Drying Oil and Natural Varnishes in Paintings: A Competition in the Metal Soap Formation

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.

In Situ Infrared and UV–Vis Ellipsometries to Probe The Varnish Ageing and Swelling of G. Bellini “Transfiguration”

“…Moreover, the artificial aging of triterpenic varnishes and resins leads to the broadening of the carbonyl band. [22,28] In parallel, a decrease of the CH characteristic bands has been reported by different authors [22][23][24][25][26][27][28][29] very visible in the spectrum of the aged model varnish. Also commented in the literature, the bands at ca.…”

“…UV exposure is known to promote oxidation processes and can help to predict the evolution of chemical, optical, and mechanical properties of systems highly sensitive to oxidation such as varnishes. [24,25] As described in the literature, [22][23][24][25][26][27][28][29] the absorbance between 3200 and 3600 cm −1 , assigned to OH bonds, increases due to oxidation reactions. Furthermore, the absorbance of the fingerprint zone increases between 1330 and 800 cm −1 due to C─O stretching absorption.…”

“…[23] The presence of Cu-carboxylated species in the varnish layer may be due to the migration of Cu-carboxylate from the paint layers or to the reaction between the resin/varnish and the Cu-based pigments. [24][25][26] Moreover, if the global intensity of the 1330-800 cm −1 zone increases with oxidation, a band at ca. 1085 cm −1 seem also present, which can be linked to an additional compound, such as cellulose remaining from the cottons.…”

An Improved Oil Painting Formation Using Advanced Image Processing

“…ere are many color image processing methods, most of which can be divided into the following two types: first, there is a component image in the color image, which can decompose the module image in the color picture, and procedure the module image to synthesize it into a complete color image to obtain a processed color image [15]. Second, there are a large number of pixels in the color image, which can be processed to get the processed image [16].…”

Modelling the Interaction between Carboxylic Acids and Zinc Oxide: Insight into Degradation of ZnO Pigments

“…The stability of oil paints is influenced by several factors such as temperature, humidity [ 1 , 2 , 3 ], and exposure to light [ 4 ], pollutants, and solvents [ 5 , 6 , 7 ]. The oil binding medium, upon exposure to oxygen, first dries via radical polymerization reactions forming a cross-linked molecular network, and then it ages through reactions of the autoxidation products with the pigment and eventually leads to paint degradation [ 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. This is a serious problem for paint conservation since degradation can irreversibly change the stability and the appearance of the paint.…”