Spectral analysis is usually thought of in relation to remote sensing data, but the Library of Congress Preservation Research and Testing Division (PRTD) has developed an application of hyperspectral imaging to analyze and preserve cultural heritage materials. “We knew ENVI was an incredibly powerful piece of software for extracting information from satellite and aerial imagery,” said Fenella France, chief of the PRTD. “We were interested in seeing what else it could do.”

Working primarily in ENVI, France and her team adapted spectral imaging tools to create noninvasive, nondestructive analytical techniques. “We’ve been able to identify and characterize colorants, inks, and paper or parchment; monitor deterioration or changes due to exhibit and other environmental conditions; assess and potentially identify previous treatments that modify chemical and spectral responses of cultural heritage materials; and detect watermarks or previous conversations on paper – all without sampling.”

France and her team use spectral imaging as a forensic-type tool on historical documents with fascinating results. Thomas Jefferson’s handwritten draft of the Declaration of Independence is one such document. “Jefferson wrote this first draft with amendments from Benjamin Franklin and John Adams, and indeed spectral imaging revealed a number of changes to the draft,” says France. There was one obvious ‘‘smudge’’ of the under-text that hinted to the presence of a previous word where the overwrite seemed to be an attempt to expunge the original writing. “This overwrite is the only location on the entire document where a hasty change seems to have been made,” said France. “Everywhere else in the draft changes are neatly crossed out and clear insertions are made, and if they were made by someone other than Jefferson, he carefully noted it in the margin.”

“Image processing of this region of the draft proved challenging since the document was laminated and the inks were spectrally very similar in composition,” said France. “We finally were able to reveal that Jefferson had originally written "fellow-subjects" and changed this to “fellow-citizens.”

Spectral imaging detects a change on the Declaration of Independence

The Library of Congress recently digitized Alexander Hamilton’s papers, and the PRTD once again got to do some sleuthing with spectral imaging. One letter that Hamilton had written to his fiancée, Emily Schuyler, had an entire paragraph crossed out. With the spectral imaging and principal component analysis in ENVI, they were able to bring out the under-text and read what he had originally written.

“It was a very sweet paragraph that Hamilton had written to her that was perhaps a little risqué for the times. The redacted paragraph said, ‘for my Betsy loves me and is acquainted with all the joys of fondness,’” said Meghan Wilson, a preservation specialist with PRTD. “This hints at the couple having had premarital relations, and the heavy redactions were made by their son after Hamilton’s death.”

Original text is revealed through spectral imaging in letter from Alexander Hamilton

France and her team also examined the Gettysburg Address. “The second page is on rough paper, and we were able to pick up fingerprints – a thumb on the front and three fingerprints on the back,” says France. “I reached out to colleagues in forensics who steam documents to pull off fingerprints and they were fascinated that we could pull fingerprints, using ENVI.” The team suspects the fingerprints belong to Lincoln and are waiting for confirmation through partial-print analysis software in combination with comparing them to prints found on his other documents (Emancipation Proclamation, inaugural addresses, etc.).

As well as providing a powerful analytical tool for noninvasive research, spectral imaging can detect changes in cultural heritage objects due to the environment, treatments, and other factors. “The goal is foremost to preserve the original object, since the recovery of obscured information and spectral characterization is not possible unless it has been preserved,” said France.