Evolution of the mechanical properties of oil paints through drying, natural aging, or moisture and temperature variations can induce internal stresses and may lead to macroscopic alteration such as crack initiation and delamination [1,2,3]. Therefore, investigating the mechanical behaviour of oil paintings and their multi-layered structures is of major interest to understand their long-term stability. In this paper we focus on the impact of moisture leading to weathering, on a set of variant paint layer build-ups using three different pigments, as well as an exemple of a ground layer stratigraphy, the latter a reconstruction of an often used 17th-century method of canvas preparation. Previous research has quantified the mechanical properties of oil paints and evaluated the influence of some pigments and binding media on the layers’ mechanical properties. They studied how chemical processes inherent to the material composition of the paint, as well as the cross-linking evolution due to natural drying results influence their mechanical behaviour evolution over time [1, 2, 4,5,6,7]. Thus, the fresh reconstructions we used in this study are expected to be softer than fully dried reconstructions and historical paintings [8].
Next to the natural aging processes of oil and pigment combinations, and inherent chemical changes, environmental conditions can also modify their mechanical behaviour [1, 4, 8,9,10]. Temperature and moisture content variations are both external parameters that may induce significant modification of the mechanical response of painting supports such as canvas or panel, as well as the various ground, paint and varnish layers, depending on their composition [11,12,13,14].
To evaluate and model the macroscopic behaviour of the whole painting stratigraphy under relatives humidity (RH) variations, Mecklenburg and colleagues [4] tested the behaviour of several oil paints and artist’s materials individually, and at fixed RH levels. For instance, they showed how a RH increase results in the decrease of the elasticity modulus of various pigment-oil systems—such as Naples Yellow, Burnt Sienna or Flake White—and illustrated the impact of RH levels on their behaviour [4]. Mecklenburg et al. then proposed Finite Element Modeling analysis to assess the response of paint build-ups.
Although the RH-dependence of the elastic modulus was considered, the influence of repeated cycles of RH has yet to be investigated. Building on this literature and to improve our insight on the behaviour’s evolution over repeated cycles of relative humidity, this research focuses on both elastic and viscous properties of oil paints prepared with different pigments, and on the impact of repeated moisture cycles. Thus, we study the consequences of moisture weathering, a process by which the materials exposed to moisture and relative humidity variations may undergo changes and evolution in their properties, leading to potential degradation. For this study, the relative humidity varies from 30 % to 85 %.
To study the influence of repeated moisture adsorption-desorption cycles on the viscoelastic properties of different paints, we prepared fresh laboratory reconstructions. With these samples, we controlled all parameters, from the pigment to binding medium compositions, the layering and preparation of the build-up, as well as the testing conditions. As the age and mechanical properties of these samples differ from historical artworks, further research and modeling will be necessary to transpose our results to historical paintings; however, our measurements provide the first steps towards an evaluation of the impact of repeated moisture variations on paint build-ups. We considered three pigments mixed in linseed oil: lead white, a basic lead carbonate, \(2 PbCO_3\).\(Pb(OH)_2\), ultramarine, a blue silicate mineral of the sodalite group \((Na,Ca)_8(AlSiO_4)_6(S,Cl,SO_4,OH)_2\), and an organic red lake pigment made of madder precipitated on a hydrated alumina substrate. To access the paint layer’s sensitivity to moisture uptake and quantify the difference in terms of mechanical behaviour, induced by the layer position within the stratigraphy, we aim to study the properties of those layers that are located between other layers in the stratigraphy. Such an analysis requires local measurements of the material moduli.
To evaluate the mechanical properties of the multi-layered paint samples, we adjusted a nanoindentation device and protocol. The complete methodology of the ferrule-top indentation proposed was described in Tiennot et al. [15]. The optomechanical device adapted to carry out the required measurements allows a depth-controlled indentation profile, and a precise analysis of both storage and loss moduli of the materials. As verified in previous research [15], the accuracy of the indentation localisation and of the Dynamic Mechanical Analysis (DMA) measurements allowed by this protocol, supports the investigation of the complete build-up structure of paintings, giving access to all layers without embedding the samples as cross-sections in polyester resin. Therefore, by using this indentation protocol, we investigate both elastic and viscous components of paint film’s behaviour, their modification over eight cycles of relative humidity, and their evolution in terms of viscosity. We can study experimentally that repeated moisture variations induce different mechanical responses of paint layers, varying with the nature of the pigment as well as with the position within the stratigraphy. The discrepancies induced by these modifications of the paints’ properties may be involved in the stress field created within the paint stratigraphy. This phenomenon and its influence on crack initiation and propagation in paintings, provide the rationale for our research.
Thus, this paper aims at illustrating the evolution of the behaviour of paint layers under repeated RH variations in terms of elastic and viscous moduli. The investigation of moisture-induced chemical processes involved in these variations, are part of a broader, future research plan. Our nanoindentation-based research is one step towards the quantification of the viscoelastic moduli’s evolution of pigment-oil systems, with respect to their nature and their position in the paint stratigraphy, and help to further improve our understanding of the micromechanics of paint layers.