# Nanoindentation of Historic and Artists' Paints
## 2 Nanoindentation Studies of Artists' Paints over the Last Two Decades  
In the last 20 years numerous research groups have used nanoindentation for the characterization of artists' paint media. The very first article discussing the results of nanoindentation for artists' paints was published by [PERSON] and [PERSON] in 2007 [5]. They used nanoindentation to measure mechanical changes of modern acrylic paints artificially aged by exposure to UV light. Although the authors did not present quantitative analysis of the obtained data, they successfully demonstrated the sensitivity of nanoindentation to surface alternations resulting from accelerating aging. Subsequent nanoindentation studies focused on quantitative evaluation of material properties through the use of model paint systems to explore and optimize the potential of the measuring technique. The storage moduli (\(E_{\text{r}}^{\prime}\)) and hardness (\(H\)) of embedded, cross-sectional samples of various artists' and commercially-available nonartists' paints (including oil-based enamel, acrylic, and alkyld paints) were measured by [PERSON] et al. [6]. Their study demonstrated the feasibility of the traditional Oliver and Pharr method [7] to analyze the mechanical properties of embedded paint samples. The authors also discussed the possible influence of pigments on the aging properties of paints, subsequently publishing a study analyzing the effects of zinc oxide, known as zinc white, on the curing and mechanical response of alkyld coatings [8]. Effects of zinc oxide pigment on the paint's properties were studied systematically at different concentrations over a wide range of time scales using three experimental techniques: dynamic mechanical analysis, quartz crystal rheometer and nanoindentation. The potential of using nanoindentation alongside other testing techniques for studying the mechanical properties of artists' paints was further explored in a recently published review article by [PERSON] et al. [9]. Nanoindentation was proven effective for evaluating mechanical changes of paint films following conservation treatments. [PERSON] et al. [10] analyzed the effect of consolidation of model wall paints (executed in three techniques: lime, lime casein and animal glue) with selected commercial synthetic binders. The experimental methodology applied in this work measured subtle differences resulting from various consolidation treatments but generally showed that they have a rather limited impact on the mechanical performance of the tested wall paints. The purpose-cast samples were also used for quantifying the effects of solvent exposure on a water-sensitive yellow ochre paint [11,12]. These authors reported an increase of paint stiffness after immersion in deionized water but an absence of changes following immersion in other tested solvents or following sponge cleaning with all tested solvents including water. These studies demonstrated how nanoindentation can help inform the selection of conservation materials and guide conservation treatments. The most important feature of nanoindentation is its ability to measure small amounts of materials, which is critical to the analysis of minute samples taken from real art objects. Micro and nanoscale indentations have been applied to the characterization of samples recovered from seventeenth and nineteenth century oil paintings [13,14], as well as medieval distemper wall paints [15]. Such tests provide unique information unattainable by other techniques on how natural aging influences the mechanical properties of the measured materials. However, all three studies emphasize the difficulty in obtaining precise and representative mechanical properties due to high material heterogeneity and small sample size. They show that selection and preparation of samples are among the crucial issues when analyzing historic paints by nanoindentation.