RICERCA ITALIANA     

Research program

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

University Co-ordinator

Università degli Studi di NAPOLI "Federico II" - SCIENZE FISICHE - NAPOLI(NA)

Research Unit Leader

Luciano Rosario Maria VICARI

Description

LC/LA is currently one of the techniques used to remove extraneous material from a substrate. Its application in the field of cultural heritage is relatively recent and suffers from the lack of a way to quantitatively evaluate the overall effect of the treatment, starting from quantitative results of specific chemical/physical analyses.
In order to achieve this main objective of the research project, research unit in Naples will actively collaborate with researchers (both physicists/chemists and cultural heritage experts) of all other research units in all phases and will have two specific tasks: execute LC/LA treatments and perform some (spectrophotometric and LIBS) analyses. Both tasks will be carried out at LOA (Laboratorio di Ottica Applicata - Applied Optics Laboratory) which is located at Physics Department of "Federico II" Naples University and also operates within the "Centro Regionale di Competenza per la Conservazione, Valorizzazione e Fruizione dei Beni Culturali ed Ambientali" (CRdC "Innova"), a Regional Centre working in the field of cultural heritage.
Task 1: sample treatment
LC/LA treatments will be carried out using a Q-wsitched Nd:YAG laser equipped with second harmonic generator so that it will be possible to operate both at fundamental (corresponding to 1064 nm wavelength) and doubled (corresponding to 532 nm wavelength) frequency. Pulse duration is about 10 ns and pulse repetition rate can be varied from single shot up to 20 pulse/sec. Beam fluence can be controlled by acting on laser emission or laser beam. It will be possible to use "true" samples from ancient artefacts and/or "specifically prepared" samples processed for artificial soiling ar ageing. In all cases some "clean" samples will be used as reference.
Task 2: sample analysis
Spectrophotometry, i.e. measurement of diffuse reflection spectrum of sample surface, allows to compute parameters able to quantitatively describe human eye perception of sample colour. It is possible to define several set of parameters, but the most used, since directly connected to visual stimuli, are L*a*b* defined by CIE (Commission Internationale de l'Eclairage) Lab colour system. Samples of interest in cultural heritage are often non uniform: in such situation it is important to evaluate the statistical distribution of Lab parameter values in reference and in treated samples in order to evaluate changes that are perceived as a colour alteration of the sample.
LIBS (Laser Induced Breakdown Spectroscopy) is performed examining the time resolved spectrum of radiation from the ionised plasma generated during ablation. It can be considered a non-destructive (or micro-destructive) technique due to the small amount of removed material. This allows to use LIBS for monitoring (during LC/LA treatment) and/or analysing (before or after LC/LA treatment) procedures. As preliminary analysis LIBS allows characterisation of extraneous material to be removed, as monitoring technique allows recognition of changes in the composition of the ablated material giving a measure of "cleaning level", as final analysis allows evaluation of LC/LA treatment. LIBS can also applied to "clean" or reference samples, in order to characterise the substrate, as a non-destructive (or micro-destructive) analysis.

Results of these analysis will be collected togeter with data obtained by other research units and compared with qualitative but global evaluation of the LC/LA effect in order to determine the relevance of each parameter in the overall evaluation of the treatment.