New article published: An innovative methodology for testing and selecting greener solvents for varnishing paintings

Fife, G., Samyn, P., Stabik, B., Chauvat, C., & Sels, H. (2025). An innovative methodology for testing and selecting greener solvents for varnishing paintings. npj Heritage Science, 13:40. https://doi.org/10.1038/s40494-025-01638-6

The publication of this work was co-funded by GoGreen, and the research was initiated and carried out within SiC’s Greener Solvent Project in collaboration with: Karel de Grote University of Applied Sciences and Arts, Department of Sciences and Technology, Salesianenlaan 90, 2660, Antwerp, Belgium; SRAL The Conservation Institute, Avenue Céramique 224, 6221 KX, Maastricht, The Netherlands; Sirris, Department of Innovations in Circular Economy and Renewable Materials, Gaston Geenslaan 8, B-3001, Leuven, Belgium.

ABSTRACT
Organic solvents remain needed for certain treatments in cultural heritage conservation and restoration, including the application of traditional and synthetic resin varnishes on paintings. This research describes a collaborative approach for selecting and testing alternative greener sustainable solvents for this purpose. With commonly used solvent xylene targeted for substitution, initial solvent suggestions were made employing the SUSSOL (sustainable solvent selection and substitution) software tool considering key physical properties of the solvents and specified safety, health and environment hazard boundaries. In parallel HSPiP (Hansen Solubility Parameters in Practice) software was employed after the experimental determination of Hansen solubility parameters and the solubility of selected resins. Fourteen alternative solutions were selected and prepared from commonly used resins for application testing on reference substrates (e.g., Leneta cards and prepared canvas test boards). This testing included (1) the assessment of essential working properties, paint film solvent sensitivity, drying and film formation, and (2) standard coating tests such as water resistance, gloss, and color measurements. Comparable reference resin solutions used as varnishes, prepared with xylene and other solvents, were simultaneously evaluated. First standardized solvent tests provided a practical assessment of the sensitivity of the paint films to the substitute and reference solvents. Varnishing tests were subsequently performed on two historical paintings in order to evaluate the working properties and esthetic qualities of the resin solutions. The novel varnish Regalrez 1094 in isoamyl acetate showed very good working properties and gave a good visual result on both paintings. The resin solution Paraloid B72 in anisole showed good visual results on both paintings, while demonstrating acceptable working properties. This study illustrated that good working properties, protective and visual film qualities could be achieved with varnishes based on typical resins in alternative solvents. Various alternative greener solvents and their mixtures can thereby be suggested for dissolving and applying these resins. The approach within this study acknowledges the individuality of an artwork, which guides the selection of the most appropriate resin solution.