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A. van Loon, K. Keune, J. Dik, D. Kohl, P. Chang, G. Schitter:
"Analytical Spectroscopy in Art and Archaeology";
Talk: Technart 2013, Amsterdam, Netherlands; 09-23-2013 - 09-26-2013; in: "Book of Abstracts - Technart 2013", (2013).

In this paper, the potential of Atomic Force Microscopy (AFM) to measure the mechanical
properties of pigmented oil paint samples at nanoscale-resolution is explored. With techniques
such as tensile testing, thermomechanical experiments or (nano)indentation, the mechanical
properties of paint films are studied in bulk, as the sum of pigment and binder. The great
advantage of AFM is that it can image the relative differences in modulus and adhesion of
individual pigment particles and the surrounding binding medium. Our goal is to relate the
mechanical properties of oil paints to their chemical properties, and to compare the effect of
different pigmentations and ageing histories on the stability of the paints.
While AFM is well known to image the surface topography of materials at the nanoscale, it is also
widely used to measure the mechanical properties through the AFM tip/sample interaction (Armini
et al., 2006). This is achieved by taking force curve measurements throughout the imaged sample
area to create a force volume mapping. By analyzing the captured force curves at various
positions using the Dejaguin-Muller-Toporov (DMT) model (Derjaguin et al, 1973), one can
determine the elastic modulus of the scanned sample with nanometer resolution. A Bruker
FastScan Dimension AFM system was used to conduct the measurements on the paint samples.
Two types of pigmented linseed oil paints, one with lead white and the other with vine black, were
selected on the basis of their differences in chemical composition. Lead white is known to form
durable oil films, due to its strong interaction with the oil, low oil absorption, and low amount of
free fatty acids (Keune et al., 2008). In contrast, carbon-based black pigments such as vine black
usually result in softer paint films, since they have little possibility of interacting with carboxylic
acid groups in the oil, they are highly oil-absorbing, and contain large amounts of free fatty acids
and oxidation products.
Samples from traditional oil paintings and oil paint reconstructions were prepared as paint crosssections
(embedded in an acrylate resin), and as paint outs glued on aluminum discs. All samples
were dry polished with Micro-Mesh polishing cloths. Sample areas of 10 to 50 square micrometers
were measured.
The resulting AFM images were successful in visualizing the heterogeneous character of the oil
paint films. They clearly outlined the pigment particles in the paints and picked up differences in
modulus and adhesion, between the various pigment particles, between the pigment and the
surrounding oil matrix, and between the various oil matrices. These differences could all be
related back to their chemical compositions. This offers great potential for future studies in the
field of cultural heritage.