Multispectral imaging provides periodic high-resolution, full-area, non-destructive and zero-contactmonitoring over both short and very long periods of time of cultural heritage materials including photographs, paintings, fabrics, documents, books, tapestries, and other works of artistic and historic importance with verylarge data sets consisting of up to ten thousand or more discrete colorimetric data points for the short-term and long-term monitoring of full-tonal-scale – generally nonlinear – colorimetric changes (including in the UV and IR regions), in a fully-time-integrated manner, that may take place over time in the full image area and in the support material (front and back). Multispectral imaging can accurately monitor rates of degradation of optical brightening agents (OBAs), and to quantify gradual yellowish or other stain formation in photographs, including albumen prints, polyethylene coated (RC) papers, and other materials. Multispectral imaging provides the ability to monitor glazed works periodically during exhibition without the necessity of removingglass or plastic sheets from their frames, while the works remain on the wall. Likewise, works housed in anoxic frames may be monitored over the long term without opening the frames. Irregularities in image deterioration and/or staining brought about by localized variations with photographic materials and their chemical processing, washing, contamination during drying selenium, gold, or other chemical toning treatments, coating and varnish layers, laminates, and other steps employed in the creation and finishing of the work, integrated with the inevitably non-uniform contact with mounting, framing, and storage materialsover time, and the effects of exposure to non-uniform lighting, environmental and “micro-climate” temperature and relative humidity conditions, can be assessed and compared over long periods of time in all areas of an image – including within very small image details. Representative times required to image an object will be given. This presentation will consider the formidable technical challenges of very-long-term monitoring in the context of the now more than 2,000-year-old Dead Sea Scrolls in Israel, and the ongoing programs to systematically multispectrally-image, monitor, and preserve the delicate parchment scrolls and scrollfragments. During the coming hundreds or many thousands of years into the future, every single part of a multispectral imaging system and the associated computers, software and data storage systems, calibration targets – and our understanding color science itself – will repeatedly become obsolete and must be replaced with new systems. Strategies that will assure a continued high degree of accuracy relative to the original measurements are proposed. Without a comprehensive multispectral monitoring program, conservators and other institutional caretakers will have little or no quantitative data concerning what has actually been happening to their collections as they age over time, and with the understanding that comes with quantitative information of how degradation may be slowed or halted by changes in display and loan policies, the use of humidity-controlled sub-zero freezer preservation, and by various other means.
References
1. Henry Wilhelm, “Monitoring the Fading and Staining of Color Photographic Prints,” Journal of the American Institute for Conservation, pp. 49-64, Vol. 21, No. 1, Fall 1981.
2. Henry Wilhelm, “Monitoring the Fading and Staining of Color Photographs in Museum and Archive Collections,” pp. 239-266 in Chapter 7 in “The Permanence and Care of Color Photographs:Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures,” by Henry Wilhelmwith contributing author Carol Brower, Preservation Publishing Company, Grinnell, Iowa, 1993.
3. Henry Wilhelm, Ken Boydston, Kabenla Armah, and Barbara C. Stahl, “Use of a Multispectral Camera System and Very Small, Comprehensive ‘Micropatch’ Test Targets for Full Tonal Scale Colorimetric Evaluation of the Permanence of Digitally-Printed Color and B&W Photographs,” Proceedings of “Imaging Conference Japan 2011,” p. 131–134, The Imaging Society of Japan, Tokyo, Japan, June 7, 2011.
4. Henry Wilhelm (Wilhelm Imaging Research, Inc.); Ken Boydston (MegaVision, Inc.); John McElhone, National Gallery of Canada); Gregory Hill (Canadian Conservation Institute), “Use of High-ResolutionMultispectral Imaging and Analysis Systems for the Long-Term Monitoring of Salted Paper Prints andEvaluation of the Intrinsic Permanence Characteristics of Contemporary Salted Paper Prints Made with a Variety of Process Variations,” presentation at the Harvard University Symposium “Salted PaperPrints: Process and Purpose,” September 14–15, 2017 at Harvard University, Cambridge, Massachusetts U.S.A.
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