Suzanne Quillen Lomax

Near Infrared Reflectance Imaging Spectroscopy to Map Paint Binders In Situ on Illuminated Manuscripts

In recent years, visible and near infrared (NIR) reflectance imaging spectroscopy, i.e., the collection of contiguous calibrated spectral images to provide the reflectance spectra for each pixel of the scene, has been applied to the study of old master paintings. Its application to the study of lightsensitive works of art, such as illuminated manuscripts, has been limited to partial pigment identification using visible electronic transitions. Here we show the potential of NIR imaging spectroscopy in the 1000 to 2500 nm (10 000– 4000 cm ) spectral region to map and identify paint binders by utilizing vibrational features associated with methylenic, CH and amide functional groups. This is demonstrated by using a novel hyperspectral NIR imaging spectrometer (1000– 2500 nm, 4.4 nm resolution) to map the use of a fat-containing paint binder (likely egg yolk) for certain compositional elements of a 15th century manuscript leaf. The use of a fatcontaining binder for manuscript illumination is surprising in itself since egg white and gum Arabic (protein and polysaccharides) are historically considered to be the binders preferred by illuminators. This study offers the opportunity to map paint binders in situ on works of art, at a macroscopic scale, for the first time. While analytical techniques using micro-samples (mg) taken from art objects can provide the most accurate identification of artists materials, there is a preference for in situ methods. Among them, site-specific tools such as X-ray fluorescence (XRF), Raman spectroscopy and fiber optics reflectance spectroscopy (FORS) can identify pigments. Production of material maps on the macroscopic scale (entire artwork) has been limited to date, while mapping on the microscopic scale has progressed rapidly and has proven to be useful. Techniques that have been used include scanning electron microscope–energy dispersive spectroscopy (SEMEDS), mid-IR (FT-IR), Raman, XRF, and luminescence spectroscopy using traditional as well as synchrotron sources. Given the 2-D nature of many works of art, the spatial information derived from macroscopic maps can provide important clues about an artist s working methods and help guide conservation choices. Hence, there is interest in the development of macroscopic mapping methods, which utilize existing analytical in situ methods such as XRF and X-ray diffraction, reflectance and luminescence spectroscopy. These methods not only provide the identification and mapping of artist s materials, but also other information such as compositional changes and layering of paint. Unlike XRF mapping, which provides elemental information and is thus limited to being used to infer inorganic pigments, X-ray diffraction and reflectance imaging spectroscopy provide information on the molecular structure of the pigment. Reflectance spectroscopy offers the capability to map also organic materials, such as dyes, and recently a plasticizer in a PVC object. To date most studies using reflectance imaging spectroscopy have relied on electronic transitions in the visible range alone for pigment identification and have been only partially successful. Improved results have been obtained by extending into the NIR range (750–1700 nm) in order to collect vibrational band overtones and combinations associated with hydroxy inorganic pigments. Extending the spectral range to 2500 nm to collect vibrational features associated with carbonate functional groups would be a further improvement. While progress has been made on mapping artists inorganic materials, the mapping of organic materials— paint binders in particular—has succeeded only on the microscopic scale utilizing mid-IR microscopes (650 to ca. 4000 cm ). While remote-sensing hyperspectral imaging cameras operating in the mid-IR exist, such instruments require exotic infrared focal planes and cooling to temper[*] Dr. P. Ricciardi, Dr. J. K. Delaney, M. Facini, Dr. S. Lomax National Gallery of Art, 6th St. and Constitution Ave. NW Washington, D.C. 20001 (USA) E-mail: j-delaney@nga.gov