3 books from a Danish library could poison their readers

Credits: webandi / Pixabay

We feel like we’re in the middle of the story The Name of the Rose. In this novel by Umberto Eco, a poisoned book contaminates anyone who reads it. However, this time, it’s a true story. 

The three books in question were found in the university library of the University of South Denmark. Their covers contained large quantities of arsenic. These books date back to the 16th and 17th centuries, and they are all history books.

Researchers were able to identify the poisoned books by running a series of tests, with the help of fluorescent X-rays. The rays were able to detect the hidden poison. 

This technique is widely used in the fields of archaeology and art, to look for chemicals that make up paints or colours.

How did these books attract the attention of the scientists?

These books were passed through X-rays because the librarians discovered that the parchment used to make the covers of the books came directly from the Middle Ages. And yes, even in the 16th century, people knew how to recycle (which was mainly a case of sparing resources at the time). Furthermore, the covers featured extracts from Roman law and canon law. 

The scientist thus wanted to read these direct testimonials from 1,000 years ago. But a layer of paint had been painted over them. The books were therefore put through X-rays to try and read the letters and be able to decode the inscriptions. The researchers wanted to separate the chemical compounds of the ink from the rest, in order to highlight the letters on the paper.

A deadly green pigment

The analysis revealed that the paint contained arsenic, one of the most toxic elements in the world. Daily exposure can lead to various symptoms and can cause cancer. The scientists believe that it was used in the form of copper acetate. 

mains symptome arsenic
Skin reaction on hands exposed to arsenic. Credits: Anita Ghosh/ reachwater (CC 2,0)

This compound was commonly used for multiple purposes, and mostly as a pigment: the size of the seeds influence the colour. The largest seeds produce a dark green, while the smallest seeds produce a lighter green. The pigment is best known for its intensity and its resistance to discolouration.

Industrial production of the pigment began in Europe in the early 19th century. The impressionist and post-impressionist painters used different versions of it to create their works. This indicates that many works displayed in museums contain this poison. 

At its peak, all types of material, even book covers and clothes, could be covered in Paris green for aesthetic reasons. Continual skin contact with the substance can lead to significant symptoms.

However during the second half of the 19th century, the toxic effects of the substance became better known. Copper acetate stopped being used as a pigment. It was still used as an insecticide and fungicide on agricultural lands and in gardens up until the middle of the 20th century.

In the case of these books, its use was not aesthetic, as the pigment is located in the bottom layers of the parchment. It is thought to have been used on the covers for its ability to repel insects.

The library is keeping the books

In order to conserve the works without risking poisoning the readers, the books have been placed in a ventilated cupboard (because of the toxic gas that could form). What’s more, digital versions of the books are going to be made in order to allow students and readers to continue to consult the works.


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