The culprits were mercury salts added in a previous restoration to ward off microbiological attacks, and sulfur, linked to Milan's atmospheric pollution or to additives used in glue.
The findings have been reported in a study by Milan's Politecnico University, published in Scientific Reports.
The Codex Atlanticus, donated to the Veneranda Biblioteca Ambrosiana library of Milan in 1637, was the subject of a major restoration carried out by the Ancient Books Workshop of the Abbey of Grottaferrata between 1962 and 1972.
The intervention ended with the creation of 12 volumes made up of 1,119 pages, to which were added some passe-partouts (glass frames) to frame the fragments.
It was in fact on these areas of around 210 pages that small stains were noted in 2006.
The restoration led by Lucia Toniolo began in 2021 and detected the presence of inorganic particles composed of mercury and sulfur, accumulated among the cellulose fibres of the passe-partouts.
The presence of mercury may be associated with the addition of an anti-vegetative salt in the mixture of glue used in the restoration at Grottaferrata, while the sulfur has been linked to atmospheric pollution or to additives used in the glue which, over time, may have triggered a reaction with the mercury salts and the formation of the stains.
ANSA
