Gray metal card cabinets line the walls of the sub-basement of the James Madison Building. In this hallway, history sits cataloged. It’s the old-fashioned way of doing things, the system that the Library of Congress used for most of the 20th century.

But through a pair of doors in that same hallway, a group of scientists in the Library’s Preservation Directorate is taking history into the 21st century.

This department has been working on preserving the country’s most important materials since 1967, and that’s still its main mission today.

But now, the directorate’s scientists can use modern technology not only to maintain artifacts but also to examine them in a way that would have been impossible just a decade ago.

This year, the Library of Congress opened the optical properties lab, which allows scientists to take a closer look at materials and track how different elements may affect their visual properties.

Within this lab sit pieces of equipment that were often designed for other purposes, from a microscope intended for use in the medical field to imaging software originally made for satellites.

But instead of looking at cells or topography, the Library’s scientists are looking for clues about the items’ history, from previous treatments that may have been applied to a document to erased words that aren’t visible to the human eye.

It’s like forensics for the Library’s collection, said Dianne van der Reyden, the Library’s director of preservation.

“These are all mysteries we’re trying to solve,” she said.

The most talked-about of these examinations was the discovery that Thomas Jefferson changed a word in the original draft of the Declaration of Independence. Instead of the word “citizens” in the phrase “our fellow citizens,” Jefferson used the word “subjects.”

Fenella France, the directorate’s researcher who made this finding, used a process called hyperspectral imaging to discover the word. She took pictures of the document with different types of light, from ultraviolet to infrared, then compared the photographs to each other. This allowed her to see markings that have been invisible to the naked eye for more than 200 years.

While the Jefferson discovery has attracted the most media attention, this barely scratches the surface of what the department works on, van der Reyden said.

Other documents the department has examined include the 1791 L’Enfant plan for the design of Washington and the Gettysburg Address. The first showed that civil engineer Pierre L’Enfant had planned where the “President’s House” and “Congress’s House” would be placed in the city. The second revealed fingerprints on the edges of the paper, which scientists believe may have belonged to Abraham Lincoln.

One of the main goals of this type of research is to share it with the academics who have studied these documents for years. By studying the materials, some questions may be answered.

“Scholars are often not aware that there are hidden pieces of information,” van der Reyden said. “They didn’t realize we could do this.”

The six scientists who work on these projects have different backgrounds, since there is no specific training for this kind of job. But it’s important for this type of research to be done because it helps the Library preserve its collection, van der Reyden said. It also sets the precedent for other museums and libraries that are looking to do this type of work on their own collections around the world.

Later this year, the department will continue to expand by opening two other labs, a chemical properties lab and a physical properties lab. Here, scientists will continue their research to better understand how to preserve materials without damaging them.

“We’re really at the beginning of all of this,” van der Reyden said. “It’s cutting edge, and we’re just at the edge.”