Researchers at the University of Southern Denmark were struggling to make sense of medieval manuscript fragments detected within the covers of three tomes dating back to the 16th and 17th centuries when they made a deadly discovery—poison, lurking in the guise of an emerald green pigment.

In an article published on The Conversation, research librarian Jakob Povl Holck and associate professor Kaare Lund Rasmussen explain that they had decided to scan the books using a series of x-ray fluorescence analyses in hopes it would enable them to read the recycled Latin, which was obscured beneath a layer of green pigment.

Instead, the analysis revealed that the layer of green was saturated in arsenic, a highly toxic substance known to irritate the stomach, intestines and lungs, as well as induce nausea, diarrhea and skin changes. The specific type of arsenic evident on the covers is known as Paris green or emerald green, as its intense hue is similar to that of the eponymous gemstone.

Prior to but particularly during the Victorian era, arsenic was shockingly commonplace. Hyperallergic’s Allison Meier reports that in addition to acting as a key ingredient in green paint and dye, the pigment was used in cosmetics, children’s toys and wallpaper. The Telegraph’s Lucinda Hawksley adds that individuals even ate vegetables sprayed with arsenic-laced insecticides, or meat dipped in the poison in an effort to stave off flies.

Although the Victorians were vaguely aware of arsenic’s poisonous propensities, many erroneously believed they would only be affected if they directly consumed it. One family learned this lesson the hard way, Meier adds, referencing an 1862 incident in which children from an east London home died after ingesting pigments found on the family’s wallpaper.

According to Atlas Obscura’s Sarah Laskow, the culprits behind these arsenic incidents were microscopic particles released by the tainted paint and snuck into unsuspecting individuals’ lungs. The pigment is also capable of emitting a poisonous gas, which in high doses, can cause death by cellular failure.

“A great deal of slow poisoning is going on in Great Britain,” Birmingham doctor William Hinds wrote in 1857, as widespread coverage of arsenic-related deaths began to turn the public away from the toxin. To this day, Britain has no laws officially prohibiting the use of arsenic to color wallpaper; however, following the realization that the pigment could, in fact, kill, Victorian tastes shifted away from the bewitching yet deadly dyes.

Unlike the Victorians and their penchant for interior decorating, Holck and Rasmussen don’t believe the pigment found in their volumes was used for aesthetic purposes. Instead, they propose that the arsenic may have been applied during the 19th century as a method of warding off insects and vermin.

To minimize physical handling and open up the unique tomes for research opportunities, the library says it plans to digitize the volumes. The physical books, meanwhile, will be housed inside of a ventilated cabinet, where each will be safely stored in separate cardboard boxes, carefully marked with safety warnings.