Abstract
Metal soaps formation is a well-known issue in oil paintings. Along the lifetime of the painting, carboxylic acids coming from drying oil (free fatty acids, acids from hydrolysis of triglycerides and from oxidation processes) can react with cations of some pigments (in particular, smalt, lead white and zinc white) forming the related carboxylic salts. As observed by many authors, the formation of these carboxylates, with the tendency to migrate and to aggregate, not only modifies the behavior and the aspect of the paint film but also complicates the cleaning approach. In previous works we have demonstrated that a similar pigment reactivity is possible even in presence of natural resins (such as colophony, dammar, mastic, etc) historically used as final varnishes on paintings. In this case, in the reactions the terpenic acids, among the main components of the resins, are involved. In this work, the carboxylates formation kinetics has been studied starting from two representative acids (palmitic and abietic) of painting oils and natural varnishes. Successively, the reactivity of the palmitic acid with the potassium abietate and of the abietic acid with the potassium palmitate has been verified. This investigation aims at clarifying in which way terpenic acids can be involved in the metal soaps reactivity confirming that also surface varnishes may play a significant role in the carboxylates formation and reactivity. It is important to keep in mind that a finishing varnish can be removed and reapplied many times during the lifetime of a painting, thus renewing the provision of reactive terpenic acids at the interface of the painted layers.
Highlights
One of the most common decay pattern of oil paintings is the formation, along the lifetime of the work of art, of metal soaps [1,2,3,4,5,6]
Along the lifetime of the painting, carboxylic acids coming from drying oil can react with cations of some pigments forming the related carboxylic salts
In previous works we have demonstrated that a similar pigment reactivity is possible even in presence of natural resins historically used as final varnishes on paintings
Summary
One of the most common decay pattern of oil paintings is the formation, along the lifetime of the work of art, of metal soaps [1,2,3,4,5,6]. The characteristic of drying oils, linseed oil in particular, is the presence of double bonds that allow the crosslinking process. These reactive sites are prone to oxidative processes that lead to the chain breaking and the formation of new acid sites [7,8,9,10,11,12]. Main results suggest that cations migrate from the pigment surface to the bulk of the binder (the drying oil) shifting from an acid site to another. The presence on surface of these protrusions and in general of metal soaps make the cleaning approach to the painting definitely complicate and delicate. Metal soaps cannot be removed from painting layers and surfaces since they are poorly soluble in traditional solvents used in conservation and, not all the conservators agree on their removal
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