# Nanoindentation of Historic and Artists' Paints
## 5 Summary  
The complexity of analyzing the mechanical properties of historic and artists' paints stems from two separate issues. First, the intrinsic properties of the pigment-binder composite material may vary due to its heterogeneity, time-dependent behavior, and sensitivity to temperature and relative humidity. Testing such materials requires a statistical approach to address a heterogeneous composition and a consistent method to establish the contact depth and, consequently, the probe-material contact area. Second, the analysis of mechanical properties may be impacted by the specific geometry of paint samples and, in the case of historic paints, their small volumes. High surface roughness combined with the substrate effect requires a careful selection of indentation depths. Examination of cross-sectional samples must also consider edge effects. Several research teams addressed these issues by proposing specific measuring protocols and use of indentation probes with various geometries. In addition, it has been shown that sample preparation is an inherently important component of the performed studies. Common among all the experimental paint studies discussed in this article are the relatively large indentation depths, up to several microns, necessary for testing heterogeneous samples with rough surfaces. Despite these large indentation depths, analysis with high spatial resolution is still needed to account for subtle effects from paint-resin interface and gradients of mechanical properties. Thus, it is particularly important to perform a CSM using a displacement amplitude that is a small fraction of the indentation depth. All nanoindentation studies of artists' paints described in this review paper employed microindentation depths with nanoscale resolution. Mechanical characterization of artists' paints requires the application of customized measurement protocols and often unconventional methods of data analysis. These are inspired by classic studies of metals, as well as less conventional examinations of polymers and biological tissues. It is hoped that drawing upon research into the latter categories will help bridge the gap between nanoindentation and AFM (commonly used on softer materials), and ultimately allow the quantification of the influence of adhesion on the measured mechanical properties of extremely soft and viscoelastic paints. Research conducted over the past two decades shows that, while nanoindentation is a well-established method in engineering and materials science, its potential for the micromechanical characterization of historic and artists' paints has yet to be fully realized in the field of cultural heritage. In principle, nanoindentation is capable of measuringproperties such as yield stress, strength, fracture toughness and residual stress, all of which are needed to model paint behavior in unstable climatic conditions or under external loads. Nanoindentation can be especially effective in tracing subtle changes of material properties. The results obtained are less affected by uncertainty in determining contact dimension and substrate/edge effect when interpreted in a comparative sense, and therefore can be an excellent source of information on the effects of treatments and preventive conservation measures. Consideration of a wide variety of existing indentation parameters, including available probe geometries, context-specific testing protocols, and the use of advanced analytical and numerical models of contact mechanics, provides a rich environment for further exploration by heritage conservation scientists. Conceptualization, M.L. ; writing--original draft preparation, M.L., N.F. ; writing--review and editing, M.L., A.B., N.F. ; visualization, N.F., A.B. ; supervision, M.L., N.F. ; All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors would like to acknowledge the support of The Getty Conservation Institute's Managing Collections Environments Initiative.