Researchers finally determined what is causing the mysterious black stains to appear on the original writings of Leonardo da Vinci.

Politecnico di Milano recently conducted an in-depth study on a particular folio of the Codex Atlanticus, one of the largest collections of Leonardo da Vinci’s drawings and writings. This study aimed to understand the origins of black stains that appeared on the modern passepartout binding Leonardo’s original folios.

The Codex Atlanticus, donated to the Veneranda Biblioteca Ambrosiana in 1637, was the subject of a major restoration carried out by the Laboratorio del Libro Antico (Laboratory for the Restoration of Ancient Books) of the Abbey of Grottaferrata between 1962 and 1972. The intervention ended with the production of 12 volumes with 1119 folios: each page has a passepartout with a panel (added by the restorers in Grottaferrata) framing Leonardo’s original fragments. Since 1997, the Codex has been stored in an environment with a strictly controlled microclimate, in accordance with paper conservation standards.

The interdisciplinary research team used non-invasive and micro-invasive analysis techniques to investigate the phenomenon and study its nature and causes. The research 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. Additionally, the presence of round inorganic nanoparticles made up of mercury and sulphur was detected, which had accumulated within the cavities formed between the cellulose fibers of the passepartout paper. Finally, using synchrotron analysis, conducted at ESRF in Grenoble, the particles were identified as metacinnabar, a mercury sulphide in an unusual black crystalline phase.

The presence of mercury could be associated with the addition of an anti-vegetative salt in the glue mixture used in Grottaferrata’s restoration techniques, which could have been applied only in certain areas of the passepartout paper, precisely where it holds Leonardo’s folio, to ensure adhesion and prevent microbiological infestations on the Codex. The presence of sulphur, on the other hand, has been linked to air pollution (in Milan in the 1970s, levels of sulphur dioxide SO2 were very high) or to the additives used in the glue, which over time would have led to a reaction with mercury salts and the formation of metacinnabar particles, responsible for the black stains.