The source of a mysterious black staining developing on what experts have called the “most important” collection of works by Leonardo da Vinci has been identified. The preservation of the Codex Atlanticus — which is housed in the collections of the Biblioteca Ambrosiana in Milan, Italy — represents a great challenge for scholars, with the recent stains a worrying development. Researchers analysed folio 843 of the Codex, and traced the staining to nano-sized particles of an uncommon mercury and sulphur mineral.
The team believes this may have originated from glue used to attach Leonardo’s original work to the paper mounts used to frame them some five decades ago.
The Codex Atlanticus underwent a major restoration between 1962 and 1972 at the Laboratory for the Restoration of Ancient Books of the Abbey of Grottaferrata, Italy.
At the end of this preservation effort, the collection was split into 12 volumes containing 1,119 individual folios, or leaves of paper.
Each page of the restored Codex features a panel — known technically as a “passepartout” — framing the original fragments of Leonardo’s works.
Since 1997, the collection has been stored in a precisely controlled environment designed to ensure the conservation of the papers.
Despite this, however, small black stains were found in 2006 on some 210 of the passepartouts of the Codex Atlanticus, from folio 600 onwards.
As experts at the Politecnico di Milano in Italy note: “This phenomenon of blackening […] has caused great concern among museum curators and scholars.”
In an effort to reduce the risk to the Codex of the stains spreading, the volumes were unbound in 2009, with the individual passepartouts now kept in folders in acid-free boxes.
Previous studies of the stains had ruled out the possibility that they were caused by microbiological deterioration.
In their new study, materials scientist Professor Lucia Toniolo of the Politecnico di Milano and her colleagues conducted a series of both non-destructive and only micro-invasive analyses on the Codex Atlanticus to determine the cause of the stains.
Their investigation began in 2021 with an initial pilot study on three of the Codex’s drawings, which included the removal and replacement of the passepartout surrounding folio 843.
Analysis of this page revealed that both starch and vinyl glue can be found in areas where the black staining is the most concentrated, right near the edges of the folio.
In addition, the team found round, inorganic, nanoparticles — each around 100–200 nanometres in diameter — that had accumulated between individual cellulose fibres in the paper of the passepartout.
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Analysis of the nanoparticles at the European Synchrotron Radiation Facility in Grenoble, France revealed that they were made out of metacinnabar — a mercury sulphide that exists in an unusual black crystalline phase.
The researchers explained: “In-depth studies on paper preservation methods have allowed us to formulate some hypotheses on the formation of metacinnabar.
“The presence of mercury could be associated with the addition of an anti-vegetative salt in the glue mixture used in the Abbey of Grottaferrata’s restoration techniques.”
The reason that the staining is only found on certain parts of the passepartout paper — specifically, where it holds Leonardo’s folio — is because this is where the glue was applied to hold the original paper and prevent microbiological infestation of the Codex.
The researchers added: “The presence of sulphur, on the other hand, has been linked to air pollution — in Milan in the 1970s, levels of sulphur dioxide were very high.”
Another hypothesis involved the additives used in the glue.
Over time, the team explained, this “would have led to a reaction with mercury salts and the formation of metacinnabar particles, responsible for the black stains.”
The full findings of the study were published in the journal Scientific Reports.
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