Application of non-invasive optical monitoring methodologies to follow and record painting cleaning processes

Abstract

The cleaning of painted artworks, i.e. the critical operation whereby materials are selectively removed from a painted surface by partial thinning or complete elimination of varnish, is one of the most debated conservation operations, being an irreversible process, which may result in chromatic and morphological variations in the painted surface. Due to ageing, the upper layer is subject to darkening and yellowing because of blanching and fading from ultraviolet exposure, dust deposition, and overpainted layers due, for instance, to restoration interventions. This degradation can either alter the original appearance of painting polychromy or cause mechanical failure of the finishes. To address these adverse conditions, a process of examination and analysis is critical to the definition and interpretation of the varnish layer. When investigating the ageing process of old paintings, it is of great importance to obtain insight into the painting technique as practiced in the past, and the first step in gaining this knowledge is, to a large extent, based on the study of the varnish film. An effective control of the process and objective evaluation of its outcome requires therefore instrumental/analytical support. The present study illustrates the successful application of non-invasive optical techniques-such as colorimetry, multispectral reflectography, laser scanning micro-profilometry, and optical coherence tomography-to the monitoring of an Italian fourteenth-century painting cleaning process. Results presented here confirm that optical techniques play a pivotal role in artwork diagnostics, especially with regard to conservation operations, while also indicating their validity when applied to the monitoring of the cleaning process.

Sustainability in art conservation: a novel bio-based organogel for the cleaning of water sensitive works of art

“…Such a stiffness allows an easy deployment and removal of the cleaning gel, without leaving any visible residue. The complex viscosity of the produced formulation steadily decreases with the frequency, as already reported for similar systems [33] while the storage and loss moduli (G′ and G″) behave as expected for gel-like systems, i.e. G′ > G″ over the investigated frequencies.…”

“…Laser scanning microprofilometry was performed directly on the painting surface to figure out if any morphological modification occurred after the treatment. This non-invasive technique allows the acquisition of topographic maps of the surface with a depth resolution of 1 μm and a lateral resolution of 20 μm, suitable for morphological investigation of a 3D surface with micrometric resolution [33]. Further investigations of the cleaning performance were achieved by analyses of fragments sampled before and after the cleaning procedures with Optical Microscope in UV-Vis light and with FTIR microscopy in ATR mode.…”

Optical devices provide unprecedented insights into the laser cleaning of calcium oxalate layers

“…The instrument works at a distance of 12 cm from the paint surface in a 45°/0°configuration according to CIE standards. [32,33] The CIELab values are promptly returned for each point of the surface (for each pixel), in our case nine L*a*b* values were computed for each step of cleaning with a preset size of 10×10 pixels (2.5×2.5 mm). The spectrophotometer Konica Minolta CM-2600d was used for the spectrophotometric survey as well.…”

Multianalytical non-invasive characterization of phthalocyanine acrylic paints through spectroscopic and non-linear optical techniques

“…Together with the above mentioned spectroscopic techniques, several other non-invasive optical methods and 3D imaging techniques have been applied to paintings in the past decades for the study of multi-layer internal structures, aiming at overcoming the detection limits imposed by the presence of highly scattering and semi-opaque materials [17][18][19][20][21][22][23][24]. Optical Coherence Tomography (OCT), originally developed for biological applications, provides high-resolution cross-sectional images of semi-transparent objects.…”