Greg Bearman et al: The Application of Spectral Imaging for monitoring changes in parchment from the environment and conservation treatments.

Parchment readily responds to the local environment and under goes a variety of changes, mostly due to humidity and temperature. These can drive two chemical reactions, denaturation and hydrolysis, that can damage collagen, the major parchment component. In addition, various conservation treatments may induce changes, some perhaps desired and others not.

We present an imaging methodology that allows the conservator to image and map out changes in a manuscript and identity local areas of that require a closer look. The approach is a combination of spectral imaging and Chemometrics and is being developed for monitoring of the Dead Sea Scrolls as part of the Leon Levy Digital Dead Sea Scrolls Library. Spectral imaging acquires the spectrum of each pixel in the image, which can be used by a wide range of analysis methods and software, many of them developed by NASA for the space program. Chemometrics is a multivariate analysis method that lets us look at across the spectra of the millions of pixels in each image and look for changes.

The Dead Sea Scrolls have been studied with spectral imaging, which has explained the differences in legibility that are typical of the scrolls. For these texts (and many others) the contrast between the ink and parchment is a function of the wavelength. For those easy to read scrolls, the ink and parchment in the visible have very different reflectance, so the eye sees contrast. For those illegible scrolls, it turns out that the parchment is much less reflective in the visible, so there is little contrast between the ink and parchment. These spectral contrast differences naturally suggest a means of looking for changes in a scroll; measure the parchment reflectance spectra with time and look for changes.

These changes in parchment and reflectance have been demonstrated for a wide range of parchment and papyrus documents as well as artificially aged samples. Our monitoring method is that we measure the reflectance with spectral imaging and then use Chemometrics to automatically interrogate the image dataset and identify areas of spectral change, which are correlated with underlying chemical or structural collagen changes. This approach puts a significant burden on the spectral imaging method, as it has to provide reliable and repeatable quantitative absolute reflectance of the parchment. Without that, it is not possible to compare parchment reflectance over time. Recent developments in technology have made acquisition of spectral imaging data easier and less expensive and we are taking advantage of those improvements. Quantitative imaging also allows for the comparison across collections, multiple holding institutions of distributed texts or from different institutions and groups.

We will show results of our method on artificially aged parchment, provide details on the system noise and performance. In addition to spectral imaging, we have used a variety of physical and chemical analysis methods in an attempt to link the spectral changes to chemical changes. Understanding the reason behind the spectral changes in critical to the conservator who has control over storage and usage environments.

Our method is well suited to “before” and “after” imaging of a text or objects during conservation treatment. It provides a wealth of information on the object at all times. By extension, the method is applicable to a wide range of cultural heritage objects; texts, frescoes, wood panel painting, art and textiles, to name a few. In addition to the conservation monitoring aspects, as a side benefit the imaging technology produces exceptionally accurate color images, significantly better than any standard Bayer filter camera on the market. This is due to the fact that we sample the visible spectrum at more wavelengths and use color standards to calibrate the color system.

The work reported was supported by The Leon Levy Dead Sea Scrolls Digital Library, a project of the Israel Antiquities Authority, headed by Pnina Shor and sponsored by the Leon Levy Foundation and the Arcadia Fund.