Formulation of Innovative Spirit Varnishes Using Eutectic Systems as Alternative Solvents

Eutectic systems have emerged as promising alternative solvents for numerous applications. However, despite their potential, their use in cultural heritage conservation remains scarce, and their suitability for spirit-type varnish formulations has not yet been comprehensively demonstrated. Therefore, this study explored the viability of eutectic systems as alternative solvents for spirit varnishes by comparing their performance with conventional solvents through physicochemical characterisation and a preliminary proof-of-concept. Following a screening process, the Menthol:Thymol (8:1) system (MT) was identified as the most suitable candidate. Owing to its relatively low volatility, formulations of dammar and mastic dissolved in MT (D-MT and M-MT, respectively) were prepared with 5 and 10 wt% ethanol (D-MT5, DMT10, M-MT5 and M-MT10). The results showed that MT-based varnishes exhibit comparable behaviour to conventional solvents, in terms of properties such as resin solubilisation capacity, rheological behaviour, viscosity and refractive index. However, MT-based varnishes showed enhanced antioxidant activity and produced homogeneous coating films with appealing optical characteristics. Conversely, drying tests revealed slower initial solvent evaporation than conventional varnishes; however, except for D-MT, all eutectic-based formulations reached ≥90% solvent evaporation after 2 days. These findings prove the feasibility of menthol-based eutectic systems as alternative solvents; however, further ecological studies are required to establish their overall sustainability.