Multi-technique characterization of Antonio Basoli’s picturesque views of Bologna: an integrated approach to 19th-century graphic heritage conservation

Antonio Basoli’s one hundred Picturesque views of the city of Bologna (1824–1836) include 95 preparatory drawings, 16 of which were executed on oil-impregnated lightweight paper now showing advanced degradation. This study aims to investigate the materials and techniques used by Basoli’s workshop and to develop an evidence-based conservation approach for these fragile works. An integrated analytical methodology combining non-invasive hyperspectral imaging (HSI), Fiber Optics Reflectance Spectroscopy (FORS) and Ion Beam Analysis (IBA) with micro-invasive SEM-EDX and FTIR-ATR spectroscopy was applied on five drawings on lightweight impregnated paper to characterize both the paper supports and drawing media. Linseed oil containing lead-based drying agents was suggested to be the impregnating substance, while iron oxide (sanguine) over metalpoint (Cu, Sb, Pb) defined the graphic media. The detection of copper–lead residues suggests that Basoli employed a direct pressure transfer technique similar to James Watt’s copying machine. Conservation treatments resulted in significant pH stabilization (from 5.35 ± 0.20 to 6.45 ± 0.33) and reduced yellowing (ΔE* = 4.9 ± 1.8) while maintaining the paper’s translucency. The results elucidate the innovative practices of Basoli’s workshop and establish a reproducible analytical and conservation methodology applicable to the preservation of nineteenth-century graphic heritage.