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Identifier

000419063

Title

Development of a photoacoustic monitoring system for the study of laser ablation processes upon the removal of encrustation from stonework

Alternative Title

Ανάπτυξη συστήματος φωτοακουστικής παρακολούθησης και μελέτης διαδικασιών αποδόμησης κατά την αφαίρεση επικαθήσεων από πέτρινα υποστρώματα με λέιζερ

Author

Παπανικολάου, Αθανασία Φ.

Thesis advisor

Χαραλαμπίδης, Δημήτριος

Reviewer

Άγγλος, Δημήτριος
Ζαχαράκης, Ιωάννης

Abstract

Laser cleaning of environmental encrustation from outdoor monuments and sculptures is a well - established technique successfully applied in the preservation of Cultural Heritage. Conservators commonly rely on their experience and visual assessment in order to set the limits for the termination of the laser cleaning process. However, this practice can jeopardize the cleaning outcome as micro-damage may occur. In this respect, the accurate and real time monitoring of laser cleaning interventions remains an open issue urging for further investigation, particularly when employing cleaning methodologies involving objects and deposition layers that present similar ablation threshold values. Thus the cleaning process becomes more demanding and the decision as regards the precise number of pulse that removes the unwanted layer gets important. In the context of this thesis, a prototype hybrid photoacoustic (PA) and optical system for the on-line monitoring of pulsed laser cleaning procedures was developed. This approach has enabled the “listening” of the sound produced during the ablation through the recording of acoustic waves in the MHz regime. The intrinsically generated PA signals combined with high resolution optical images provide the opportunity to follow the cleaning process accurately and in real time. Technical simulation samples have been used to demonstrate the potential of this novel technique, while the real time monitoring of the cleaning process has been successfully implemented, for the individual use of IR and UV wavelengths, as well as, their simultaneous use in various energy fluence ratios. This state of the art approach has allowed the precise determination of the critical number of laser pulses that eliminate the encrustation layer, and most importantly enables the understanding of the involved ablation mechanisms. Specifically, variations at the amplitude of the PA signal have been used as an indicator of the different laser ablation processes and, by means of cross correlation analysis, they enabled the detection of changes to the irradiated surfaces. The final irradiated surface was also examined with analytical techniques (SEM, profilometry and reflectance measurements) aiming to evaluate and assess the cleaning result. In addition, through the study of the generated PA signal it was possible to calculate the relative temperature ratio rise (T_IR / T_UV), induced on the material during the laser ablation and thus associate it with the involved mechanisms. The results from the above study will be presented with the aim to demonstrate the successful monitoring for the unwanted material removal, as well as, to explain the dominant ablation mechanisms for each wavelength upon the laser cleaning of encrustation from stonework and thus to discuss the role of the two-wavelength laser cleaning methodology to avoid discoloration phenomena.

Language

English

Subject

Black crust

Laser cleaning

Marble

Issue date

2018-11-23

Collection

School/Department--School of Sciences and Engineering--Department of Physics--Post-graduate theses

Type of Work--Post-graduate theses

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