Secrets that have been hidden in medieval manuscripts for centuries were revealed using two imaging techniques that, when combined, created a brand-new imaging technique.
An interdisciplinary team of Northwestern University researchers combined visible hyperspectral imaging and x-ray fluorescence to develop a new nondestructive imaging technology that revealed 500-year-old text. This technology allows historians to access medieval writings that have been hidden inside ancient book bindings for centuries.
Between the 15th and 18th centuries, bookbinders recycled bindings from medieval parchments into new binding materials for printed books. Scholars have been aware that books from this time period may have hidden fragments of earlier writings, but they have never been able to access them.
"For generations, scholars have thought this information was inaccessible, so they thought, 'Why to bother?'" said Marc Walton, senior scientist at the Northwestern University-Art Institute of Chicago Center for Scientific Studies (NU-ACCESS). "But now computational imaging and signal processing advances open up a whole new way to read these texts."
The book that sparked the start of the study is a copy of Greek poet Hesiod’s Works and Days from 1537. Northwestern purchased this book in 1870, and it is the only copy remaining that has the original slotted parchment binding. The binding is what originally sparked the interest of Northwestern librarians, but the suggestion that there is more writing beneath the parchment of the book board is what sparked new questions.
When Northwestern researchers further studied the binding, they noticed that the bookbinder had tried to remove the writing on the book board through some kind of washing or scraping. Despite this, the book board has retained two ghostly columns of writing surrounded by marginal comments that were still visible through the parchment on the book’s front and back covers.
"The ink beneath degraded the parchment, so you could start to see the writing," Emeline Pouyet, the first author of the study, said. "That is where the analytical study began."
Pouyet and Marc Walton, senior scientist at Northwestern, originally used a visible light hyperspectral imaging technique to view the writing, but this technique didn’t work due to the parchment’s irregular degradation. The techniques made the writing clearer but not clear enough for Richard Kieckhefer, a Northwestern professor and historian, to read. Next, they tried to use x-ray fluorescence imaging with a portable instrument to view the text. This provided information about the ink composition, but the text was still unreadable because of poor spatial resolution.
Walton and Pouyet set out to find a more powerful imaging source. They sent the book to the Cornell High Energy Synchrotron Source (CHESS) in Ithaca, New York. A team there used a bright x-ray source and fast detection system for full imaging of the main text and marginalia comments in the whole bookbinding. Researchers sent the more clearly imaged writing back to Kieckhefer, who immediately recognized the 6th-century Roman Law code with interpretive notes referring to Canon Law in the margins.
Walton and Pouyet believe that the parchment may have been originally used in a university where Roman Law was studied as the basis for understanding Canon Law. This was common practice in the Middle Ages. It is possible that the writing was covered and recycled because the information was outdated. At the time, society had struck down the Roman law and started to implement the church code.
"When you have the right tool, the analysis is a lot easier," Walton said. "But the problem is that you can't always bring priceless books to an often out-of-reach synchrotron beamline. We wanted to be able to use our lab-based instruments to do this sort of work."
The team then contacted Northwestern computer science professors Dr. Aggelos Katsaggelos and Dr. Oliver Cossairt to help explore new ways to image the book.
"We had to develop new methods of doing the analysis that we wouldn't otherwise have had," Walton said. "There's nothing that exists off-the-shelf that we can use to simply image this book and read the hidden writing."
"There is a vast number of wavelengths in the electromagnetic spectrum, and each wavelength has its advantages and disadvantages," said Katsaggelos, the Joseph Cummings Professor of Electrical Engineering and Computer Science. "Some of them can penetrate deeper into the specimen; some of them have better resolution, and so on."
Katsaggelos used a machine-learning algorithm developed by his team and discovered the best way to view the text is to use a fusion of two imaging techniques. His team combined visible hyperspectral imaging, which uses wavelengths within the visible light spectrum to provide high spatial resolution, and x-ray fluorescence imaging, which provides high-intensity resolution. The algorithm told the researchers the relative contribution of each modality in order to produce the best image. This combination allowed the team to successfully read the cover of the book.
Katsaggelos’ data fusion image was so clear it rivaled an image of the main text that was produced by the powerful x-ray beams at CHESS. The mystery of Hesiod’s "Works and Days" bookbinding may have been solved, but the work doesn’t stop there.
"We've developed the techniques now where we can go into a museum collection and look at many more of these recycled manuscripts and reveal the writing hidden inside of them," Walton said. "This is really the start of a much larger initiative."
This research was published in the journal Analytica Chimica Acta.