Forensic examination of a fragmentary funerary portrait in the collection of the Harvard art museums.

The Harvard Art Museums' collection includes six Egyptian funerary portraits of the Roman period. These portraits are all that remains of the funerary equipment of individuals whose bodies were carefully prepared for burial and the afterlife. One example, depicting a man, is particularly complicated, broken into multiple fragments which have been glued down onto a board. The in-depth study of the portrait used a combination of non-invasive techniques, including X-radiography, infrared-, ultraviolet- and visible-induced luminescence imaging, and X-ray fluorescence spectroscopy to identify and locate particular pigments, binders and other artist materials, without needing to take a sample. Targeted sampling, informed by the imaging process, was then undertaken for additional analysis through the use of cross-sections, scanning electron microscopy with energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy, Raman spectroscopy, radiocarbon dating, and lead isotope ratio analysis. This study identified a core group of three fragments in the center of the portrait that comprise much of the face and neck, tunic, and part of the hair. The remaining 15 fragments contain most of the background, parts of the hair, and the proper left eye and tunic, and are distinct from the central group of fragments. Analysis suggests these fragments were reused from other ancient funerary portraits, and whilst it was not possible to connect any of these added fragments to one another, a potential workshop connection between the central fragments and three added fragments can be suggested based on a study of the composition of the lead white pigment, and similarities in painting technique.

Application of Uniform Manifold Approximation and Projection (UMAP) in spectral imaging of artworks.

This study assesses the potential of Uniform Manifold Approximation and Projection (UMAP) as an alternative tool to t-distributed Stochastic Neighbor Embedding (t-SNE) for the reduction and visualization of visible spectral images of works of art. We investigate the influence of UMAP parameters-such as, correlation distance, minimum embedding distance, as well as number of embedding neighbors- on the reduction and visualization of spectral images collected from Poèmes Barbares (1896), a major work by the French artist Paul Gauguin in the collection of the Harvard Art Museums. The use of a cosine distance metric and number of neighbors equal to 10 preserves both the local and global structure of the Gauguin dataset in a reduced two-dimensional embedding space thus yielding simple and clear groupings of the pigments used by the artist. The centroids of these groups were identified by locating the densest regions within the UMAP embedding through a 2D histogram peak finding algorithm. These centroids were subsequently fit to the dataset by non-negative least square thus forming maps of pigments distributed across the work of art studied. All findings were correlated to macro XRF imaging analyses carried out on the same painting. The described procedure for reduction and visualization of spectral images of a work of art is quick, easy to implement, and the software is opensource thus promising an improved strategy for interrogating reflectance images from complex works of art.