Near-infrared spectroscopic imaging in art conservation: investigation of drawing constituents

AbstractThe remote-sensing technique of spectroscopic imaging has been adapted to the non-destructive examination of works of art.The principleof near-infrared reflectance spectroscopic imaging is explained, and our instrumentation for art examination described. The technique allowsthe art materials to be distinguished by their composition, and under-drawings revealed. The initial results indicate that even over limitedwavelength ranges (650–1040 nm) and with relatively coarse spectral resolution (10 nm) a number of pigments can be distinguished on thebasis of variations in spectral properties such as spectral slope and the presence or absence of absorption bands. Software adapted from theremote-sensing image-processing field has been used to successfully map areas of different brown and black pigments across a drawing.Non-destructive identification of pigments can be used to address issues of attribution, age dating, and conservation.An additional advantageof this technique is that it can be performed off-site using portable instrumentation, and under relatively benign lighting conditions. Thetechnique has been applied to the examination of a 15th-century drawing,Untitled (The Holy Trinity), in the collection of the Winnipeg ArtGallery. Multivariate image analysis produced a set of principal component (PC) images highlighting different materials’ aspects of thedrawing. A color composite image produced from the PC images provided a direct visualization of the compositional characteristics of thework. Features of the under-drawing have been exposed, and its material tentatively identified as charcoal, by comparison with reference data.© 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.