This paper describes the application of the Er:YAG laser for the cleaning of a 17th-century polychrome wood sculpture which was originally opulently gilded, but had been totally overpainted around the nineteenth century. A series of diagnostic analyses and laser tests have been performed for a comprehensive understanding of the artistic-technique and the state of preservation, and for controlling the laser cleaning. Cross-sections of minute fragments and samples collected by the Er:YAG laser allowed us to understand the complex stratigraphy. PY-GC-MS, SEM-EDX, and XRF analyses highlighted the challenge of this cleaning work. It has been considered appropriate to remove gradually the overpaints on the clothing of the Virgin in order to uncover the original gilded portion, although it might be extremely degraded. The Er:YAG laser ablation allowed the selective removal of different layers of multi-colored overpaint and of the thick and resistant past restoration infilling which covered the thin degraded original layers.

There are very few documented instances of the use of an Er:YAG laser in the cleaning of stone artefacts; however, the cases reported in the literature have shown its effectiveness in the removal of surface organic patinas and lichens. A Roman marble funerary urn (67-100 AD) of archaeological importance was covered with an intractable surface layer that obscured its delicate decorative carving. Chromatographic mass spectrometric and Raman spectroscopic techniques allowed the characterisation of those inorganic and organic materials that had undergone photo-oxidation and biological changes, resulting in the thick encrustation. An Er:YAG laser at 2940nm, with a pulse length of 300 mu sec, was used for cleaning the urn. Preliminary tests were performed with fluences ranging from 0.7 to 6.4J/cm(2) and in the presence of a variety of wetting agents. The laser cleaning resulted in a sufficiently clean marble surface with the thick deposit of unwanted material removed. Finally, the sensitivity of the method allowed for a compact layer of calcium oxalate to be retained on the marble's surface to help protect it.

The Er:YAG laser has proven particularly efficient in cleaning procedures of works of art. The removal of the superficial deposits is achieved through melting, thermal decomposition and evaporation. However, the energy absorbed by vibrational modes is dissipated as heat, increasing the temperature of the surface coating that could cause damage on the object. The aim of this study was to evaluate the temperature increase induced by a Er:YAG MonaLaser (LLC., Orlando, FL, USA). To that purpose, we designed a dedicated device to perform the tests in an inert atmosphere or with a wetting agent, to measure the radiant energy per laser pulse. Tests were carried out both on graphite, which absorbs IR radiation and showed a very intense flash emission, and on different kind of samples representative of materials with different levels of conductivity and thermal diffusivity. Results obtained showed that the temperature increase in the irradiated surface depends on the substrate but never causes the damage of the organic and inorganic material. The use of a solvent as wetting agent has been also tested.

A cleaning method based on an Er:YAG laser system at 2.94 mu m, highly absorbed by OH bonds, was tested for removal of over-paintings, varnishes and patina top-layers from various painted surfaces, including laboratory paint models and old paintings. The aim was to evaluate the efficiency, selectivity and safety of the laser cleaning method using various pulse energies and various OH containing wetting agents to enhance the efficacy and limit the penetration of the laser beam. A large number of paint models were prepared with known characteristics (type and number of layers, thickness, composition) simulating old masters' techniques. A set of diagnostic controls was designed to study the effects of the laser radiation on the surface components, including morphological, optical and chemical examination and analyses. The aim was also to compare the laser method with the traditional solvent based procedures. Thresholds of safe energy were found for each type of surface layer such as varnishes and over-paintings. The results confirmed the suitability of the Er:YAG laser when used by qualified and expert conservators, especially in combination with traditional chemical and mechanical cleaning methods. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

An analysis of surface ablation by laser exposure of art objects as part of the conservation process indicates that heat diffusion from the site of laser exposure may be minimized by proper selection of wavelength and exposure duration. A model for unwanted material removal with a laser has been developed taking account of the threshold phenomenon of ablation as a function of wavelength, and exposures at 2.94 mm by an Er:YAG laser with short duration pulses is compared with those from a Nd:YAG (1 064 and 532 nm), CO2 (10.6 mu m) and the ultraviolet excimer laser at 193 nm. Thermal diffusion is minimized by taking advantage of the large amount of heat removed by the phase change of water into steam. This model suggests that for bulk removal at strongly absorbed wavelengths, many short pulses are better than continuous exposures. The selection of the Er: YAG laser allows the use of hollow glass waveguides of high flexibility, which are commercially available, as delivery systems. Examples of successful removal are given for contaminants overlying a Madonna's gold leaf halo and the heavy dark accumulation of soot on an unvarnished oil painting, as well as for thick synthetic adhesive on canvas. (C) 2000 Editions scientifiques et medicales Elsevier SAS

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Adele de Cruz, Susanne A. Hauger, and Myron L. Wolbarsht, "The Role of Lasers in Fine Arts Conservation and Restoration," Optics & Photonics News 10(7), 36-40 (1999) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article