of feasibility are documented in scientific papers submitted to international reviews in the Cultural Heritage field. In particular, the metallic alloy composition of a gun found in the seabed near S. Cataldo (Lecce) was determined through the identification of the characteristic atomic emissions coming from the superficial layer of the piece of ordnance. A comparative analysis of the emission spectra recorded before and after the gun restoration has proved the success of cleaning by means of traditional restoration techniques and has shown that the LIBS can be used as a useful tool for diagnostic in order to test the effectiveness of the artwork restoration. Moreover LIBS has been used to determine the composition of different parts of the bust of St. Gregory the Armenian, in the diocese of Nardò (in province of Lecce, Italy). The investigations have shown that all pieces are covered by polluted layers, rich mainly in calcium and easily removable by laser ablation. The performed analyses revealed an unexpected result: the decorative coating of the stole was realised in gold, instead of the supposed brass. Therefore, there are both the competences to manage the described project and the instrumentation to perform the proposed activity. The research consists in the treatment of samples different of historical-artistic interest by means of not conventional techniques (laser ablation) and in the analysis of the plasma produced during the laser irradiation by LIBS in order to reach a better understanding of the involved mechanisms. The final aim of the project will be to give to the restorers a technical-scientific base to choose the best conditions for the laser cleaning process.In particular, the activity will be developed through the following stages:- identification of materials of particular historical-cultural interest- arrangement of the experimental instrumentation present in the Laboratory of Radiation Physics of Physics Department of the University of Lecce- execution of the treatments by laser ablation on samples in different experimental conditions in order to remove the pollution products - use of LIBS to monitor the sample treatment- study of the sample characteristics as a function of the experimental parametersThe first phase of the project, consisting in the identification of the materials of historical-cultural interest for laser cleaning, will be carried out from the research group of the Department of the Cultural Heritage. The opportune material selection will be agreed with the competent organism for the preservation of the Cultural Heritage. According to the different materials, the researchers of the Physics Department will arrange the instrumentation, already present in the Laboratory of Radiation Physics.The second phase of the project represents the executive phase in which the laser ablation and cleaning will be performed and the emission spectroscopy technique will be used to control the processes and to verify the cleaning results. For this reason, it will become necessary to program a systematic work regarding the laser-material interaction.Joining the competences of the researchers of the Department of Cultural Heritages with the know-how of the Physicists of the Laboratory of Radiation Physics, different sets of samples will be considered: (i) not polished samples as reference data, (ii) artificial treated samples, (iii) naturally dirtied samples. In this way it will be possible to compare data from different samples. Moreover the ablation will be carried out in different experimental conditions, changing opportunely the characteristic parameters of the laser treatment (wavelength, frequency, fluence, etc).An UV pulsed excimer with a pulse duration of about 20 ns will be used for the ablation/cleaning. The possibility to use a multigas laser, working with different mixtures, will allow the study of the laser-matter interaction process versus different wavelengths (308 nm for XeCl laser, 248 nm for KrF laser, 193 nm for ArF laser). The laser fluence, that is the energy for sample surface unity, is another important parameter. The systematic increase of the fluence will let to determine an eventual damaging threshold of the artwork and the range of proper values for the cleaning process. Finally, it will be interesting to analyze how the cleaning mechanism is affected by the laser repetition rate and the pulse number.The ablation/cleaning process will be controlled systematically by LIBS. Such technique allows the characterisation of the chemical species typical of the dirty layer (e. g., the lines of calcium, a polluting element frequently recorded in dirty layer spectra) through the identification of the emission lines of the chemical species in the plasma produced as laser-material interaction effect. Through the control of the identified line intensity in real time it is possible to monitor on-line the cleaning process, to give an indication about the treatment times and to verify the obtained results. The study of the sample characteristics as a function of the experimental parameters will allow, therefore, the determination of the optimal parameters to remove the superficial layers by laser cleaning.Therefore, from a careful analysis and a study of the experimental results in the second phase of the project, a detailed classification of the materials and the interventions suitable for the single material will be performed in order to organize a technical-scientific database for the choice of the best conditions.The 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...). Such knowledges will stimulate the promotion of a scientific debate about the application of the new intervention methods on manufactured objects, increasing the interest also in the Faculty Course of Cultural Heritage and in the Interfaculty Course of "Technologies for the Cultural Heritage".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.
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