This is a guest post by Cindy Connelly Ryan, Preservation Science Specialist in the Preservation Research and Testing Division. Her research areas include the light sensitivity of inks, dyes and organic colorants, deterioration and stabilization of verdigris and iron gall ink, technical study of collection items, and re-creating obsolete historic artists’ materials from old recipes.
Selecting items for an exhibition involves many curatorial decisions – how it relates to the exhibit theme, how it might fit into the planned space based on size and shape, similarities and differences from other pieces in the show, and how engaging viewers will find it. Preservation factors are considered as well – does the item require mending, re-matting, or other stabilization to be safely handled and exhibited, and will it be at risk of damage from prolonged exposure to light? To answer that last question, the Preservation Research and Testing Division (PRTD) is here to help!
One of the more specialized analytical methods used by PRTD, micro-fade testing (MFT) measures the light sensitivity of materials. Before the introduction of this technique in the late 1990s, to estimate a collection item’s risk for light damage, museum scientists needed to exhaustively identify all the inks, paints, dyes, adhesives, and substrates present, then predict each materials’ sensitivity to light through lengthy exposure studies on mockups. With MFT, micro-scale light exposure tests are done directly on the item, in a minimally invasive manner. Small changes in surface reflectance during testing, invisible to the eye even through a microscope, are readily measured by a spectrophotometer. This 10-minute test gives us a clear picture of how the material will react to light exposure, and the information obtained ensures that collection items can be confidently and safely exhibited or loaned.
PRTD has been using MFT since 2007, testing maps, manuscripts, books, banners, photographs, prints, autographs, comic books, feathers, fabrics, and more, coming to us from multiple divisions of the Library. Some of these pieces have required a little creativity in adapting the instrument’s setup to safely bring the regions of interest under the light beam, due to their size or three-dimensional format. One recently examined piece took a whole village’s worth of creative thinking, pre-planning, and inter-division cooperation. This blog post shares that item’s story.
This mid-18th C. Dpag bsam ʾkhri shing (Buddha Shakyamuni) thangka was gifted to the Asian Division of the Library of Congress in 2010 by His Holiness the 14th Dalai Lama Bstan-ʼdzin-rgya-mtsho. The thangka is in excellent condition, particularly given its age. Head of Paper Conservation Yasmeen Khan suspected the textile elements incorporated fugitive dyes, and possibly some of the paints as well. The analysis of such a large item, and even the process of bringing Buddha to the spectroscopy laboratory, held challenges both foreseen and unexpected.
When secured to an A-frame cart, the thangka in its storage box is taller than a standard door frame and requires a team of 3 experienced handlers to maneuver it from its ‘home’ in the Asian Division to the preservation labs. The piece can’t be tilted on its side during transport or analysis, as the thangka, its cover silk, and brocade pendants are all suspended from the top of a supporting board – it must stay upright, or flat. Every doorway and hallway threshold requires unstrapping the box from the A-frame, hand-carrying it through the threshold, then re-securing the thangka before proceeding.
Would it fit into the elevator to travel down to sub-basement of the Madison building? Fit through the door into the lab? Clear the drop ceiling over the MFT’s bench? And then what? We couldn’t lay the piece flat on the lab bench, which is far too small. Adding a second table, with modifications, would support the travel box, and we could reach one edge this way, but limits to the optical fibers’ length would prevent reaching many regions of interest.
With half a dozen sketches of possible configurations in hand, and the route with the fewest doorways mapped out, conservator Sonja Reid and I did a test-drive of our plans with a cardboard mockup. Happily we found the thangka in its box would juuuust clear the ceiling (though not the sprinklers); once unboxed, there would be enough vertical clearance for the silk cover to be tucked up; and the traveling frame could cozy up to the lab bench without (completely) blocking access to the instrument components the analyst needs to reach during testing. Re-assembling the instrument to work vertically and at an angle matched to the piece on its travel frame would give us access across the full width of the thangka and about 50 cm vertical range across the center of the painting.
Our worries somewhat calmed, testing day was scheduled, and the move arranged with the curator. We took advantage of newly installed dimmable LED fixtures to lower the light levels in the testing lab for the day, to reduce overall light exposure. Once safely settled in the lab, the cover silk and hanging pendants were tested with the piece still flat in its box, using the extra table for support. Then the cover silk was arranged, and the thangka carefully lifted back onto the travel frame. Buddha’s serenity seeped into us all as the day unfolded smoothly.
Normally a boom arm is used to move the MFT head to each analysis site, to minimize handling of collection items. But in this case, positioning required moving both the instrument and the thangka into approximate alignment, then a three-person collaboration to adjust angles and positions to focus the 0.2 mm diameter light beam onto the surface within a fraction of a mm vertical tolerance. A co-worker who came by to snap a few photos for us found himself drafted for the rest of the day to assist with minute 1- and 2- mm position adjustments to the travel frame’s wheel chocks, while Sonja focused on the thangka and adjustments to the overall alignment, and I adjusted the MFT head position and focus while looking through the camera.
We quickly discovered that, while safely supporting the piece, the mounting board and travel frame combination was vibration-sensitive. Anyone walking near the piece during the 10-minute data collection period caused a few microns of movement of the thangka, enough to affect the focus and thus cause visible jumps in the spectral data. Adding to this, most of the thangka floats slightly above the surface of the backing – the lower roller holds it away from the mounting board, and it tended to ‘breathe’ slightly with shifts in air currents in the room.
Despite the challenges of the alignment process, over a very full day we were able to successfully test the light sensitivity of all the elements of interest on the thangka, and as a bonus, documented the use of an uncommon purple plant-based colorant in the painting. Many of the textile elements did indeed prove to be highly sensitive to light, making us particularly glad we could provide concrete findings that will enable this very special piece to be exhibited safely.
You can see this item in person when the upcoming exhibition Collecting Memories: Treasures from the Library of Congress opens in late 2023 and read more about the MFT that PRTD did in preparation for the exhibit here and here.
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