Don’t lick your hands: Arsenic pigments used in 16-17th c. bookbinding

Original article: Poisonous books: analyses of four sixteenth and seventeenth century book bindings covered with arsenic rich green paint 

Living during the time of a global pandemic we have heard health guidelines to wash our hands regularly and to not touch our faces, especially when in public. Based on the findings in this paper, it’s also a good policy to maintain in libraries, museums, and archives with rare, historic books. 

Delby et al. investigated four books made in the 16–17th century, which are a part of the library collections at the University of Southern Denmark and the Smithsonian Institution. These books were initially selected due to the use of recycled 12th century Latin manuscripts as part of the bookbinding process. In order to penetrate the green paint layer found on the covers of the books and read the underlying text from the older manuscripts, the researchers performed micro x-ray fluorescence (μ-XRF), an elemental analysis technique, and were surprised to find arsenic in the upper paint layer. This discovery led to a full-scale investigation into the source of arsenic as well as what risks the books potentially present to researchers and librarians who may handle the books, since arsenic and arsenic-based compounds are known to be toxic for humans when ingested. 

Samples of the green pigment were cut from the inside covers of the books for exploring the potential arsenic presence. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used on cross sections of these samples to determine the spatial distribution of the elements identified with the μ-XRF. Figure 1 shows one of the book samples and elemental maps to correlate the distribution of elements with the sample. Arsenic found in the samples was localized to the green paint layers of the books. Fourteen other elements were observed and attributed to either the preparation of the parchment (sodium, magnesium, calcium, aluminium, strontium) or as trace elements from the pigment preparation (copper, lead, mercury, silver, barium, gold, iron, rubidium, antimony).

Cross section of a sample from a 17th century book cover with LA-ICP-MS elemental maps shown below.
Figure 1. Sample from the book KLR-11972 Herlufsholm 186.6. The top image is an optical photo of the sample cross section taken in polarized light. The images below show the corresponding LA-ICP-MS data, specifically how each element of interest maps back onto the sample.

Another prominent element seen with μ-XRF was sulfur, making a potential candidate for the arsenic source in the green paint the yellow mineral orpiment (As2S3) mixed with a blue colorant. X-ray diffraction and Raman spectroscopy confirmed the presence of orpiment, and Raman and mass spectrometry were used to confirm the presence of the organic blue dye indigo (C16H10N2O2) in the green paint layers. This combination was a common mixture for a green paint during medieval times, also being seen in Japanese woodblock prints and Egyptian mummies. A previous bite highlights arsenic-based pigments and discusses chemical and photodegradation observed in some Dutch paintings.

Confirming the components of the green pigment was important, but how do these results translate to the safety of the librarians handling and housing the books in their institutions? Most of the danger associated with orpiment comes from reactions with water to form arsenic and arsine.  Since the main sources of exposure would come from interacting with the books, air and wipe samples were collected by librarians handling two orpiment-containing books in a chemical fume hood to investigate potential arsenic and arsine exposure. Air samples were collected to measure the ambient conditions and the personal exposure of the librarian. Wipe samples from the blotter paper used underneath the books and from the gloved hands of the librarian determined if arsenic could be easily transferred to another surface. 

The air samples showed arsenic exposure levels were below the Personal Exposure Level set by OSHA and the Recommended Exposure Levels (REL) of ACGIH and NIOSH, but the wipe samples showed arsenic levels above the REL of 387 μg/m2. Based on this finding, the recommended handling practices include placing blotting paper underneath the book to minimize surface transfer and the use of gloves and washing hands after handling the books. The books are also stored in boxes with warning labels in well-ventilated areas of the libraries to prevent the reaction of the orpiment with ambient moisture. 

This investigation into the toxicity of these 16–17th century books highlights the importance of object characterization and assessment, especially when the objects have potentially harmful components present. The researchers are using these results to begin investigating some underlying art history questions, including why this particular combination of orpiment and indigo was used to create a green paint and whether it was applied by a single person or multiple people at a bookbinding workshop. To learn more about the confirmation and potential toxicity of this pigment palette, read the paper or check out this blog or video from the researchers.  

All figures were adapted from the article by Delby et al. under the terms of the Creative Commons Attribution 4.0 International License.

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