How to approach long-term monitoring of chemical dynamics in oil paintings?

Long-term condition monitoring of works of art can provide new insights into object-specific deterioration mechanisms. Detecting change over time allows us to determine whether deterioration is active, to investigate its cause and to establish the efficacy of conservation interventions. However, long-term condition monitoring poses both logistical and technical challenges. To address the latter, a 6-month pilot study using model systems has been performed to investigate the approach to long-term monitoring of chemical dynamic processes in oil paintings. The focus was placed on metal soap protrusions: a condition phenomenon encountered in oil paintings that results from dynamic chemical pigment-binder interactions. Eight portable non- or minimally invasive examination technologies available via the MOLAB facility of IPERION HS were used to detect change in model systems. These model systems were designed to form lead soap protrusions in situ in a short time frame by including reactive components in their stratigraphy, providing changes on a scale more typical of years or decades in real paint systems. Raking light imaging or commercial colorimetry did not provide sufficient resolution for detecting small-scale changes associated with lead soap protrusions. X-radiography with consistent acquisition parameters in combination with a form of automated recognition of protrusions was found to provide a relatively accessible method for monitoring changes in the spatial distribution of protrusions. 3D techniques such as optical coherence tomography and micro-profilometry were found to be suitable for detecting change in lead soap protrusions, provided that they reach sufficient spatial resolution in the plane of a paint layer (≤20 μm) and depth (≤2–3 μm). Acoustic microscopy was found to provide insufficient spatial resolution for this purpose. More specificity for lead soaps was provided by techniques that couple high resolution 2D or 3D imaging to spectral information, such as micro-profilometry coupled to VIS-NIR spectroscopy.