A Critical Review on the Analysis of Metal Soaps in Oil Paintings

Abstract

Up to 70 % of the oil paintings conserved in collections present metal soaps, which result from the chemical reaction between metal ions present in the painted layers and free fatty acids from the lipidic binders. In recent decades, conservators and conservation scientists have been systematically identifying various and frequent conservation problems that can be linked to the formation of metal soaps. It is also increasingly recognized that metal soap formation may not compromise the integrity of paint so there is a need for careful assessment of the implications of metal soaps for conservation. This review aims to critically assess scientific literature related to commonly adopted analytical techniques for the analysis of metal soaps in oil paintings. A comparison of different analytical methods is provided, highlighting advantages associated with each, as well as limitations identified through the analysis of reference materials and applications to the analysis of samples from historical paintings.

A Comprehensive and Systematic Diagnostic Campaign for a New Acquisition of Contemporary Art—The Case of Natura Morta by Andreina Rosa (1924–2019) at the International Gallery of Modern Art Ca’ Pesaro, Venice

“…ATR-FTIR measurements allowed also for the detection of metal soaps; these did not result in the characteristic aggregates and eruptions but are likely homogeneously distributed throughout the paint layers [69]. Carboxylic acids may have derived from oil ageing and have combined with alkaline earths or heavy metals present in the artwork, resulting in metal soaps [70][71][72]. On the other hand, metal soaps might have been part of the commercial paints formulations with the purpose of better dispersing the pigments in the medium or of lowering the price of the product [70,71].…”

Multi-Analytical Assessment of Bodied Drying Oil Varnishes and Their Use as Binders in Armour Paints

“…Additional changes in the shape of the ν OH can be seen with the growth of bands at 1550 cm −1 , as has been reported in metal carboxylate formation. An increase in absorption at 1425 cm −1 can be attributed to the oxidation of double bonds and the formation of v(C-O) [34].…”

“…Heritage 2021, 4 FOR PEER REVIEW 11 as has been reported in metal carboxylate formation. An increase in absorption at 1425 cm −1 can be attributed to the oxidation of double bonds and the formation of v(C-O) [34].…”

Long-chain mercury carboxylates relevant to saponification in oil and tempera paintings: XRPD and ssNMR complementary study of their crystal structures

“…1–4 Nevertheless, the recently increased concern regarding metal soaps has been awakened in conservation science since these substances, being spontaneously formed in paintings, can induce serious undesirable changes in the appearance and stability of precious works of art. 5–7…”

“…, metal cations from pigments react with free fatty acids from binders), in the light of current knowledge, the metal soaps formed in an oil paint undergo a complicated evolution comprising stages such as ionomers, amorphous metal carboxylate complexes, crystalline soap phases and consequently re-mineralized complex degradation products. 6,8 The final stages of saponification can manifest themselves variously, for example, by the formation of aggregates, protrusions, surface crusts, delamination or increased transparency of paintings, affecting both the appearance and the structural integrity of painted works of art. 1,5,6,9…”