Aplicación de un láser Nd:YAG para retirar grafiti de la caliza Liozinfluencia de los parámetros láser (longitud de onda, fluencia y número de pulsos) y la composición del grafiti

  1. Santiago Pozo-Antonio
  2. Nuria Antonio Fontán
Journal:
Cadernos do Laboratorio Xeolóxico de Laxe: Revista de xeoloxía galega e do hercínico peninsular

ISSN: 0213-4497

Year of publication: 2021

Issue: 43

Pages: 17-40

Type: Article

DOI: 10.17979/CADLAXE.2021.43.0.8751 DIALNET GOOGLE SCHOLAR lock_openOpen access editor

More publications in: Cadernos do Laboratorio Xeolóxico de Laxe: Revista de xeoloxía galega e do hercínico peninsular

Abstract

Recently laser ablation as a tool for cleaning heritage stones has experienced important improvements due to the mechanical contactless, the absence of residues, its gradualness and its selectivity. Once a laser equipment has been selected, optimization of the laser parameters (i.e. wavelength, fluence and number of pulses) has to be carried out. In this work, the influence of these parameters on the extraction of two compositionally different graffiti paints from the Portuguese limestone Lioz using a nanosecond Nd: YAG laser was evaluated for the first time. Unpainted and painted surfaces were treated with different wavelengths (infrared at 1064nm or ultraviolet at 355nm), fluences and number of pulses. Initially, the damage threshold was identified, and then, the most suitable conditions for the extraction of an alkyd blue graffiti and a polyethylene silver paint were detected. The resulting surfaces were evaluated by means of stereomicroscopy, spectrophotometry, roughness measurement and scanning electron microscopy.As a general result, after the optimization of the removal, differences were not identified for the results obtained by both wavelengths. However, the most influential factor in the efficacy was the graffiti paint composition. The silver paint was the most difficult graffiti to clean since after the laser application with the subsequent extraction of the aluminum sheets (inorganic pigment in this paint), a C-rich translucent layer was found on the surface. Although damage thresholds were established at 0,4 and 0,8 J.cm-2 for 355 and 1064 nm respectively, the fluences that allowed better results in terms of graffiti extraction were higher than these thresholds. For each wavelength, the optimal fluence was different depending on the composition of paint: i) for blue paint, ultraviolet radiation (at 355 nm) achieved a satisfactory cleaning with 0,5 J.cm-2 and 10 pulses and infrared radiation (at 1064 nm) achieved it with 1,3 J.cm-2 and 25 pulses, and ii) for the silver paint, although the total extraction of the C-rich translucent layer was not achieved, the best efficiency levels were identified with 0,5 J.cm-2 and 5 pulses for 355 nm and 0,9 J.cm-2and 5 pulses for 1064 nm. In this work, for the first time, the optimization of the graffiti cleaning on Lioz limestone, widely used in the Portuguese cultural heritage built with rock, is carried out. Therefore, this research is the necessary information that every conservator-restorer should consult before undertaking a laser cleaning of Lioz rock.

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Data source: Dialnet