Research program
Evaluation of effects of laser Cleaning/Ablation tretment on materials of interest in cultural heritage
University Co-ordinator
Università degli Studi di BRESCIA -
INGEGNERIA MECCANICA - BRESCIA(BS)
Research Unit Leader
Roberta PEDRAZZANI
Description
The whole project consists in the following 4 stages:1.choice of samples, pre-characterization and instrumentation setting2.treatment of laser cleaning/ablation3.treated sample characterization4.results analysisThe program of this Unit has been divided in only two stages(in order to simplify its structure) which include points 1,3 and 4 to which this Unit will take part in an active way.Stage 1 is focussed on criteria for choosing the samples and on the choice of proper analytical techniques, accordingly with sample origin. Biological analyses are also added, so as to investigate possible biodeterioration phenomena. Stage 2, however, is focussed on samples characterization after cleaning treatments and on the discussion of results, aimed at assessing proper techniques for each studied sample.The aim of the Unit is to support by chemical, structural, and biological characterization the development of new approaches for the recovery of monuments and manufacts with historical, artistic, and cultural values based on laser cleaning techniques. The characterization of the samples must be accurate and it is mandatory to consider complementary analysis to evaluate the sample before and after the treatments. The Unit will be in charge to analyze the materials and to follow the recovery process for the optimization of the parameters for the cleaning. STAGE 1For each specific material, the characterization of encrustation before treatment is mandatory for choosing proper parameters, such as energy flux, approximate number of pulses and applied overlap between adjacent spots for achieving an efficient treatment of the sample. The patinas and crusts coating the surface of samples chosen for the LC/LA test will be characterized to understand their possible origins : (1) treatments for aesthetic and/or protective purposes, (2) biologically induced deposits produced by microflora living on the surface, with or without reaction with the substrate, (3) relationship between atmospheric agents, such as SOx, leading to sulphation and the formation of gypsum crusts, and (4) dry or wet deposition of atmospheric particles. A combination of factors will be considered in the interpretation of such deposits. Several monuments of different stone types will be chosen with a range of microenvironmental conditions, so as to obtain an overview of crusts and patinas. Samples will be collected from monuments of different stone and metals which have not been restored or cleaned yet. Sampling could be carried out with the aid of scaffolding mounted for restoration work or by using mechanical devices such as hydraulic towers. Samples will be taken from several parts of the monuments, each submitted to different rainfall conditions and with different spatial orientation. Special care will be taken to collect samples from similar microclimatic conditions and to different exposures to the sun, to the position in comparison to the sea and wind, to the presence and the intensity of the vehicular traffic.The samples will be analyzed by applying classical and innovative techniques in order to obtain different sets of information on the coloured coatings for a comparison of results. When possible, the techniques should be non-destructive, efficient, and rapid to investigate materials and their state of deterioration.Several techniques will be systematically applied to a large number of samples from each site. In particular, optical and SEM analyses of fresh and polished sections, chemical analyses by EDX, X-ray diffraction (XRD) of flat surfaces and micro-XRD2 analysis of scratched powder from each individual patina will be performed. Additionally, several other techniques, such as IR microscopy, microraman will be used on selected samples. If the samples will have coloured patinas, measurements of their diffuse spectral reflectance will be systematically made and colour coordinates (CIE 1931) will be calculated from these spectra. X-ray diffraction (XRD) is commonly recognized as a fundamental tool to obtain information on crystalline phases of materials. In the Unit four different diffractometers are available. During the last years two new diffraction techniques have been developed: glacing incidence X-ray diffraction (GIXRD) and microdiffraction with bidimensional detector (micro-XRD2). Thanks to IR microscope, microraman, microXRF, and electron microscopy with EDX detector, also available in the Unit, it will be possible to well identify pigments and all the other phases with a spatial resolution of few microns or less. The MiniSIMS instrument, available in the next future in the Unit, will allow also the detailed characterization of surface before and after the laser cleaning treatment, in order to verify the chemical composition of the surface and the possible contaminants. The thickness of each patina will be correlated to the occurrence of specific environmental conditions allowing deposition phenomena. In general, the microstructure of the patinas and their interaction with the substrate can be related to textural properties, diagenetic processes, and climatic parameters occurring during their formation. On the basis of the experimental results, a model sequence will be described based mainly on the mineral composition. Microbiological analyses will be carried out in parallel, on samples exhibiting typical biodeterioration appearance, in order to assess the presence of biodeteriogens. The main groups of microorganisms (total heterotrophic bacteria, N and S oxidizing bacteria, fungi, algae) responsible for cultural heritage damage will be investigated by means of standard methods.STAGE 2The cleaning process should be carried out gradually and selectively, in order to avoid the formation of harmful by-products (e.g., soluble salts) and surface alterations (e.g., microfractures, abrasions) which may accelerate the deterioration, owing to the increased specific surface area. In the cleaning of stone artworks there are two major needs: 1) removal of accumulated pollutants, black particles, etc., for maintaining the aesthetics of artwork and increasing its lifetime and 2) partial preservation of gypsum, whose internal layer retains on its surface all the potential remains of carved details. In comparison with conventional cleaning techniques, laser-assisted cleaning offers a high degree of control and selectivity, especially when utilizing self-limiting phenomena or interfacing with on-line analytical techniques.Thus, cleaning should be carried out at the lowest practical fluX so that the more selective mechanisms operate. An initial examination into the effect of laser cleaning on samples will be undertaken. Treated laser-cleaned surfaces will be examined by the techniques previously described (XRD, microXRD2, IR microscopy, microraman, microXRF, SEM, EDX, ecc.). The effects of laser cleaning on the samples will be investigated by comparing experiments obtained from laser-cleaned samples with those obtained from a clean reference sample.During the project, a young researcher will be educated to use the techniques for characterize the historical artworks and monuments.
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