The Preservation Research and Testing Division (PRTD) is equipped with many wonderful analytical tools to investigate the Library’s vast collection. Material identification is key for preservation; knowing what materials make up a particular object will guide preservation policies and treatments. PRTD has several different laboratories, including a non-invasive optical properties lab, which is generally where collections items come to be examined. The optical properties lab includes a wide range of spectroscopic instrumentation, imaging techniques, and microscopes. More recent spectroscopic instrumentation purchases have included relatively small and portable instruments to supplement the larger, benchtop instruments typically associated with scientific analyses. Portable equipment makes it possible to take analyses to the collection items, wheeling our instruments to the Rosenwald room, Kislak collection, secure storage vaults, conservation, Jefferson galleries, and even to the labs of our colleagues at other institutions.
In general terms, optical spectroscopy encompasses a range of techniques that involve bouncing light off items or areas of interest and collecting the light that is reflected back to a detector in the instrument. Spectroscopy aids in material identification because, depending on the color (or wavelength) of the light, different information can be gleaned from the light that bounces back. As an example, imagine a piece of paper painted with a red spot. Illuminating the red spot with x-rays gives information about the metallic elements in the paint, infrared light can identify the molecular vibrations in the paint’s chemical bonds, while ultraviolet and visible light highlights the electronic structure of the paint’s chemical compounds. In other words, these various spectroscopic techniques can tell us almost everything we want to know about this red paint. To illuminate this example further, let’s pretend our sample is vermilion painted in animal glue on a piece of paper (and also pretend you don’t know what the red paint is, but you desperately want to). X-ray fluorescence will reveal the presence of mercury and sulfur, infrared light will find the nitrogen and hydrogen bonds that make up the protein portion of animal glue, as well as all the carbon and water peaks associated with a wood-based paper, and ultraviolent and visible light will be able to measure a distinct bandgap for vermilion, as it is a semi-conductor pigment. In other words, we would have identified the pigment and its binding medium without physically sampling the paint on the object.
Another form of optical spectroscopy that we use to look at dyes, pigments, paints, etc. is Raman spectroscopy. In Raman spectroscopy, the light doesn’t exactly bounce back to the detector, but rather scatters off of the chemical bonds in the samp