How did the Dead Sea Scrolls—documents written on parchment, papyrus and bronze— survive almost 2,000 years in caves near the Dead Sea? Researchers have been intrigued, in particular, by one document called the Temple Scroll, a 25-foot-long parchment that still maintains a bright white surface. Now, a new study of the scroll is elucidating some of the methods that kept it intact for millennia.

Back in 1947, a Bedouin shepherd searching for a lost sheep found a series of caves in the limestone cliffs above the Dead Sea near Qumran. Inside, he came across clay jars in which a handful of scrolls had been stuffed. In the following decades, further excavations in the area uncovered around 900 scrolls written in Hebrew, Aramaic and Greek, dating from the 3rd century B.C. to the 1st century A.D. Some scrolls contained content from the Bible while others were non-biblical religious texts. It’s believed the scrolls were hidden in the caves by a sect called the Essenes, to protect the texts from destruction by the Roman Empire, though that theory has recently come under scrutiny.

To understand how the Temple Scroll survived all those years, a research team was given access to a 1-inch fragment of the parchment—itself just 1/250th of an inch thick—from The Shrine of the Book, a museum in Jerusalem that holds the scroll. Besides being super long, the scroll is unusual in several ways, Nicola Davis at The Guardian reports: The text is written on the flesh side of the skin, which is uncommon. The thin parchment may be an animal skin that has been split in two. And the text is written on a thick layer containing lots of inorganic minerals pressed into the collagen.

The team tested the parchment’s chemical composition and mapped it in high resolution using specialized techniques. “These methods allow us to maintain the materials of interest under more environmentally friendly conditions, while we collect hundreds of thousands of different elemental and chemical spectra across the surface of the sample, mapping out its compositional variability in extreme detail,” coauthor James Weaver of the Wyss Institute at Harvard University explains in a press release.

What they found were some unexpected chemicals, in particular salts that do not come from the Dead Sea region. The paper appears in the journal Science Advances.

In ancient times, parchment was made from animal hide that had the hair and tissue removed through enzymatic treatments before it was scraped down and stretched. After it dried, the hide was sometimes prepped further using salts. While other scrolls from the region were prepared with salt derived from the Dead Sea, the proportion of sulfur, sodium, and calcium on the Temple Scroll don’t match salt from the area, meaning the salt came from elsewhere, though the team does not yet know where. Maria Temming at Science News reports that the scroll also contains gypsum, glauberite and thenardite, which aren’t found in the area either. “Sometimes you find a lot of inorganic components on these scrolls or fragments, and they probably came from the caves,” says coauthor Admir Masic, an MIT research scientist. In this case, however, the minerals were not present in the caverns.

Co-author Ira Rabin of Hamburg University in Germany tells Davis that while the mineral coating is unusual, it’s consistent with the western tradition of parchment preparation in which parchments are untanned or lightly tanned. In the eastern tradition, parchment hides are completely tanned. Temming reports that a similar coating was found on a few other Dead Sea Scrolls as well, meaning that it’s possible that the prepared parchment was being imported into the area.

Figuring out how the parchment was made will not only shed some light on the Temple Scroll; it could also help document researchers of all sorts spot forgeries and aid in document conservation. “This study has far-reaching implications beyond the Dead Sea Scrolls. For example, it shows that at the dawn of parchment making in the Middle East, several techniques were in use, which is in stark contrast to the single technique used in the Middle Ages,” Rabin says in the press release. “The study also shows how to identify the initial treatments, thus providing historians and conservators with a new set of analytical tools for classification of the Dead Sea Scrolls and other ancient parchments.”

The most important finding to come out of the study, however, is the confirmation that the mineral coatings on the parchment are hygroscopic, meaning that they easily absorb moisture from the air. That moisture can degrade the underlying parchment, meaning conservators need to pay special attention to humidity levels, since even small fluctuations could cause a scroll that survived the last 2,000 years of history in a forgotten jar to finally crumble inside a high-tech museum.