Researchers uncovered the deadly tomes by accident. Initially, the professors X-rayed the books to try to determine whether any old Latin texts had been used in the binding. “The library had previously discovered that medieval manuscript fragments, such as copies of Roman law and canonical law, were used to make their covers,” Josh Povi Holck and Kaare Lund Rasmussan wrote in the Conversation. “It is well documented that European bookbinders in the 16th and 17th centuries used to recycle older parchments.”
But their scans revealed something else: The green paint covering the works contained the poison arsenic.
The researchers don't suspect nefarious ends. Arsenic-tinted green paint was common through the 19th century, even after people understood that the element was poisonous to ingest. For a while, arsenic was considered safe enough to use as dye for stamps meant to be licked, and in dresses and ballgowns, according to Atlas Obscura. It was not until the early 20th century that scientists understood arsenic could be dangerous when inhaled and that it could give off a poisonous gas.
The scientists theorize that the poison paint was used to protect the books from insects and vermin.
Other books have been found to be tainted by arsenic. As Atlas Obscura explained: “One of the most dangerous books ever created was meant to warn against exactly this danger. In the 1870s, an American doctor tried to raise awareness of the hazards of arsenic-laced wallpaper by creating a book of potentially poisonous samples and sending it around to libraries. The intent was to help people identify dangerous wallpaper in their homes, not to poison librarians. Today, only four copies of that book still exist, and they’re treated very carefully.”
For now, the books have been given a new home. Each is stored in a separate cardboard box marked with safety labels. The boxes will be stored in a ventilated cabinet. Meanwhile, specialists will work to digitize each book so that researchers won't need to physically handle them.