The following is a guest post by Anna Darden, intern in the Preservation Research & Testing Division (PRTD).

Not so long ago, LeVar Burton graced TV screens on *Reading Rainbow *with his emphasis on building reading comprehension and instilling a life-long love of reading for me and others in my generation. Reading is usually reserved to books, Instagram posts, the occasional magazine, and hopefully, a “Stop” sign. However, in the cultural heritage field, the process of reading can take on a different meaning and can require a bit of chemistry.

A small subset of the pigment swatch cards which were scanned throughout this internship. Photo credit: Anna Darden, 2025.

In the Preservation Research and Testing Division at the Library of Congress, there have been several interns tasked with helping build a spectral reference database for pigments. The division has over 500 historical and modern reference pigments, with over 100 having swatch cards painted out in common mediums. These swatch cards are used when duty calls and a request is made to identify a pigment in an artwork, object, or whatever interesting item stands out to a curator from the collection.

Pigment identification can be done in a non-destructive manner through spectroscopic techniques. Spectroscopy is the study of the absorption and emission of light and other radiation by matter. Everything in our universe has its own spectroscopic footprint which can be seen in the unique signals that are produced from a scan. Still, there’s a bit of a problem: for that footprint to be useful, we need to have a reference. Pigments degrade over time which can cause these identifier signals to shift and change in intensity. Other factors like types of mediums and supports such as papers and wood paneling, etc. can also affect these signals. The more references available to a conservation scientist, the more accurate identification of pigments will be for collection items. Comparing reference scans to scans taken from objects is one way conservation scientists begin to “read” cultural heritage items!

Over the past several months, I have been performing scans on reference pigments using spectroscopic techniques including Fourier Transform Infrared (FTIR), X-Ray Fluorescence (XRF), and Fiber Optics Reflectance Spectroscopy (FORS). These three techniques each have their own advantages: FTIR is useful in identifying organic compounds and their structures, XRF can detect heavy metals, and FORS can accurately identify colorants. All these methods are non-destructive and are typically accessible to other cultural heritage institutions. At the beginning of my internship, I scanned the most common pigments that researchers come across at the Library. These scans provided a baseline characterization of the pigments which would allow researchers to reference them, if needed.

Anna Darden takes a FTIR scan of a pigment swatch card. Photo credit: Meghan Hill, 2025.

Then, I went on and tested for variations between the pigment swatch cards. Each card has 5 replicates so that some remain preserved, and others can be artificially aged to mimic natural aging over hundreds of years. But before aging occurs, it was important to confirm that each replicate had a spectrally consistent baseline to ensure that each card can be scientifically compared to one another. Luckily for the swatch cards, FTIR, XRF, and FORS all confirmed that there was no variation between the pigment swatch cards for each color. The graph below shows a FORS scan of Orpiment in Gum Arabic binding medium. The peaks from the five Orpiment swatch cards are all aligned with no outliers. This indicates that all five swatch cards are chemically the same. Given this conclusion, artificial aging can now be conducted to analyze how the spectra of each pigment changes due to degradation.

Graph of a FORS spectra of Orpiment in which Wavelength (nm) is plotted versus Reflectance. The red line represents swatch card B, the green as swatch card C, the dark blue as swatch card D, the light blue as swatch card E, and the pink as swatch card F.

In addition to these variant tests, I performed baseline scans of more modern (18th and 19th century) pigments from manufacturer sample cards. I also took scans of these pigment swatches on parchment which is another common material found in the Library’s collection.

Anna Darden takes an XRF scan of a pigment on a sheet of parchment. Photo credit: Meghan Hill, 2025.

In between scans of pigment cards, I’ve also had the privilege of witnessing colleagues in PRTD work on collection objects. During my first week, I assisted on scanning a Tibetan Astrological manuscript which has proven to be a very complicated item with lots of different pigments. I’ve also had the chance to see the L’Enfant Map be imaged with multispectral imaging (MSI) and had the opportunity to assist on a few scans of first-edition copies of Ulysses. Even more exciting, was having the opportunity to perform FTIR scans on portraits drawn by Charles de Saint-Mémin of George Washington and Michael Leib.

It’s also been wonderful to know that for some of these objects, my reference scans have already been useful in identifying pigments. Having the opportunity to see these objects up-close and be able to assist and observe the talented researchers in PRTD has been a once in a lifetime experience.

Image of a Tibetan astrological manuscript page being analyzed using XRF. Photo credit: Anna Darden, 2025.

I would like to thank everyone in PRTD for welcoming me and supporting me throughout this project. I’ve learned so much over the past six months, and I am grateful for this opportunity. I’d like to specifically thank my mentor, Meghan Hill, for supplying me with plenty of pigment swatches for scanning, objects to “oo” and “awe” at, and for the running joke of “Don’t tell me you’re already finished.” I have truly enjoyed this experience and am proud to say that I’ve played a small part in helping future conservation scientists more easily

Subscribe to the blog— it’s free! — learn how the largest library in the world preserves the coolest stuff in the world.