Solid-state NMR for the study of Asger Jorn’s paintings

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

A molecular-level understanding of the structure of the polymeric network formed upon the curing of air-drying artists' oil paints still represents a challenge. In this study we used a set of analytical methodologies classically employed for the characterisation of a paint film-based on infrared spectroscopy and mass spectrometry-in combination with solid state NMR (SSNMR), to characterise model paint layers which present different behaviours towards surface cleaning with water, a commonly applied procedure in art conservation. The study demonstrates, with the fundamental contribution of SSNMR, a relationship between the painting stability and the chemical structure of the polymeric network. In particular, it is demonstrated for the first time that a low degree of cross-linking in combination with a high degree of oxidation of the polymeric network render the oil paint layer sensitive to water.

Profiles of paint layer samples obtained in the fringe field of a high field magnet by means of very short broadband frequency-modulated pulses

In this article, we describe the acquisition of depth profiles, in particular of paint layers, in the static gradient of a high field magnet, providing a superior sensitivity. The main objective are reference profiles that help to understand scans made with noninvasive unilateral nuclear magnetic resonance (NMR), which often suffers from poor signal-to-noise ratio when working with real samples. Various technical aspects like the coil geometry and the limit of resolution are investigated. A major advancement is the use of frequency-modulated pulses that are very broadband and at the same time very short (25 μs). The latter is necessary to allow the acquisition of a CPMG echo train of old, rigid paint material. Despite being far from adiabatic, they provide uniform excitation and refocusing over 1 MHz, which corresponds to about 400 μm with the used gradient. We show that the uniformity is even sufficient to obtain biexponential relaxation profiles. With these tools, a paint sample from a restoration campaign is analyzed with different contrast criteria: The original and two layers from former restoration attempts can be visualized, and furthermore, the relaxation profiles allow to study the migration of plasticizing molecules.

New Insights to Characterize Paint Varnishes and to Study Water in Paintings by Nuclear Magnetic Resonance Spectroscopy (NMR)

Paintings are complex multi-layered systems made of organic and inorganic materials. Several factors can affect the degradation of paintings, such as environmental conditions, past restoration works and, finally, the type of painting technique and the art materials used over the centuries. The chemical–physical characterization of paintings is a constant challenge that requires research into and the development of novel analytical methodologies and processes. In recent years, solvents and water-related issues in paintings are attracting more attention, and several studies have been focused on analyzing the interaction between water molecules and the constitutive materials. In this study, recent applications applying different NMR methodologies were shown, highlighting the weakness and the strength of the techniques in analyzing paintings. In particular, the study of water and its diffusive interactions within wall and oil paintings was performed to prove how the portable NMR can be used directly in museums for planning restoration work and to monitor the degradation processes. Furthermore, some preliminary results on the analysis of varnishes and binders, such us linseed oil, shellac, sandarac and colophony resins, were obtained by 1H HR-MAS NMR spectroscopy, highlighting the weakness and strengths of this technique in the field of conservation science.