Scientists have discovered the chemical culprits behind stains that appeared in 2006 on some pages of the Codex Atlanticus, one of the most extensive and fascinating collections of drawings and writings by Leonardo da Vinci: 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.