Complementary use of Optical Coherence Tomography (OCT) and Reflection FTIR spectroscopy for in-situ non-invasive monitoring of varnish removal from easel paintings

Abstract

Higlights:• Reflection FTIR spectroscopy and OCT, when used together, enhance insight into the details of cleaning of easel paintings • FTIR complements OCT structural images by providing crucial information on the chemical composition of superficial layers • OCT complements the FTIR spectral data with information on the thicknesses of layers, both removed and left • This integrated approach increases the safety of the cleaning treatment. AbstractAlthough the solvent removal of unwanted layers (e.g. aged varnishes, overpaints, oxalate patinas, dirt from easel paintings is one of the most frequently performed restoration treatments it is questioned regarding its controllability and safety to the artwork. There is thus a need for developing suited diagnostic methodologies able to inform restorers on the chemical, optical and morphological effects of cleaning. To this aim, we have explored the possibility of complementary use of Optical Coherent Tomography (OCT) and reflection FTIR. On the one hand, the OCT technique provides cross-sectional images with varnish layers visible and thus permits for measurement of their thicknesses. Combining many cross-sections into 3D data provides insight into the varnish thickness distribution over a given area of the paintings. On the other hand, reflection FTIR allows for a chemical characterization of the surface compounds revealing the nature of the varnish and monitoring its removal. Artificially aged model samples and two historical paintings were investigated at different steps of solvent cleaning tests and the results of the two techniques compared highlighting the benefits of their combined exploitation.

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].…”

“…The ability of OCT to visualize varnish layers makes it a convenient tool to monitor a varnish removal with chemical treatment and/or laser ablation qualitatively [36,37] and quantitatively [33,39,40]. An application of OCT for quantitative assessment of varnish removal by swabbing is experimentally challenging because it is difficult to avoid micro-displacements of the object between sequential measurements caused by the contact with a cotton swab.…”

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)

“…The historical easel painting investigated herein, entitled Floral painting (Fig. 1a), belongs to the Rijksmuseum study collections of Amsterdam and was selected among a set of paintings donated to science within a specific research task (WP7.1a: Diagnostic Strategies for Assessing the Cleaning of Paintings) in the framework of the IPE-RION CH project [19,22].…”

“…Reflection FT-IR spectroscopy from a complex and optically thick surface (as that of a painting) generally involves the collection of both diffuse (from the volume) and specular (from the surface) reflection with a variable and unpredictable ratio that basically depends on the roughness of the examined surface as well as on the optical properties of the investigated materials; as a result reflection FT-IR spectra generally show distorted profiles that cannot be properly corrected by software. Nevertheless, they are still able to provide valuable information on the surface chemical composition ( [19] and references therein).…”

“…For this purpose, in the present study, a non-invasive approach based on the complementary use of optical coherence tomography (OCT) and reflection FT-IR spectroscopy has been used on an historical easel painting subjected to KrF excimer laser treatments, in order to develop a methodology for in situ assessment of the treated surface, thus enabling optimization of operative laser cleaning parameters. Strengths and limits of the integrated use of OCT and FT-IR were already proved for assessing the progressive removal of unwanted layers from both model paints and historical paintings by solvent cleaning procedures [19]. Specifically, OCT allowed obtaining the stratigraphic information (i.e.…”