AIC's 47th Annual Meeting

In pursuit of accessible tools for non-invasive examination, Brooklyn Museum (BKM) conservators experimented with using multiband reflectance image subtraction (MBRIS) to characterize materials in the collection. This imaging technique can visualize and localize materials that have a pronounced difference in reflectance across defined spectral bands, producing a map of an object’s surface. As pioneered by Webb et al., MBRIS utilizes a camera that captures monochromatic images and combines one near infrared image and one visible light image in digital post-processing. The technique was adapted for multiband imaging equipment already in use at BKM. As few references to MBRIS exist in the literature, BKM conservators not only evaluated the information gained from its application, but also investigated the technique itself, refining variables in image capture and processing to optimize results and create representative images. An overview of the broad applications of the MBRIS protocols developed at BKM will be presented in addition to a discussion of the challenges associated with equipment set up and image processing. Prompted by the exhibition “Infinite Blue,” MBRIS has been used at BKM to localize the presence of indigo dye on both Egyptian and Andean textiles, indigo-containing paints on Egyptian cartonnage, and indigo-containing Maya blue pigment on ancient figurines, as well as to characterize lapis lazuli components of Afghani, Chinese, Russian, and Egyptian objects. MBRIS has also helped distinguish indigo from Prussian blue on Japanese woodblock prints and visualize areas of retouching carried out on a Colonial American portrait. As part of the Getty initiative Ancient Panel Paintings: Examination, Analysis and Research (APPEAR), BKM conservators conducted a focused study on the use of indigo-containing paints in six Romano-Egyptian funerary portraits, which provided the opportunity to refine the BKM imaging protocols. The BKM APPEAR study assessed MBRIS in the context of analytical results obtained through fiber optics reflectance spectroscopy, Raman spectroscopy, and X-ray fluorescence spectroscopy, as well as in concert with X-radiographs and a broader suite of multiband images. The imaging guidelines established during the APPEAR project explicated and reduced inconsistencies observed in initial MBRIS images arising from variations in the relative exposures of the near infrared and visible light captures. Further investigation into MBRIS processing carried out using softwares such as Adobe Photoshop, ImageJ, and GIMP, demonstrated that exposure adjustment post-capture can yield unrepresentative results and that multiple functions employed during post-processing affect the final MBRIS output. BKM conservators are also experimenting with a new approach to generating MBRIS images that would preserve information about both the magnitude and the sign of the values calculated in post-processing. Preliminary imaging of known pigment samples in the BKM reference collection indicates that MBRIS could be used to characterize additional colorants or media on cultural heritage objects. The overall success of this MBRIS imaging project suggests the potential to develop protocols involving different combinations of filters and light sources for the characterization of other materials that have pronounced changes in reflectance across the ultraviolet, visible, and infrared spectral regions.