Quantifying Change in Ethnographic Materials with Biocolorants and Structural Colors

The amount of research that has been carried out investigating the risks of light damage to fine art materials is very small, but the amount directed at ethnographic and natural history materials is even smaller. More might have been done had not the fundamental properties of many of these products been easier to characterize.

Unlike paints and dyes, biological colors are divided into those created by biopigments and those produced by a handful of physical phenomena that constructively and/or destructively interfere with reflected light. Structural colors are notoriously difficult to measure in a repeatable manner. They cause an angular dependence to color that gives butterfly wings and peacock feathers their enormous range of appearance effects.

One technique that has been exploited by biologists to measure the color of birds employs the bidirectional reflectance distribution function (BRDF). In this technique, overall color appearance can be described by a series of incident and reflected light beams with small solid angles. In a feather this can be achieved using a multi-angle series of incident and reflectance measurements, but since the organization of feather structure also includes large macro-scale features (e.g., feather barbs and barbules), rotational freedom about those features also need to be incorporated into any function. Measurement of BRDF at different incident wavelengths is thus exceedingly burdensome and to gain realistic experimental insights into light damage the approach needs to be simplified. It then becomes an important decision which subset of solid angles and barbule orientations imbed the best discrimination between structural and biopigment changes when estimating overall light sensitivity. This question has never been resolved in conservation research and is part of our project goals. The current recommendations on feather ISO blue wool sensitivities are, at best, conjectural. Some of the literature even permits a skewed interpretation based upon a few uniquely sensitive members of the superset of feather colorant systems that are possibly atypical and unique.

Our overall goal is to better identify the light-sensitivity of anthropological, fine art, and natural history artifacts composed in part, or totally, of feathers. Co-directing this project is Ellen Pearlstein, associate professor of Information Studies, UCLA/Getty Master's Program in Conservation of Ethnographic and Archaeological Materials, Cotsen Institute of Archaeology.

Last updated: July 2010