An in-depth study by the Politecnico di Milano, published in Scientific Reports, has uncovered the cause of a number of black spots that appeared several years ago on the Codex Atlanticus, one of the most extensive and fascinating collections of Leonardo da Vinci’s sketches and writings.
The stains particularly affect the modern panel – known technically as the passepartout – that binds and frames Da Vinci’s original papers in the codex preserved at the Veneranda Biblioteca Ambrosiana library in Milan. The interdisciplinary research team coordinated by Lucia Toniolo, Professor of Material Science and Technology at the Politecnico di Milano, used a series of non-invasive and micro-invasive analysis techniques to examine and understand the nature and causes of the phenomenon of blackening that has been observed on some 210 pages of the Codex since 2006 and has caused great concern among museum curators and conservators, as well as scholars.
The research by the Politecnico di Milano focused on folio 843 of the Codex and, combining hyperspectral photoluminescence analysis, UV fluorescence imaging, with micro-ATR-IR imaging, revealed the presence of starch glue and vinyl glue in the areas where the staining is most concentrated, right at the edge of the folio.
In addition, the presence of rounded inorganic nanoparticles composed of mercury and sulphur was detected within the cavities formed between the cellulose fibres of the passepartout paper. These particles were identified as metacinnabar, a mercury sulphide in an unusual black crystalline phase.
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 linked to the addition of an anti-vegetative salt to the glue mixture used during the restoration of the Codex in the 1960s and 70s, which could have been applied in only certain areas of the passepartout paper, precisely where it holds Da Vinci’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 (sulphur dioxide (SO2) levels were very high in Milan in the 1970s) 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.
