Fig. 1

a Schematic indication of physicochemical processes that occur simultaneously during cleaning operations, altogether defined as solvent action. After and during the dissolution of varnish (orange), the paint matrix (yellow) expands in size (shaded yellow) due to the uptake of solvent (blue). The relative thickness of the layers indicated does not represent the actual thickness. b–e Schematic examples of possible structure in the polymeric binding medium in oil paint. b Aged ionomeric binding medium with metal carboxylates and high degree of cross-linking, c cracked ionomeric binding medium with metal carboxylates and high degree of cross-linking (shown crack diameters are not to scale), d ionomeric binding medium with metal carboxylates and low degree of cross-linking, e highly oxidised binding medium with intermediate degree of cross-linking and a high concentration of free carboxylic acid groups