Aging of Natural Resins in Presence of Pigments: Metal Soap and Oxalate Formation

Abstract

The degradation process involving the formation of metal soaps in drying oils is a well-known problem due to the reaction of cations coming from pigments reacting with free fatty acids from the oil. A similar behavior has been observed during an experiment concerning the aging of different natural resins (shellac, dammar, and colophony) in the presence of common historic pigments (smalt, ochre, umber, azurite, lead white, zinc white, and titanium white). In the case of natural resins, the terpenic acids react with the cations in the pigments. Each resin shows a different reactivity and affinity to different cations related to the complex mixture of terpenic structures and acids contained in natural resins. So far it has not been possible to identify one or more specific acids preferentially involved in this “saponification” process. Colophony seems to be the most reactive resin, while zinc white and smalt appear to be the most reactive pigments. The explanation for the different reactivity of the acid components of the three resins is unclear. Studies on acidity of different resins, free acid mobility, and pigment reactivity are ongoing. Generally, the presence of a pigment has been proven to modify the stability of resins that have undergone accelerated aging. Mixtures of resins and pigments have been exposed to natural and different artificial aging conditions (photooxidative, thermal, thermo-oxidative, and wet cycles). Moreover, it has been observed that some pigments, particularly smalt and zinc white, favor the simultaneous formation of oxalates. The formation of oxalates and different degradation products from natural resins in the presence of pigments is particularly important, as it deeply affects the removability of varnishes and, more generally, the cleaning processes. Furthermore, it permanently modifies the interface between painting and varnish layers as well as the aesthetic properties of the paint surfaces.