  A significant number of oil paintings produced by Georgia O'Keeffe
(1887-1986) show surface protrusions of varying width, up to several hundreds
of microns. These protrusions are similar to those described in the art
conservation literature as metallic soaps. Since the presence of these
protrusions raises questions about the state of conservation and long-term
prospects for deterioration of these artworks, a 3D-imaging technique,
photometric stereo using ultraviolet illumination, was developed for the
long-term monitoring of the surface-shape of the protrusions and the
surrounding paint. Because the UV fluorescence response of painting materials
is isotropic, errors typically caused by non-Lambertian (anisotropic)
specularities when using visible reflected light can be avoided providing a
more accurate estimation of shape. As an added benefit, fluorescence provides
additional contrast information contributing to materials characterization. The
developed methodology aims to detect, characterize, and quantify the
distribution of micro-protrusions and their development over the surface of
entire artworks. Combined with a set of analytical in-situ techniques, and
computational tools, this approach constitutes a novel methodology to
investigate the selective distribution of protrusions in correlation with the
composition of painting materials at the macro-scale. While focused on
O'Keeffe's paintings as a case study, we expect the proposed approach to have
broader significance by providing a non-invasive protocol to the conservation
community to probe topological changes for any relatively flat painted surface
of an artwork, and more specifically to monitor the dynamic formation of
protrusions, in relation to paint composition and modifications of
environmental conditions, loans, exhibitions and storage over the long-term.
