39th Annual Meeting – Book and Paper Session, June 3rd “A Comprehensive In-Situ Approach for the Analysis of Illuminated Manuscripts and Drawings: Exploring the Synergy Between Imaging Spectroscopy, FORS, XRF, and High-Resolution Multispectral Infrared Reflectography”

Paula Ricciardi, John K. Delaney, Lisha D. Glinsman, Mathieu Thoury, and Michelle Facini of the National Gallery of Art used several analytical methods to study the media used in the Praying Prophet, a manuscript cutting illuminated by Lorenzo Monaco c. 1410/1413.

Up front, let me say that analytical techniques such as image spectroscopy, XRF and FORS analysis are way outside my balliwack, so please feel free to comment on my post to correct any missing or mis-stated information. My feelings will certainly not be hurt!

So, the crew at NGA used a Si-CCD camera with color-corrected lenses and their specialized set up allowed them to reduce the light levels needed to capture their imagery, thus making the analysis safer and easing their conservator’s mind. They captured several images in the 400-950nm (visible through near-infrared range). They compiled the images into an image cube for analysis, calibrating it to black and white standards that allowed them to gather both reflectance and luminescence information. This allowed them to differentiate between a wider range of visually similar pigments.

Image spectroscopy alone cannot identify a pigment, however, so they also did XRF (which identifies elements such as copper, arsenic and lead in paints) and FORS analysis (which identifies different light absorption bands). One particular example she gave of the the need for both XRF and FORS analysis was the presence of copper in a green area of the miniature. Since copper was a common material used to make green pigment, the XRF identification of copper in that area was not surprising – but FORS analysis showed absorption bands that indicated the presence of azurite. So contrary to expectation, the green was not copper based, but azurite + a yellow pigment!

Even more intriguingly, when analyzing the vermilion robes of the prophet, they found absorption bands that did not match any of the pigments in their database. Curious, they collaborated with a conservator to mock up samples of vermilion pigment in a variety of different binders (egg glair, gum arabic, egg yolk). They found that the mystery absorption bands matched egg yolk. (This is exciting, as they are the first to map any sort of binder using FORS analysis).

Egg yolk was a slightly surprising find, as egg yolk is not well known to be a binder used in illuminated manuscripts. The team was able to find a passage from an early 15th century instruction manual on illuminating manuscripts that indicated egg yolk should only be used as a binder only when painting bodies (and specifically not in writing or when painting flowers). But the team knew that Lorenzo Monaco was also a panel painter (a media better known for egg tempura), so they wanted to dig deeper. Was he using egg yolk because of his panel painting experience? Was he the only one to use egg yolk in illuminated manuscripts? Their analysis found egg yolk was consistently used in the robes of figures throughout the miniature and in other illuminations by Monaco. Most of these robes were vermilion, but even the green robes showed the egg yolk absorption bands. Areas outside figures (such as decorated initials, foliage, etc) did not show evidence of egg yolk, although the robes of figures hidden away in decorative foliage did – suggesting that the difference in use was not just a matter of master painter vs. workshop painter.

When the team turned their attentions to other illuminated manuscripts of the era, they were unable to find egg yolk present, even when the artists’ responsible were also known panel painters. Obviously, more research needs to be done to further explore this area, but one of the most telling points of the talk (for me, anyway) came during the question/answer section. One conservator noted that while the talk did not make her rethink her treatment decisions based on concerns about the items themselves, it did make her think about how her consolidant decisions might muddy the waters for future investigators. This is a very good point, and one I will certainly be mulling over as I consider my own treatment decisions. Although, to be honest, the thought of inspiring some as-of-yet unborn scientist to curse my name is just a bit tempting.