Nanoindentation of softening modern oil paints

Abstract

A methodology was developed to address challenges associated with the use of nanoindentation to characterize small volumes of soft-matter thin films, encountered in many research fields including heritage conservation. The methodology is based on the use of a flat-punch indenter, whose radius is so dimensioned to be able to predict the effect of substrate rigidity using an existing solution independently of the film’s Poisson’s ratio and frictional characteristic. The film’s mechanical response is analyzed in a minimally strained condition at the onset of full contact to fulfill the solution’s assumption of linear viscoelasticity. The indentation response prior to full contact is corrected for the substrate effect using a model of structural compliance coupled with the solution. Homogeneity of the film material, further assumed by the solution, can thus be established based on its constant loss tangent response while deforming linear viscoelastically prior to full contact. This methodology was applied to minute volumes of bone black oil paints sampled from modern paintings by Karel Appel and Asger Jorn, across a range of relative humidities. The methodology not only differentiated between viscoelastic behaviors of these paints from the same period, but also facilitated the discovery that they possess softening points below the target humidity range of museums. The latter finding convincingly explains why these paints exhibit conservation problems even in a standard climate-controlled museum environment. The methodology can be applied to polymer coatings and thin biological tissues, whose Poisson’s ratios and frictional characteristics are not exactly known.