Reversibility in the restoration of fresco paintings: a comparative study of nanosilica-based chromatic reintegrations

• Reversibility of chromatic reintegrations based on a colloidal dispersion of nanosilica particles (SiO₂·nH₂O) as binder. • Mechanical cleaning achieved full removal but damage to the mortar substrate. • Chemical cleaning left residues and caused pitting damage to the mortar substrate. • Laser ablation showed the best balance between removal efficiency and substrate preservation. • Pigment composition determined laser response, with oxide-based paints being the most susceptible. This study evaluates the reversibility of inpainting restoration work on wall paintings using a colloidal dispersion of nanosilica particles (SiO₂·nH₂O) as a binder. Three paint removal methods—mechanical scraping, laser ablation, and a gel-based chelating agent—were tested. To replicate real-life conservation challenges, laser and chemical treatments were also applied to fresco mock-ups designed to simulate historical paintings. Mock-ups were prepared using pigments such as Egyptian blue, lapis lazuli, ultramarine blue, chromium green, and mars red mixed with the nanosilica binder (chromatic reintegration) and applied a fresco (historical painting). Analytical techniques including stereomicroscopy, colour spectrophotometry, hyperspectral imaging, optical and scanning electron microscopies and vibrational spectroscopies were used to assess effectiveness and side effects. The results highlight the challenges of achieving full reversibility, with laser ablation proving to be the most reliable and controllable approach, although its application varies according to the chemistry of the pigments in the adjacent fresco paint. Laser parameters such as pulse duration should also be considered as they can cause mechanical damage to the underlying mortar. This study supports more sustainable and ethically responsible practices in wall painting restoration.