Revealing the Distribution of Metal Carboxylates in Oil Paint from the Micro‐ to Nanoscale

Abstract

Oil paints comprise pigments,d rying oils,a nd additives that together confer desirable properties,but can react to form metal carboxylates (soaps) that maydamage artworks over time.T oo btain information on soap formation and aggregation, we introduce an ew tapping-mode measurement paradigm for the photothermal induced resonance (PTIR) technique that enables nanoscale IR spectroscopyand imaging on highly heterogenous and rough paint thin sections.PTIR is used in combination with m-computed tomography and IR microscopytodetermine the distribution of metal carboxylates in a23-year old oil paint of knownformulation. Results show that heterogeneous agglomerates of Al-stearate and aZ ncarboxylate complex with Zn-stearate nano-aggregates in proximity are distributed randomly in the paint. The gradients of zinc carboxylates are unrelated to the Al-stearate distribution. These measurements open an ew chemically sensitive nanoscale observation windowo nt he distribution of metal soaps that can bring insights for understanding soap formation in oil paint.

Substrate-mediated hyperbolic phonon polaritons in MoO3

“…With PTIR, light absorption in a sample is measured by transducing the photo-induced thermal expansion of the sample directly beneath the probe tip of an atomic force microscope [37]. Notably, PTIR can sense sample composition far below the exposed surface [38][39][40], at depths exceeding 1 µm [33,41], with its signal proportional to the local sample absorption coefficient [42], enabling easy comparison of PTIR spectra with far-field IR databases [40,42,43]. These characteristics enable broad applications in materials science [44][45][46] and biology [47][48][49][50][51].…”

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

“…Other FTIR applications for the study of metal soaps involve the use of synchrotron light in combination with micro‐FTIR spectroscopy (μSR‐FTIR)[ 33 , 105 ] and micro‐FTIR spectroscopy combined with X‐ray micro‐tomography and photothermal induced resonance. [ 65 , 106 ] Romano and co‐workers performed multivariate analysis on micro‐FTIR data from cross‐sections characterised by zinc soaps formation. They were able to detect a zinc carboxylate gradient in the cross‐sectional view of the gel material, where the edges were rich in zinc carboxylates, and the centre contained quantities of unreacted linseed oil.…”

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

“…With time, hydrolysis of ester bond takes place 9 – 11 and, depending on the nature of the pigment and additives, formation of metal soaps occurs 12 . As a result, the organic molecular composition of an oil paint layer evolves from polyunsaturated triglycerides to a significantly more complex system, whose composition evolves over years, even centuries, entailing the simultaneous presence of free fatty acids, free dicarboxylic acids, mono-, di- and triglycerides, and cross-linked fractions 2 , 3 , 5 , 7 , 10 , 11 , 13 – 17 . In addition, when metal soaps are formed, carboxyl moieties are bound to metal cations, in variable proportions depending on the age of the paint, the environmental conditions 18 , 19 , and the nature of the pigment 20 , leading to the formation of free metal soaps and ionomer-like networks 21 – 23 .…”