The role of the polymeric network in the water sensitivity of modern oil paints

Jacopo La Nasa,Ilaria Bonaduce,Klaas Jan Van Den Berg,Bronwyn Ormsby,Ilaria Degano,Aviva Burnstock,Judith Lee

doi:10.1038/s41598-019-39963-z

Abstract

Spectroscopic and mass spectrometric analytical techniques were used to characterise two naturally aged Winsor & Newton (W&N) Winsor Green (phthalocyanine green, PG7) artists’ oil colour paint swatches dating to 1993 and 2003. Infrared and Energy Dispersive X-ray (EDX) analysis indicated that the swatches were of closely similar composition, yet the swatch from 2003 was water-sensitive whilst the swatch from 1993 was not. Water-sensitivity is a conservation challenge associated with significant numbers of modern oil paintings and this study aimed to further develop our understanding of the molecular causes of water sensitivity. SEM elemental mapping of samples taken from both swatches provided no indication for the formation of epsomite – a known cause of water sensitivity in some modern oil paintings. Liquid chromatography coupled with mass spectrometry (HPLC-MS) and gas chromatography coupled with mass spectrometry (GC-MS) also revealed very similar qualitative-quantitative composition in terms of unbound and esterified medium fractions. The polymeric network was investigated using analytical pyrolysis. A combination of flash pyrolysis coupled with gas chromatography mass spectrometry (Py-GC-MS) together with evolved gas analysis mass spectrometry (EGA-MS) revealed that the polymeric material was relatively more abundant in the non-water-sensitive paint. This is the first multi-analytical study that has demonstrated a correlation between water-sensitivity and the degree of polymerisation of the oil medium; independent of other known causes of water-sensitivity.

Highlights

There are many examples of unvarnished water sensitive twentieth and twenty-first century oil paintings, which limit or preclude the use of water or protic solvents for surface cleaning treatment and the removal of polar coatings
The results suggest that many water-sensitive paints contain a relatively high content of extractable dicarboxylic acids in comparison to non-water sensitive paints – such as those containing zinc oxide or lead white
It has been demonstrated that, for paints pigmented with three pigment types, differences in the severity of water-sensitivity of paints made with the same type of pigment did not appear to relate to: the total or free dicarboxylic acid content, metal soaps of free fatty and dicarboxylic acids, and the overall degree of hydrolysis

Summary

Introduction

There are many examples of unvarnished (uncoated) water sensitive twentieth and twenty-first century (modern) oil paintings, which limit or preclude the use of water or protic solvents for surface cleaning treatment and the removal of polar coatings. Polymerised fraction metal soaps of diacids that are connected to a covalently bound polymeric network, only via ester bonds fatty acid and nonsaponified diacids that are connected to a covalently bound polymeric network, only via ester bonds cross-linked material (fatty and diacids) that would remain covalently bound (C-C and C-O-C bounds), even if hydrolysis of all ester bounds present in the binder occurred of ester bonds[6] This led to the hypothesis that the degree of crosslinking, the polarity of the polymeric network, and, depending on the pigment, the nature of the ionomeric network were likely to influence the development and severity of water-sensitivity; which has been explored further in this study

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