Review of the use of NMR spectroscopy to investigate structure, reactivity, and dynamics of lead soap formation in paintings

Abstract

Heavy metal carboxylate or soap formation is a widespread deterioration problem affecting oil paintings and other works of art bearing oil‐based media. Lead soaps are prevalent in traditional oil paintings because lead white was the white pigment most frequently chosen by old masters for the paints and in some cases for the ground preparations, until the development of other white pigments from approximately the middle of the 18th century on, and because of the wide use of lead‐tin yellow. In the latter part of the 19th century, lead white began to be replaced by zinc white. The factors that influence soap formation have been the focus of intense study starting in the late 1990s. Since 2014, nuclear magnetic resonance (NMR) studies have contributed a unique perspective on the issue by providing chemical, structural, and dynamic information about the species involved in the process, as well as the effects of environmental conditions such as relative humidity and temperature on the kinetics of the reaction(s). In this review, we explore recent insights into soap formation gained through solid‐state NMR and single‐sided NMR techniques.

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

“…A detailed and precise review on the use of NMR to study structure, reactivity and dynamics of lead soaps has been recently written by Catalano et al. [111] Their review deals with many applications of solution‐state NMR spectroscopy, high‐resolution NMR (HR NMR), and solid‐state NMR (ssNMR). [111] …”

“… [111] Their review deals with many applications of solution‐state NMR spectroscopy, high‐resolution NMR (HR NMR), and solid‐state NMR (ssNMR). [111] …”

“… [65] In addition, single‐sided NMR can be used to perform in situ and non‐invasive analysis to monitor the penetration depth, the chemical and physical effects of solvents, solutions, and suspensions, when various surface‐cleaning techniques are applied. [111] …”

The stability of paintings and the molecular structure of the oil paint polymeric network

“…Indeed, SSNMR is particularly useful to dissect the structural and dynamic properties of polymeric materials at the molecular scale 48 – 51 . NMR techniques have been used to characterise and monitor a range of Cultural Heritage materials 52 – 55 and have proven extremely valuable in the study of metal soaps 35 , 56 , 57 , and have occasionally been used to characterise the organic fractions of oil paintings. As thoroughly discussed elsewhere 35 , 58 , mobile fractions in drying oils may be analysed by liquid-state NMR 59 , 60 , complementing MS based studies, and swollen-state NMR may help to assess the insoluble fraction of the oil paint layers 61 .…”

“…NMR techniques have been used to characterise and monitor a range of Cultural Heritage materials 52 – 55 and have proven extremely valuable in the study of metal soaps 35 , 56 , 57 , and have occasionally been used to characterise the organic fractions of oil paintings. As thoroughly discussed elsewhere 35 , 58 , mobile fractions in drying oils may be analysed by liquid-state NMR 59 , 60 , complementing MS based studies, and swollen-state NMR may help to assess the insoluble fraction of the oil paint layers 61 . Ultrafast spinning SSNMR spectroscopy was used in recent research on two case study paintings, highlighting the potential of this technique to access all fractions—mobile and immobile—within the paint layers 58 .…”