Laser cleaning of paintings: in situ optimization of operative parameters through non-invasive assessment by optical coherence tomography (OCT), reflection FT-IR spectroscopy and laser induced fluorescence spectroscopy (LIF)

Abstract

Nowadays the use of laser technology as a highly precise tool for the cleaning of paintings is gaining ground. The development of a non-invasive analytical protocol aimed at thorough assessment of the treated surfaces and real time monitoring of the laser cleaning action is thus becoming imperative. This ensures that no side effects (e.g. discoloration, darkening, blackening) will occur on the painting surfaces due to laser ablation. In the present study the potential of the combined use of optical coherence tomography (OCT) and reflection FT-IR spectroscopy for in situ non-invasive assessment of laser cleaning procedures has been investigated on a historical easel painting donated to science. Specifically, OCT and FT-IR analyses were carried out before and after each cleaning test in order to carefully assess the condition of the painting surfaces upon their irradiation with a KrF excimer laser and evaluate the removal of weathered and/or non-original materials (i.e. waxes, aged natural varnishes, oxalates, deposition/migration compounds). This specific laser radiation, operating in the ultraviolet region (λ = 248 nm), is highly absorbed by coating materials (i.e. oxidized natural varnishes) and thus high selectivity and control of the cleaning process can be pursued. A systematic integration of the information on the chemical changes of the molecular composition of the painting surface provided by reflection FT-IR spectroscopy with the visualization of the stratigraphical changes by OCT allowed the evaluation of the sensitivity of the painting materials to different operative parameters of the laser treatment, such as laser fluence and number of pulses. In parallel, laser induced fluorescence (LIF) spectroscopic measurements, recorded with the same laser beam at significantly attenuated energy density values, were exploited to provide information on the fluorescence properties of the irradiated varnish surfaces. The combination of the results obtained by OCT and FT-IR complemented with those from LIF measurements are discussed with the aim of developing an efficient methodology for assessing in situ the irradiated painting surfaces after the laser cleaning treatments and, thus, setting the basis for a future monitoring scheme that would ensure optimum cleaning interventions.

OCT for Examination of Cultural Heritage Objects

“…The most common restoration technique used for this purpose is with the use of solvents. OCT, especially if used together with FTIR spectroscopy examination, is capable of accurate assessment of the cleaning process [33,34]. Some tests of this kind were also performed during the aforementioned examination campaign of van Gogh's "Sunflowers" [35].…”

Cryogenics as an Advanced Method of Cleaning Cultural Heritage: Challenges and Solutions

“…In the case of contemporary paintings without a protective layer, we see abundant studies that advocate finding a solution to the cleaning of this type of surface. The research of physical, chemical, mechanical and biological methods has advanced towards gelled, soapy, emulsifying and ionic systems [3,[9][10][11][12], including photoablation [13][14][15][16], with the idea of reducing the risks associated with the treatment of this type of artwork [7], in addition to the reduction of toxicity and environmental impacts. The study of phenomena caused by the use of solvents, such as leaching or extraction of soluble polymeric substances, continues to provide information on the effect that these solubilizing agents have on the pictorial films and is also linked to the washing procedures and elimination of non-volatile residues deposited by the cleaning system [17,18].…”

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

“…The reliable assessment of the cleaning result in-situ has been investigated using a variety of cutting-edge analytical techniques, most of the times with encouraging results 2,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . Nevertheless, the on-line and real time monitoring of the laser cleaning progress remains a critical open issue that attracts the interest of the scientific community working in the field of heritage science.…”

“…In this respect, the sensitivity of the technique as regards the physicochemical properties change of the involved materials, the geometry of the object to be cleaned, the time necessary for the assessment of the recorded data, the required spatiotemporal resolution, as well as, the portability and cost of the experimental equipment are particularly important parameters. Laser induced breakdown spectroscopy (LIBS) 2,[6][7][8][9][10][11] , laser induced fluorescence (LIF) 7,11,12 , multispectral and hyperspectral imaging [13][14][15] , speckle imaging 16 , digital holographic speckle interferometry 17,18 and optical coherence tomography (OCT) 19,20 are some of the analytical techniques that have been considered for the effective assessment of the treated surfaces, as well as, the in-situ evaluation of the cleaning progress. In all these studies the final aim was to monitor in real time the laser restoration interventions in a variety of applications, such as the removal of environmental crust from stone, the cleaning of salts and other deposits from wall paintings, and the elimination of aged varnish from wooden artefacts and easel paintings.…”