A multi-analytical approach for the identification of surface whitening phenomena in contemporary oil painting and its application to metal soaps

This research presents a multi-analytical approach for the characterization of the degradation products that form white hazes on paint surfaces. The surface of Untitled (1971), an oil painting by Santos Iñurrieta (1950–2023), one of the most noteworthy contemporary Basque artists, was chosen as a representative example. The appearance of surface whitening phenomena is a major issue in oil painting conservation and their identification represents a challenge since their origin can be very diverse. In order to characterize the white substance over Untitled (1971) and understand its formation, a global examination was performed, including radiography and UV induced fluorescence, followed by the analysis of different types of microsamples according to the diagnostic technique applied: paint fragments, cross-sections or surface scrapings. The methodology included the application of several techniques on the aforementioned samples: Digital Microscopy (DM), Optical Microscopy (OM), Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDX), X-ray Computed Tomography (CT), micro X-Ray Fluorescence (µ-ED-XRF), Raman Spectroscopy, Attenuated Total Reflection-Fourier Transform Infrared spectroscopy (ATR-FTIR), X-ray Diffraction (XRD), Gas Chromatography-Mass Spectrometry (GC–MS) and X-ray Photoelectron Spectroscopy (XPS). The proposed approach resulted in the identification of lead soaps (palmitate and stearate) as the primary cause for the whitening of the painting surface. Additionally, it provided information of the materials and techniques employed by the artist and revealed that the lead source was probably a lead drier. Thus, this work is a step forward in order to deal with one of the most significant problems currently being addressed in contemporary art conservation, providing relevant information for the efficient removal of the different whitening phenomena.