Portable macro-FTIR scanner for advanced analysis of paintings and monitoring of cleaning operations

Abstract

In the last decades, there has been a marked increase in interest in External Reflection Fourier Transform Infrared spectroscopy for non-invasive and in-situ analysis of paintings, aimed at identifying constituent materials and degradation products without damaging artifacts. For the same reasons, it has also been successfully used to assess cleaning and conservation treatments directly on artwork surfaces during restoration. Within the CHANGES project, this research focuses on recent advancements in external reflection FTIR spectroscopy, utilizing a macro-FTIR scanner for fully automated chemical mapping with Pearson’s coefficient correlation maps. The scanner was employed to identify the paint material and study the distribution of degradation products present on the panel painting “Madonna con Bambino e altri episodi sacri” by Arcangelo di Cola (15th century). In particular, measurements were taken to monitor the progress of two cleaning gel systems during restoration. A 4 × 11 cm² area of the painting was scanned before and after cleaning to assess the thinning of oxidized degradation products and oil-based retouching from the Madonna’s blue mantle. The macro-FTIR scanner allowed automated acquisitions of about 1200 points per map, and material distribution was visualized through false-color correlation maps based on the Pearson correlation coefficient. These maps revealed detailed pigments distribution across glazes, highlights, and shadowed areas, offering valuable evidence of the artist’s meticulous technical approach. Pre-cleaning maps revealed oxalates as degradation products, likely originating from an old restoration, and possibly oil-based repainting; this finding was confirmed by paint fragment analysis through macro-ATR-FTIR imaging system. Post-cleaning maps showed a significant reduction in calcium oxalate and oil retouching, particularly achieved with one of two cleaning systems, thereby confirming the gel’s effectiveness in removing unwanted surface layers and degradation products. The macro-FTIR proved effective in simultaneously identifying and mapping organic and inorganic compounds, even distinguishing overlapping layers in some cases. Moreover, the use of correlation maps proved useful for comparing and assessing different degrees of cleaning performance, representing a significant new advancement in the real-time analytical evaluation for users without advanced chemometric expertise, during conservation treatments. These results support further research on an experimental setup to predict the risk of painting degradation by monitoring the extent of superficial deposits.