Folio 843 of Codex Atlanticus, Veneranda Biblioteca Ambrosiana, Milan
Key points:
- Black stains appeared a few years ago on the modern passepartout that binds Leonardo da Vinci's original folios.
- Researchers detected inorganic nanoparticles made of mercury and sulfur.
- They have multiple hypotheses as to how the stains formed, including salt in the restoration glue and air pollution in Milan.
In 2017, small black stains appeared on the passepartout that binds the Codex Atlanticus, one of the most extensive collections of Leonardo da Vinci's drawings and writings. As described in a new study published in Scientific Reports, researchers recently used a series of non-invasive and micro-invasive analysis techniques to investigate the phenomenon and study its nature and causes.
The blackening, observed on about 210 pages of the Codex from folio 600 onwards, has caused great concern among museum curators and scholars.
Previous studies had ruled out that the stains resulted from microbiological deterioration processes. Research at the Politecnico di Milano began in 2021 during an initial pilot project on three Codex drawings, which included the removal and replacement of the passepartout of folio 843.
Combining hyperspectral photoluminescence imaging and UV fluorescence imaging with micro-ATR-IR imaging revealed the presence of starch glue and vinyl glue located in the areas where the staining is most concentrated, right near the edge of the folio.
In addition, the researchers found inorganic nanoparticles about 100 to 200 nanometers in diameter made up of mercury and sulfur, which had accumulated within the cavities formed between the cellulose fibers of the passepartout paper. Using synchrotron analysis, the researchers identified the particles as metacinnabar, a mercury sulfide in an unusual black crystalline phase.
The researchers say the presence of mercury could be associated with the addition of an anti-vegetative salt in the glue mixture used in the 2009 restoration techniques. The presence of sulphur, on the other hand, has been linked to air pollution—in 1970s Milan levels of sulphur dioxide were very high. Alternatively, the researchers say the black stains could be the result of the additives used in the glue, which over time would have led to a reaction with mercury salts and the formation of metacinnabar particles.