The importance of the optical properties of the materials involved in laser cleaning: the case of calcium oxalate and albumen patinas on oil paintings

The presence of calcium oxalate patinas on easel paintings is well documented in the literature, as they clearly affect the overall visual perception of an artwork due to their chalky appearance. Their formation mechanisms have been widely debated, with calcium oxalate believed to form either from constituent materials or due to previous conservation treatments. Traditional cleaning approaches involving mechanical methods, aqueous systems, solvents, chelating agents and enzymes can produce unsatisfactory results or be overly intrusive. Removing calcium oxalate films could damage the original paint layers, making this task challenging. This study investigates the effectiveness of different laser systems for thinning or removing calcium oxalate and albumen patinas from oil paintings. The initial case study focused on an 18th-century oil painting titled «Christ on the Cross», recently treated at the conservation laboratories of the Academy of Fine Arts in L’Aquila. A widespread patina of calcium oxalate and albumen was present on the artwork. This layer formation was ascribed to mineralisation of a white egg-based varnish applied during a previous restoration following a strong cleaning. The patina was partially thinned using a 2940 nm Er:YAG laser system, usually employed for calcium oxalate removal, followed by application of a new varnish layer to reduce the whitish visual effect of the residues and saturate the paint layer. Building on this case study findings, this research further investigated laser cleaning options by testing other laser ablation regimes applicability, such as ultraviolet light wavelengths. Mock-ups were created to simulate the painting stratigraphy, incorporating various paint layers (e.g. Cadmium Yellow, Raw Umber and Ultramarine Blue), that were then covered with an artificial calcium oxalate-albumen patina. The laser tests involved mock-ups irradiation with the fundamental beam (1064 nm), the second (532 nm) and third (355 nm) harmonics of a Q-switched Nd:YAG laser, along with a 248 nm beam emitted from a KrF excimer laser. Both systems emitted nanosecond-duration pulses, while a longer pulses regime was investigated using a Short Free Running (SFR) Nd:YAG system with pulses of up to 130 μs. Reflectance-derived absorbance spectroscopy identified the absorption characteristics of the undesired patina and optical microscopy (OM) was used to evaluate the laser cleaning tests and to analyse changes occurred to the surface after irradiation. Additionally, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) and Raman spectroscopies were employed to identify any remaining patina and investigate potential alterations to the paint. Irradiation at 1064 nm and 532 nm wavelengths was found to damage the paint layers, as was the 355 nm wavelength, despite its superior performance in patina ablation. Our research has identified the 248 nm KrF system, which has not been previously reported for this type of application, as a promising alternative for cleaning oxalate patinas when conventionally Er:YAG systems prove to be unsatisfactory. The proposed methodology combines controlled, laser-assisted patina reduction and traditional solvent application to effectively clean the surface while preserving the paint layer integrity, thereby advancing the current state of research.